U.S. patent application number 11/286103 was filed with the patent office on 2006-08-31 for monitoring.
This patent application is currently assigned to BIL Solutions Limited.. Invention is credited to Christopher Henry Orr.
Application Number | 20060193421 11/286103 |
Document ID | / |
Family ID | 33548719 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060193421 |
Kind Code |
A1 |
Orr; Christopher Henry |
August 31, 2006 |
Monitoring
Abstract
The method includes providing the volume of waste in a
monitoring space; providing a support, the support being provided
with a plurality of detectors for radioactive material; monitoring
the volume of waste for radioactive material, in one or more parts,
to give a monitoring result; and correcting the monitoring result
for geometry and/or attenuation to give a corrected result using a
correction factor. The correction factor is obtained by a method
that includes providing a simulation of an equivalent volume of
waste free of radioactive material in an equivalent monitoring
space, with equivalent detectors; providing a known activity
radioactive source at one or more positions in the volume of waste;
determining the detector response to the source in a position, a
comparison of the response and known activity contributing to a
correction function for that equal volume, one or more such
correction functions providing the correction factor.
Inventors: |
Orr; Christopher Henry;
(Cumbria, GB) |
Correspondence
Address: |
WORKMAN NYDEGGER;(F/K/A WORKMAN NYDEGGER & SEELEY)
60 EAST SOUTH TEMPLE
1000 EAGLE GATE TOWER
SALT LAKE CITY
UT
84111
US
|
Assignee: |
BIL Solutions Limited.
|
Family ID: |
33548719 |
Appl. No.: |
11/286103 |
Filed: |
November 22, 2005 |
Current U.S.
Class: |
376/156 |
Current CPC
Class: |
G01T 1/167 20130101;
Y02E 30/30 20130101 |
Class at
Publication: |
376/156 |
International
Class: |
G21G 1/00 20060101
G21G001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 23, 2004 |
GB |
0425736.6 |
Claims
1. A method of monitoring a volume of waste for radioactive
material, the method comprising: providing the volume of waste in a
monitoring space; providing a support, the support being provided
with a plurality of detectors for radioactive material; monitoring
the volume of waste for radioactive material, in one or more parts,
to give a monitoring result; and correcting the monitoring result
for geometry and/or attenuation to give a corrected result using a
correction factor; wherein the correction factor is obtained by a
method comprising: providing a simulation of an equivalent volume
of waste free of radioactive material in an equivalent monitoring
space, with equivalent detectors; providing a known activity
radioactive source at one or more positions in the volume of waste;
and determining the detector response to the source in a position,
a comparison of the response and known activity contributing to a
correction function for that equal volume, one or more such
correction functions providing the correction factor.
2. A method according to claim 1 in which the simulation is a
computer simulation.
3. A method according to claim 1 in which the simulation is a
physical simulation.
4. A method according to claim 1 in wherein the correction factor
is obtained by a method comprising: providing an equivalent volume
of waste free of radioactive material in an equivalent monitoring
space, with an equivalent support provided with equivalent
detectors; dividing in one or more parts, the volume of waste into
a plurality of equal volumes; providing a known activity
radioactive source at one or more positions in one or more of the
equal volumes; and monitoring the detector response to the source
in a position in one of the equal volumes, a comparison of the
response and known activity contributing to a correction function
for that equal volume, one or more such correction functions
providing the correction factor.
5. A method according to claim 4 in which one of the equal volumes
is a core equal volume, the core equal volume being that volume to
which the detectors are least sensitive.
6. A method according to claim 4 in which the other equal volumes
are defined in layers around the core equal volume.
7. A method according to claim 4 in which the core equal volume is
defined in terms of the volume bounded by a given minimum distance
from all the detectors.
8. A method according to claim 5 in which the other equal volumes
are defined as that volume which is all between a first given
minimum distance and a second given minimum distance from all the
detectors, the first and second distances varying for the different
equal volumes.
9. A method according to claim 4 in which the shape of the equal
volumes is defined relative to that location separated from all of
the detectors by the greatest distance.
10. A method according to claim 9 in which the greatest distance is
the greatest distance through the waste and/or container
therefor.
11. A method according to claim 9 in which the equal volume
containing that location is the core equal volume.
12. A method according to claim 4 in which the method is repeated
with a number of different known activity sources.
13. A method according to claim 12 which includes the use of a
Cs137 source and a Co60.
14. A method according to claim 4 in which the positions are evenly
distributed throughout an equal volume.
15. A method according to claim 4 in which between 5 and 20
positions are provided for each equal volume.
16. A method according to claim 4 in which one or more of the
positions are at locations equidistant from the boundary of the
equal volume with the next equal volume out and the boundary of the
equal volume with the next equal volume in.
17. A method according to claim 4 in which one or more of the
positions are at locations equidistant from the boundary of the
equal volume with the next equal volume out and the location with
the greatest minimum distance from all detectors.
18. A method according to claim 4 in which one or more of the
positions are at locations equidistant from the boundary of the
equal volume with the next equal volume in and the outside boundary
of the equal volume.
19. A method according to claim 4 in which the positions are
accessed using one or more tubes provided in the waste.
20. A method according to claim 19 in which a tube provides access
to one or more of the equal volumes.
21. A method according to claim 4 in which all equal volumes are
considered using a source.
22. A method according to claim 4 in which the comparison is of the
calculated or monitored response without the waste present compared
with the waste present.
23. A method according to claim 4 in which the correction factor is
a combination of all the correction functions.
24. A method according to claim 4 in which a single correction
factor for each part or segment is provided.
25. A method according to claim 1 in which the simulation accounts
for one or more of: detector type, detector sensitivity, detector
positions relative to each other, detector positions relative to
the monitoring space, field of view positions and shape, container
shape, container material, container position within the monitoring
space, waste shape, waste material, waste position within the
monitoring space.
26. A method according to claim 1 in which the simulation accounts
for one or more of: the factors involved in the detection of
emissions due to issues of geometry, the factors involved in the
detection of emissions due to issues of attenuation, the factors
involved in the detection of emissions due to issues of detector
performance, the factors involved in the detection of emissions due
to apparatus performance.
27. A method according to claim 1 in which the simulation includes
introducing one or more simulated sources to one or more positions
within the waste and consider the detector responses thereto.
28. A method according to claim 27 in which sensitivity values are
derived from the detector response.
29. A method according to claim 2 in which the simulation is
verified using the method of claim 3.
30. Apparatus for monitoring a volume of waste for radioactive
material, the apparatus comprising a support, the support being
provided with a plurality of detectors for radioactive material; a
monitoring space in which the volume of waste is provided; and a
data processor for calculating a monitoring result, for correcting
the monitoring result for geometry and/or attenuation to give a
corrected result and for providing a correction factor for
correcting the monitoring result to give the corrected result;
wherein the correction factor is obtained by a method comprising:
providing a simulation of an equivalent volume of waste free of
radioactive material in an equivalent monitoring space, with
equivalent detectors; providing a known activity radioactive source
at one or more positions in the volume of waste; and determining
the detector response to the source in a position, a comparison of
the response and known activity contributing to a correction
function for that equal volume, one or more such correction
functions providing the correction factor.
31. Apparatus according to claim 30 in which the simulation is a
computer simulation.
32. A method according to claim 30 in which the simulation is a
physical simulation.
33. Apparatus according to claim 30 wherein the correction factor
is obtained by a method comprising: providing an equivalent volume
of waste free of radioactive material in an equivalent monitoring
space, with an equivalent support provided with equivalent
detectors; dividing in one or more parts, the volume of waste into
a plurality of equal volumes; providing a known activity
radioactive source at one or more positions in one or more of the
equal volumes; and monitoring the detector response to the source
in a position in one of the equal volumes, a comparison of the
response and known activity contributing to a correction function
for that equal volume, one or more such correction functions
providing the correction factor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of United Kingdom Patent
Application No. 0425736.6 filed on Nov. 23, 2004, the disclosure of
which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. The Field of the Invention
[0003] This invention concerns improvements in and relating to
monitoring, particularly, but not exclusively in relation to waste,
including radioactive material, within containers.
[0004] 2. The Relevant Technology
[0005] In a variety of situations volumes of radioactive waste need
to be consigned to storage facilities. As a part of that process
there is a need to quantify the activity of the radioactive
waste.
SUMMARY OF THE INVENTION
[0006] The present invention has amongst its aims to provide a more
accurate measurement of the radioactive material in containers
and/or to productivity and throughput for a monitoring process
and/or to increase safety for a monitoring process, for instance by
reducing the dose to operators.
[0007] According to a first aspect of the present invention we
provide apparatus for monitoring a volume of waste for radioactive
material, the apparatus comprising: [0008] a support, the support
extending around at least two, preferably at least three, sides of
a monitoring space, the support being provided with a plurality of
detectors for radioactive material; [0009] a mover, the mover
providing relative movement between the support and the monitoring
space.
[0010] According to a second aspect of the present invention we
provide a method of monitoring a volume of waste for radioactive
material, the method comprising: [0011] providing a volume of waste
in a monitoring space; [0012] providing a support extending around
at least two, preferably at least three, sides of the monitoring
space, the support being provided with a plurality of detectors for
radioactive material; [0013] providing the support at a first
position relative to the monitoring space by use of a mover and
monitoring at least a part of the volume of waste for radioactive
material from the first position; [0014] providing the support at
one or more further positions relative to the monitoring space by
use of the mover and monitoring at least a part of the volume of
waste for radioactive material from each of the further positions,
the at least a part of the volume of waste being at least partially
different between positions.
[0015] The first and/or second aspects of the present invention may
include any of the features, options or possibilities set out
elsewhere in this document, including in particular from amongst
the following.
[0016] The monitoring may provide information on the radioactive
material present. The information may be expressed relative to one
or more thresholds. The information may indicate the waste is Low
Level waste. The information may be expressed as an activity and/or
activity level, particularly a measurement thereof. The activity
may be a total activity. The activity level may be a total activity
level. The activity level may be the activity level of one or more
isotopes. The information may be a mass of radioactive material the
information may be the specific activity.
[0017] The volume of waste may be provided in a container. The
container may be of a standard size from amongst one or more
standard sizes. The container may be an ISO FREIGHT container. The
container may be a half height ISO FREIGHT container. The container
may be a third height ISO FREIGHT container. The container may be
rectilinear in shape. The container may be between 2 and 10 metres
long, for instance 5 to 7 metres long. The container may be between
1 and 6 metres wide, for instance 2 to 3.5 metres wide. The
container may be between 1 and 6 metres high, for instance between
2 and 3.5 metres high or between 1 and 2 metres high. The container
may contain over 10 tonnes of waste. The container may have two
side walls, two end walls and a base. The container may be open at
the top. A lid for the container may be provided.
[0018] The waste may be uncompactable waste. The Waste may be waste
that in a high force compactor has a volume that cannot be reduced
by more than 30% compared with the initial volume. The waste may
include one or more of ferrous metal, steel, aluminum, wood, soil,
concrete, rubble, plastics, pipes and the like. The waste may have
a bulk density of between 0.4 and 1.7 gcm-3 or between 0.7 and 1.5
gcm-3.
[0019] The waste may be formed of one or more different waste
types. The waste may be formed of one or more different zones, a
different zone containing or being formed from a waste type. A
waste type may be defined by the material it is formed from, for
instance soil. A waste type may be defined by the source of that
material, for instance waste arising from neutron activation.
[0020] The radioactive material may include one or more isotopes
with emissions of gamma energy between 400 and 1200 keV. The
radioactive material may include Cs137 and/or Co60. The support may
be a framework or gantry. Preferably the framework has a first
portion and a second portion at substantially 90o to the first
portion. Preferably the framework has a third portion at
substantially 90o to the second portion. The first portion and
third portion are preferably parallel to one another. Preferably a
U-shaped support is provided.
[0021] The support may extend below the monitoring space and above
the monitoring space. The support above and below the monitoring
space may be linked or may be separate. The support may extend up
either side of and above the monitoring space. The support may
extend above and below the monitoring space and up one or both
sides thereof. The support may extend up either side and below the
monitoring space.
[0022] The monitoring space may be the volume occupied by the
waste, preferably in a container, during monitoring. The monitoring
space may be above a surface on which the container rests during
monitoring. The monitoring space may be a volume which is
substantially rectilinear. The cross-section of the monitoring
space is preferably smaller than the cross-section of the support,
the cross-sections being considered perpendicular to the direction
of movement.
[0023] One or more detectors, preferably one, may be provided on
one side of the support, for instance on a portion of the support.
One or more detectors, preferably one, may be provided on the other
side of the support. One or more, preferably a plurality, more
preferably at least four, detectors may be provided on the top
and/or the bottom of the support. The detectors may be regularly
spaced down the sides and/or across the top and/or bottom of the
support.
[0024] One or more detectors, preferably one, may be provided to
one side of the monitoring space. One or more detectors, preferably
one, may be provided to the other side of the monitoring space. One
or more detectors, preferably a plurality, more preferably at least
four, detectors may be provided across the top and/or across the
bottom of the monitoring space. A detector may be provided at a mid
height position relative to the monitoring space on one or both
sides of the monitoring space. A detector may be provided at a mid
height position relative to the height of a volume of waste and/or
container therefor on one or both sides of the monitoring space.
The detectors provided above and/or below the monitoring space may
be evenly spaced across the monitoring space. The detectors
provided above and/or below a volume of waste and/or container
therefor may be evenly spaced across the volume of waste and/or
container therefor.
[0025] The positions of one or more detectors on the support may be
adjustable, for instance to be suitably positioned for different
waste volumes.
[0026] The detectors may be identical to one another. The detectors
are preferably gamma detectors. The detectors may include NaI
crystals. The detectors may be collimated. A collimator defining a
conical field of view may be provided. The detectors may
particularly be provided according to the third and/or fourth
aspects below.
[0027] One or more further detectors may be provided which are
differently configured to the detectors. The one or more further
detectors may particularly be provided according to the seventh
and/or eighth aspect below.
[0028] The mover may be provided as a part of the support. The
mover may comprise a motor to propel the support. The mover may
move the support along one or more rails. A rail to either side of
the monitoring space may be provided. Preferably the mover can move
the support from a position in front of one end of the monitoring
space to a position beyond the other end of the monitoring
space.
[0029] The mover may be provided as a part of the monitoring space.
In particular, the mover may act on the volume of waste, more
preferably a container containing the waste, to move the waste past
the support. Preferably the mover can move the waste from a
position in front of the support, past the support to a position
beyond the support.
[0030] The relative movement of the support and the monitoring
space may be provided by moving the support past the monitoring
space and/or the monitoring space past the support.
[0031] The first monitoring position may position the support
closest to a first part of the volume of waste. The first part of
the waste may be that part of the waste at one edge of the volume
of waste and/or at one edge of the container therefor, particularly
the lead edge relative to the direction of movement.
[0032] The monitoring may include the detection of one or more
types of emission. A type of emission may be an emission of a
particular energy. A type of emission may be an emission from a
particular isotope. Preferably emissions from Cs137 and Co60 are
monitored. The emissions monitored may be corrected for attenuation
and/or geometry. The emissions may particularly be corrected
according to the fifth and/or sixth aspects below. The counts from
the detectors may be considered together. The counts from the
detectors may be considered separately, and in particular according
to the ninth and/or tenth aspects below.
[0033] The one or more further monitoring positions may position
the support closest to one or more further parts of the volume of
waste. The first of the one or more further parts of the volume of
waste may be the further part next to the first part. Preferably
the further parts are monitored in sequence, preferably progressing
from the first part towards the further part of the volume furthest
therefrom.
[0034] According to a third aspect of the present invention we
provide apparatus for monitoring a volume of waste for radioactive
material, the apparatus comprising: [0035] a support, the support
being provided with a plurality of detectors for radioactive
material; [0036] the detectors being provided with a collimator to
define a first field of view; [0037] the detectors being provided
with a further collimator to define a second field of view which,
in at least one configuration of the further collimator, is a part
of the first field of view, the further collimator being adjustable
to provide different configurations thereof and the second field of
view including at least a part of a monitoring space for the volume
of waste.
[0038] According to a fourth aspect of the present invention we
provide a method of monitoring a volume of waste for radioactive
material, the method comprising: [0039] providing a volume of waste
in a monitoring space; [0040] providing a support, the support
being provided with a plurality of detectors for radioactive
material, the detectors being provided with a collimator to define
a first field of view, the detectors being provided with a further
collimator to define a second field of view which, in at least one
configuration of the further collimator, is a part of the first
field of view, the further collimator being adjustable to provide
different configurations thereof; [0041] monitoring the volume of
waste for radioactive material in the second field of view.
[0042] The third and/or fourth aspects of the present invention may
include any of the features, options or possibilities set out
elsewhere in this document, including in particular from amongst
the following.
[0043] The support may extend around at least three sides of the
monitoring space. The support may be moveable relative to the
monitoring space.
[0044] The detectors may be identical to one another. The detectors
are preferably gamma detectors. The detectors may include NaI
crystals. The collimators may define a conical field of view as the
first field of view.
[0045] One or more of the further collimators may be formed of a
plurality of elements. One or more of the further collimators may
comprise a first plate and a second plate. Preferably the plates
are planar, most preferably the plane is perpendicular to the axis
of the first field of view and/or the second field of view. The
elements may have opposing edges. Preferably the edges are parallel
to one another. Preferably the elements are slidably mounted,
potentially directly or indirectly on the support.
[0046] The second field of view may be a sectioned cone. The
cross-section of the second field of view, preferably perpendicular
to the axis thereof, may be defined by a pair of opposing parallel
edges linked by opposing parts of a circle. Preferably the parallel
edges are defined by the further collimator. Preferably the part
circles correspond to part of the first field of view. The second
field of view may remove a section from the first field of view,
preferably an equivalent section from each side.
[0047] Preferably the different configurations of the second field
of view are provided by varying the separation of a plurality of
elements, particularly the separation between two plates. The
separation of the elements may be varied along an axis. The axis
may be parallel to the axis of the relative movement of the support
and the monitoring space.
[0048] The configuration of the second field of view is preferably
the same during the monitoring of a volume of waste. The
configuration of the second field of view may be varied between the
monitoring of one volume of waste and the monitoring of a different
volume of waste. The configuration may be varied where the size of
the volume of waste varies between monitorings. The configuration
may be varied where the waste type varies between one monitoring
and a further monitoring.
[0049] The plurality of detectors may be provided with equivalent
first fields of view. The plurality of detectors may be provided
with equivalent second fields of view. Preferably the second fields
of view overlap between adjacent detectors. Preferably the
detectors along the top and/or bottom have second fields of view
which overlap perpendicular to the direction of relative movement
between the support and monitoring space. Preferably the parallel
edges of the second fields of view for adjacent detectors are
aligned with one another, ideally this applies to detectors on the
top and/or bottom and/or one or both sides.
[0050] Preferably the second fields of view include a segment of
the monitoring space and/or the volume of waste. The segment may be
wedge shaped. The volume of waste may be monitored using four or
more segments and preferably using ten to twenty segments.
[0051] The second field of view may be narrower parallel to the
direction of relative movement than perpendicular to the direction
of relative movement. The second field of view may be a parallel
sided slice. The second field of view may be non-parallel sided
slice, such as a wedge.
[0052] According to a fifth aspect of the present invention we
provide apparatus for monitoring a volume of waste for radioactive
material, the apparatus comprising: [0053] a support, the support
being provided with a plurality of detectors for radioactive
material; [0054] a monitoring space in which the volume of waste is
provided; [0055] a data processor for calculating a monitoring
result, for correcting the monitoring result for geometry and/or
attenuation to give a corrected result and for providing a
correction factor for correcting the monitoring result to give the
corrected result; [0056] wherein the correction factor is obtained
by a method comprising: [0057] providing an equivalent volume of
waste free of radioactive material in an equivalent monitoring
space, with an equivalent support provided with equivalent
detectors; [0058] dividing in one or more parts, the volume of
waste into a plurality of equal volumes; [0059] providing a known
activity radioactive source at one or more positions in one or more
of the equal volumes; [0060] monitoring the detector response to
the source in a position in one of the equal volumes, a comparison
of the response and known activity contributing to a correction
function for that equal volume, one or more such correction
functions providing the correction factor.
[0061] According to a sixth aspect of the present invention we
provide a method of monitoring a volume of waste for radioactive
material, the method comprising [0062] providing the volume of
waste in a monitoring space; [0063] providing a support, the
support being provided with a plurality of detectors for
radioactive material; [0064] monitoring the volume of waste for
radioactive material, in one or more parts, to give a monitoring
result; [0065] correcting the monitoring result for geometry and/or
attenuation to give a corrected result using a correction factor;
[0066] wherein the correction factor is obtained by a method
comprising: [0067] providing an equivalent volume of waste free of
radioactive material in an equivalent monitoring space, with an
equivalent support provided with equivalent detectors; [0068]
dividing in one or more parts, the volume of waste into a plurality
of equal volumes; [0069] providing a known activity radioactive
source at one or more positions in one or more of the equal
volumes; [0070] monitoring the detector response to the source in a
position in one of the equal volumes, a comparison of the response
and known activity contributing to a correction function for that
equal volume, one or more such correction functions providing the
correction factor.
[0071] The fifth and/or sixth aspects of the present invention may
include any of the features, options or possibilities set out
elsewhere in this document, including in particular from amongst
the following.
[0072] The volume of waste may be monitored in one or more parts,
with the monitoring performed on the parts being combined to give
the monitoring result. The one or more parts may be segments. The
volume may be monitored using four or more parts and preferably
using ten to twenty parts. The monitoring of each part may be
corrected for geometry and/or attenuation separately, with the
corrected monitorings being combined to give the corrected result.
The monitoring for each part may be combined and then corrected for
geometry and/or attenuation to give the corrected result.
[0073] The equivalent volume of waste is preferably equivalent in
terms of the volume of waste and/or the type of waste and/or the
material forming the waste and/or the density of the waste and/or
the container holding the waste and/or attenuation properties. The
equivalent monitoring space is preferably equivalent in terms of
the size and/or shape and/or volume and/or position of the support
relative to the monitoring space. Preferably the equivalent support
is equivalent in terms of its shape and/or configuration and/or
size and/or position relative to the monitoring space. Preferably
the equivalent detectors are equivalent in terms of the detector
type and/or the number of detectors and/or the detector position
and/or the collimator and/or the further collimator and/or the
configuration of the further collimator.
[0074] The volume of equivalent waste may be monitored in one or
more parts, preferably equivalent to the one or more parts of the
waste. The one or more parts of the equivalent waste may be
segments. The volume of equivalent waste may be monitored using
four or more parts and preferably using ten to twenty parts.
[0075] The shape of the equal volumes may be different from one
another. The shape of the equal volumes may be different for
different detector positions and/or detector configurations. The
shape of the equal volumes may be different for different numbers
of detectors.
[0076] The shape of the equal volumes may be defined relative to
the location separated from all of the detectors by the greatest
distance, particularly the greatest distance through the waste
and/or container therefor. The equal volume containing that
location may be the core equal volume. The core equal volume may be
defined in terms of the volume bounded by a given minimum distance
from all the detectors and/or a given distance from that location
and/or a shape centred on that location. The core equal volume may
be that volume to which the detectors are least sensitive. The
other equal volumes may be defined in layers around the core equal
volume. The other equal volumes may be defined as that volume which
is all between a first given minimum distance and a second given
minimum distance from all the detectors, the first and second
distances varying for the different equal volumes.
[0077] The method may be repeated with a number of different known
activity sources. Preferably the method is performed using a Cs137
source and/or a Co60.
[0078] The positions may be evenly distributed throughout an equal
volume. Preferably positions are provided evenly distributed
throughout all the volumes. Between 5 and 20 positions may be
provided for each equal volume and/or each segment.
[0079] One or more of the positions may be at locations equidistant
from the boundary of the equal volume with the next equal volume
out and the boundary of the equal volume with the next equal volume
in. One or more of the positions may be at locations equidistant
from the boundary of the equal volume with the next equal volume
out and the location with the greatest minimum distance from all
detectors. One or more of the positions may be at locations
equidistant from the boundary of the equal volume with the next
equal volume in and the outside boundary of the equal volume. One
of the positions may be at the location with the greatest minimum
distance between it and all the detectors. The positions may be
representative of different positions within an equal volume.
[0080] The positions may be accessed using one or more tubes
provided in the waste. A tube may provide access to one or more of
the positions. A tube may provide access to one or more of the
equal volumes. The tubes may be filled with waste apart from at the
positions occupied by the source or sources. The tubes are
preferably provided with an open end. A closure for the open end
may be provided. The open end may be provided at the top and/or one
or more sides and/or bottom of the waste and/or container.
[0081] Preferably all equal volumes are considered using a source.
Preferably a plurality, ideally all, of the positions within an
equal volume are provided with sources whilst the detector response
is monitored. Preferably a plurality, ideally all, of the same
position equal volumes in the segments are provided with sources
whilst the detector response is monitored. Preferably the approach
is repeated separately for each of the equal volumes. The response
of one or more, preferably all, of the detectors may be monitored.
Preferably the approach is repeated for a plurality of different
sources.
[0082] The detector response for each detector may be considered
separately and/or with the other detectors.
[0083] The comparison may be of the calculated or monitored
response without the waste present compared with the waste
present.
[0084] The correction function may relate to the sensitivity of one
or more, preferably all, the detectors to activity in the equal
volume involved.
[0085] The correction factor may be a combination of all the
correction functions. A single correction factor for each part or
segment may be provided. A single correction factor for the volume
may be provided. Different correction factors may be used for
different parts and/or segments of the volume.
[0086] Preferably the radioactive material in the waste is assumed
to be homogeneously distributed therein. Preferably the waste is
assumed to be homogeneous in nature.
[0087] The sensitivity determined for one or more equal volumes may
be too low to be detected or statistically significantly detected
in the monitoring time period used. Where such a sensitivity is
deemed to apply, the monitored result may be deemed to have
detected no emissions from that equal volume or volumes. The
monitored result, more preferably the corrected result, may be
deemed to be the emissions from the other equal volumes only. The
monitored result, more preferably the corrected result, may be
increased proportionately to the proportion that the equal volumes
deemed to be undetected represent of the total volume.
[0088] According to a seventh aspect of the present invention we
provide apparatus for monitoring a volume of waste for radioactive
material, the apparatus comprising [0089] a support, the support
being provided with a plurality of detectors for radioactive
material; [0090] a monitoring space in which the volume of waste is
provided; [0091] a data processor for calculating a monitoring
result, for correcting the monitoring result for geometry and/or
attenuation to give a corrected result and for providing a
correction factor for correcting the monitoring result to give the
corrected result; [0092] wherein the correction factor is obtained
by a method comprising: [0093] providing a simulation of an
equivalent volume of waste free of radioactive material in an
equivalent monitoring space, with equivalent detectors; [0094]
providing a known activity radioactive source at one or more
positions in the volume of waste; [0095] determining the detector
response to the source in a position, a comparison of the response
and known activity contributing to a correction function for that
equal volume, one or more such correction functions providing the
correction factor.
[0096] According to an eighth aspect of the present invention we
provide a method of monitoring a volume of waste for radioactive
material, the method comprising [0097] providing the volume of
waste in a monitoring space; [0098] providing a support, the
support being provided with a plurality of detectors for
radioactive material; [0099] monitoring the volume of waste for
radioactive material, in one or more parts, to give a monitoring
result; [0100] correcting the monitoring result for geometry and/or
attenuation to give a corrected result using a correction factor;
[0101] wherein the correction factor is obtained by a method
comprising: [0102] providing a simulation of an equivalent volume
of waste free of radioactive material in an equivalent monitoring
space, with equivalent detectors; [0103] providing a known activity
radioactive source at one or more positions in the volume of waste;
[0104] determining the detector response to the source in a
position, a comparison of the response and known activity
contributing to a correction function for that equal volume, one or
more such correction functions providing the correction factor.
[0105] The seventh and/or eighth aspects of the present invention
may include any of the features, options or possibilities set out
elsewhere in this document, including in particular from amongst
the following.
[0106] Preferably the simulation is a computer simulation.
[0107] The simulation may account for one or more of: detector
type, detector sensitivity, detector positions relative to each
other, detector positions relative to the monitoring space, field
of view positions and shape, container shape, container material,
container position within the monitoring space, waste shape, waste
material, waste position within the monitoring space.
[0108] The simulation may be performed using the Monte Carlo
Neutrons and Photons modelling package/
[0109] The simulation may account for the factors involved in the
detection of emissions due to issues of geometry and/or due to
issues of attenuation and/or due to issues of detector and/or
apparatus performance.
[0110] Preferably the simulation includes introducing one or more
simulated sources to one or more positions within the waste and
consider the detector responses thereto. Preferably sensitivity
values are derived from the detector response. The positions used
in the simulation could be homogeneously dispersed sources
throughout the waste.
[0111] A simulation may be verified using the apparatus and/or
method of the fifth and/or sixth aspects of the invention.
[0112] The simulation may be a physical simulation. The simulation
may include the provision of an equivalent support and/or
equivalent detector positions. The simulation may include dividing,
in one or more parts, the volume of waste into a plurality of equal
volumes. The known activity radioactive source may be a simulated
source. Preferably the simulation includes the simulation of the
source at one or more positions in one or more of the equal
volumes. The detector response may be determined by monitoring,
particularly for a physical simulation. The detector response may
be calculated as part of the simulation, particularly for a
computer simulation.
[0113] According to a ninth aspect of the present invention we
provide apparatus for monitoring a volume of waste for radioactive
material, the apparatus comprising [0114] a support, the support
being provided with a plurality of first detectors for radioactive
material; [0115] the first detectors being provided with one or
more collimators to define their fields of view, the fields of view
including at least a part of a volume of waste; [0116] a further
detector provided with one or more collimators to define a further
Me detector field of view, the further detector field of view
including within it that location in the at least a part of the
volume of waste which has the greatest minimum distance from all
the first detectors.
[0117] According to a tenth aspect of the present invention we
provide a method of monitoring a volume of waste for radioactive
material, the method comprising [0118] providing a volume of waste
in a monitoring space; [0119] providing a support, the support
being provided with a plurality of first detectors for radioactive
material, the detectors being provided with one or more collimators
to define their field of view, their fields of view including at
least a part of the volume of waste; [0120] providing a further
detector provided with one or more collimators to define a further
detector field of view, the further detector field of view
including within it that location in the at least a part of the
volume of waste which has the greatest minimum distance from all
the first detectors; [0121] monitoring the volume of waste for
radioactive material.
[0122] The ninth and/or tenth aspects of the present invention may
include any of the features, options or possibilities set out
elsewhere in this document, including in particular from amongst
the following.
[0123] The first detectors are preferably the detectors of
elsewhere in this document.
[0124] The at least a part of the volume of waste with the fields
of view of the first detectors may be a segment.
[0125] The further detector may be of the same type as the first
detectors. The further detector may be more sensitive than the
first detectors. Preferably the further detector is provided with a
different collimator or collimators to the first detectors. The
field of view of the further detector may be more restricted. One
or more further detectors may be provided.
[0126] The further detector or detectors field of view preferably
includes within it that part of the volume of waste which is
furthest from the first detectors. The further detector or further
detectors field of view preferably includes within it that equal
volume which contains the waste furthest from the first
detectors.
[0127] According to a eleventh aspect of the present invention we
provide apparatus for monitoring a volume of waste for radioactive
material, the apparatus comprising [0128] a monitoring space;
[0129] a support, the support being provided with a plurality of
detectors for radioactive material; [0130] a data processor, the
data processor calculating a monitored result which includes
information on one or more directly detected isotopes, the data
processor calculating a calculated result from the monitored
result, the calculated result including information on one or more
not directly detected isotopes, the data processor selecting a
function for use in the calculation of the calculated result, the
function being selected from a memory and being selected according
to one or more characteristics of the monitored result.
[0131] According to a twelfth aspect of the present invention we
provide a method of monitoring a volume of waste for radioactive
material, the method comprising [0132] providing a volume of waste
in a monitoring space; [0133] providing a support, the support
being provided with a plurality of detectors for radioactive
material; [0134] monitoring the volume of waste for radioactive
material to obtain a monitored result which includes information on
one or more directly detected isotopes; [0135] calculating a
calculated result from the monitored result, the calculated result
including information on one or more not directly detected
isotopes, the calculation of the calculated result using a
function, the function being selected according to one or more
characteristics of the monitored result.
[0136] The eleventh and/or twelfth aspects of the present invention
may include any of the features, options or possibilities set out
elsewhere in this document, including in particular from amongst
the following.
[0137] The monitored result may be the count and/or count rate
and/or activity detected by the detectors. The monitored result may
be uncorrected. The monitored result may be corrected for
attenuation and/or geometry.
[0138] The information on the one or more directly detected
isotopes may include their count and/or count rate and/or activity.
The information may include the ratio of information for two or
more energies. The two or more energies may be energies of the same
isotope, but are preferably energies of different isotopes. The
information may include a ratio involving Cs137 and Co60, such as
the ratio between the count and/or count rate and/or activity at
one energy for one of the isotopes to the count and/or count rate
and/or activity at another energy for the other of the
isotopes.
[0139] The calculated result may be the count and/or count rate
and/or activity for all radioactive material in the volume of waste
or a part thereof. The calculated result preferably accounts for
isotopes which can be monitored directly and those which
cannot.
[0140] The function may be one or more ratios. The ratio may be the
ratio of the count and/or count rate and/or activity for a directly
detected isotope to another isotope. The another isotope may be
directly detectable or not. The function may be a set of ratios.
Preferably the function relates all of the not directly detectable
isotopes to one or more directly detected isotopes. The function
may be a fingerprint, preferably a fingerprint for a waste type.
Two or more functions may be provided, each preferably containing
one or more ratios, preferably with one or more different ratios.
The ratios may differ it terms of the isotopes they relate to one
another, or more preferably in terms of the difference in the ratio
of a detected to an not detected isotope they detail. The function
may be related to a waste type. The function may be related to a
source of waste. The function may be related to the nature of the
environment the waste was exposed to. The function may be different
for neutron activation waste to fission product and/or fuel pin
leakage waste.
[0141] The characteristic the function is selected on may be based
upon the ratio of one isotope in the monitored result to another
and/or upon the ratio of the count and/or count rate and/or
activity at one energy in the monitored result to another and/or
upon the level of one or more isotopes and/or energies in the
monitored result. The function may particularly be selected based
upon the ratio of counts and/or count rate and/or activity
indicative of Cs137 to Co60 or vice versa.
[0142] It is particularly preferred that the volume of waste be
considered in a plurality of parts. Preferably the method is
applied individually to each part. Thus one function may be used
for one part, with another function being used for the adjoining
part and so on.
[0143] The plurality of parts may be a plurality of segments. A
segment may be that part of the waste within the fields of view of
the detectors at a first position for the support and/or at a given
configuration for the collimators of the detectors. A segment may
be subdivided with the method being applied to each resulting
sub-part.
[0144] A part may extend for the full depth and width of a volume
of waste, but less than its full length. A part may extend for a
fraction of the full depth and/or width of a volume of waste and
less than its full length. A part may be a fraction of slice
through the volume of waste, for instance a quarter of a slice.
[0145] A part may be a small fraction of the volume of waste, for
instance equal to or less than 0.1% thereof, preferably equal to or
less than 0.01% thereof. The method may be applied independently
with respect to each of these parts. The monitored results from
each of the detectors may be considered independently in such an
approach. The monitored results may be subjected to deconvolution
and/or tomography to provide the monitored result for each
part.
[0146] The results of the application of the methods to the parts
may be combined, preferably summed, to give an overall result.
[0147] According to a thirteenth aspect of the present invention we
provide apparatus for monitoring a volume of waste for radioactive
material, the apparatus comprising [0148] a monitoring space;
[0149] a support, the support being provided with a plurality of
detectors for radioactive material; [0150] a data processor, the
data processor calculating a monitored result, determining the
level of radioactive material in one or more parts of the volume of
waste and comparing the level of radioactive material in one of the
parts with the level of radioactive material in another part.
[0151] According to a fourteenth aspect of the present invention we
provide a method for monitoring a volume of waste for radioactive
material, the method comprising [0152] providing a volume of waste
in a monitoring space; [0153] providing a support, the support
being provided with a plurality of detectors for radioactive
material; [0154] monitoring the volume of waste for radioactive
material to obtain a monitored result; determining the level of
radioactive material in one or more parts of the volume of waste;
[0155] comparing the level of radioactive material in one of the
parts with the level of radioactive material in another part.
[0156] The thirteenth and/or fourteenth aspects of the present
invention may include any of the features, options or possibilities
set out elsewhere in this document, including in particular from
amongst the following.
[0157] The monitored result and/or level of radioactive material
may be the count and/or count rate and/or activity. The monitored
result and/or level of radioactive material may be uncorrected. The
monitored result and/or level of radioactive material may be
corrected for attenuation and/or geometry.
[0158] It is particularly preferred that the volume of waste be
considered in more than two parts. Preferably the method is applied
individually to a plurality of pairs of parts.
[0159] The parts may be a segments. A segment may be that part of
the waste within the fields of view of the detectors at a first
position for the support and/or at a given configuration for the
collimators of the detectors. A segment may be subdivided with the
method being applied to each resulting sub-part as the part.
[0160] A part may extend for the full depth and width of a volume
of waste, but less than its full length. A part may extend for a
fraction of the full depth and/or width of a volume of waste and
less than its full length. A part may be a fraction of slice
through the volume of waste, for instance a quarter of a slice.
[0161] A part may be a small fraction of the volume of waste, for
instance equal to or less than 0.1% thereof, preferably equal to or
less than 0.01% thereof. The method may be applied with respect to
each of these parts. The monitored results from each of the
detectors may be considered independently in such an approach. The
monitored results may be subjected to deconvolution and/or
tomography to provide the monitored result for each part.
[0162] The method and/or apparatus may be used to establish whether
there is non-uniformity of activity distribution in the volume of
waste.
[0163] Preferably the activity for a part is established using the
detectors which monitor that part.
[0164] Preferably differences in the activity between parts are
used as an indicator of non-uniform activity being present. The
apparatus and/or method may provide an indication as to the actual
activity involved and/or an indication as to the location
thereof.
[0165] The apparatus and/or method may involve a set of views of
the waste to be taken, preferably from a variety of directions.
This may include that being repeated at a variety of different
positions relative to the waste, for instance along the waste.
Imaging of the activity and its position may be provided.
[0166] The method and/or apparatus may involve recording the output
from each detector at each measurement segment separately and
perform a deconvolution thereon. Tomography may be used on the data
set, for instance to give a voxelated image of the activity
distribution. The volume in the container would be broken down into
a thousands or more parts or voxels. The activity associated with
each part or voxel may be obtained. The differences in the activity
of one part or voxel compared with one or more of the others can be
established. A range of activities may be defined, with activities
falling within that range being accepted as evenly distributed
activity and/or with activities outside that range being deemed
non-uniform activity distribution. The actual level of activity for
a part or voxel and/or the position of that part or voxel can be
provided.
[0167] According to a fifteenth aspect of the present invention we
provide apparatus for monitoring a volume of waste for radioactive
material, the apparatus comprising [0168] a monitoring space;
[0169] a support, the support being provided with a plurality of
first detectors for radioactive material; [0170] a radioactive
transmission source; [0171] a transmission detector for
transmissions which have interacted with the volume of waste;
[0172] a data processor for comparing one or more attenuation
characteristics of the volume of waste according to the
transmission detector with one or more attenuation characteristics
used in the monitoring.
[0173] According to a sixteenth aspect of the present invention we
provide a method of monitoring a volume of waste for radioactive
material, the method comprising [0174] providing a volume of waste
in a monitoring space; [0175] providing a support, the support
being provided with a plurality of first detectors for radioactive
material; [0176] monitoring at least a part of the volume of waste
for radioactive material; [0177] providing a radioactive
transmission source; [0178] providing a transmission detector for
transmissions which have interacted with the volume of waste;
[0179] comparing one or more attenuation characteristics of the
volume of waste according to the transmission detector with one or
more attenuation characteristics used in the monitoring.
[0180] The fifteenth and/or sixteenth aspects of the present
invention may include any of the features, options or possibilities
set out elsewhere in this document, including in particular from
amongst the following.
[0181] The transmission source is preferably provided on an
opposing side of the volume of waste to the transmission detector.
The transmission source may be below the volume of waste, above or
to a side thereof. One or more transmission sources may be
provided.
[0182] The transmission detector may be of the same or a different
type to the first detectors. The transmission detector may be
provided with a different collimator and/or field of view to the
first detectors. One or more detectors for transmissions may be
provided.
[0183] Preferably the emissions have interacted with the volume of
waste by passing through from one side to another. The emissions
may have interacted by entering and returning from the volume of
waste, either themselves or in terms of a form of emission they
cause.
[0184] Preferably the emissions interact with, ideally pass
through, that part of the volume of waste which is furthest from
the first detectors. Preferably the field of view of the
transmission detector includes that part of the volume of waste
which is furthest from the first detectors. The emissions may
interact with, ideally pass through, that equal volume which
contains the waste furthest from the first detectors. The field of
view of the transmission detector preferably includes that equal
volume which contains the waste furthest from the first
detectors.
[0185] Transmissions may be passed through a plurality of, and
preferably all, parts of the volume of waste to reach one or more
transmission detectors. Different paths for the transmission
emissions through the volume may be provided by one or more of the
following: moving the transmission source and the detector it is
paired with to a variety of positions; moving the transmission
source so that it is "aimed" at a detector along a different path;
moving the transmission source to a different position, with the
source pairing with a different detector already in position; using
multiple transmission sources paired with multiple detectors.
[0186] Preferably the volume of waste is exposed to the
transmission source at a different time to the monitoring. The
transmission detector may be one of the first detectors.
[0187] The one or more attenuation characteristics of the volume of
waste according to the transmission detector may be the extent of
attenuation, for instance along a particular path, and/or the
variation in attenuation along one path compared with another
and/or the variation in attenuation compared with a preset value,
such as that used in the assumptions for the monitoring.
[0188] The one or more attenuation characteristics maybe determined
by considering the change in energy caused by the volume of waste
on two or more energies emitted by the transmission source. Two or
more of the energies may arise from the same isotope of the
transmission source. Two or more of the energies may arise from a
different isotope of the transmission source, for instance one from
Co and one from Cs, for instance Cs at 662 keV and/or Co 1331
keV.
[0189] The one or more attenuation characteristics used in the
monitoring may include the characteristic that attenuation is
homogeneous throughout the volume of waste and/or that the waste is
homogeneous.
[0190] The comparison may establish that the approach taken to
attenuation in the monitoring was appropriate. The comparison may
establish that the approach taken to attenuation in the monitoring
was inappropriate.
[0191] The results of the transmission detector and/or the
comparison may be made available to the apparatus and/or method
operator. The results of the transmission detector and/or the
comparison may only be made available to one or more third parties,
for instance verification parties or the disposal facility
party.
[0192] The one or more attenuation characteristics of the volume of
waste according to the transmission detector from one volume of
waste may be further compared with those from one or more further
volumes of waste. The further comparison may reveal attenuation
characteristics for waste volumes over time and/or variations
between the attenuation characteristics for waste and the assumed
characteristics.
[0193] The results of the further comparison may be made available
to the apparatus and/or method operator. The results of the
transmission detector and/or the comparison may only be made
available to one or more third parties, for instance verification
parties or the disposal facility party.
[0194] According to a seventeenth aspect of the invention we
provide apparatus for monitoring a volume of waste for radioactive
material, the apparatus comprising [0195] a monitoring space;
[0196] a support, the support being provided with a plurality of
first detectors for radioactive material; [0197] a data processor,
the data processor determining the ratio of the level of emissions
at two different energies with predetermined ratio information, the
comparison providing information on the attenuation properties of
the volume of waste.
[0198] According to an eighteenth aspect of the invention we
provide a method for monitoring a volume of waste for radioactive
material, the method comprising [0199] providing a volume of waste
in a monitoring space; [0200] providing a support, the support
being provided with a plurality of first detectors for radioactive
material; [0201] monitoring at least a part of the volume of waste
for radioactive material; [0202] monitoring at least two different
energies of emissions from the waste; [0203] comparing the ratio of
the level of emissions at the two different energies with
predetermined ratio information, the comparison providing
information on the attenuation properties of the volume of
waste.
[0204] The seventeenth and/or eighteenth aspects of the present
invention may include any of the features, options or possibilities
set out elsewhere in this document, including in particular from
amongst the following.
[0205] The predetermined information may be obtained from reference
materials and/or calculations and/or the consideration of
unattenuated sources to establish the ratio of the activity of the
lower energy to the higher energy of the pair. The predetermined
information may relate to the materials and/or densities and/or
geometry expected to apply to the volume of waste.
[0206] A decrease in the level of the lower energy relative to the
higher energy is preferably representative of higher
attenuation.
[0207] Preferably the comparison establishes whether the
attenuation is at the expected level and/or within the expected
range and/or at the level used in the assumptions for the
calculations and/or within the range used in the assumptions for
the calculations. The comparison may suggest that the attenuation
is higher than expected and/or is higher than is possible for
accurate monitoring. The comparison may result in the expression of
a measure of the level of attenuation. The comparison may provide a
further correction factor for this attenuation.
[0208] The approach preferably uses a pair of energies from the
same isotope. More than two energies can be considered.
[0209] It is possible to combine this method with the type of
transmission based investigation detailed in the aspects provided
above, for instance to provide information on the position of the
increased attenuation and/or to provide a still further correction
factor which accounts for the extent and position of the increased
attenuation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0210] Various embodiments of the invention will now be described,
by way of example only, and with reference to the accompanying
drawings in which:--
[0211] FIG. 1 is a perspective view showing an instrument according
to one embodiment of the present invention;
[0212] FIG. 2 is a perspective view of the detector gantry of the
instrument of FIG. 1;
[0213] FIG. 3 is a schematic illustration of the fields of view of
one of the detectors;
[0214] FIG. 4 schematically illustrates the segmented approach
taken in the present invention;
[0215] FIG. 5 schematically illustrates the equal volume
calibration approach taken according to the present invention, in
respect of one detector configuration; and
[0216] FIG. 6 schematically illustrates the allocation of different
fingerprints to different segments.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0217] During the operation and decommissioning of nuclear power
stations and other plant involved in the nuclear fuel cycle low
level waste, LLW, in particular is generated. LLW is that waste
having a level of contamination with radioactive material above the
level where it can be released for disposal in civil waste sites
and below that where it is classified intermediate level waste and
so different approaches need to be taken to its storage. LLW is
usually sent to medium to long term disposal facilities. The
facilities for LLW, are usually limited in terms of the total
volume of material they can accommodate and the total amount of
radioactive material they can accommodate. Uncompactable LLW is a
particularly high consumer of available volume as its volume cannot
be significantly reduced for storage. Such wastes include steel,
soil, concrete, pipes and the like. Uncompactable waste is normally
defined as that which in a High Force compactor cannot be reduced
in volume by more than 30% compared with the initial volume.
[0218] The more accurate the measurement of radioactivity that can
be made, the greater the extent to which the facility's total
amount of radioactivity can be used up. Less accurate measurements
result in a greater margin of error having to be provided for and
this may result in the facility's total amount of radioactivity
being used up by phantom activity. This leads to underutilisation
of the disposal facility, shortens the life thereof and hastens the
date at which a new facility needs to be made available. The
licensing, in particular, of a new facility is a massive, time
consuming and expensive undertaking.
[0219] Efforts to measure LLW using NDA have to date concentrated
on compactable LLW. These approaches are not suitable for
uncompactable LLW for a number of reasons. Approaches for
uncompactable LLW which have been accepted to date, involve the use
swabs to sample activity and/or health physics dose rate probes or
contamination probes to measure activity. These approaches are
problematic from an accuracy point of view, are not suited to
considering the waste in bulk and require highly trained health
physics staff who are in demand and hence have a high associated
cost.
[0220] As well as these issues, such waste is often filled into ISO
FREIGHT containers and half height ISO FREIGHT containers which are
then transported and stored at the LLW disposal facilities. Such
containers are of a very substantial size and contain a substantial
thickness of material, impacting further on the accuracy and ease
with which measurements can be made.
[0221] Techniques which use a single detector have been established
to face problems in terms of significant unevenness of response
when measuring large containers. A simple detector placed alongside
the container produces an inaccurate measurement due to: the wide
range of distances between any sources and the detector; the wide
range of absorbers between any sources and the detector; the non
uniform spatial response of the detector.
[0222] To address such issues, according to the present invention,
and referring to FIG. 1, a half ISO container 1 is shown positioned
for investigation. A half ISO container 1 is around 6 m long, 2.6 m
wide and 1.3 m high. Loaded with uncompactable material they
contain around 16 to 22 tonnes of waste.
[0223] The container 1 is seemingly loaded with soil 3 in one part,
poles 5 in another and limestone chippings 7 in another. The
container 1 is straddled by a gantry 9. The legs 11 of the gantry 9
have wheel sets 13 which cooperate with rails 15 on either side of
the container 1. The gantry 9 can be moved from a first measurement
position, as shown, to a second measurement position closer to the
viewer, then a further still closer to the viewer and so on. The
gantry is static during measurement.
[0224] The gantry 9 accommodates four gamma detectors 17 evenly
spaced across the top of the container 1. A single gamma detector
19 is provided on each of the legs 11 of the gantry 9. The
detectors 17, 19 are generally low resolution gamma detectors. Such
apparatus allows effective measurement of such containers for the
first time, as the non uniformity of response is greatly reduced
with an array of detectors. Furthermore, this is achieved with
realistic operating times and with greatly reduced dose to the
operators when compared with the previous health physic based
approaches. The movement of the gantry past the container, or in an
alternative form, the movement of the container past the gantry,
means that the prohibitively expensive provision of detectors to
cover the whole container at once is avoided.
[0225] In a particularly preferred embodiment an array of gamma
detectors are provided above the container 1 and an array of gamma
detectors are provided below the container 1. Optional side
detectors may also be provided.
[0226] As more clearly seen in FIG. 2, the detectors 17, 19 each
include a detector crystal (usually NaI), a collimator 21 which
define a first conical field of view and shutters 23 which are
slidably mounted and can be used to reduce the first field of view
down to a second field of view. As shown in FIG. 3, the first
conical field of view 25 is truncated at its sides 27 by the
shutters 23 to give the second field of view 29. The second fields
of view 29 of this type, when combined for the various detectors
represent one approach to considering the container 1 and waste it
contains in a series of parts or segments 40 of the type shown in
FIG. 4.
[0227] Benefits in considering the container 1 using such a
collimator design are obtained. The field of view can be controlled
as desired and hence the segment dimensions and shape can be
controlled. This in turn allows flexibility on the size and shape
of container being monitored.
[0228] A generally slice like segment is useful for the further
analysis of the contents of the container as described later. A
segment 40 is in effect considered for each measurement position,
which in the case illustrated in FIG. 4 means the container is
considered by means of five segments 40a, 40b, 40c, 40d, 40e. In
practice ten to twenty segments would normally be used, but the
thickness would vary when considering containers of different
depths and/or when different waste types are being considered. All
the segments 40 used to consider a particular container 1 have the
same thickness 42 measured in the direction of travel of the gantry
between measurement positions. The same depths 44 and widths 46 are
present because of the bounds of the container 1. To present the
different segments 40 to the detectors 17, 19 the investigation
method includes moving the gantry 9 to the first measurement
position and then on to a second measurement position and so on.
The process is repeated for the other segments 40 over the course
of the investigation run. A time period of around 6 hours enables
an ISO FREIGHT container 1 to be considered.
[0229] The measured amounts of activity for the measured isotopes
in a segment 40 is corrected for the effects of attenuation and
geometry, as discussed below. The corrected amounts for each of the
segments 40 of the container 1 are then summed to give the total
amount for these isotopes. From these measured results for certain
isotopes the difficult to measure nuclides and overall activity can
be obtained using one or more fingerprints for the waste in the
container. A fingerprint is the ratio of the various isotopes to
one another and is determined by a variety of physical/chemical
analyses on a source of waste. A fingerprint generally applies to
waste from a particular waste stream/source over a long period of
time. The measured and fingerprint inferred activity information is
used to complete the activity catalogue for the container. This
activity information is then supplied to the storage site operator
for their records and use.
[0230] The approach taken in the present invention to account for
the effects of geometry and/or attenuation is to apply a correction
factor to the monitored results to give a corrected result. Two
principle ways for obtaining the correction factor are
envisaged.
[0231] In either approach, some basic characteristics of the waste
are taken into account.
[0232] The typical waste types present in uncompactable waste
generally have a density of 1 gcm-3 and a density range of 0.7
gcm-3 to 1.2 gcm-3 covers most containers 1 encountered. Two key
constituents of the radioactive materials encountered are Cs137 and
Co60 which each present useful characteristic energy emissions.
Both are strong gamma emitters, but the densities encountered in
uncompactable waste give rise to significant attenuation of the
emissions, so preventing all the emissions reaching the detectors
17, 19. The extent depends upon the type of waste present and
density of the waste. Usefully, the gamma attenuation coefficients
for most common waste materials are very similar between 400 and
1200 keV. This includes such material as steel, wood plastics,
aluminum copper etc, but excludes shielding such as lead.
[0233] In either approach, some basic assumptions about the
characteristics of the waste are made. These are, that the activity
in a container is assumed to being evenly distributed at throughout
and that the waste is evenly distributed in terms of its
attenuation effects. Techniques and approaches for verifying the
applicability of these assumptions are discussed later.
[0234] The first approach of the present invention for obtaining
the correction factor to take account of attenuation and/or
geometry effects involves the determination of the correction
factor using a mock up of the container and waste type of
interest.
[0235] A container of the container type of interest is loaded with
representative waste, free of radioactive material, to provide a
calibration container 50, FIG. 5. The volume of waste is divided
into a number of segments 52 corresponding to the number that would
be used to analyse the container if the waste in it were unknown.
In keeping with the non-limiting description above, therefore, five
segments, 52a, 52b, 52c, 52d, 52e, are used.
[0236] Each segment 52 is in turn divided into a number of volumes,
each volume being equal in volume to the others. Between four and
eight equal volumes are useful for this purpose, with four being
used in the illustration of FIG. 5.
[0237] The shape of the four equal volumes 54, 55, 56, 57, will be
different with different detector configurations and/or numbers of
equal volumes. Different volume shapes would apply if detectors
were present below the container and/or absent from one side, for
instance. In the four detectors 17 above, one detector 19 to each
side detector configuration, previously discussed, this gives the
four equal volumes 54, 55, 56, 57, represented in FIG. 5. The
determination of the shape of the equal volumes 54, 55, 56, 57,
preferably starts with the core volume 54, that is the equal volume
including the hardest to measure parts of the overall volume. In
the illustrated example, this is the central bottom part of the
volume as that is the greatest distance from the detectors. The
subsequent equivalent volumes 55, 56, 57, are in effected layered
around the core volume and reflect increasingly easy to measure
equal volumes. Thus in general the outer quarter volume 57 is the
surface incorporating volume close to the detectors.
[0238] In the preferred form of the invention mentioned above, the
provision of bottom and top detectors means that the hardest to
measure volume is that at the centre.
[0239] This use of segments and equal volumes provides a useful
structure for the next part of the approach to obtaining the
correction factor.
[0240] A series of reentrant tubes 59 are provided leading from the
outside of the container 50 into the waste. The tubes 59 allow
access to a series of positions 60 within the waste. With respect
to the core volume 54 a series of positions 60 are provided,
distributed throughout the core volume 54, but generally with an
equal distance X between the position 60 and point 62 furthest from
the detectors and the boundary 64 between the core volume 54 and
the next equal volume 55. With respect to the equal volume 56, the
positions 60 are again evenly distributed and at the mid point
between the boundary with equal volume 55 and equal volume 57. The
same tubes 59 can be used to access more than one of the positions
60.
[0241] The tubes 59 are used to provide access for the provision of
a known activity source at the positions 60. With the sources in
the positions 60 for one of the equal volumes, this enables
measurement of the detectors response to that activity, with the
activity being subjected to the geometry and attenuation effects of
that position in the waste. As a result, a sensitivity factor for
that volume can be obtained. It is desirable to provide sources in
the matching equal volume of at least the adjacent segments, and
preferably all of the segments, during the measurements for that
equal volume. Around 5 to 20 sources per equal volume are desirable
to fully reflect emissions from that volume. It is also desirable
to perform the process using a Cs137 source and using a Co60
source.
[0242] Repeating the process for each of the equal volumes 54, 55,
56, 57, allows the sensitivity factors for all the volumes to be
obtained. In effect the proportion of emissions from each volume
which are detected may be established. The individual sensitivities
(correction functions) may then be combined to give a single
correction factor for geometry and attenuation that covers the
whole of that segment. Separate correction factors are possible for
all the different segments forming the overall volume. Separate
correction factors are possible for different type segments forming
a volume (for instance, one correction factor applying in respect
of the two end segments, with a second correction factor applying
in respect of all the intermediate segments--perhaps as a
reflection of the different volumes of the segments in these two
different type segments--the wedge shaped view of the intermediate
segment may be fully occupied by waste, with the end segment being
partially free of waste). In the simplest approach the same
correction factor may be applied throughout the volume of waste,
that is to say the same correction factor is used for each segment.
The correction factor obtained can then be applied to each unknown
volume of waste as it is monitored.
[0243] In pursuing this approach, due to the decreasing sensitivity
of the detectors to emissions from deep within the volume (given
the need to keep monitoring times practical) there may be
situations in which sensitivity to emissions from the core equal
volume 54 is too low for meaningful detection. In that case, the
assumption that the activity is evenly distributed is used. In
effect the three outer volumes are corrected using their
sensitivities/correction functions and this result equals the
activity for 3 out of the 4 volumes or in effect is 75% of the
total activity. Of course, the fraction detected, corrected and
then scaled up will depend upon the number of equal volumes used
and the number of those for which the sensitivity is too low to be
meaningful (for instance: 2 out of 8 equal volumes; 1 out of 10
equal volumes etc).
[0244] In tests the approach has been established to give
calculated Cs137 and Co60 activity values, for worst case
locations, that are <1% of the upper LLW waste limit at 90%
confidence. Assuming uniform distribution of activity, then without
correction, the worst case scenarios vary in response by a factor
of 10 to 40. On the same basis, but with correction of the above
mentioned type, variations by a factor of 3 only are obtained. This
applies to both metal waste and concrete rubble waste
situations.
[0245] The second approach of the present invention for obtaining
the correction factor to take account of attenuation and/or
geometry effects involves the determination of the correction
factor using a computer simulation of the container and waste type
of interest.
[0246] In this approach details of the detector type, detector
sensitivity, detector positions relative to each other and to the
monitoring space, field of view positions and shape, container
shape, container material, container position within the monitoring
space, waste shape, waste material, waste position within the
monitoring space and the like are fed to a computer model. Monte
Carlo Neutrons and Photons is one available package which can be
set up to simulate such a situation and receive the necessary
information. The information generally relates to issues of
geometry (positions, views etc), to issues of attenuation
(material, distances etc) and detector performance (sensitivity,
collimation etc).
[0247] Having formed a model of the whole container it is possible
to introduce one or more simulated sources to one or more positions
within the waste and consider the detector responses thereto and
hence again derive sensitivity values. The number and positions of
the sources could follow the useful structure provided by the equal
volume approach above (sources at mid points in an equal volume).
The number and positions of the sources could be quite different,
however, to reflect the versatility a simulation provides. Thus
sensitivity to homogeneously dispersed sources could be simulated
and/or a far larger number of sources could be simulated.
[0248] The overall result is the same, however, in that a
correction factor is obtained which can be used to correct the
monitored results on an unknown waste to give corrected results
fully reflecting all the activity present.
[0249] In many cases it will be desirable to verify the simulation
using a real world mock up of the type described above.
[0250] In the above mentioned embodiments the detectors are
provided over and to the sides of the container 1. As a result, the
problem volume (that furthest from the detectors) is at the centre
bottom thereof, core volume 54. To address this one or more further
detector could be positioned under the container, potentially
without the side detectors. This would improve the accuracy of the
measurement (by increasing the sensitivity of the system), change
the configuration of the volumes used (the core volume would be
more centrally positioned) and reduce the extent to which the core
volume was a problem (the chances of a volume being too far from at
least one of the detectors for there to be enough sensitivity is
reduced).
[0251] In a further embodiment, it is possible to use a different
detector configuration to look at the core volume. This can be
achieved using a detector for which the field of view is more
narrowly defined by its collimator(s) and with that field of view
centred on the hardest volume. As a result, that detector's counts
relate more specifically to the hardest volume, with counts from
elsewhere excluded, and so allow a more accurate accounting for
radioactivity in that volume.
[0252] As an extension of the above mentioned approaches a number
of techniques can be used where non-uniformity of waste source (and
hence applicable fingerprint) is likely or suspected.
[0253] As mentioned above, the measured results for certain
isotopes can be used to give the results for the difficult to
measure isotopes (and as a result the total activity) through the
use of a fingerprints applicable to that waste. A fingerprint is
the ratio of the various isotopes to one another and is determined
by a variety of physical/chemical analyses on a source of waste. A
further advantage of the segment based approach discussed above is
that, as illustrated in FIG. 6, specific fingerprints can be
assigned to each of the segments 70a, 70b, 70c, 70d, 70e, so as to
further increase the accuracy of the investigation.
[0254] In practice, a low number of fingerprints may be applicable
to the situation being investigated as only a low number of
different waste types are involved. To take an example of a nuclear
power plant, the waste type is either one with Co60 activity as the
dominant form due to the waste type arising from neutron activation
or is one in which Cs137 activity is the dominant form because the
waste type arose as a result of fuel pin failures or fission
product release.
[0255] Because of this, consideration of the measurements from the
detectors when considering one segment 70a may suggest Co60
predominating and thus a first waste and thus a first fingerprint
(Type A) applying. Consideration of another segment 70b may suggest
Cs137 predominating and thus a second waste type and thus a second
fingerprint (Type B) as being the most applicable. The fingerprint
allocated to each of the segments is then used in the calculation
of the difficult to measure isotopes for that segment 70. The
overall container result is then a combination of the results for
the different segments 70 summed together.
[0256] In other situations, other measurement results may suggest
other fingerprints. Either way, segment specific correction is
provided.
[0257] In an extension of this approach, the results from
individual detectors 17, 19 or combinations of less than all the
detectors 17, 19 can be considered to break the segments 70 up
still further. Thus with respect to segment 70c this could be
broken up into quadrants 72a, 72b, 72c, 72d and the nature of the
activity measured for each quadrant could lead to a fingerprint
specific to the waste in each. For instance, the signals from the
side detector 19 and first of the top detectors could be used to
interpret quadrant 72a and assign a specific fingerprint, Type A,
to it. Side detector 19 along might be used for quadrant 72c at the
bottom and so lead to the assignment of a Type B fingerprint.
[0258] Extending this principle still further, it is possible to
record the output from each detector at each measurement segment
separately and perform a deconvolution exercise, e.g. tomography,
on the data set to give a voxelated image of the activity
distribution. In effect the volume in the container would be broken
down into many thousands of voxels. The activity associated with
each small volume element, voxel, is obtained as a result and so an
appropriate fingerprint specific to each voxel can be assigned and
used in the calculation of the total activity etc for it. The
activity for all the voxels would then be summed to give the total
for the container. The voxel level approach has advantages as the
voxels are discrete and without overlap, so allowing more precise
allocation of fingerprints and avoiding the need for any adjustment
for overlap.
[0259] As an extension of the above mentioned approaches a number
of techniques can be used where non-uniformity of activity
distribution is suspected and/or needs to be verified. In general
these are variations of techniques mentioned above, but adapted to
put them to a different purpose.
[0260] In a first of the techniques, the results from individual
detectors 17, 19 or combinations of less than all the detectors 17,
19 can be considered to break the segments 70 up, for instance,
into quadrants 72a, 72b, 72c, 72d as discussed above. Rather than
considering the activity measured for a preponderance of Cs137 or
Co60 in respect of each quadrant, the approach considers the amount
of activity from the quadrant. For instance, the signals from the
side detector 19 and first of the top detectors could be used to
interpret quadrant 72a and the activity arising from it. The side
detector 19 might be used for quadrant 72c at the bottom and the
activity from it and so on. Differences in the activity between
quadrants would point towards non-uniform activity being present.
Furthermore, a rough indication as to the actual activity involved
and/or a rough indication as to its location could be provided.
[0261] The movement of the gantry relative to the waste in effect
allows a set of views of the waste to be taken from a variety of
directions (top, bottom, side) and with that being repeated at a
variety of different positions along the waste. Imaging of the
activity and its position is thus provided.
[0262] In the second such approach, it is possible to record the
output from each detector at each measurement segment separately
and perform a deconvolution exercise, e.g. tomography, on the data
set to give a voxelated image of the activity distribution, instead
of the isotope which is in preponderance. In effect the volume in
the container would be broken down into many thousands of voxels.
The activity associated with each small volume element, voxel, is
obtained as a result and so differences in the activity of one
voxel compared with the others can be established at a very
detailed level. A range of activities may be defined, with
activities falling within that range being accepted as evenly
distributed activity and with activities outside that range being
deemed non-uniform activity distribution. The detail can of course
be considered in terms of the actual level of activity for a voxel
and/or the position of that voxel can be considered in detail.
[0263] As an extension of the above mentioned approaches a number
of techniques can be used where non-uniformity of attenuation
within the waste is suspected and/or needs to be verified.
Non-uniformity of attenuation is a different issue to
non-uniformity of waste source. as different sources may have
different emissions associated with them, but still have the same
attenuation effect as the bulk of the waste, the matrix, is the
same (wood, soil etc). A number of forms of the invention for
investigating attenuation variation are possible.
[0264] In a first form, the adaptation addresses the issue that
even if detectors are provided below the container and are used to
examine the core (hardest/problem) volume, a core volume will
remains somewhere within the container. As a verification issue, or
for other reasons, the attenuation properties of the core volume
could be checked using this adaption. Either using another detector
type, or potentially using one of the same detectors, the pairing
of the detector with a transmission source is useful. The impact of
the core volume on transmissions from the source can be established
by comparing the transmission emissions with the detected emissions
are they have interacted with the volume of waste. The source could
be provided to one side with the detector to the other or with one
below and one above the container. This information on the effect
of the waste on the emissions would warn if high shielding material
was present in the core volume and hence the results were
unreliable as any emissions for that volume would be shielded more
than expected. Furthermore, such a situation being found to occur
over a number of runs on different containers would provide useful
information that the loading of the containers was not occurring in
the right way. For instance, dense waste was being dumped first in
the bottom centre for ease of handling, but with this being the
least desirable position for this type of waste for accuracy of
measurement due to overall attenuation effects, together with the
geometry effects. The information from the another detector type
could be shared with the waste handler/apparatus operator or be fed
only to verification personnel.
[0265] Once the above example only uses a transmission source to
investigate the core volume, because high shielding material there
would be of particular concern, the possibility exists to consider
the whole of the container and its volume of waste in this way. To
achieve this the effects of the waste lying along a number of paths
through the volume are monitored. A variety of ways of considering
the multiple paths exist. It is possible to move the transmission
source and the detector it is paired with to a variety of
positions. It is possible to move the transmission source so that
it is "aimed" at the detector along a different path. It is
possible to move the transmission source to a different position,
with the source pairing with a different detector already in
position (particularly where the detectors already exist in such
positions because they were used for the initial consideration of
the waste). It is possible to use multiple transmission sources
paired with multiple detectors.
[0266] In a second of these approaches, it is possible to monitor
and analyse two or more different energy emissions and consider the
impact of the attenuation effects of the waste volume upon them. If
two such energies are considered, then it is possible from
reference materials and/or consideration of unattenuated sources to
establish the ratio of the activity of the lower energy to the
higher energy of the pair. Attenuation will effect the lower energy
emissions to a greater extent than the higher energy emissions. The
lower and higher energies may come from the same isotope or from
different isotopes, for instance 662 keV from caesium and 1331 keV
from cobalt, particularly when the fingerprint of the waste is
known. For the expected materials and densities expected for the
volume of waste, it is possible to establish the extent of
variation in the ratio that will occur. If higher levels of
attenuation occur than expected and/or are desirable for accurate
monitoring using the techniques of this document, then this will
preferentially reduce the lower energy compared with the higher
energy emissions of the pair. Hence a ratio between the two outside
of the range of ratios expected reveals that the attenuation is
greater than expected.
[0267] The approach can be extended to expressed a measure of the
level of attenuation based upon the ratio encountered. The approach
can be extended further to provide a further correction factor for
this attenuation.
[0268] The approach preferably uses a pair of energies from the
same isotope so as to be confident of the unattenuated ratio and
hence the impact of attenuation. More than two energies can be
considered.
[0269] It is possible to combine this warning of non-uniform
attenuation with the type of transmission based investigation
detailed above to provide information on the position of the
increased attenuation. This can be extended to a still further
correction factor which accounts for the extent and position of the
increased attenuation.
[0270] The solutions the present invention provides are beneficial
in many respects. The technology used is based on components which
have been well tested in other uses and are established as
reliable. The approach is easy to use and is highly accurate. The
operation is flexible in terms of the different waste stream types
or fingerprints which can be accommodated and the range of
different waste item types which can be accommodated. This is
achieved using a reasonable assay time, for instance suited to over
night use. The data generated is useful for the immediate
interpretation of the waste and for subsequent analysis. Finally,
the approach allows cost savings in terms of the personnel who are
needed to make the determination (existing approaches require
health physics personnel who are in relatively short supply and are
hence expensive) as the approach does not require highly
specialised personnel. High levels of cost savings are also
provided through the increased utilisation of storage sites.
[0271] The present invention may be embodied in other specific
forms without departing from its spirit or essential
characteristics. The described embodiments are to be considered in
all respects only as illustrative. The scope of the invention is,
therefore, indicated by the appended claims rather than by the
foregoing description. All changes which come within the meaning
and range of equivalency of the claims are to be embraced with
their scope.
* * * * *