U.S. patent application number 10/084425 was filed with the patent office on 2002-07-04 for radioactive gas measurement apparatus and failed fuel detection system.
This patent application is currently assigned to Hitachi, Ltd.. Invention is credited to Izumi, Shigeru, Kaihara, Akihisa, Kitaguchi, Hiroshi, Matsui, Tetsuya, Yamagoshi, Atsushi.
Application Number | 20020084420 10/084425 |
Document ID | / |
Family ID | 18568659 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020084420 |
Kind Code |
A1 |
Kitaguchi, Hiroshi ; et
al. |
July 4, 2002 |
Radioactive gas measurement apparatus and failed fuel detection
system
Abstract
To provide a radioactive gas measurement apparatus that is
simply constructed and can efficiently measure Xe-133 in a
radioactive gas on-line under the condition that the radioactive
gas is mixed with interference N-13, an apparatus is provided for
measuring a radiation emitted from Xe-133, including an
anticoincidence counter circuit 13 that conducts counting if it
receives an output of a main detector 1 when it does not receive
outputs of scintillation detectors 2 and 9, and a gate circuit 14,
a plate-shaped semiconductor detector is used as the main detector
1, and a material not emitting a characteristic X ray in the range
from 70 to 90 keV is used for a shielding structure. In particular,
the thickness of the semiconductor detector 1 is set to fall within
a range from 2 mm to 7 mm, thereby improving the analysis
precision.
Inventors: |
Kitaguchi, Hiroshi; (Tokyo,
JP) ; Yamagoshi, Atsushi; (Tokyo, JP) ; Izumi,
Shigeru; (Tokyo, JP) ; Matsui, Tetsuya;
(Tokyo, JP) ; Kaihara, Akihisa; (Tokyo,
JP) |
Correspondence
Address: |
CROWELL & MORING LLP
INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
Hitachi, Ltd.
|
Family ID: |
18568659 |
Appl. No.: |
10/084425 |
Filed: |
February 28, 2002 |
Current U.S.
Class: |
250/370.01 |
Current CPC
Class: |
G01T 1/205 20130101 |
Class at
Publication: |
250/370.01 |
International
Class: |
G01T 001/24 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2000 |
JP |
2000-046190 |
Claims
What is claimed is:
1. A radioactive gas measurement apparatus, comprising: a radiation
detection system having a main detector and a sub-detector that are
arranged at positions diametrically opposed to each other with
respect to a sampling chamber, into or out of which a radioactive
gas flows, and a shield for shielding a background radiation
surrounding the detectors; and an anticoincidence counter circuit
in a measuring circuit, in which a particular radiation emitted
from the radioactive gas is measured with an anticoincidence count
processing using signals of both the detectors, wherein the main
detector is a plate-shaped semiconductor detector having a
thickness less than a diameter of a surface thereof orthogonal to
the thickness direction.
2. A radioactive gas measurement apparatus, comprising: a main
detector and a first sub-detector having the shape of a well and
surrounding the main detector that are arranged at one of two
positions diametrically opposed to each other with respect to a
sampling chamber, into or out of which a radioactive gas flows; a
second sub-detector arranged at the other of the two positions; and
an anticoincidence counter circuit as a measuring circuit, in which
a particular radiation emitted from the radioactive gas is measured
with an anticoincidence count processing using signals of the main
detector and two sub-detectors, wherein the main detector is a
detector made from a plate-shaped semiconductor having a thickness
less than a diameter of a surface thereof orthogonal to the
thickness direction.
3. The radioactive gas measurement apparatus according to claim 1
or 2, wherein the thickness of said plate-shaped semiconductor
detector is between 2 mm and 7 mm.
4. The radioactive gas measurement apparatus according to claim 1
or 2, wherein said shield for shielding the background radiation is
made of a material that does not emit a characteristic X ray within
a range of energy from 70 to 90 keV inclusive.
5. A failed fuel detection system, wherein radiation intensity
emitted from Xe-133 contained in an off-gas in a reactor condensate
system is measured by the radioactive gas measurement apparatus
according to one of claims 1 to 4, the measurement values are
collected on the time series, and the resulting time-series data is
analyzed to detect a fuel failure in a reactor.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a radioactive gas
measurement apparatus for measuring radiation of a radioactive gas
and a failed fuel detection system, and in particular, to a
radioactive gas measurement apparatus and a failed fuel detection
system suitable for measuring Xe-133 emitted when a fuel failure
occurs in a reactor.
[0003] 2. Description of the Prior Art
[0004] Detection of a fuel failure in a reactor is accomplished by
detecting a radioactive substance in a reactor water or in a gas.
In Japanese Patent Laid-Open No. 7-218638, for example, a failed
fuel detector that detects the concentration of I-131 in a reactor
water is disclosed. This failed fuel detector is configured to
measure the concentration of I-131, which is an index for failed
fuel detection, by suppressing the effect of a nuclide emitting
annihilation gamma rays contained in the reactor water.
[0005] FIG. 9 shows an example of a conventional failed fuel
detector intended to measure radioactive gas. In this conventional
failed fuel detector, a delay tank 41 is provided in a discharge
pipe 40 for radioactive gas (primarily containing a bleed air in a
reactor condensate system and referred to as an off-gas), and a
sampling chamber 43 and an ionization chamber detector 44 both
enclosed by a lead shield 42 are provided downstream of the delay
tank to monitor the radiation intensity level of the radioactive
gas. An index for the failed fuel detection is the concentration of
Xe-133 in the radioactive gas. The delay tank 41 is provided
because nitrogen-13 contained in a gas in quantity (N-13, having a
half-life of 10 minutes and produced in a (p, .alpha.) reaction of
0-16) interferes with the measurement of the index in the
radioactive gas, and without a measure against nitrogen, it is
difficult to accurately measure the Xe-133 indicative of the fuel
failure. Specifically, this is due to the fact that N-13 emits
annihilation gamma rays of 511 keV and the low-energy gamma rays
(81 keV) of Xe-133 are hidden in the Compton background thereof.
Thus, in order to reduce N-13, a residence time of about 1 hour in
the delay tank 41 is provided to remove N-13 before measuring the
Xe-133 by the radiation level monitor 44 in the ionization
chamber.
[0006] Furthermore, in Japanese Patent Laid-Open No. 62-6199
("Off-Gas Monitor"), there is disclosed a method for determining a
quantitative value of Xe-133 by detecting the intensity of gamma
rays in the off-gas with a NaI detector and a CaTe detector and
processing the value with a computer. In addition, in Japanese
Patent Laid-Open No. 3-138593 ("Exhaust Gas Radiation Monitoring
Apparatus"), there is disclosed a method in which gamma rays are
detected after N-13 is removed from an exhaust gas by taking
advantage of the fact that the ion thereof is a negative ion.
[0007] In order to detect a fuel failure, an index in a reactor
water or a gas needs to be measured quickly and precisely.
Therefore, failed fuel detection is desirably conducted by
monitoring gas, which exhibits the index earlier than a reactor
water. However, in the conventional example shown in FIG. 9,
measurement is conducted on the gas after passing through the delay
tank, so that the failed fuel detection can only be conducted after
about 1 hour. In addition, since a level monitor, such as an
ionization chamber, is used for measuring radiation, accurate
identification (analysis of nuclide) of Xe-133 is impossible.
Besides, the technique described in Japanese Patent Laid-Open No.
62-6199 involves an attenuation pipe for attenuating the
radioactivity of the radioactive material having a short half-life
before measuring gamma rays of a gas, so that the detection is
delayed. According to the technique described in Japanese Patent
Laid-Open No. 3-138593, a delay in the detection of gamma rays of
Xe-133 due to removal of N-13 is avoided, but the size of the
apparatus becomes large.
[0008] Alternatively, the failed fuel detector intended for a
reactor water described in Japanese Patent Laid-Open No. 7-218638
may be applied to a radioactive gas. In such a case, however, there
is a large quantity of annihilation gamma rays of 511 keV from N-13
launched into a main detector (detector for measuring Xe-133) and a
Compton scattering component, so that the precision of the analysis
of Xe-133 is relatively significantly degraded. This is because the
energy of the gamma rays emitted from Xe-133 is 81 keV, which is
lower than the energy of the gamma rays emitted from I-131, which
is the index for the failed fuel detection in terms of reactor
water, of 364 keV. In other words, this is because the lower the
energy of an index, the more significant the effect of the Compton
scattering component is.
SUMMARY OF THE INVENTION
[0009] An object of the present invention is to provide a
radioactive gas measurement apparatus that can measure a
concentration of Xe-133 in a radioactive gas in a short time, with
a simple construction, under the condition that the radioactive gas
is mixed with interference N-13.
[0010] According to the present invention, there is provided a
radioactive gas measurement apparatus, comprising: a radiation
detection system having a main detector and a sub-detector that are
arranged at positions diametrically opposed to each other with
respect to a sampling chamber, into or out of which a radioactive
gas flows, and a shield for shielding a background radiation
surrounding the detectors; and an anticoincidence counter circuit
in a measuring circuit, in which a particular radiation emitted
from the radioactive gas is measured with an anticoincidence count
processing using signals of both the detectors, characterized in
that the main detector is a plate-shaped semiconductor detector
having a thickness less than a diameter of a surface thereof
orthogonal to the thickness direction.
[0011] Furthermore, according to the present invention, there is
provided a radioactive gas measurement apparatus, comprising: a
main detector and a first sub-detector having the shape of a well
and surrounding the main detector that are arranged at one of two
positions diametrically opposed to each other with respect to a
sampling chamber, into or out of which a radioactive gas flows; a
second sub-detector arranged at the other of the two positions; and
an anticoincidence counter circuit as a measuring circuit, in which
a particular radiation emitted from the radioactive gas is measured
with an anticoincidence count processing using signals of the main
detector and two sub-detectors, characterized in that the main
detector is a detector made from a plate-shaped semiconductor
having a thickness less than a diameter of a surface thereof
orthogonal to the thickness direction.
[0012] Furthermore, according to the present invention, in the
above-described radioactive gas measurement apparatus, the
thickness of the plate-shaped semiconductor detector is between 2
mm and 7 mm.
[0013] Furthermore, according to the present invention, in the
above-described radioactive gas measurement apparatus, the shield
for shielding the background radiation is made of a material that
does not emit a characteristic X ray within a range of energy from
70 to 90 keV inclusive.
[0014] Furthermore, according to the present invention, there is
provided a failed fuel detection system, characterized in that
radiation intensity emitted from Xe-133 contained in an off-gas in
a reactor condensate system is measured by the above-described
radioactive gas measurement apparatus, the measurement values are
collected on the time series, and the resulting time-series data is
analyzed to detect a fuel failure in a reactor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows a construction of a radioactive gas measurement
apparatus according to this invention;
[0016] FIG. 2 is a time chart of measurement of Xe-133;
[0017] FIG. 3 is a graph showing a relationship between a thickness
of a detection layer and an absorption amount of gamma rays in a
main detector;
[0018] FIG. 4 is a graph showing a relationship between a thickness
of a detector and an analysis error of Xe-133;
[0019] FIG. 5 shows an example of a gamma-ray spectrum of a
radioactive gas measured by the apparatus shown in FIG. 1;
[0020] FIG. 6 shows an enlarged spectrum for a Xe-133 detection
region;
[0021] FIG. 7 shows a modification of the apparatus according to
this invention;
[0022] FIG. 8 shows a construction of a radioactive gas sampling
chamber; and
[0023] FIG. 9 shows a conventional failed fuel detector.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] An embodiment of the present invention will be described
below with reference to the drawings. FIG. 1 shows a construction
of a radioactive gas measurement apparatus according to this
invention, in which a plate-shaped main detector (a semiconductor
detector) 1 for detecting Xe-133 is provided in a well-type NaI
(T1) scintillation detector 2 for anticoincidence counting. A
signal line 4 of the main detector 1 and a signal line 4' of the
scintillation detector 2 (an output of a photomultiplier tube 3)
are drawn out via a bent hole 6 formed in a rear portion of a lead
shield 5. The bent hole 6 is intended to prevent background
radiation from entering from the outside. A sampling chamber 8,
into or out of which the radioactive gas to be measured flows, is
disposed in front of the main detector 1 via a collimator 7 of the
shield 5. At a position 180 degrees opposite to the scintillation
detector 2 with respect to the sampling chamber 8, another
scintillation detector 9 including a photomultiplier tube 3' is
provided via a collimator 7' of a similar shield 5'. As in the case
of the side of the main detector 1, a signal line 10 is drawn out
via a bent hole 6'.
[0025] The collimator 7 of the main detector 1 and the collimator
7' on the side of the scintillation detector 9 are provided within
the range allowing for the sampling chamber 8 (within the range of
angle defined by lines a and b in FIG. 1). This arrangement is
intended to prevent the annihilation gamma rays (511 keV) emitted
from N-13 in the sampling chamber one in each of the direction 180
degrees opposite to each other from being detected by only one of
the main detector 1 and the scintillation detector (sub-detector) 9
so that they are always detected by both the detectors when they
are detected. This arrangement is realized by adjustment of the
depth or aperture of the collimators 7'.
[0026] Next, a construction of a signal processing circuit will be
described. Each of the output signal lines 4, 4' and 10 of the main
detector 1, the scintillation detector 2 and the opposing
scintillation detector 9, respectively, has a preamplifier 11 and a
linear amplifier 12, and is connected to an anticoincidence counter
circuit 13 downstream of the amplifiers. In addition, the output of
the amplifier 12 in the signal line 4 of the main detector 1
branches off from the line leading to the anticoincidence counter
circuit 13 and connected to a gate circuit 14. The anticoincidence
counter circuit 13 outputs a gate signal G to the gate circuit 14
if both of signals S2 and S3 obtained by amplifying the outputs of
the scintillation detectors 2 and 9, respectively, don't exist when
the circuit is synchronized with a signal S1 obtained by amplifying
the output of the main detector 1. The gate circuit 14 inputs the
input signal S1 to a pulse-height analyzer 15 only when it receives
the gate signal G. Based on the result of the pulse-height analysis
by the analyzer 15, an analyzer 16 identifies Xe-133, and then a
fuel failure determination apparatus consisting of a data processor
17 and a display 18 determines the presence of a fuel failure and
displays the result. The detection level of the fuel failure is
determined according to the value of the statistical error .sigma.
(square root of the actual count value) of the count value of
Xe-133 under the condition of no fuel failure. That is, the
detection is based on whether the count value of Xe-133 is higher
than the statistical error .sigma. or not. Alternatively, the
criterion may be 2.sigma. or 3.sigma.. As for the selection of the
criterion k.sigma., a small constant k is selected when the
sensitivity of the failure determination is to be increased, and a
large constant k is selected when the precision of the failure
determination is to be increased.
[0027] FIG. 2 is a time chart showing an example of the
above-described measurement of Xe-133. At time A, the main detector
land other detectors 2, 9 simultaneously detect gamma rays, and the
gate signal G is not output. Similarly, at time B, the detection
signal S2 of the detector 2 is detected in synchronization with the
detection signal S1 of the main detector 1, so that the gate signal
G is not output. At time C, the signals of the detectors 2 and 9
are not detected in synchronization with the detection signal S1 of
the main detector 1, and the gate signal G is output. In this way,
Xe-133 is measured by taking advantage of Compton suppression
(removal of Compton scattered rays) including annihilation gamma
rays. This measurement allows the annihilation gamma rays of N-13
discharged from the sampling chamber, or Compton components of the
annihilation gamma rays and other gamma rays to be eliminated, so
that only the gamma rays solely launched into the main detector 1
can be efficiently measured without the need to physically remove
components such as N-13.
[0028] FIG. 3 shows a relationship between a thickness of a
detection layer and an absorption amount of gamma rays (detection
sensitivity) in the main detector 1 for cases where the energy of
the gamma rays is 81 keV (Xe-133) and 511 keV (N-13), in which the
material of the detector is germanium. As for the absorption amount
a of gamma rays of 81 keV, the sensitivity becomes saturated when
the detector thickness is on the order of 5 mm. That is, even if
the thickness of the detector is further increased, the sensitivity
cannot be further increased. As for the absorption amount .beta. of
gamma rays of 511 keV, the sensitivity continues to be increased
even after the detector thickness exceeds 40 mm. However, each of
the absorption amounts .alpha. and .beta. represents a value scaled
according to the saturation value thereof, which corresponds to the
detection efficiency. In addition, the scale shown on the right in
FIG. 3 shows an absorption ratio R of gamma rays (511 keV/81 keV).
From this relationship, it can be seen that a thinner detector
results in a lower absorption ratio R, so that it is more
advantageous for the measurement of the gamma rays of 81 keV.
[0029] Next, the degree to which the thickness of the detector can
be reduced will be described. On the assumption that reference
character Np denotes the intensity (concentration) of the gamma
rays of 81 keV from Xe-133 in a measurement region, reference
character Nb denotes the background intensity (primarily containing
Compton tails of annihilation gamma rays (511 keV) from N-13), and
the main detector 1 has the characteristics shown in FIG. 3, the
analysis error .DELTA. of a gamma rays spectrum in a pulse-height
analysis is represented by:
Formula 1
.DELTA.=(.alpha..multidot.N.sub.p+2
.beta..multidot.N.sub.b).sup.1/2/(.alp- ha..multidot.N.sub.p)
[0030] Here, if an amount .zeta. equivalent to the reciprocal of
the signal-to-noise ratio is defined as:
Formula 2
.zeta.=N.sub.b/N.sub.p
[0031] then the analysis error .DELTA. is given by:
Formula 3
.DELTA.=(.alpha..multidot.N.sub.p+2.beta..multidot..zeta..multidot.N.sub.b-
).sup.1/2/(.alpha..multidot.N.sub.p)
[0032] FIG. 4 shows the analysis error A calculated from Formula 3
using .zeta. as a parameter, after determining the detection
efficiencies .alpha. and .beta. of the germanium semiconductor
detector on the assumption that the value Np is fixed and the
detector thickness is variable.
[0033] From this drawing, the optimum range of the detector
thickness is from 2 mm to 7 mm. When the thickness is less than 2
mm, the detection efficiency of Xe-133 itself is extremely reduced,
so that the measurement time needs to be extended in order to
maintain the level of analysis precision. In addition, it can be
seen that, in this relationship, when the concentration of N-13,
which is defined as the background noise here, is increased so that
the value of .zeta. is increased, the effect thereof becomes
remarkable. Furthermore, it can be seen that the ratio of N-13 to
Xe-133 contained in the off-gas maybe equal to or higher than 1000
(.zeta.=1000), and a plate-shaped detector having a thickness
within the range from 2 mm to 7 mm is most preferably used as the
main detector 1 of this invention. In addition, even if the
detector thickness is on the order of 30 mm, increasing the SIN
enables the analysis precision to be improved compared with a
conventional detector having greater thickness. In this regard, the
"plate-shaped detector" herein refers to a detector having a ratio
(t/L) between the thickness t and a value L (diameter in the case
of a circular detector, or length of a diagonal line in the case of
a rectangular detector) is equal to or less than 1.
[0034] FIG. 5 shows an example of a gamma-ray spectrum of a
radioactive gas measured by the apparatus according to this
invention. FIG. 5 shows an untreated spectrum 20 obtained by the
main detector 1, and a spectrum 21 obtained by subjecting the
scintillation detectors 2 and 9 to the Compton suppression. In the
spectrum 21, the annihilation gamma rays 22 from N-13 is reduced,
the Compton component 23 in the Xe-133 detection region 23 (energy
range from 70 to 90 keV) is reduced by one or more order of
magnitude, and thus the spectrum including Xe-133 (81 keV) is
distinguished, which indicates that the measurement error of Xe-133
is improved by a factor of 3 or more.
[0035] The spectrum analysis of low-energy gamma rays (81 keV) from
Xe-133 is seriously affected by the characteristic x ray, as well
as the Compton component. In particular, the lead shield typically
used emits the characteristic X rays of K.alpha. (75 keV),
K.alpha.2 (72.8 keV), K.beta.1 (84.9 keV), and K.beta.2 (87.3 keV),
which seriously affect the analysis of Xe-133 (81 keV). Therefore,
the apparatus for measuring a radioactive gas advantageously
includes a shield made of a material that does not emit the
characteristic X ray in the Xe-133 detection region such as iron,
copper, or stainless steel, or has a lining (denoted by reference
numeral 19 in FIG. 1) made of such a material. For example, if the
material is iron, the thickness of the lining is preferably equal
to or more than 5 mm.
[0036] FIG. 6 shows an enlarged spectrum for the Xe-133 detection
region 23 in FIG. 5. The spectrum in the region 23 in FIG. 5
includes both the characteristic X ray from the lead shield and the
gamma rays of 81 keV from Xe-133. Eliminating the characteristic X
ray by the above-described shield results in the spectrum 24, which
is obviously different from the spectrum in the region 23. In this
way, the analysis precision and sensitivity of Xe-133 can be
improved by one or more order of magnitude. This has the effect of
significantly improving the performance of the failed fuel
detection based on the detection of Xe-133 as an index. In
addition, since the failed fuel detection apparatus includes no
delay tank for attenuating N-13, it is excellent as an on-line
apparatus.
[0037] FIG. 7 shows a modification of the apparatus according to
this invention, which has a simple construction in which only the
scintillation detector 30 is provided at a position opposed to the
main detector 1 with respect to the sampling chamber 8 for the
radioactive gas. With this construction, the detection sensitivity
of Xe-133 is somewhat sacrificed. However, since unlike the
apparatus in FIG. 1, the well-type scintillation detector 2
surrounding the main detector 1 is not provided, the whole
detection section including the shielding structure can be
downsized.
[0038] FIG. 8 shows a construction of a radioactive gas sampling
chamber. In a suitable construction, the sampling chamber 8
comprises a thin detection window intended to efficiently detect
the low energy gamma rays from Xe-133 on the side of the main
detector 1. If the detection window is made of iron or stainless
steel, the thickness thereof is desirably on the order of 0.5
mm.
[0039] While in the above description, the Ge semiconductor
detector is used for the main detector, it may be replaced with
various semiconductor detectors such as those made of CdTe, CZT, or
GaAs. Furthermore, besides NaI (T1), the scintillation detectors 2
and 9 for anticoincidence count processing may have a construction
including various scintillators such as a plastic scintillator,
BGO, or CsI. Also, a construction including a semiconductor
detector made of Ge or the like as the detector for anticoincidence
count processing provides the same effect. A fuel failure can be
detected with reliability at an early stage by using the radiation
detection apparatus thus constructed to measure the concentration
of Xe-133 in the off-gas in the reactor condense system, collecting
the time-series data thereof, and monitoring the collected
data.
[0040] According to the present invention, a radioactive gas
measurement apparatus with a high performance that can efficiently
measure Xe-133 (81 keV) in a radioactive gas containing N-13 with a
simple construction can be provided.
* * * * *