U.S. patent application number 10/122819 was filed with the patent office on 2003-10-16 for system and method for transferring spent nuclear fuel from a spent nuclear fuel pool to a storage cask.
Invention is credited to Agace, Stephen J., Singh, Krishna P..
Application Number | 20030194042 10/122819 |
Document ID | / |
Family ID | 28041128 |
Filed Date | 2003-10-16 |
United States Patent
Application |
20030194042 |
Kind Code |
A1 |
Singh, Krishna P. ; et
al. |
October 16, 2003 |
SYSTEM AND METHOD FOR TRANSFERRING SPENT NUCLEAR FUEL FROM A SPENT
NUCLEAR FUEL POOL TO A STORAGE CASK
Abstract
An apparatus, transfer cask, system, and method for defueling a
nuclear reactor and transferring spent nuclear fuel from a spent
nuclear fuel to a storage cask for long terms storage. In one
aspect, the invention is an apparatus for use in transferring a
canister of spent nuclear fuel from a transfer cask to a storage
cask, the apparatus comprising a radiation absorbing shield
surrounding a portion of a hole through which the canister can
pass; means for securing the apparatus to the top surface of the
storage cask; means for securing the bottom surface of the transfer
cask to the apparatus; wherein the transfer cask securing means and
the storage cask securing means are positioned on the apparatus so
that when the apparatus is secured to both the transfer cask and
the storage cask, the cavity of the transfer cask, the hole, and
the cavity of the storage cask are substantially aligned; and means
for moving the bottom lid in a horizontal direction once the bottom
lid is unfastened from the bottom surface. In another aspect the
invention is a transfer cask with a sealable bottom lid. In yet
another aspect, the invention is system comprising the above
described apparatus, transfer cask, and a storage cask. In still
another aspect, the invention is a method of using the system of
the present invention to defuel a nuclear reactor and transfer the
spent nuclear fuel form a spent nuclear fuel pool to a storage
cask.
Inventors: |
Singh, Krishna P.; (Palm
Harbor, FL) ; Agace, Stephen J.; (Maple Shade,
NJ) |
Correspondence
Address: |
Michael B. Fein
Cozen O'Connor
1900 Market Street
Philadelphia
PA
19103
US
|
Family ID: |
28041128 |
Appl. No.: |
10/122819 |
Filed: |
April 12, 2002 |
Current U.S.
Class: |
376/261 |
Current CPC
Class: |
Y10T 403/1608 20150115;
Y10T 403/1633 20150115; Y10T 403/1683 20150115; Y02E 30/30
20130101; G21F 7/005 20130101; Y10T 403/1666 20150115; Y10T
403/1641 20150115; Y10T 403/1691 20150115; G21C 19/32 20130101;
Y10T 403/16 20150115; Y10T 403/1616 20150115; Y10T 403/1624
20150115; Y10T 403/1658 20150115; Y10T 403/1649 20150115; G21F 5/12
20130101; G21F 5/008 20130101; Y10T 403/1674 20150115 |
Class at
Publication: |
376/261 |
International
Class: |
G21C 019/00 |
Claims
What is claimed is:
1. An apparatus for use in transferring a canister of spent nuclear
fuel from a transfer cask to a storage cask, the transfer cask
having a bottom surface, a bottom lid adapted to be secured to and
unfastened from the bottom surface, and a cavity adapted for
receiving the canister, the storage cask having a top surface and a
cavity adapted for receiving the canister, the apparatus
comprising: a radiation absorbing shield surrounding a portion of a
hole through which the canister can pass; means for securing the
apparatus to the top surface of the storage cask; means for
securing the bottom surface of the transfer cask to the apparatus;
wherein the transfer cask securing means and the storage cask
securing means are positioned on the apparatus so that when the
apparatus is secured to both the transfer cask and the storage
cask, the cavity of the transfer cask, the hole, and the cavity of
the storage cask are substantially aligned; and means for moving
the bottom lid in a horizontal direction once the bottom lid is
unfastened from the bottom surface.
2. The apparatus of claim 1 wherein the horizontal moving means is
adapted to move the bottom lid between an open and closed
position.
3. The apparatus of claim 2 wherein when the horizontal moving
means receives the bottom lid and moves the bottom lid to the open
position, the bottom lid together with the radiation absorbing
shield substantially surround and enclose the hole.
4. The apparatus of claim 3 wherein the bottom lid is circular and
the radiation absorbing shield is U-shaped comprising a
semi-circular portion and a pair of substantially parallel legs,
the diameter of the bottom lid being substantially equal to a
perpendicular distance between the legs.
5. The apparatus of claim 3 wherein the horizontal moving means
comprises a slidable tray and the radiation absorbing shield
comprises low friction tracks on which the slidable tray may
slide.
6. The apparatus of claim 1 comprising a top plate and a bottom
plate, the top and bottom plates having an opening through which
the canister can pass, the openings aligned with the hole.
7. The apparatus of claim 6 wherein the horizontal moving means
comprises a slidable tray and the bottom plate comprises low
friction tracks on which the slidable tray may slide.
8. The apparatus of claim 1 comprising means for lowering the
bottom lid of the transfer cask in a controlled manner onto the
horizontal moving means.
9. The apparatus of claim 8 wherein the lowering means are one or
more hydraulic or pneumatic lifters.
10. The apparatus of claim 9 wherein the pneumatic or hydraulic
lifters are located on the horizontal moving means.
11. The apparatus of claim 1 wherein the storage cask securing
means is a plurality of bolt holes, the apparatus being secured to
the top surface of the storage cask by extending bolts through the
plurality of bolt holes and threadily engaging threaded holes
located on the top surface of the storage cask.
12. The apparatus of claim 1 wherein the transfer cask securing
means is a plurality of threaded holes, the transfer cask being
secured to the apparatus by extending bolts through holes located
on the bottom surface of the transfer cask and threadily engaging
the plurality of threaded holes.
13. The apparatus of claim 1 wherein the radiation absorbing shield
is substantially U-shaped.
14. The apparatus of claim 1 wherein the radiation absorbing shield
is constructed of concrete, lead, on steel.
15. The apparatus of claim 1 wherein the transfer cask securing
means and storage cask securing means are positioned on the
apparatus so that the apparatus can be secured to and unfastened
from both the transfer cask and storage cask simultaneously.
16. A transfer cask for transporting a canister of spent nuclear
fuel from a spent nuclear fuel pool to a storage cask comprising: a
cylindrical body having a top surface, a bottom surface, and a
cavity adapted for receiving the canister, the bottom surface
comprising means for securing and unfastening a bottom lid and
means for securing to a mating device, the top surface comprising
means to secure a cask lid; a bottom lid, the bottom lid acting as
a floor for the cavity when secured to the bottom surface; a cask
lid; a bottom seal positioned between the bottom lid and the bottom
surface; and an annulus seal at or near the top surface of the
cylindrical body and positioned between the canister and the
cylindrical body when the canister is resting in the cavity.
17. The transfer cask of claim 16 wherein when the bottom lid is
secured to the bottom surface, a hermetic seal is formed.
18. The transfer cask of claim 16 wherein the means for securing
the bottom lid and means for securing to the mating device are
positioned on the bottom surface so that the bottom lid can be
unfastened and removed from the bottom surface while the transfer
cask is secured to a mating device.
19. The transfer cask of claim 16 wherein the bottom lid is
circular having a circumference and the bottom surface is circular
having a circumference, the circumference of the circular bottom
lid being smaller than the circumference of the bottom surface.
20. The transfer cask of claim 16 wherein the bottom surface of the
transfer cask is formed by a bottom flange.
21. The transfer cask of claim 20 wherein the means for securing a
bottom lid is a plurality of bottom lid bolt holes and the bottom
lid comprises a plurality of threaded holes, the bottom lid being
secured to the bottom flange by extending bolts through the bottom
lid bolt holes and threadily engaging the threaded holes of the
bottom lid.
22. The transfer cask of claim 20 wherein the means for securing to
a mating device is a plurality of mating device connection holes,
the transfer cask being secured to a mating device by extending
bolts through the mating device connection holes of the bottom
flange and threadily engaging threaded holes located on the mating
device.
23. The transfer cask of claim 20 wherein the bottom flange is
circular having an outer perimeter and the means for securing to
the mating device are closer to the outer perimeter than the means
for securing the bottom lid.
24. The transfer cask of claim 16 wherein the bottom seal comprises
a gasket fitted in a groove on the bottom lid.
25. The transfer cask of claim 16 wherein the annulus seal is a
circular gasket.
26. A system for transferring spent nuclear fuel from a spent
nuclear fuel pool to a storage cask comprising a fuel canister, a
transfer cask, a storage cask, and an apparatus according to claim
1; the storage cask comprising a top surface, means for securing
the apparatus, and a cavity adapted for receiving the canister; the
transfer cask comprising a bottom surface, a bottom lid adapted to
be secured and unfastened to the bottom surface, means for securing
to the apparatus, and a cavity adapted for containing the
canister.
27. The system of claim 26 wherein the transfer cask comprises a
bottom seal positioned between the bottom lid and the bottom
surface and an annulus seal positioned between the canister and the
transfer cask when the canister is contained in the transfer cask
cavity.
28. The system of claim 26 wherein the apparatus's transfer cask
securing means and storage cask securing means are positioned on
the apparatus so that the apparatus can be secured to and
unfastened from both the transfer cask and storage cask
simultaneously.
29. The system of claim 26 wherein the transfer cask's means for
securing to the apparatus are positioned on the transfer cask so
that the bottom lid can be unfastened and removed from the bottom
surface while the transfer cask is secured to the apparatus.
30. A method for transferring spent nuclear fuel from a reactor to
a storage cask comprising: submersing a transfer cask having a
removable bottom lid and a cavity containing a canister into a
spent nuclear fuel pool; placing spent nuclear fuel in the
canister; securing the apparatus of claim 1 to a storage cask
having a cavity adapted for receiving the canister; removing the
transfer cask from the pool and securing the transfer cask to the
apparatus; unfastening the bottom lid and horizontally moving the
bottom lid with the apparatus; and lowering the canister from the
transfer cask into the cavity of the storage cask.
31. The method of claim 30 comprising: securing a lid to the
canister after placing the spent nuclear fuel in the canister;
placing the transfer cask down in a staging area and preparing the
canister for dry storage; and securing a cask lid to the transfer
cask.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to the field of transporting and
storing spent nuclear fuel and specifically to transferring spent
nuclear fuel from a spent nuclear fuel pool to a storage cask.
[0002] In the operation of nuclear reactors, it is customary to
remove fuel assemblies after their energy has been depleted down to
a predetermined level. In the commercial nuclear industry, fuel
assemblies are typically an assemblage of long, hollow, zircaloy
tubes filled with enriched uranium. Upon depletion and subsequent
removal, spent nuclear fuel is still highly radioactive and
produces considerable heat, requiring that great care be taken in
its packaging, transporting, and storing. Specifically, spent
nuclear fuel emits extremely dangerous neutrons and gamma photons.
It is imperative that these neutrons and gamma photons be contained
at all times.
[0003] In defueling a nuclear reactor, the spent nuclear fuel is
removed from the reactor and placed in a canister that is submerged
in a spent nuclear fuel pool. The pool facilitates cooling of the
spent nuclear fuel and provides radiation shielding in addition to
that which is supplied by the canister. However, the canister alone
does not provide adequate containment of the radiation. As such, a
loaded canister cannot be removed or transported from the spent
nuclear fuel pool without additional radiation shielding. Because
it is preferable to store spent nuclear fuel in a "dry state," the
canister must eventually be removed from the spent nuclear fuel
pool. As such, apparatus that provide additional radiation
shielding during the transport and long-term storage of the spent
nuclear fuel are necessary.
[0004] In state of the art facilities, this additional radiation
shielding is achieved by placing the loaded canisters in large
cylindrical containers called casks. There are two types of casks
used in the industry today, storage casks and transfer casks. A
transfer cask is used to transport canisters of spent nuclear fuel
from location to location while a storage cask is used to store
spent nuclear fuel in the "dry state" for long periods of time.
Both transfer casks and storage casks have a cavity adapted to
receive a canister of spent nuclear fuel and are designed to shield
the environment from the radiation emitted by the spent nuclear
fuel.
[0005] Storage casks are designed to be large, heavy structures
made of steel, lead, concrete and an environmentally suitable
hydrogenous material. However, because the focus in designing a
storage cask is to provide adequate radiation shielding for the
long-term storage of spent nuclear fuel, size and weight are often
secondary considerations (if considered at all). As a result, the
weight and size of storage casks often cause problems associated
with lifting and handling. Typically, storage casks weigh
approximately 150 tons and have a height greater than 15 ft. As
such, a common problem associated with storage casks is that they
are too heavy to be lifted by most nuclear power plant cranes.
Another common problem is that storage casks are too large to be
placed in spent nuclear fuel pools. Thus, in order to store a
canister of spent nuclear fuel in a storage cask, the canister must
be removed from the pool, prepared in a staging area, and
transported to the storage cask. Adequate radiation shielding is
needed throughout all stages of this transfer procedure.
[0006] Removal from the storage pool and transport of the loaded
canister to the storage cask is infacilitated by a transfer cask.
In facilities utilizing transfer casks to transport loaded
canisters, an empty canister is placed into the cavity of an open
transfer cask. The canister and transfer cask are then submerged in
the storage pool. As each assembly of spent nuclear fuel is
depleted, it is removed from the reactor and lowered into the
storage pool and placed in the submerged canister (which is within
the transfer cask). The loaded canister is then fitted with its
lid, enclosing the spent nuclear fuel and water from the pool
within. The canister and transfer cask are then removed from the
pool by a crane and set down in a staging area to prepare the spent
nuclear fuel for storage in the "dry state." Once in the staging
area, the water contained in the canister is pumped out of the
canister. This is called dewatering. Once dewatered, the spent
nuclear fuel is dried using a suitable process such as vacuum
drying. Once dry, the canister is back-filled with an inert gas
such as helium. The canister is then sealed and the canister and
the transfer cask are once again lifted by the plant's crane and
transported to an open storage cask. The transfer cask is then
placed atop the storage cask and the canister is lowered into the
storage cask.
[0007] Because it is imperative that the loaded canister is not
directly exposed to the environment during the step of lowering the
canister from the transfer cask into the storage cask, transfer
casks have bottoms that can be withdrawn so that that the canister
can be lowered directly into the storage cask. In prior art
transfer casks, a rectangular compartment is attached to the bottom
of the transfer cask. Within this rectangular compartment are two
retractable sliding plates. When closed, these retractable plates
act as the floor of the transfer cask's cavity on which the loaded
canister rests. When fully retracted, the retractable plates leave
an unobstructed path leading from the transfer cask to the storage
cask through which the canister can be lowered. While the
retractable plates and rectangular compartment provide radiation
shielding for the canister as it passes between the transfer cask
and the storage cask, this transfer cask design and transfer
procedure have a number of deficiencies.
[0008] First off, it should be noted that the external surface of a
loaded canister is in continuous contact with the ambient air after
it is placed in a storage cask. Thus, it is desirable that the
external surface of the canister remain free of any radioactive
contamination. However, because it is virtually impossible to seal
the retractable plates because of the hardware (rollers, tracks,
etc.) required to make the plates retractable, the retractable
plates of prior art transfer casks are ineffective in preventing
the intrusion of pool water (which may contain radioactive
particulates in emulsion) into the space between the canister's
external surface and the walls of the transfer cask cavity. As
such, the external surface of the canister can become contaminated.
In order to deal with this threat of contamination, power plants
employ a variety of measures such as continuously flushing the
space with clean water from an external source. Such measures
greatly complicate the process of fuel loading in the pool, leading
to additional fuel loading time, added cost, and added risk to the
operations staff who must work above the pool.
[0009] Second, as mentioned above, the transfer of the canister
from the transfer cask to the storage cask occurs in a
configuration where the transfer cask is stacked atop the storage
cask. Because of the size of the transfer cask and storage cask,
this stack can be quite tall, reaching heights of over thirty-five
feet. Therefore, physical stability is a matter of concern,
especially if a seismic event were to occur. As such, it is
preferable to secure the transfer cask and the storage cask
together to make the stack more robust. However, the presence of
the retractable plate assembly at the bottom of the transfer cask
precludes the design opportunity to configure a fastening detail.
As a result, prior art transfer cask designs result in the
undesirable situation where the transfer cask and the storage cask
are stacked without being physically unconnected to each other.
[0010] Third, the retractable door assembly (including the
retractable plates and the rectangular compartment) is quite heavy,
reaching weights in excess of 12,000 lbs. As such, the area where
radiation shielding is most needed, namely the cylindrical body of
the transfer cask, must be made lighter to accommodate the heavy
bottom region in order to remain within the lifting capacity of the
power plant crane. Because the amount of radiation shielding
provided by the transfer cask's cylindrical body is directly
proportional to its weight, the heavy retractable door assembly
results in a reduced amount of radiation shielding.
[0011] Fourth, the hardware of the retractable door assembly, such
as the rollers and tracks, require lubricant or grease to work
properly. Submersing this lubricant in the pool can result in the
undesirable result of contaminating the pool water.
[0012] Finally, prior art transfer cask designs utilizing the
retractable door assembly may not fit into the spent fuel pools of
some nuclear power plants. This problem results because the
rectangular compartments often have a large footprint which is
necessitated by the presence of the retractable plates.
SUMMARY OF THE INVENTION
[0013] These and other problems are solved by the present invention
which in one aspect is an apparatus for use in transferring a
canister of spent nuclear fuel from a transfer cask to a storage
cask, the transfer cask having a bottom surface, a bottom lid
adapted to be secured to and unfastened from the bottom surface,
and a cavity adapted for receiving the canister, the storage cask
having a top surface and a cavity adapted for receiving the
canister, the apparatus comprising: a radiation absorbing shield
surrounding a portion of a hole through which the canister can
pass; means for securing the apparatus to the top surface of the
storage cask; means for securing the bottom surface of the transfer
cask to the apparatus; wherein the transfer cask securing means and
the storage cask securing means are positioned on the apparatus so
that when the apparatus is secured to both the transfer cask and
the storage cask, the cavity of the transfer cask, the hole, and
the cavity of the storage cask are substantially aligned; and means
for moving the bottom lid in a horizontal direction once the bottom
lid is unfastened from the bottom surface.
[0014] It is preferable that the horizontal moving means be adapted
to move the bottom lid between an open and closed position. When
the horizontal moving means is in the open position, an
unobstructed path is formed between the cavity of the transfer
cask, through the hole of the mating apparatus, and into the cavity
of the storage cask. When in the closed position, the horizontal
moving means is in a position to receive the bottom lid of the
transfer cask. Preferably, when the horizontal moving means
receives the bottom lid and moves the bottom lid to the open
position, the bottom lid together with the radiation absorbing
shield substantially surround and enclose the hole, the hole being
unobstructed. Also preferably, the bottom lid is circular and the
radiation absorbing shield is U-shaped comprising a semi-circular
portion and a pair of substantially parallel legs, the diameter of
the bottom lid being substantially equal to a perpendicular
distance between the legs. The horizontal moving means can comprise
a slidable tray and the radiation absorbing shield can comprise low
friction tracks on which the slidable tray may slide.
[0015] Preferably, the apparatus further comprises a top plate and
bottom plate, the top and bottom plates having an opening through
which the canister can pass, the openings substantially aligned
with the hole. In this embodiment, the horizontal moving means
comprises a slidable tray and the bottom plate comprises low
friction tracks on which the slidable tray may slide.
[0016] Moreover, it is preferable for the apparatus to further
comprise means for lowering the bottom lid of the transfer cask in
a controlled manner onto the horizontal moving means when the
transfer cask is secured to the apparatus and the bottom lid is
unfastened. These lowering means can be one or more pneumatic or
hydraulic lifters and can be located directly on the horizontal
moving means.
[0017] The apparatus's means for securing the apparatus to the
storage cask can be a plurality of bolt holes wherein the apparatus
is secured to the top surface of the storage cask by extending
bolts through the plurality of bolt holes and threadily engaging
threaded holes located on the top surface of the storage cask.
Additionally, the apparatus's means for securing the transfer cask
can be a plurality of threaded holes, the transfer cask being
secured to the apparatus by extending bolts through holes located
on the bottom surface of the transfer cask and theadily engaging
the plurality of threaded holes of the apparatus.
[0018] Preferably, the radiation absorbing shield is substantially
U-shaped and is constructed of concrete or lead. Also, preferably,
the means for securing the apparatus to the storage cask and the
means for securing the apparatus to the storage cask are positioned
on the apparatus so that the apparatus can be secured to and
unfastened from both the transfer cask and storage cask
simultaneously.
[0019] In another aspect, the invention is a transfer cask for
transporting a canister of spent nuclear fuel from a spent nuclear
fuel pool to a storage cask comprising a cylindrical body having a
top surface, a bottom surface, and a cavity adapted for receiving
the canister, the bottom surface comprising means for securing and
unfastening a bottom lid and means for securing to a mating device,
the top surface comprising means to secure a cask lid; a bottom
lid, the bottom lid acting as a floor for the cavity when secured
to the bottom surface; a cask lid; a bottom seal positioned between
the bottom lid and the bottom surface; and an annulus seal at or
near the top surface of the cylindrical body and positioned between
the canister and the cylindrical body when the canister is resting
in the cavity.
[0020] Preferably, when the bottom lid is secured to the bottom
surface, a hermetic seal is formed. It is also preferable that the
means for securing the bottom lid, and means for securing to the
mating device, be positioned on the bottom surface so that the
bottom lid can be unfastened and removed from the bottom surface
while the transfer cask is secured to a mating device. If the
bottom lid and bottom surface are circular, this can be
accomplished by the circumference of the circular bottom being
smaller than the circumference of the bottom surface.
[0021] Also, preferably, the bottom surface of the transfer cask is
formed by a bottom flange. In this embodiment, the means for
securing the bottom lid can be a plurality of bottom lid bolt holes
wherein the bottom lid would comprise a plurality of threaded
holes, the circular bottom lid being secured to the bottom flange
by extending bolts through the bottom lid bolt holes and threadily
engaging the threaded holes of the circular bottom lid.
Additionally with respect to this embodiment, the means for
securing to a mating device can be a plurality of mating device
connection holes, the transfer cask being secured to a mating
device by extending bolts through the mating device connection
holes of the bottom flange and threadily engaging threaded holes
located on the mating device. The bottom flange can be circular
having an outer perimeter wherein the means for securing to the
mating device are closer to the outer perimeter than the means for
securing the bottom lid.
[0022] The bottom seal can be a gasket fitted in a groove on the
bottom lid. Moreover, the annulus seal can be a circular
gasket.
[0023] In yet another aspect, the invention is a system for
transferring spent nuclear fuel from a spent nuclear fuel pool to a
storage cask comprising a fuel canister, a transfer cask, a storage
cask, and an apparatus as described above; the storage cask
comprising a top surface, means for securing the apparatus, and a
cavity adapted for receiving the canister; the transfer cask
comprising a bottom surface, a bottom lid adapted to be secured and
unfastened to the bottom surface, means for securing to the
apparatus, and a cavity adapted for containing the canister.
[0024] In regards to the system, it is preferable that the transfer
cask comprise a bottom seal positioned between the bottom lid and
the bottom surface and an annulus seal positioned between the
canister and the transfer cask when the canister is contained in
the transfer cask cavity. It is also preferable that the
apparatus's transfer cask securing means and storage cask id
securing means are positioned on the apparatus so that the
apparatus can be secured to and unfastened from the transfer cask
and storage cask simultaneously. Finally, the transfer cask's means
for securing to the apparatus are preferably positioned on the
transfer cask so that the bottom lid can be unfastened and removed
from the bottom surface while the transfer cask is secured to the
apparatus.
[0025] In still another aspect, the invention is a method for
transferring spent nuclear fuel from a reactor to a storage cask
comprising submersing a transfer cask having a removable bottom lid
and a cavity containing a canister into a spent nuclear fuel pool;
placing spent nuclear fuel in the canister; securing the apparatus
of claim 1 to a storage cask having a cavity adapted for receiving
the canister; removing the transfer cask from the pool and securing
the transfer cask to the apparatus; unfastening the bottom lid and
horizontally moving the bottom lid with the apparatus; and lowering
the canister from the transfer cask into the cavity of the storage
cask. It is preferable that this method further include the steps
of securing a lid to the canister after placing the spent nuclear
fuel in the canister; placing the transfer cask down in a staging
area and preparing the canister for dry storage; and securing a
cask lid to the transfer cask.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a perspective view of a prior art transfer cask
having a retractable door assembly.
[0027] FIG. 2 is a perspective view of a prior art transfer cask
having a retractable door assembly placed atop a storage cask with
a canister of spent nuclear fuel being lowered from the prior art
transfer cask into the storage cask.
[0028] FIG. 3 is a perspective view of an embodiment of the
transfer cask of the present invention, a transfer cask having a
sealable bottom lid.
[0029] FIG. 4 is a top view of an embodiment of a bottom lid used
to hermetically seal the bottom of the transfer cask of the present
invention.
[0030] FIG. 5 is a perspective view of an embodiment of the
transfer cask of the present invention partially in section.
[0031] FIG. 6 is a perspective view of an embodiment of the
transfer cask of the present invention partially in section and
loaded with a canister of spent nuclear fuel.
[0032] FIG. 7 is a perspective view of an embodiment of the
apparatus of the present invention, a cask mating device wherein
the mating device's slidable tray is in a closed position.
[0033] FIG. 8 is a perspective view of an embodiment of the
apparatus of the present invention, a cask mating device wherein
the slidable tray is in an open position.
[0034] FIG. 9 is a perspective view of an embodiment of the system
of the present invention, a transfer cask with a circular bottom
lid, a mating device, and a storage cask, wherein the system is in
a stacked arrangement.
[0035] FIG. 10 is a perspective view of an embodiment of the system
of the present invention in the stacked arrangement partially in
section wherein the transfer cask's bottom lid has been unfastened
and lowered onto the mating device's slidable tray which is in the
closed position.
[0036] FIG. 11 is a perspective view of an embodiment of the system
of the present invention in the stacked arrangement partially in
section wherein the slidable tray is in the open position and the
canister is being lowered into the storage cask.
[0037] FIG. 12 is a perspective view of an embodiment of the system
of the present invention in the stacked arrangement partially in
section wherein the slidable tray is in the open position and the
canister is fully lowered into the storage cask.
[0038] FIG. 13 is a flowchart of an embodiment of the method of the
present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 illustrates a prior art transfer cask 10 having
cylindrical body 11 and a retractable door assembly 12. In order to
shield the environment from the radiation emitted by a canister of
spent nuclear fuel once it is placed within cavity 13, cylindrical
body 11 is typically constructed of a gamma absorbing material such
as lead and a suitable hydrogenous material. Retractable door
assembly 12 comprises rectangular compartment 14. Rectangular
compartment 14 forms space 15 in which retractable plates 16, 17
(FIG. 2) are located. Prior art transfer cask 10 further comprises
cask lid 18 having lid hole 19.
[0040] Referring to FIG. 2, once prior art transfer cask 10 is
loaded with a canister 20 of spent nuclear fuel, transfer cask 10
is positioned above and set atop storage cask 21. This is done in
order to facilitate the transfer of canister 20 from transfer cask
10 to storage cask 21. However, as discussed in detail above, prior
art transfer cask 10 is not secured to storage cask 21 during this
process, transfer cask 10 merely rests atop storage cask 21. Once
prior art transfer cask 10 is placed atop storage cask 21,
retractable plates 16, 17 are moved to an open position.
Retractable plates 16, 17 comprise rollers that require lubricant
in order to move properly. Moving retractable plates 16, 17 to the
open position results in an unobstructed path being formed between
the cavity of transfer cask 10 and the cavity of storage cask 21.
As such, canister 20 can be lowered by a crane 22 from prior art
transfer cask 10 into storage cask 21 for permanent storage. As
discussed above, prior art transfer cask 10 has a number of
deficiencies.
[0041] FIG. 3 illustrates an embodiment of the present invention,
transfer cask 30 having sealable bottom lid 32. Transfer cask 33
comprises a cylindrical body 31, circular bottom lid 32, and cask
lid 33. Cask lid 31 comprises cavity hole 34 and a plurality of
cask lid bolt holes 43 circumferentially located around cask lid
31. Cavity hole 34 facilitates access to cavity 40 (FIG. 5) which
is necessary to perform certain canister transfer operations.
Cylindrical body 31 comprises bottom flange 35, top flange 36,
drain valve 37, fill hole 38, and crane handles 39. Bottom flange
35 comprises a plurality of bottom lid bolt holes 41 and a
plurality of mating device connection holes 42, both
circumferentially located around bottom flange 35. For the reasons
discussed below, mating device connection holes 42 are positioned
closer to the outer perimeter 55 of bottom flange 35 than bottom
lid bolt holes 41.
[0042] Referring to FIG. 4, circular bottom lid 32 comprises a
plurality of threaded holes 51 and circular groove 52. Circular
groove 52 is adapted to receive circular gasket 53 (FIG. 5).
Circular bottom lid 32 is preferably constructed of carbon steel
and is of a thickness that provides adequate radiation shielding.
Threaded holes 51 do not extend through the entire thickness of
circular bottom lid 32. Moreover, circular bottom lid 32 can be a
single circular plate or can be multiple circular plates welded or
otherwise fastened together. As illustrated, circular bottom lid 32
comprises two circular plates welded together (FIG. 5).
[0043] Referring to FIG. 5, cylindrical body 31 of transfer cask 30
is constructed so as to provide adequate radiation shielding for a
canister 50 (FIG. 6) of spent nuclear fuel placed within cavity 40.
Cylindrical body 31 comprises cylindrical inner shell 43. Inner
shell 43 forms cavity 40 within which canister 50 (FIG. 6) can be
placed when cask lid 33 is removed. Bottom lid 32 acts as the floor
of cavity 40 when secured. Cylindrical body 31 further comprises
cylindrical outer shell 44 which is concentric with and surrounds
inner shell 43. Both inner shell 43 and outer shell 44 are made
from carbon steel. Inner shell 43 and outer shell 44 are welded to
top flange 36 and bottom flange 35, forming an annulus 45 that is
capable of holding radiation absorbing material such as concrete,
lead, or steel. Lead is preferred because it most effectively
provides gamma shielding for the radioactive spent nuclear fuel
once it is placed within cavity 40.
[0044] Cylindrical body 31 further comprises jacket shell 46.
Jacket shell 46 is concentric with and surrounds outer shell 44.
Jacket shell 46 has top surface 47. The bottom of jacket shell 46
is welded to the top of bottom flange 35 while top surface 47 is
welded to outer shell 44, forming a second annulus 48, referred to
herein as "jacket 48." Jacket 48 is adapted for receiving a neutron
absorbing liquid such as water, which provides a layer of neutron
shielding for the radioactive spent nuclear fuel once it is placed
in cavity 40. In order to facilitate easy filling and draining of
jacket 48, jacket shell 46 comprises one or more drain valves 37
and one or more fill holes 38.
[0045] Additionally, transfer cask 30 comprises a plurality of
radial plates (not shown) that extend radially from outer shell 44
to jacket shell 46. The radial plates are circumferentially located
around transfer cask 30. Each radial plate is welded on one side to
outer shell 44 and to jacket shell 46 on the other side. The radial
plates act as fins for improved heat conduction.
[0046] In the illustrated embodiment, bottom flange 35 forms the
bottom surface of cylindrical body 31. Circular bottom lid 32 is
secured to bottom flange 35 by extending bolts 49 through bottom
lid bolt holes 41 (FIG. 3) and threadily engaging corresponding
threaded holes 51 (FIG. 4) located on circular bottom lid 32. As a
result, cavity 40 is formed wherein circular bottom lid 32 acts as
a floor. Before circular bottom 32 is secured to bottom flange 35,
circular gasket 53 is fitted circular groove 52 (FIG. 4). Upon
securing circular bottom lid 32 to bottom flange 35 by sufficiently
tightening bolts 49, circular gasket 52 hermetically seals the
bottom of cavity 40. As mentioned earlier bottom flange 35 further
comprises mating device connection holes 42 located closer to outer
perimeter 55 than bottom lid bolt holes 41. By positioning mating
device connection holes 42 sufficiently closer to outer perimeter
55 than bottom lid bolt holes 41, transfer cask 30 can be secured
to mating device 70 (FIG. 7) even when circular bottom lid 32 is
secured to bottom flange 35.
[0047] In the illustrated embodiment, top flange 36 forms the top
surface of cylindrical body 31. Top flange 36 comprises a plurality
of circumferentially located threaded holes 57. Cask lid 33 is
secured to cylindrical body 31 by extending bolts 54 through cask
lid holes 43 (FIG. 3) and threadily engaging the threaded holes 57
of top flange 36. Cask lid 33 is constructed of concrete and carbon
steel, so as to provide radiation shielding for the enclosed
canister 50 (FIG. 6). Cask lid 33 also comprises lid handles 56 for
facilitating the lifting and removing of cask lid 33.
[0048] Referring to FIG. 6, cavity 40 (FIG. 5) is adapted to
receive a canister 50 when cask lid 33 is removed from cylindrical
body 31. In the illustration, canister 50 is already placed into
cavity 40 and cask lid 33 is secured to top flange 36. When
canister 50 is in cavity 40, a small annulus (not labeled) is
formed between inner shell 43 and the external wall of canister 50.
This small annulus is a result of the diameter of canister 50 being
slightly smaller than the diameter of cavity 40. As discussed
earlier, gasket 53 hermetically seals the bottom of cavity 40 when
circular bottom lid 32 is secured to bottom flange 35. In order to
hermetically seal the top of cavity 40 when canister 50 is placed
therein, annulus seal 59 is positioned between top flange 36 and
the top of the external surface of canister 50. This results in the
small annulus being hermetically sealed from the top in addition to
the bottom. As such, the external surface of canister 50 is not
exposed to pool water when transfer cask 30 and canister 50 are
lowered into the pool as described below.
[0049] Finally, handles 39 facilitate crane 60 to engage, lift, and
transport transfer cask 30 throughout the defueling, transfer, and
storage procedures.
[0050] FIG. 7 illustrates an embodiment of the apparatus of the
present invention, a cask mating device 70 for use in transferring
a canister of spent nuclear fuel from the transfer cask of the
present invention to a storage cask. In the illustrated embodiment,
cask mating device 70 comprises top plate 71, bottom plate 72,
radiation absorbing shield 73, and slidable tray 74. Bottom plate
72 and top plate 71 are constructed of carbon steel and are
respectively welded to radiation absorbing shield 73 which
comprises substantially U-shaped steel walls filled with a
radiation absorbing material such as concrete.
[0051] Top plate 71 comprises a plurality of threaded holes 75, a
plurality of guide extrusions 76, and opening 80. Threaded holes 75
extend into radiation absorbing shield 73 and are used to secure
transfer cask 30 (FIG. 9) to mating device 70. Threaded holes 75
are positioned near and partially surround opening 80. Opening 80
is adapted so that it is large enough so that canister 50 (FIG. 6)
can pass through, 80 but small enough so that bottom flange 35
(FIG. 6) can rest on top plate 71 without falling into opening 80.
Top plate 71 further comprises guide extrusions 76 which help
correctly position transfer cask 30 (FIG. 9) atop mating device 70
when transfer cask 30 is being lowered onto and secured
thereto.
[0052] Bottom plate 71 comprises a plurality of storage cask
connection holes 77, low friction tracks 78, and opening 81 (FIG.
8). Opening 81 is substantially aligned with opening 80 and adapted
to be large enough so that canister 50 (FIG. 6) can pass through.
Cask connection holes 77 are located in recesses in radiation
absorbing shield 73. Storage cask connection holes 77 are used to
secure mating device 70 to the top surface of storage cask 90 (FIG.
9). In the illustrated embodiment there are three cask connection
holes 77 (although only one is visible). Bottom plate 72 further
comprises low friction tracks 78 for guiding the horizontal
movement of slidable tray 74. Low friction tracks 78 are
constructed so as to not require lubricant or grease in order for
slidable tray 74 to slide thereon. Specifically, low friction
tracks 78 are constructed of steel and comprise roller bearings
contained within steel guides, wherein only the roller bearings
contact slidable tray 74. Slidable tray 74 has guides (not shown)
welded to the bottom of slidable tray 74 to engage the roller
bearings and control the sliding motion.
[0053] As will be described in more detail below, slidable tray 74
comprises a plurality of pneumatic lifters 79 for controlled
lowering of circular bottom lid 32 (FIG. 4). Moreover, slidable
tray 74 comprises elevated ring 81 which is adapted to receive
circular bottom lid 32 (FIG. 4) and stabilize circular bottom lid
32 when it is resting on slidable tray 74. Slidable tray 74 is
constructed of steel and is capable of horizontal movement between
a closed and an open position.
[0054] In FIG. 7, slidable tray 74 is in the closed position. When
slidable tray 74 is in the closed 20 position, slidable tray 74
covers opening 81 (FIG. 8) and is positioned so as to be capable of
receiving circular bottom lid 32 (FIG. 4) when transfer cask 30 is
secured to mating device 70 (FIG. 9).
[0055] Referring to FIG. 8, slidable tray 74 is in the open
position. When slidable tray 74 is in the open position, slidable
tray 74 does not obstruct opening 81. As such, canister 50 can pass
from cavity 40 of transfer cask 30, through hole 83 and openings
80, 81, and into cavity 91 of storage cask 90 (FIG. 11). Radiation
absorbing shield 73 partially surrounds hole 83 through which
canister 50 can pass.
[0056] FIG. 13 is a flowchart of an embodiment of the method of the
present invention. The steps of FIG. 13 will be described in detail
below using mating device 70 and transfer cask 30. Specifically,
the steps of FIG. 13 will be discussed in relation to FIGS. 9-12
whenever possible.
[0057] In defueling a nuclear reactor and storing the spent nuclear
fuel according to the method of the present invention, initially
cask lid 33 is not secured to cylindrical body 31 of transfer cask
30 and canister lid 58 is not secured to canister 50. Open canister
50 is then lowered into cavity 40 of open transfer cask 30 wherein
circular bottom lid 32 is secured to bottom flange 35. Transfer
cask 30 (having open canister 50 within cavity 40) is then
submerged into a spent nuclear fuel pool, completing step 1300 of
FIG. 13. Once transfer cask 30 is fully submerged and resting at
the bottom of the spent nuclear fuel pool, spent nuclear fuel is
removed from the reactor as necessary and placed into open canister
50, completing step 1310 of FIG. 13. Once canister 50 is fully
loaded with spent nuclear fuel, canister lid 58 is secured to
canister 50, sealing both pool water and the spent nuclear fuel
within canister 50. As such, step 1320 of FIG. 13 is completed.
[0058] At this point, transfer cask 30 (and loaded canister 50) are
ready to be removed from the pool. However, before this occurs
mating device 70 is secured to storage cask 90 (FIG. 9). Referring
to FIG. 9, mating device 70 is secured to top surface 92 of storage
cask 90 by positioning mating device 70 on top surface 92 so that
cask connection holes 77 (FIG. 7) line up with threaded holes 93
(FIG. 10) located on top surface 92. Bolts 94 are then extended
through cask connection holes 77 threadily engaging threaded holes
93. Moreover, at this point, slidable tray 74 of mating device 70
is in the closed position (see FIG. 7). As such, step 1330 of FIG.
13 is completed.
[0059] Once step 1330 has been performed (or possibly during or
after), crane 60 (FIG. 6) completes step 1340 of FIG. 13 by lifting
transfer cask 30 (having loaded canister 50 in cavity 40) from the
pool. Transfer cask 30 is then set down in a staging area where the
pool water is pumped out of canister 50, the spent nuclear fuel is
allowed to dray, and the canister is backfilled with an inert gas
such as helium and then resealed. Canister 50 is now ready for dry
storage and step 1350 of FIG. 13 is completed.
[0060] At this point cask lid 33 (FIG. 3) is secured to cylindrical
body 31 as described above, completing step 1360. Closed transfer
cask 30 is then lifted by crane 60 and positioned above mating
device 70 which is secured to storage cask 90. Once transfer cask
30 is positioned above mating device 70, crane 60 lowers transfer
cask 30 down onto mating device 70 (see FIG. 9). As transfer cask
30 is being lowered onto top plate 71 of mating device 70,
extrusion guides 76 help guide transfer cask 30 to its proper
resting position. Transfer cask 30 is positioned so that the mating
device connection holes 42 (FIG. 3) on bottom flange 35 line up
with threaded holes 75 (FIG. 7) of mating device 70. Once properly
positioned, bolts 95 are extended through mating device connection
holes 42, threadily engaging threaded holes 75. As such, step 1370
of FIG. 13 is completed.
[0061] Referring to FIG. 10, once mating device 70 is properly
secured to both storage cask 90 and transfer cask 30, cavity 40
(FIG. 5) of transfer cask 30, hole 83 and openings 80, 81 (FIG. 8)
of mating device 70, and cavity 91 of storage cask 90 are
substantially aligned. Once properly secured together, circular
bottom lid 32 is unfastened from bottom flange 35 by removing bolts
49 (FIG. 5). Pneumatic lifters 79 (FIG. 7) engage circular bottom
lid 32 and lower circular bottom lid 32 onto slidable tray 74
within circular ring 81.
[0062] Referring to FIG. 11, once slidable tray 74 receives
circular bottom lid 32, slidable tray 74 is moved to the open
position as defined above. As such, slidable tray 74 slides on low
friction tracks 78, horizontally removing circular bottom lid 32 so
that a clear path through which canister 50 can pass from transfer
cask 30 into storage cask 90 is formed. Thus, step 1380 of FIG. 13
is completed. Moreover, when circular bottom lid 32 and slidable
tray 74 are moved to the open position, radiation absorbing shield
73 combined with circular bottom lid 32 substantially enclose the
space between transfer cask 30 and storage 90 through which
canister 50 will pass. In the illustrate embodiment, this is
accomplished by designing U-shaped radiation shield 73 so that the
diameter of circular bottom lid 32 is substantially equal to the
perpendicular distance between the legs (i.e. the straight portions
of the U-shape) of radiation shield 73. This design allows canister
50 to be lowered into storage cask 90 without radiation
contaminating the outside environment in unacceptable levels.
[0063] Upon a clear path being formed between cavity 40 of transfer
cask 30 and cavity 91 of storage cask 90, canister 50 is lowered
from cavity 40 into cavity 91 until canister 50 is fully within
storage cask 90 (FIG. 12). This lowering process is performed by
crane 60. Crane 60 engages canister handles 59 located on canister
lid 58 through cavity hole 34. In this way, crane 60, completes
step 1390 of FIG. 13.
[0064] The foregoing discussion discloses and describes merely
exemplary embodiments of the present invention. As will be
understood by those skilled in this art, the invention may be
embodied in other specific forms without departing from the spirit
or essential characteristics thereof. Specifically, mating device
70 can be constructed so as not to include top and bottom plates
71, 72. In this embodiment, slidable tray 74 would slide on low
friction trucks 78 which would be located on the interior of
radiation shield 73. In such an embodiment, the storage cask and
transfer cask are secured directly to the radiation shield.
Accordingly, the disclosure of the present invention is intended to
be illustrative, but not limiting, of the scope of the invention,
which is set forth in the following claims.
* * * * *