U.S. patent application number 13/213389 was filed with the patent office on 2013-02-21 for nuclear reactor refueling methods and apparatuses.
The applicant listed for this patent is Mathew W. Ales, Lewis A. Walton. Invention is credited to Mathew W. Ales, Lewis A. Walton.
Application Number | 20130044850 13/213389 |
Document ID | / |
Family ID | 47712668 |
Filed Date | 2013-02-21 |
United States Patent
Application |
20130044850 |
Kind Code |
A1 |
Walton; Lewis A. ; et
al. |
February 21, 2013 |
NUCLEAR REACTOR REFUELING METHODS AND APPARATUSES
Abstract
Refueling of a nuclear reactor (40) includes removing a fuel
assembly (10). The removal method includes lowering a lifting tool
(80) of a crane (44) onto a top of the fuel assembly. The lowered
lifting tool including a plurality of downwardly extending elements
(82) that surround and vertically overlap a portion (74) of a
control rod assembly (70) extending above the top of the fuel
assembly. The downwardly extending elements are locked with
corresponding mating features (26) at the top of the fuel assembly
to connect the lifting tool with the fuel assembly. The connected
fuel assembly is moved into a spent fuel pool (42) using the crane,
and the lifting tool is disconnected from the top of the fuel
assembly by unlocking the downwardly extending elements from the
corresponding mating features at the top of the fuel assembly.
Inventors: |
Walton; Lewis A.; (Forest,
VA) ; Ales; Mathew W.; (Forest, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Walton; Lewis A.
Ales; Mathew W. |
Forest
Forest |
VA
VA |
US
US |
|
|
Family ID: |
47712668 |
Appl. No.: |
13/213389 |
Filed: |
August 19, 2011 |
Current U.S.
Class: |
376/264 |
Current CPC
Class: |
G21C 19/10 20130101;
G21C 19/205 20130101; Y02E 30/30 20130101 |
Class at
Publication: |
376/264 |
International
Class: |
G21C 19/10 20060101
G21C019/10 |
Claims
1. A method comprising: performing refueling of a nuclear reactor,
the refueling including removing a fuel assembly from a reactor
core of the nuclear reactor by a removal method including:
connecting a lifting tool of a crane with a top of the fuel
assembly, the lifting tool comprising an assembly of downwardly
extending elements, the connecting including locking lower ends of
the downwardly extending elements with respective mating features
located at a top and periphery of the fuel assembly; moving the
fuel assembly connected with the lifting tool into a spent fuel
pool using the crane; and releasing the lifting tool from the top
of the fuel assembly, the releasing including unlocking the lower
ends of the downwardly extending elements from the respective
peripherally located mating features at the top and periphery of
the fuel assembly.
2. The method of claim 1, wherein the removal method does not
include removing a control rod assembly (CRA) that is inserted in
the fuel assembly and wherein the CRA vertically overlaps the
lifting tool when the fuel assembly is connected with the lifting
tool.
3. The method of claim 2, wherein the overlap between the CRA and
the lifting tool is at least one-half of the vertical height of the
lifting tool.
4. The method of claim 2, wherein the overlap between the CRA and
the lifting tool is at least one-half of the vertical height of the
downwardly extending elements of the lifting tool.
5. The method of claim 4, wherein the downwardly extending elements
comprise downwardly extending vertical rods or bars.
6. The method of claim 1, wherein the removal method does not
include removing a control rod assembly (CRA) that is inserted in
the fuel assembly and wherein the lower ends of the downwardly
extending elements of the lifting tool surround an upper end of the
inserted CRA when the fuel assembly is connected with the lifting
tool.
7. The method of claim 1, wherein the peripherally located mating
features at the top and periphery of the fuel assembly are selected
from a group consisting of protrusions, openings, and recesses.
8. The method of claim 1, wherein the locking comprises: rotating
at least the lower ends of the downwardly extending elements to
lock the lower ends disposed over or inside the respective mating
features with the respective mating features.
9. The method of claim 8, wherein the lower ends of the downwardly
extending elements and the respective mating features define
lockable bayonet or J-lock couplings.
10. A method comprising: performing refueling of a nuclear reactor,
the refueling including removing a fuel assembly having a control
rod assembly (CRA) inserted in the fuel assembly from a reactor
core of the nuclear reactor by a removal method including: lowering
a lifting tool of a crane onto a top of the fuel assembly, the
lowered lifting tool including a plurality of downwardly extending
elements that surround and vertically overlap a portion of the CRA
extending above the top of the fuel assembly; locking the
downwardly extending elements of the lowered lifting tool with
corresponding mating features at the top of the fuel assembly in
order to connect the lifting tool with the fuel assembly; moving
the fuel assembly connected with the lifting tool into a spent fuel
pool using the crane; and disconnecting the lifting tool from the
top of the fuel assembly in the spent fuel pool by unlocking the
downwardly extending elements from the corresponding mating
features at the top of the fuel assembly.
11. The method of claim 10, wherein the downwardly extending
elements of the lowered lifting tool vertically overlap the portion
of the CRA extending above the top of the fuel assembly by at least
one-half of a vertical height of the downwardly extending
elements.
12. The method of claim 10, wherein the corresponding mating
features at the top of the fuel assembly are selected from a group
consisting of protrusions, openings, and recesses.
13. The method of claim 10, wherein the locking comprises: rotating
at least lower ends of the downwardly extending elements to lock
the downwardly extending elements with the corresponding mating
features at the top of the fuel assembly.
14. The method of claim 10, wherein the locking comprises engaging
bayonet or J-lock couplings between the downwardly extending
elements and the corresponding mating features at the top of the
fuel assembly.
15. The method of claim 10, wherein the downwardly extending
elements do not pivot during the locking.
16. The method of claim 10, wherein the plurality of downwardly
extending elements consists of at least three downwardly extending
elements.
17. The method of claim 10, wherein the plurality of downwardly
extending elements consists of N downwardly extending elements
where N is at least four.
18. An apparatus comprising: a lifting tool including: an upper end
configured for attachment with a crane; and a plurality of
downwardly extending elements surrounding an open central region
disposed below the upper end, lower ends of the downwardly
extending elements being configured to mate with mating features at
the top of a nuclear fuel assembly.
19. The apparatus of claim 18, further comprising: the nuclear fuel
assembly including the mating features at the top of the nuclear
fuel assembly.
20. The apparatus of claim 19, further comprising: a control rod
assembly (CRA) inserted in the nuclear fuel assembly with an upper
end of the CRA extending out of the top of the nuclear fuel
assembly; wherein the open central region of the lifting tool that
is surrounded by the plurality of downwardly extending elements is
configured to receive the upper end of the CRA when the lower ends
of the downwardly extending elements mate with the mating features
at the top of the nuclear fuel assembly.
21. The apparatus of claim 20, wherein the downwardly extending
elements of the lifting tool comprise downwardly extending rods or
bars.
22. The apparatus of claim 21, wherein the downwardly extending
rods or bars are oriented vertically.
23. The apparatus of claim 21, wherein the plurality of downwardly
extending rods or bars consists of at least three downwardly
extending rods or bars.
24. The apparatus of claim 21, wherein the plurality of downwardly
extending rods or bars consists of N downwardly extending rods or
bars where N is at least four.
Description
BACKGROUND
[0001] The following relates to the nuclear reactor arts,
electrical power generation arts, nuclear reactor control arts,
nuclear electrical power generation control arts, and related
arts.
[0002] Nuclear reactors employ a reactor core comprising a critical
mass of fissile material, such as a material containing uranium
oxide (UO.sub.2) that is enriched in the fissile .sup.235U isotope.
The fuel rod may take various structural configurations, for
example including fissile material as pellets embedded in a ceramic
matrix or so forth. To promote safety, it is conventional to
assemble the core as rods containing the fissile material. A set of
rods is preassembled to form a fuel assembly. Preferably, the mass
of fissile material in the fuel assembly remains below critical
mass. The fuel assemblies are shipped to the reactor site, and are
installed in a grid in the reactor pressure vessel to form the
reactor core. To prevent a premature chain reaction, suitable
neutron absorbing material is provided during installation, for
example by inserting neutron-absorbing control rods into the fuel
assemblies before they are brought together in the pressure vessel,
and by omitting the neutron moderator (e.g., water ambient) if
employed.
[0003] With reference to FIGS. 1 and 2, an illustrative example of
such an assembly is shown. FIG. 1 shows an illustrative fuel
assembly 10 including a set of fuel rods 12 secured together with a
controlled spacing by mid-spacer grid elements 14 and by end-spacer
grid elements 16, 18. In the illustrative example, the fuel rods 12
form a 17.times.17 array. The fuel assembly 10 is typically
substantially elongated, and is shown in part in FIG. 1 with an
indicated gap G. The fuel assembly 10 also suitably includes other
elements, such as control rod guide tubes or thimbles 20 through
which neutron-absorbing control rods may pass. One or more of these
or similar tubes or thimbles may also serve as instrumentation
conduits for in-core sensors. Upper and lower nozzle plates 22, 24
may be provided to facilitate coupling of control rods,
instrumentation bundles, or so forth into or out of the fuel
assembly 10. The illustrative upper and lower nozzle plates 22, 24
include respective upper and lower alignment pins 26, 28 at the
corners of the respective nozzle plates 22, 24 for facilitating
alignment of the fuel assemblies during installation in the reactor
core.
[0004] FIG. 2 shows the assembled reactor core 30, including a
closely packed grid of fuel assemblies 10 disposed in a core former
32. In FIG. 2, a control rod assembly (CRA) is fully inserted into
each fuel assembly 10. In the view of FIG. 2, only an upper support
element 34 of the CRA is visible extending above each corresponding
fuel assembly 10. The upper support element of each CRA may in be a
conventional spider or (as in FIG. 2) a larger element (see
"Terminal Elements for Coupling Connecting Rods and Control Rods in
Control Rod Assemblies For a Nuclear Reactor", U.S. Ser. No.
12/862,124 filed Aug. 24, 2010, which is incorporated herein by
reference in its entirety, for some illustrative examples). The
illustrative reactor core 30 includes sixty nine (69) fuel
assemblies, although in general more or fewer fuel assemblies may
be included.
[0005] The reactor core has a designed lifetime, typically in a
range of a year to a few years. The core lifetime is controlled by
the reduction in fissile material caused by operation of the
nuclear chain reaction. To continue operation, a refueling
operation must be performed, in which the spent fuel assemblies are
removed and replaced by new fuel assemblies. Typically, this
entails shutting down the reactor, opening the pressure vessel and
removing any components in order to gain overhead access to the
fuel assemblies, and removing the fuel assemblies with the
assistance of a crane. To enable coupling with the fuel assembly,
each fuel assembly is typically fitted with a box structure with
leaf springs mounted on top of the box, or a plate-and-post
structure with preloaded helical coil springs mounted between the
posts. The fuel assembly is lifted by a grappling mechanism that
engages the fixed top plate of the box structure or the movable top
plate of the plate-and-post structure via hooks that swing
laterally under the top plate in four orthogonal directions. In box
designs, the hooks swing outward to engage the top plate of the
box, while in plate-and-post designs the hooks swing inward to
engage the top plate.
[0006] These refueling approaches have substantial disadvantages.
The swinging motion of the grappling hooks calls for a large
working space proximate to the top of each fuel assembly. However,
this working space is constrained by the presence of closely
adjacent neighboring fuel assemblies in the array disposed in the
core former. Moreover, if the CRA is left fully inserted into the
fuel assembly during refueling (which is desirable to maintain
suppression of the neutron population in the fuel assembly during
the refueling process), then either the spider must be removed
entirely (a process entailing individually detaching each of the
numerous control rods from the spider), or the spider must be of
sufficiently low profile to enable the grappling hooks to operate
above the spider.
[0007] Disclosed herein are improvements that provide various
benefits that will become apparent to the skilled artisan upon
reading the following.
BRIEF SUMMARY
[0008] In one aspect of the disclosure, a method comprises
performing refueling of a nuclear reactor. The refueling includes
removing a fuel assembly from a reactor core of the nuclear
reactor. The removal method includes: connecting a lifting tool of
a crane with a top of the fuel assembly, the lifting tool
comprising an assembly of downwardly extending elements, the
connecting including locking lower ends of the downwardly extending
elements with respective mating features located at a top and
periphery of the fuel assembly; moving the fuel assembly connected
with the lifting tool into a spent fuel pool using the crane; and
releasing the lifting tool from the top of the fuel assembly, the
releasing including unlocking the lower ends of the downwardly
extending elements from the respective peripherally located mating
features at the top and periphery of the fuel assembly.
[0009] In another aspect of the disclosure, a method comprises
performing refueling of a nuclear reactor. The refueling includes
removing a fuel assembly having a control rod assembly (CRA)
inserted in the fuel assembly from a reactor core of the nuclear
reactor. The removal method includes: lowering a lifting tool of a
crane onto a top of the fuel assembly, the lowered lifting tool
including a plurality of downwardly extending elements that
surround and vertically overlap a portion of the CRA extending
above the top of the fuel assembly; locking the downwardly
extending elements of the lowered lifting tool with corresponding
mating features at the top of the fuel assembly in order to connect
the lifting tool with the fuel assembly; moving the fuel assembly
connected with the lifting tool into a spent fuel pool using the
crane; and disconnecting the lifting tool from the top of the fuel
assembly in the spent fuel pool by unlocking the downwardly
extending elements from the corresponding mating features at the
top of the fuel assembly.
[0010] In another aspect of the disclosure, an apparatus comprises
a lifting tool including an upper end configured for attachment
with a crane, and a plurality of downwardly extending elements
surrounding an open central region disposed below the upper end,
lower ends of the downwardly extending elements being configured to
mate with mating features at the top of a fuel assembly of a
nuclear reactor core.
[0011] In another aspect of the disclosure, an apparatus comprises:
a nuclear fuel assembly including mating features at a top of the
nuclear fuel assembly; and a lifting tool including an upper end
configured for attachment with a crane and a plurality of
downwardly extending elements surrounding an open central region
disposed below the upper end, lower ends of the downwardly
extending elements being configured to mate with the mating
features at the top of the nuclear fuel assembly.
[0012] In another aspect of the disclosure, an apparatus comprises:
a nuclear fuel assembly including mating features at a top of the
nuclear fuel assembly; a control rod assembly (CRA) inserted in the
nuclear fuel assembly with an upper end of the CRA extending out of
the top of the nuclear fuel assembly; and a lifting tool including
an upper end configured for attachment with a crane and a plurality
of downwardly extending elements surrounding an open central region
disposed below the upper end, lower ends of the downwardly
extending elements being configured to mate with the mating
features at the top of the nuclear fuel assembly. The open central
region of the lifting tool that is surrounded by the plurality of
downwardly extending elements is configured to receive the upper
end of the CRA when the lower ends of the downwardly extending
elements mate with the mating features at the top of the nuclear
fuel assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The invention may take form in various components and
arrangements of components, and in various process operations and
arrangements of process operations. The drawings are only for
purposes of illustrating preferred embodiments and are not to be
construed as limiting the invention.
[0014] FIGS. 1 and 2 show a nuclear fuel assembly and a nuclear
reactor core, respectively, according to the prior art.
[0015] FIG. 3 shows a diagrammatic perspective view of a nuclear
reactor and selected associated components.
[0016] FIG. 4 shows an exploded perspective view of the pressure
vessel of the nuclear reactor of FIG. 3.
[0017] FIG. 5 shows an exploded perspective view of the lower
vessel portion of the pressure vessel of FIG. 4 including selected
internal components.
[0018] FIG. 6 shows a perspective view of a nuclear fuel assembly
with the fuel rods omitted to reveal the control rod guide tubes or
thimbles, with a control rod assembly (CRA) positioned in a
withdrawn position above the fuel assembly.
[0019] FIG. 7 shows a perspective view of a nuclear fuel assembly
with a control rod assembly (CRA) inserted in the fuel
assembly.
[0020] FIG. 8 shows an isolated perspective view of the upper
support element of the CRA of FIGS. 6 and 7.
[0021] FIG. 9 shows an enlarged perspective sectional view of the
CRA focusing on the upper support element and showing a J-lock
coupling between the connecting rod and the CRA.
[0022] FIG. 10 diagrammatically shows a refueling process flow
including those portions related to unloading spent nuclear fuel
assemblies from the reactor.
[0023] FIGS. 11, 12, 13, 14, 15, 15A, 16, and 16A show perspective
views (with partial cutaway in the case of FIGS. 15A and 16A) of
various operations of the process flow of FIG. 10.
[0024] FIG. 17 shows an illustrative embodiment of the lifting tool
including diagrammatically indicated motors for rotating the lower
ends of the downwardly extending elements of the lifting tool to
engage the locks.
[0025] FIGS. 18-20 diagrammatically show overhead views of three
nuclear fuel assembly embodiments each with an inserted control rod
assembly (CRA) and showing the peripherally located mating features
at the top of the fuel assembly for connecting with the lifting
tool.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] With reference to FIGS. 3-5, an illustrative nuclear reactor
is shown. FIG. 3 shows the nuclear reactor 40 in conjunction with a
diagrammatically indicated spent fuel pool 42 and a
diagrammatically indicated crane 44. FIG. 4 shows an exploded view
of the pressure vessel of the nuclear reactor of FIG. 3. The
pressure vessel includes a lower vessel portion 50, an upper vessel
portion 52, and a skirt or support structure 54. In the
illustrative arrangement, the pressure vessel is mounted vertically
(as shown) with at least part of the lower vessel portion 50
disposed below ground level. The bottom of the skirt or support
structure 54 is at ground level and supports the pressure vessel
and/or biases the pressure vessel against tipping. In the
illustrative example of FIG. 3, the spent fuel pool 42 is a
below-ground pool containing water and optional additives such as,
by way of illustrative example, boric acid (a soluble neutron
poison). FIG. 5 shows an exploded perspective view of the lower
vessel portion 50 including selected internal components. The lower
vessel 50 contains the nuclear reactor core comprising the core
former 32 and an array of fuel assemblies 10 (only one of which is
shown by way of example in FIG. 5). The reactor core is disposed in
and supported by the core former 32 which is in turn disposed in
and supported by a core basket 56, which may include radiation
shielding, optional emergency coolant tubing (not shown), or so
forth.
[0027] The illustrative nuclear reactor includes upper internals 58
which include wholly internal control rod drive mechanism (CRDM)
units. In the illustrative example, the upper internals 58 are
supported by a mid-flange 60 that also forms a structural joint of
the pressure vessel (being disposed at the junction between the
lower and upper vessel portions 50, 52). Alignment between the fuel
assemblies 10 and the upper internals 58 is suitably provided by
the upper alignment pins 26 at the corners of the upper nozzle
plates 22 of the fuel assemblies 10. These pins 26 are designed to
accommodate the differential thermal expansion between the fuel
assembly 10 and the reactor internals 58 and the fuel assembly
growth due to irradiation without losing engagement.
[0028] The illustrative nuclear reactor is a thermal nuclear
reactor employing light water (H.sub.2O) as a primary coolant that
also serves as a neutron moderator that thermalizes neutrons to
enhance the nuclear reaction rate. Alternatively, deuterium dioxide
(D.sub.2O) is contemplated as the coolant/moderator. The primary
coolant optionally contains selected additives, such as optional
boric acid which, if added, acts as a neutron poison to slow the
reaction rate. The pressure vessel suitably includes a cylindrical
central riser or other internal compartments or structures (details
not shown) to guide circulation of the primary coolant in the
pressure vessel. The primary coolant circulation may be natural
circulation caused by the heating of the primary coolant in the
vicinity of the reactor core, or may be assisted or generated by
illustrative primary coolant pumps 62 also mounted via the
mid-flange 60.
[0029] Although not illustrated, in some embodiments the nuclear
reactor is intended to generate steam. Toward this end, primary
coolant heated by the reactor core flows through a primary loop
that is in thermal communication with a secondary coolant loop
through which secondary coolant flows. Heat transfer from the
primary loop to the secondary loop heats the secondary coolant and
converts it to steam. The thermally coupled primary/secondary
coolant loops thus define a steam generator. In some embodiments,
the steam generator is external to the pressure vessel, while in
other embodiments the steam generator is internal to the pressure
vessel, for example mounted in the upper pressure vessel portion 52
in some contemplated embodiments. The steam may for example, be
employed to drive a turbine of a generator of an electrical power
plant, thus generating electrical power from the nuclear
reaction.
[0030] The illustrative nuclear reactor is of a type generally
known as a pressurized water nuclear reactor (PWR), in which the
primary coolant (water) is maintained in a superheated state during
normal operation. This is suitably accomplished by maintaining a
steam bubble located at the top of the upper vessel portion 52 at a
desired pressure during normal reactor operation. Alternatively,
the nuclear reactor could be configured as a boiling water reactor
(BWR) in which the primary coolant is maintained in a boiling
state.
[0031] The illustrative nuclear reactor 40 and other components,
e.g. spent fuel pool 42 and diagrammatically represented crane 44,
is shown as an example. Numerous variations are contemplated. For
example, the pressure vessel can have other portioning, such as
having a removable top or "cap" section, and can have access
manways provided at various points for maintenance or so forth. In
some embodiments the entire pressure vessel may be located
underground. Similarly, while the illustrative spent fuel pool 42
is below-ground and surrounds the lower vessel portion 50, more
generally the spent fuel pool can be located anywhere within
"reach" of the crane 44, and may in some embodiments be
above-ground (or, conversely, may be buried deep underground with
suitable access from above). The reactor 40 and auxiliary
components 42, 44 are typically housed in a concrete or steel
containment structure, which is also not shown. The crane 44 is
diagrammatically shown, and may in general have any suitable
configuration that provides the desired horizontal and vertical
travel, lifting capacity, and so forth while fitting within the
containment structure. Some suitable crane configurations include
an overhead crane configuration, a gantry crane configuration, a
tower or hammerhead crane configuration, or so forth.
[0032] With continuing reference to FIGS. 3-5 and with further
reference to FIGS. 6-9, reactivity control is suitably achieved
using a control rod assembly (CRA) 70 associated with each fuel
assembly 10. FIG. 6 shows an illustrative fuel assembly with the
fuel rods omitted, denoted by reference number 10'. By omitting the
fuel rods for illustrative purposes, the diagrammatic element 10'
reveals that the control rod guide tubes or thimbles 20 through
which neutron-absorbing control rods may pass extend through the
entire (vertical) height of the fuel assembly. Corresponding
control rods 72 of the CRA 70 are shown in the fully withdrawn
position in FIG. 6 (that is, fully withdrawn out of the guide tubes
or thimbles 20). The CRA 70 also includes upper support element 74
that secures the bundle of control rods 72 together in a pattern
matching that of the guide tubes or thimbles 20. The upper support
element 74 may be a conventional spider; in the illustrative
example, however, the upper support element 74 is a larger element
intended to provide various benefits such as a longer (vertical)
length over which to secure the upper ends of the control rods 72,
and optionally increased mass for the CRA 70. The illustrative
upper support element 74 is shown in isolation in FIG. 8, and in
side sectional view in FIG. 9. The illustrative upper support
element 74 is further described in "Terminal Elements for Coupling
Connecting Rods and Control Rods in Control Rod Assemblies For a
Nuclear Reactor", U.S. Ser. No. 12/862,124 filed Aug. 24, 2010,
which is incorporated herein by reference in its entirety. FIG. 7
shows the CRA 70 fully inserted into the fuel assembly 10. It will
be noted in FIG. 7 that a portion of the CRA 70, including at least
the upper support element 74, extends above the top of the fuel
assembly 10 in the fully inserted position.
[0033] With continuing reference to FIGS. 6-9, the CRA 70 is
inserted into the fuel assembly 10 (as per FIG. 7), or withdrawn
from the fuel assembly 10 (as per FIG. 6) in order to control the
reaction rate of reactivity of the reactor core. The control rods
72 comprise a neutron-absorbing material--accordingly, as the
control rods 72 are inserted further into the fuel assembly 10 the
reaction rate is reduced. In the fully inserted position (FIG. 6)
the reaction is typically extinguished entirely. A connecting rod
76 is employed in order to raise or lower the CRA 70. As
illustrated in FIGS. 6, 7, and 9, the lower end of the connecting
rod 76 is connected with the upper support element 74 of the CRA
70. The opposite upper end of the connecting rod 76 is not
illustrated, but is connected with a suitable control rod drive
mechanism (CRDM) unit. In the illustrative embodiment (see FIG. 5)
the CRDMs are wholly internal and are part of the upper internals
58 contained within the pressure vessel. Alternatively, the CRDMs
may be mounted externally above the pressure vessel (as is typical
in a PWR) or externally below the pressure vessel (as is typical in
a BWR), with the connecting rods passing through suitable vessel
penetrations to connect with the corresponding CRA.
[0034] With returning reference to FIGS. 3-5, the reactor core has
a sufficient quantity of fissile material to support reactor
operation for a designed operational time period, which is
typically of order one to a few years, although shorter or longer
designed periods are also contemplated. Thereafter, the nuclear
reactor 40 is refueled and then restarted. Toward this end, the
crane 44 includes or is operatively connected with lifting tool 80
that is designed to connect with one of the fuel assemblies. During
refueling, the crane 44 operating in conjunction with the lifting
tool 80 transfers spent fuel assemblies out of the lower vessel 50
and deposits the spent fuel assemblies in the spent fuel pool 42.
By way of diagrammatic illustration, FIG. 3 shows several spent
fuel assemblies 10.sub.spent which have been transferred into the
spent fuel pool 42. (It should be noted that while the illustrative
spent fuel pool 42 is below-ground and surrounds the lower vessel
portion 50, more generally the spent fuel pool can be located
anywhere within "reach" of the crane 44, and may in some
embodiments be above-ground.) The crane 44 operating in conjunction
with the lifting tool 80 also transfers (i.e., loads) new fuel
assemblies into the lower vessel 50, and more particularly into the
core former 32.
[0035] With reference to FIGS. 10-16, the refueling process is
described. In an operation S1, the reactor is shut down preparatory
to the refueling. The shutdown S1 includes inserting each CRA 70
into its corresponding fuel assembly 10, producing the inserted
configuration shown in FIG. 7. A suitable time delay is allowed in
order for the reactor to cool down to a sufficiently low
temperature to allow opening of the pressure vessel. Some primary
coolant may also be removed from the pressure vessel in order to
reduce the water level. In an operation S2 (see also FIGS. 3-5),
the upper vessel portion 52 is removed (for example, using the
crane 44). The effect of the operation S2 is to provide access to
the (now spent) fuel assemblies 10 disposed in the core former 32.
In an operation S3, for each fuel assembly 10 the connecting rod 76
is detached from the corresponding CRA 70 so as to leave the
combination of the fuel assembly 10 and the inserted CRA 70, as
shown in FIG. 11.
[0036] With brief reference to FIG. 9, a suitable approach for
performing the removal S3 of the connecting rod 76 is described. In
this embodiment, the lower end 76.sub.L of the connecting rod 76
terminates in a bayonet or (illustrated) J-lock coupling that is
designed to lock into a mating receptacle 76.sub.M (see FIG. 8) of
the upper support element 74 of the CRA 70. The perspective
sectional view of FIG. 9 shows the lower end 76.sub.L of the
connecting rod 76 in the locked position biased by a spring SS
against a retaining feature RR inside the mating receptacle
76.sub.M of the CRA upper support element 74. Thus, by pressing the
connecting rod 76 downward against the bias of the spring SS and
rotating the connecting rod 76 to disengage from the retaining
feature RR, the connecting rod 76 is released from the CRA upper
support element 74. More generally, a bayonet, J-lock, or other
"quick-release" type rotatable coupling can be employed to enable
the operation S3 to be quickly performed, with the "groove" and
"pin" or other retaining combination being variously disposed
(e.g., with the groove on the connecting rod and the pin or pins on
the CRA receptacle, or vice versa). Some further illustrative
description is set forth in "Terminal Elements for Coupling
Connecting Rods and Control Rods in Control Rod Assemblies For a
Nuclear Reactor", U.S. Ser. No. 12/862,124 filed Aug. 24, 2010,
which is incorporated herein by reference in its entirety. Although
a quick-release approach is advantageous, it is also contemplated
to employ a different approach for performing the operation S3--for
example, the connecting rod may be permanently connected with the
CRA (for example, by a weld or the like), and the operation S3 may
entail cutting the connecting rod at a point at or near its
junction with the CRA.
[0037] With continuing reference to FIG. 10, after completion of
the operation S3 the resulting unit includes the fuel assembly 10
with the CRA 70 inserted, with a top portion of the CRA 70
including the upper support element 74 extending above the top of
the fuel assembly 10. This is illustrated in FIG. 11. In an
operation S4 (see also FIG. 12), the lifting tool 80 is lowered
onto the top of the fuel assembly 10. As seen in FIG. 12, the
lifting tool 80 includes an upper end 81 configured for attachment
with the crane 44. In the illustrative lifting tool 80, the upper
end 81 includes a loop for attachment with the cable or arm of the
crane 44. The lifting tool 80 also includes a plurality of
downwardly extending elements 82, namely four downwardly extending
rods or bars 82 in the illustrative example, that surround and
vertically overlap the portion of the CRA 70 extending above the
top of the fuel assembly 10 (e.g., the upper support element 74).
The illustrative downwardly extending elements 82 are vertical rods
or bars that are aligned such that lower ends 82.sub.L of the
downwardly extending elements 82 of the lowered lifting tool 80
align with respective peripherally located mating features at a top
and periphery of the fuel assembly 10. In the illustrative
embodiment, the upper alignment pins 26 of the fuel assembly 10
located at the corners of the upper nozzle plate 22 also serve as
the mating features 26 (namely lifting pins 26 in the illustrative
example) at a top and periphery of the fuel assembly 10. However,
other mating features are also contemplated. For example, the
mating features can be protrusions, openings, or recesses at a top
and periphery of the fuel assembly.
[0038] The mating features (e.g., lifting pins 26) are designed to
be weight-bearing such that the entire fuel assembly 10 can be
raised upward by lifting on the mating features. In the case of the
illustrative fuel assembly 10, this is accomplished by constructing
the upper and lower nozzle plates 22, 24, the control rod guide
tubes or thimbles 20, and the spacer grid elements 14, 16, 18 as a
welded assembly of steel or another suitable structural material
(best seen as the structure 10' in FIG. 6). The lifting pins 26 at
a top and periphery of the fuel assembly 10 are secured to the
upper nozzle plate 22 by welding, a threaded connection, a
combination thereof, or another suitably load-bearing
connection.
[0039] With continuing reference to FIG. 10 and with further
reference to FIGS. 14, 15, 15A, 16, and 16A, in an operation S5 the
lowered lifting tool 80 is connected with the top of the fuel
assembly 10. The connection operation S5 includes locking the lower
ends 82.sub.L of the downwardly extending elements 80 with the
respective peripherally located mating features, e.g. lifting pins
26, at the top and periphery of the fuel assembly 10. In the
illustrative approach (see FIGS. 14, 15, 15A, 16, and 16A), the
locking operation is performed by rotating at least the lower ends
82.sub.L of the downwardly extending elements 80 to lock the lower
ends disposed over (as illustrated) or inside the respective
lifting pins 26 with the respective lifting pins 26. Toward this
end, the lower ends 82.sub.L and the respective lifting pins 26
define a lockable bayonet coupling. FIG. 14 shows an enlarged view
of one of the lower ends 82L aligned with and being lowered over
the respective lifting pin 26. In this view a groove 86 in the
lifting pin 26 is visible, as well as a narrowed portion 88 of the
lifting pin 26. These features 86, 88 are designed to cooperate
with a recess 90 in the lower end 82.sub.L with a narrowed region
92 to form a rotationally engaging lock. FIG. 15 shows an enlarged
view of the lower end 82L fully lowered over the lifting pin 26.
FIG. 15A shows the view of FIG. 15 with partial cutaway of the
lower end 82.sub.L to reveal internal components of the (unlocked)
locking configuration. FIG. 16 shows an enlarged view of the lower
end 82L after a rotation of about 90.degree.. This rotation causes
the narrowed region 92 to move into the groove 86 to form the lock.
FIG. 16A shows the view of FIG. 16 with partial cutaway of the
lower end 82.sub.L to reveal internal components of the (locked)
locking configuration.
[0040] In other embodiments, other rotationally locking
"quick-release" configurations can be employed. For example, in
another contemplated embodiment the J-lock coupling shown in FIG. 9
for coupling the connecting rod 76 with the CRA upper support
element 74 can be used in coupling the lower end of the downwardly
extending rod or bar with a mating recess at the top and periphery
of the fuel assembly. Another rotationally locking quick-release
configuration contemplated for use in the lower ends of the
downwardly extending elements of the lifting tool are threaded
connections. In this embodiment, the lower ends have threads that
mate with threaded holes located at the top periphery of the
nuclear fuel assembly. The locking in this case is a frictional
lock obtained by rotating the lower ends to thread into the
threaded holes until a designed torque is reached.
[0041] With reference to FIG. 17, in any embodiment employing a
rotational lock, the downwardly extending elements, or at least
their lower ends, should include motorized rotation capability. In
an illustrative example shown in FIG. 17, each downwardly extending
rod or bar 82 includes a diagrammatically indicated motor 94
providing the motorized rotation of the lower end 82.sub.L.
Although FIG. 17 illustrates a separate motor 94 for each
downwardly extending rod or bar 82, in other embodiments it is
contemplated to employ a single motor that drives rotation of all
lower ends via a suitable drive train (e.g., geared rotating shafts
or the like). It is also noted that since the lifting tool 80 is
not disposed inside the pressure vessel except when the reactor is
shut down, the lifting tool 80 (including the motors 94) does not
need to be rated for operation at the operating temperature of the
nuclear reactor. The motors 94 should be robust against immersion
in the primary coolant and in the fluid of the spent fuel pool 42
(see FIG. 3), for example by being hermetically sealed.
[0042] While various embodiments of rotational locks (e.g., bayonet
or J-lock couplings) are disclosed herein, other types of locks,
including non-rotational locks, are also contemplated. For example,
in another contemplated embodiment the locks may employ motorized
clamps that clamp onto respective mating features at the top of the
fuel assembly.
[0043] With returning reference to FIG. 10, in an operation S6 the
fuel assembly 10 connected with the lifting tool 80 is moved into
the spent fuel pool 42 using the crane 44. In an operation S7 the
lifting tool 80 is released from the top of the fuel assembly. The
release operation S7 includes unlocking the lower ends 82.sub.L of
the downwardly extending elements 82 from the respective
peripherally located mating features (e.g. lifting pins 26) at the
top and periphery of the fuel assembly 10. In the illustrative
embodiment, this entails rotating the lower ends 82.sub.L in the
opposite direction to that used in the locking operation and then
lifting the unlocked lifting tool 80 upward away from the spent
fuel assembly now residing in the spent fuel pool 42. Other
unlocking operations may be employed depending upon the nature and
configuration of the locking coupling.
[0044] Since the reactor core typically includes a number of fuel
assemblies 10 (see the example of FIG. 2 in which the reactor core
30 includes sixty nine fuel assemblies). Accordingly, after the
release operation S7, an operation S8 is performed in which the
next fuel assembly to be unloaded is selected, and the process
repeats beginning at operation S4. Once all fuel assemblies have
been unloaded, an operation S9 is performed in which the lifting
tool 80 is parked in a storage location. Alternatively, if new fuel
is to be loaded into the reactor, operations analogous to
operations S4, S5, S6, S7, S8 are performed to pick up new fuel
assemblies from a loading dock or other source location and place
the new fuel assemblies into the core former 32, followed by
performing control rod reattachment (analogous to operation S3),
replacement of the upper vessel portion 52 (analogous to operation
S2), and restarting the reactor (analogous to operation S1, and
optionally further including performing various integrity or safety
checks prior to the restart). Note that these analogous loading
operations are not shown in FIG. 10. Additionally, the reloading
may include performing other maintenance such as replacing the
connecting rods or other internal reactor components, various
inspection and/or cleanup operations, or so forth.
[0045] An advantage of the lifting tool 80 is that it accommodates
a CRA inserted into the fuel assembly 10 that extends substantially
above the top of the fuel assembly 10. Because no swing action is
required to engage the lifting mechanism; the fuel assembly can be
lifted even when most or all of the inboard volume above the fuel
assembly is occupied by the upper portion 74 of the inserted CRA.
The peripherally arranged downwardly extending elements 80
accommodate the exposed portion of the CRA by surrounding the
exposed upper end of the inserted CRA (e.g., the upper support
element 74) when the fuel assembly 10 is connected with the lifting
tool. The downwardly extending elements 82 surround an open central
region disposed below the upper end 81 of the lifting tool 80, such
that the open central region can accommodate the upward extension
of the inserted CRA out of the top of the fuel assembly 10. In this
way, the CRA vertically overlaps the lifting tool 80 when the fuel
assembly 10 is connected with the lifting tool 80 (see FIG. 13). In
some embodiments the overlap is at least one-half of the vertical
height of the lifting tool 80. In some embodiments the overlap
between the CRA and the lifting tool 80 is at least one-half of the
vertical height of the downwardly extending elements 82 of the
lifting tool 80.
[0046] With reference to FIGS. 18-20, it is to be appreciated that
the fuel assemblies, CRA, and lifting tool can have various
geometries. FIG. 18 shows the illustrative geometry of the fuel
assembly 10, which has a rectangular cross-section when viewed from
above as per FIG. 18, with the CRA including the upper support
element 74 inserted in illustrative FIG. 18. FIG. 19 illustrates a
hexagonal fuel assembly 22.sub.H having six sides, with the same
CRA including the same upper support element 74 inserted. In this
embodiment there are six mating features 26.sub.H located at a top
and periphery of the fuel assembly. The illustrative six mating
features 26.sub.H are the same as the lifting pins 26 of the fuel
assembly 10. The corresponding lifting tool (not shown) suitably
includes six downwardly extending elements, e.g. six downwardly
extending rods or bars, arranged in a hexagonal pattern to mate
with the respective six lifting pins 26.sub.H. Finally, as a
further example, FIG. 20 illustrates a triangular fuel assembly
22.sub.T having three sides, with a conventional spider 74.sub.T
with six branches serving as the upper support element of the CRA.
In this embodiment there are three mating features 26.sub.T, which
in this embodiment are embodied as recesses or openings 26.sub.T.
The corresponding lifting tool (not shown) suitably includes three
downwardly extending elements, e.g. three downwardly extending rods
or bars, arranged in an equilateral triangular pattern to mate with
the respective three openings 26.sub.T. In general, the geometry of
the fuel assembly preferably promotes a closely packed
arrangement.
[0047] The preferred embodiments have been illustrated and
described. Obviously, modifications and alterations will occur to
others upon reading and understanding the preceding detailed
description. It is intended that the invention be construed as
including all such modifications and alterations insofar as they
come within the scope of the appended claims or the equivalents
thereof.
* * * * *