Patent Number: 051981831
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) The present invention shown in FIG. 1 is an apparatus for close packing of nuclear fuel assemblies. A nuclear fuel assembly (10) has an array of fuel rods (12) evenly spaced within an envelope (13) with narrow spaces (14) and wider spaces (16) between the fuel rods (12). Close packing of nuclear fuel assemblies (10) is accomplished by inserting a plate (18), having an effective amount of neutron absorbing material, between fuel rods (12) within the nuclear fuel assembly (10). It is preferred that the plate (18) is placed between an outer row (20) and a next outer row (22) of fuel rods (12). Placement of the plate (18) may be between any rows of the nuclear fuel assembly (10) where there are no interfering guide tubes (such as may be found in boiling water reactor assemblies). The number of plates (18) placed within an assembly (10) may depend upon the burnup status or reactivity of the fuel. Fresh fuel may require that plates (18) completely surround and enclose the center of the nuclear fuel assembly. Spent fuel may require plates on one side or a few plates (18) on all sides appropriate for the activity of the spent fuel. The plates (18) may be of any thickness but preferably have a thickness (24) permitting insertion between nuclear fuel rods (12) within the nuclear fuel assembly (10). Moreover, it is preferred that the plate (18) has a width and a length permitting insertion between grid spacers and other structural elements within said nuclear fuel assembly. The apparatus may further include a releasable lock (30) for securing the plate (18) within the nuclear fuel assembly (10). The releasable lock (30) may be of any type but it is preferred to be operable under water using spent fuel handling tools. These tools are required to reach up to 40 ft. underwater manipulated by an operator on a platform above the water surface. Hence, the operator has poor visibility of the nuclear fuel assemblies beneath the water surface and must rely on the feel of a tool in knowing whether a task is successfully completed. Therefore, it is preferred that the releasable lock (30) is attached to the plate (18) and actuated by a simple linear or rotary motion. The releasable lock (30) may have flexible elements, rigid elements, or a combination of rigid and flexible elements. It is further preferred that the releasable lock (30) remain within the fuel assembly envelope (13) when engaged thereby permitting a fuel assembly (10) to touch either a container wall or another fuel assembly. Maintaining the releasable lock (30) within the envelope (13) further prevents catching or snagging the releasable lock (30) on other structures during handling of the fuel assembly (10). Releasable locks (30) with a flexible element include but are not limited to locks having a spring or a pressurized element. The releasable lock (30) may be a spring clip that engages a nuclear fuel rod when the plate (18) is inserted into the nuclear fuel assembly. The force required to engage the clip would be felt by the operator. A further embodiment of a flexible releasable lock (30) comprises a flexible element compressibly inserted through a narrow space (14) between nuclear fuel rods then expanded into a wide space (16) between nuclear fuel rods. Flexible elements include but are not limited to tapered spring elements, springs attached to tapered elements, and spiral springs. Flexible elements further include pressurized and energized elements including but not limited to pneumatic cylinders, hydraulic cylinders, and electrical solenoids. An advantage of flexible elements is that the additional force required to actuate them provides the feel that the operator needs to know the status of the lock. Releasable locks (30) having rigid elements may also be used. A preferred embodiment of a rigid releasable lock (30) is illustrated in FIGS. 2 and 3. The rigid releasable lock (30) in these figures comprises an elongated member (32) having a first end (34) and a second end (35). The first end (34) has a key (36), and the second end (35) is rotatably attachable to the plate (18). The key (36) may be of any shape, but is preferably shaped to interface with a remotely operated handling tool to rotate the elongated member (32) for locking and releasing the plate (18). A locking disk (40) is attached on the elongated member (32) between the key (36) and said second end, (35) and has a thickness (42) less than the narrow space (14) between said fuel rods (12) and has a width (46) greater than the narrow space (14). The thickness (42) of the locking disk (40) permits the locking disk (40) to be inserted between nuclear fuel rods (14), then locked between them by rotating the locking disk (40) within the wide space (16) with the key (36) so that the width (46) of the locking disk (40) prevents its removal from between the nuclear fuel rods (14). The rigid releasable lock (30) may be secured in either a locked position or an open position with a retainer (47). In a preferred embodiment, the retainer (47) comprises at least one ball detent (48) mounted on an end of the plate (18) near the key (34) with the ball (48) in contact with the key (34). The key (34) is held in position when the ball (48) rests in a depression (50) on the key (34). A further embodiment of a retainer (47) is a slotted key with a flat spring. The flat spring is mounted on an end of the plate (18) with a surface pressing on the key (34). The slots in the key (34) may be tapered and rounded to facilitate reversible actuation of the releasable lock (30). Upon rotation of the key (34), a slot will align with the flat spring thereby restraining the key (34) from rotating until additional torque is applied. The plate (18) may be of any neutron absorbing material, but it is preferred to use a structural metal, compatible with the water chemistry in storage pools such as aluminum or stainless steel, alloyed or clad with a neutron absorbing material including but not limited to boron, cadmium, and hafnium. A plate (18) has an effective amount of neutron absorbing material when the concentration of the alloying neutron absorbing material is within standard alloying practice of, for example, up to 4 weight percent boron in aluminum or up to 2 weight percent boron in stainless steel. The structural or cladding metal may be any metal compatible with the water chemistry in spent fuel storage pools including but not limited to aluminum, and stainless steel. The thickness (22) of the plate (18) may depend upon the amount of neutron absorbing material, but is preferred to be about the same as plates currently used between nuclear fuel assemblies (10). By using plates (18) which are similar to those already in use, the resulting control of neutrons will also be similar. In operation, close packing of nuclear fuel assemblies (10) is permitted by placing a plate (18) having an effective amount of neutron absorbing material between nuclear fuel elements (12) within a nuclear fuel assembly (10). Placing a plate (18) within a nuclear fuel assembly (10) allows multiple nuclear fuel assemblies to touch, thereby minimizing the volume necessary for storing or transporting multiple nuclear fuel assemblies. The plate (18) may be releasably locked within the nuclear fuel assembly (10) thereby ensuring that the plate (18) stays in place. While a preferred embodiment of the present invention has been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.