Patent Publication Number: US-2011051883-A1

Title: Rack systems and assemblies for fuel storage

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/238,590, filed on Aug. 31, 2009, and U.S. Provisional Patent Application No. 61/260,719, filed on Nov. 12, 2009, the disclosures of which are hereby expressly incorporated by reference. 
    
    
     BACKGROUND 
     Racks are generally used to store fresh (new) nuclear fuel assemblies and spent (irradiated) nuclear fuel assemblies at nuclear reactor sites, for example, either in a dry storage area (for new fuel assemblies) or in a spent fuel pool area (for new and irradiated fuel assemblies). The dimensions of the fuel pool are generally standardized, depending upon the nuclear reactor type. 
     Previously designed rack assemblies are not optimized for efficient containment and storage of new and irradiated fuel assemblies. In that regard, the previously designed racks are complicated to fabricate because they use a significant number of weld points to create the frame assembly. Moreover, previously designed rack assemblies do not provide a substantially continuous neutron-absorbing shield between each compartment of the rack assembly to decrease the risk of criticality. 
     Thus, there exists a need for an improved rack assembly design that has a simplified design with a minimized number of weld points, yet a strong frame assembly for receiving and containing a plurality of individual fuel assemblies. Moreover, there exists a need for an improved rack assembly design having a continuous neutron-absorbing shield between each compartment of the rack assembly for improved criticality control. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     In accordance with one embodiment of the present disclosure, a rack assembly for nuclear fuel assemblies is provided. The rack assembly generally includes a frame assembly and a container assembly including a plurality of individual fuel containers designed to contain individual fuel assemblies, wherein the individual fuel containers are received within and supported by the frame assembly. The rack assembly further includes a shielding assembly including at least one of an inner shielding assembly comprising a substantially continuous shield between the individual fuel containers and an outer shielding assembly comprising a substantially continuous shield around at least a portion of the outer surfaces of the rack assembly. 
     In accordance with another embodiment of the present disclosure, a rack assembly for nuclear fuel assemblies is provided. The rack assembly generally includes a frame assembly including top and bottom frame portions and a plurality of vertical frame supports and a container assembly including a plurality of individual fuel containers designed to contain individual fuel assemblies, wherein the individual fuel containers are received within and supported by the top and bottom frame portions of the frame assembly. The rack assembly further includes a shielding assembly including a substantially continuous shield between the individual fuel containers, wherein the shielding assembly is supported by the frame assembly. 
     In accordance with another embodiment of the present disclosure, a rack assembly for nuclear fuel assemblies is provided. The rack assembly generally includes a frame assembly including top and bottom frame portions and a plurality of structural support grids and a container assembly including a plurality of individual fuel containers designed to contain individual fuel assemblies, wherein the individual fuel containers are received within and supported by the frame assembly. The rack assembly further includes a shielding assembly including a substantially continuous shield between the individual fuel containers, wherein the shielding assembly is supported by the container assembly. 
     In accordance with another embodiment of the present disclosure, a rack storage system for nuclear fuel assemblies is provided. The rack storage system generally includes first and second rack assemblies, the first and second rack assemblies each comprising a frame assembly, a container assembly including a plurality of individual fuel containers designed to contain individual fuel assemblies, wherein the individual fuel containers are received within and supported by the frame assembly, and a shielding assembly including a substantially continuous shield between the individual fuel containers. 
     In accordance with another embodiment of the present disclosure, a rack storage system for nuclear fuel assemblies is provided. The rack storage system generally includes at least one first rack assembly, the first rack assembly comprising a frame assembly, a container assembly including a plurality of individual fuel containers designed to contain individual fuel assemblies, wherein the individual fuel containers are received within and supported by the frame assembly, and a shielding assembly including a substantially continuous shield between the individual fuel containers. The rack storage system further includes at least one second rack assembly, the second rack assembly comprising a frame assembly and a container assembly including a plurality of individual fuel containers designed to contain individual fuel assemblies, wherein the second rack assembly does not include a shielding assembly. 
     In accordance with another embodiment of the present disclosure, the shielding assembly of the rack assembly includes a neutron-shielding material for criticality control. 
     In accordance with another embodiment of the present disclosure, the shielding assembly of the rack assembly includes material selected from the group consisting of boron carbide-aluminum metal matrix composites, natural or enriched boron aluminum alloy, boron stainless steel alloy, aluminum clad boron carbide cements, BORAI®neutron absorber material, manufactured by CERADYNE, INC., aluminum, and stainless steel. 
     In accordance with another embodiment of the present disclosure, the inner shielding of the rack assembly is supported by the frame assembly. 
     In accordance with another embodiment of the present disclosure, the inner shielding of the rack assembly is supported by the container assembly. 
     In accordance with another embodiment of the present disclosure, the outer shielding of the rack assembly is supported by at least one of the container assembly and the frame assembly. 
     In accordance with another embodiment of the present disclosure, the inner shielding of the rack assembly comprises a plurality of intersecting plates configured along the x- and y-axes of the rack assembly. In accordance with another embodiment of the present disclosure, the intersecting inner plates are configured to interface with one another without substantially interrupting the substantially continuous shield between the individual fuel containers. In accordance with another embodiment of the present disclosure, the intersecting inner plates are slotted for interfacing with one another. 
     In accordance with another embodiment of the present disclosure, the inner shielding assembly comprises two plates between each of the individual fuel containers. 
     In accordance with another embodiment of the present disclosure, the outer shielding assembly comprises a plurality of plates configured along the z-axis of the rack assembly. 
     In accordance with another embodiment of the present disclosure, the outer shielding assembly is secured in the z-axis of the rack assembly by connection bands. 
     In accordance with another embodiment of the present disclosure, the frame assembly includes first and second frame portions, each of the first and second frame portions comprising a plurality of compartments for receiving individual fuel containers, wherein the first frame portion is spaced a predetermined distance from the second frame portion. 
     In accordance with another embodiment of the present disclosure, the frame assembly includes one or more support grids intermediate the first and second frame portions. In accordance with another embodiment of the present disclosure, the support grids have apertures that align with the plurality of compartments in the first and second frame portions. 
     In accordance with another embodiment of the present disclosure, the first and second frame portions include gap cells between adjacent compartments for receiving at least a portion of the shielding assembly. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this disclosure will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a top view of a rack storage system for a spent fuel pool area in accordance with one embodiment of the present disclosure; 
         FIG. 2  is a perspective view of a first rack assembly for use in the rack storage system of  FIG. 1 , in accordance with one embodiment of the present disclosure; 
         FIG. 3  is a perspective view of a frame assembly for the first rack assembly of  FIG. 2 ; 
         FIG. 4  is an exploded view of a first rack assembly of  FIG. 2 , including an outer shielding assembly; 
         FIG. 5  is a perspective view of an inner shielding assembly of the first rack assembly of  FIG. 2 ; 
         FIG. 6  is an exploded view of the inner shielding assembly of  FIG. 5 ; 
         FIG. 7  is a perspective view of a second rack assembly for use in, for example, the rack storage system of  FIG. 1 , in accordance with another embodiment of the present disclosure; 
         FIG. 8  is a perspective view of a frame assembly for the first rack assembly of  FIG. 7 ; 
         FIG. 9  is an exploded view of a first rack assembly of  FIG. 7 , including an outer shielding assembly; 
         FIG. 10  is a perspective view of a first rack assembly for use in the rack storage system of  FIG. 1 , in accordance with another embodiment of the present disclosure; 
         FIG. 11  is a perspective view of a frame assembly for the first rack assembly of  FIG. 10 ; and 
         FIG. 12  is an exploded view of a first rack assembly of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     A rack storage system  10  for a spent fuel pool area constructed in accordance with one embodiment of the present disclosure may be best understood by referring to  FIG. 1 . The rack storage system  10  includes two regions, Region  1  and Region  2 , for receiving respective first and second rack assemblies  20  and  120  (see  FIGS. 2 and 5 ). 
     The first rack assembly  20 , designed for use in Region  1 , generally includes compartments that are configured for holding high reactivity nuclear fuel assemblies, for example, fresh nuclear fuel assemblies. The second rack assembly  120 , designed for use in Region  2 , generally includes compartments that are closer together for holding low reactivity nuclear fuel assemblies, for example, spent nuclear fuel assemblies. The first rack assembly  20  compartments are generally larger in size than the second rack assembly  120  compartments because the Region  1  rack assemblies  20  are designed to accommodate neutron shielding materials, which are not required in Region  2  rack assemblies  120 . It should be appreciated, however, that the smaller Region  2  rack assembly  120  may also hold fresh fuel if configured in a checkerboard configuration, such that fresh fuel assemblies are not contained in adjacent compartments. 
     It should be appreciated that terms used in the specification defining the orientations of the rack assemblies  20  and  120 , such as “top”, “bottom”, “vertical”, and “horizontal” are not intended to be limiting. These terms are used to simplify the description of the illustrated rack assemblies; however, it should be appreciated that the rack assemblies may be used in other orientations besides the illustrated orientations. 
     Although the rack storage system  10  is shown in a specific configuration for a spent fuel pool area in  FIG. 1 , it should be appreciated that other rack storage systems, dimensions, and configurations are also within the scope of the present disclosure. For example, suitable systems may also be configured for a dry storage area having a different system configuration than a spent fuel pool area. In addition, individual rack assemblies  20  and  120  may be configured having various numbers of compartments. As non-limiting examples, in the illustrated embodiment of  FIG. 1 , the second rack assemblies  120  are shown as having 9×10, 8×8, and 7×10 compartment configurations. 
     A first rack assembly  20  designed for use in Region  1  will now be described in greater detail with reference to  FIGS. 2-6 . A second rack assembly  120  designed for use in Region  2  will be described in greater detail below with reference to  FIGS. 7-9 . 
     Referring to  FIGS. 2-4 , the first rack assembly  20  includes a frame assembly  22  (see, in particular,  FIG. 3  for a skeleton view of the frame assembly  22 ), a container assembly  24  including a plurality of individual fuel containers  26 , shown as tubes, for receiving nuclear fuel assemblies (not shown) in a desired orientation and spacing from one another, and a shielding assembly including at least one of an inner shielding assembly  28  and an outer shielding assembly  32  for maintaining criticality control within the rack assembly  20  and within the rack storage system  10 . 
     Referring to  FIG. 3 , the frame assembly  22  will now be described in greater detail. The frame assembly  22  provides a frame for the containment of the container assembly  24  (see  FIG. 4 ). In the illustrated embodiment, the frame assembly  22  is an external frame assembly  22 , in that its components are primarily located on the exterior surfaces of the rack assembly  20 . In contrast, the container assembly  24  is primarily on the interior of the rack assembly  30 . As non-limiting examples, the components of the frame assembly  22  may be constructed from metal, such as aluminum, stainless steel, and alloys thereof. 
     The frame assembly  22  includes first and second frame portions  40  and  42 , wherein each of the first and second frame portions  40  and  42  include grid structures having apertures defining respective pluralities of compartments  44  and  46 . Both the first and second frame portions  40  and  42  are configured for receiving and supporting the first and second end portions  62  and  64  of the individual fuel containers  26  (see  FIG. 4 ). In that regard, the first and second frame portions  40  and  42  maintain the individual fuel containers  26  in a grid orientation and therefore maintain suitable spacing between the individual fuel containers  26 . In the illustrated embodiment, the first frame portion  40  is a top frame portion, and the second frame portion  42  is a bottom frame portion. 
     As a non-limiting example, the first and second frame portions  40  and  42  in the illustrated embodiment have grid structures resembling an egg-crate design that defines respective pluralities of discrete compartments  44  and  46 . The egg-crate designs are configured to be stackable when used in conjunction with an inner shielding assembly  28 , as described in greater detail below. However, it should be appreciated that other designs are also within the scope of the present disclosure. 
     It should be appreciated that the second frame portion  42  may be configured to have compartments  46  that taper from a first cross-sectional area to a smaller second cross-sectional area. With such a tapering cross-sectional area, fuel assemblies (not shown) or their containers  26  cannot be removed or cannot fall through the compartments  46  in the second frame portion  42 . On the other hand, the compartments  44  in the first frame portion  40  may have a constant cross-sectional area, such that fuel assemblies (not shown) can be inserted and/or removed from the compartments  44  with ease. It should be appreciated that the containers  26  and the compartments  44  and  46  of the respective first and second frame portions  40  and  42  generally have open ends so that water can freely flow through the containers  26  when the rack assembly  20  is in a spent fuel pool area. 
     As seen in the illustrated embodiment of  FIGS. 3 and 4 , the discrete compartments  44  and  46  of the first and second frame portions  40  and  42  are surrounded by gap cells  48  which allow for suitable spacing between fresh fuel assemblies to prevent criticality. The gap cells  48  of the first and second frame portions  40  and  42  may further receive and support inner shielding plates  30  of the inner shielding assembly  28 , such that the inner shielding assembly  28  is supported by the frame assembly  22 . In that regard, at least a portion of inner shielding plates  30  may be positioned to interface with the gap cells  48  of the first and second frame portions  40  and  42  to form discrete and shielded longitudinal compartments for the individual fuel containers  26 . The shielded longitudinal compartments may extend the length of the first rack assembly  20  from the first frame portion  40  to the second frame portion  42  (see  FIG. 4 ). The properties of the inner shielding plates  30  and the configuration of the inner shielding plates  30  in the gap cells  48  and extending between the first and second frame portions  40  and  42  will be described in greater detail below. 
     The frame assembly  22  further includes frame supports  50 , which extend longitudinally between the first and second frame portions  40  and  42 . The frame supports  50  allow the first and second frame portions  40  and  42  to be spaced a predetermined distance from one another. In the illustrated embodiment, the frame supports  50  extend vertically between the first (top) and second (bottom) frame portions  40  and  42  at each of the four corner edges of the rack assembly  20 . In that regard, a first end  52  of the frame support  50  is coupled to the first frame portion  40  and a second end  54  of the frame support  50  is coupled to the second frame portion  42 . However, it should be appreciated that other suitable frame support configurations are also within the scope of the present disclosure. For example, the frame supports need not be corner supports, and may extend between the first and second frame portions  40  and  42  along the sides of the rack assembly  20 , and not necessarily at the corner edges. 
     In addition, the frame assembly  22  includes connection bands  60  that extend between adjacent frame supports  50 . The connection bands  60  provide reinforcement to the rack assembly  20 , but require minimum weld or connection points in the overall fabrication of the frame assembly  22 . In the illustrated embodiment, the bands  60  extend horizontally between adjacent vertical frame supports  50 . However, it should be appreciated that other suitable band configurations are also within the scope of the present disclosure. For example, suitable bands may extend crosswise from the first end  52  (e.g., the top end) of a first frame support  50  to the second end  54  (e.g., the bottom end) of a second frame support  50 . The advantage of using such bands  60  is to improve the integrity of the frame assembly  22  and the overall rack assembly  20 , as well as to support and secure the outer shielding assembly  32  so that outer shielding plates  34  may be positioned to enclose the outer walls of the first rack assembly  20 , as described in greater detail below. 
     Referring to  FIG. 4 , the container assembly  24  including a plurality of individual fuel containers  26  will now be described in greater detail. These individual fuel containers  26  are designed to hold the individual fuel assemblies (not shown) in the first rack assembly  20 . As seen in  FIG. 4 , the individual fuel containers  26  are configured to be inserted into the individual compartments  44  of the first frame portion  40  and extend to the individual compartments  46  in the second frame portion  42 . In the illustrated embodiment, the individual fuel containers  26  are tubular having a square cross-sectional shape. However, it should be appreciated that other tubular containers having other cross-sectional shapes, for example, including, but not limited to, round, oval, or polygonal, are also within the scope of the present disclosure. 
     In the illustrated embodiment, the individual fuel containers  26  are configured to extend the full height of the assembly  20 , such that a first end  62  of the container  26  is received by the first compartment  44  of the first frame portion  40  and a second end  64  of the container is received by the second compartment  46  of the second frame portion  42 . By extending the full height of the assembly  20 , the individual fuel containers  26  can be contained in the frame assembly  22  without requiring additional frame support members besides the first and second frame portions  40  and  42  and the frame supports  50 . Using this construction, the rack assembly  20  relies on the rigidity of the individual fuel containers  26  to structurally complement the frame assembly  22 . 
     As non-limiting examples, the individual fuel containers  26  may be constructed from metal, such as aluminum, stainless steel, and alloys thereof. In one embodiment, the individual fuel containers  26  are manufactured from extruded aluminum. In another embodiment, the individual fuel containers  26  are manufactured by welding individual plates together. In the illustrated embodiment, the individual fuel containers  26  are maintained in a substantially vertical orientation by being received in corresponding individual compartments in the first and second frame portions  40  and  42 . However, it should be appreciated that other orientations, such as a horizontal orientation, are also within the scope of the present disclosure. 
     Referring to  FIG. 4 , the shielding assembly will now be described in greater detail. The shielding assembly generally includes an inner shielding assembly  28  and an outer shielding assembly  32 . In the illustrated embodiment of  FIGS. 2-6 , the inner shielding assembly  28  includes inner shielding plates  30 , and the outer shielding assembly  32  includes outer shielding plates  34 . Suitable shielding plates, whether inner  30  or outer  32 , may be neutron-shielding and/or neutron-absorbing material for criticality control. Suitable exemplary materials include, but are not limited to, boron carbide-aluminum metal matrix composites, natural or enriched boron aluminum alloy, boron stainless steel alloy, aluminum clad boron carbide cements, BORAI® neutron absorber material, manufactured by CERADYNE, INC., aluminum, stainless steel, and other similar materials. 
     As discussed above, the gap cells  48  in the first and second frame portions  40  and  42  are configured to receive and/or interface with the inner shielding plates  30 . In the illustrated embodiment, the inner plates  30  extend substantially horizontally through the assembly  20  along respective x- and y-axes of the assembly to align with the gap cells  48  in the first and second frame portions  40  and  42 . Such shielding plates create a shield between each individual fuel container  26  of the rack assembly  120  for criticality control. 
     Referring now to  FIGS. 5 and 6 , the inner shielding assembly  28  will be described in greater detail. In that regard, the intersections of adjacent horizontal x- and y-axes inner plates  30  will now be described. X-axis inner plates are labeled  30   a , and y-axis inner plates are labeled  30   b . Intersecting x- and y-axes inner plates  30   a  and  30   b  are configured to interface with one another to provide a continuous shield between individual fuel containers  26 . In the illustrated embodiment, the inner plates  30   a  and  30   b  include slots  66   a  and  66   b  so that perpendicularly oriented inner plates  30   a  and  30   b  can interface at their respective slots  66   a  and  66   b  and stack up upon each other to provide a substantially continuous shield in the horizontal z- and y-axes. Moreover, the grids of plates are configured to stack on top of each other to provide a substantially continuous shield along the vertical z-axis. Such stacking results in a substantially continuous shield between individual fuel containers  26  to effectively absorb neutrons and prevent criticality. 
     In the illustrated embodiment, the inner shielding assembly  28  includes two plates for increased shielding. In that regard, each of the inner shielding plate  30   a  and  30   b  has two layers of shielding material, the layers being spaced a predetermined distance from one another. Such distance may be the same width as the width of the gap cells  48  in the first and second frame portions  40  and  42 . 
     In previously designed racks, the individual shielding plates are rigidly attached or fastened to a portion of the sides of the individual fuel containers. For example, see U.S. Pat. No. 5,361,281, issued to Porowski, the disclosure of which is expressly incorporated by reference. Because the shielding plates are rigidly attached to only a portion of the sides of the individual fuel containers, it is not possible to provide a continuous shield, like the shield provided by the intersecting and stacking inner plates  30  of the present disclosure. While such rigidly attached plates provide some neutron shielding protection, the previously designed plates do not provide a continuous shield to effectively prevent criticality in the rack assembly. 
     In addition to inner plates  30 , an outer shielding assembly  32  including outer shielding plates  34  may be positioned around the outer perimeter of the rack assembly  20  for enhanced criticality properties. In the illustrated embodiment, the outer shielding plates  34  are positioned in a substantially vertical orientation along the outer perimeter of the rack assembly  20 . In one embodiment, the outer plates  34  include neutron-shielding and/or neutron-absorbing material for criticality control. 
     It should be appreciated that, in one embodiment, the outer plates  34  are designed to extend the full height of the assembly  20  from the first frame portion  40  to the second frame portion  42 . However, their width may vary depending on the manufacturing parameters of the rack assembly  20 . Moreover, it should be appreciated that the outer plates  34  may, like the inner plates  30 , have two layers of shielding material. 
     Returning to  FIG. 1 , the illustrated embodiment shows a rack storage system for a spent fuel pool area including first and second rack assemblies  20  (Region  1 ) and  120  (Region  2 ). When a side of a Region  1  assembly  20  faces a wall W of the fuel pool, the assembly  20  may be configured without an outer plate  34  on the side facing the pool wall W because neutron shielding is not required at the pool wall W. 
     Referring to  FIG. 4 , the outer plates  34  and the individual fuel containers  26  can be maintained in their substantially vertical orientation by being contained between the outer walls of the first and second frame portions  40  and  42  and the individual fuel containers  26  positioned along the outer perimeter of the rack assembly  20 . In that manner, the outer plates  34  may extend at least from the first frame portion  40  to the second frame portion  42 , such that the first end  70  of the outer plate  34  can be received within a compartment  44  and the first frame portion  40  and the second end  72  of the outer plate  34  can be received within a compartment  46  the second frame portion  40 . As mentioned above, connection bands  60  are attached to the frame supports  50  to hold the outer plates  34  in place. In the illustrated embodiment, the connection bands  60  are horizontally oriented between adjacent frame supports  50 . 
     By using connection bands  60  and first and second frame portions  40  and  42  to hold the individual fuel containers  26  and outer plates  34  in place, the rack assembly  20  has strong frame assembly  22  integrity, but can be fabricated with minimal weld points. 
     Still referring to  FIG. 4 , the base  78  and feet  80  of the rack assembly  20  are shown. The feet  80  are designed to be adjustable to accommodate an uneven standing surface, for example, on the bottom of the pool upon which the rack assembly  20  may stand. 
     Referring to  FIGS. 7-12 , other embodiments of rack assemblies are shown. Referring to  FIGS. 7-9 , an embodiment of a rack assembly  120  designed for Region  2  is shown. This embodiment is substantially similar to the embodiment  20  described above. In this embodiment, however, the first and second frame portions  140  and  142  are not configured with gap cells between the walls of the first and second frame portions  140  and  142  defining the individual compartments  144  and  146 . Moreover, the rack assembly  120  of the present embodiment does not include an inner shielding assembly. 
     Even though the rack assembly  120  does not include an inner shielding assembly, it may include an outer shielding assembly, like the Region  1  rack assembly  20 . In that regard, the Region  2  rack assembly  120  may include a plurality of outer plates  134  positioned around the outer perimeter of the assembly  120  for enhanced criticality properties. Returning to  FIG. 1 , the illustrated embodiment shows the individual rack assemblies  20  (Region  1 ) and  120  (Region  2 ) in the pool configuration. Like the Region  1  assemblies  20 , Region  2  assemblies  120  do not require an outer plate on the sides facing the pool walls W. 
     Referring to  FIGS. 10-12 , another embodiment of a rack assembly  220  designed for Region  1  is shown. Like the rack assembly  120 , this embodiment is also substantially similar to the embodiment  20  described above. In this embodiment, however, the frame assembly  222  of the rack assembly  220  includes a plurality of support grids  290  to provide enhanced structural support, for example, to withstand a seismic event (see  FIG. 11 ). Moreover, the inner shielding assembly is not a stand alone system, as seen in rack assembly  20  shown in  FIGS. 2-6 . In contrast, the inner shielding assembly is incorporated into the container assembly, by being affixed to the surfaces of the individual fuel containers  226 . 
     Referring to  FIGS. 11 and 12 , the support grids  290  are coupled to the frame supports  250  intermediate the first and second frame portions  240  and  242 . In the illustrated embodiment, three support grids  290  are shown; however, it should be appreciated that any number of support grids  290  is within the scope of the present disclosure. Each of the support grids  290  are configured with x-axis and z-axis separators  292  that create a plurality of apertures  294 . Suitable separators  292  may be round, square, rectangular solids or hollow, or of any other suitable configuration. The apertures  294  align with the compartments  244  and  246  in the first and second frame portions  240  and  242  to hold the fuel containers  226  in place. Because of this alignment, the support grids  290  provide enhanced structural support to the frame assembly. Specifically, the support grids  290  are designed to receive the load from the containers  226 , for example, during a seismic event. 
     Because the support grids  290  do not allow for a substantially continuous inner shielding assembly, as seen in the illustrated embodiment of  FIGS. 2-6 , the inner shielding assembly is incorporated into the containers  226 , such that the inner shielding assembly is supported by the container assembly. In that regard, each container  226  includes shielding material on all surfaces, creating a continuous inner shielding assembly around the containers  226 . Therefore, because there is shielding material on adjacent surfaces of adjacent containers  226 , there is also a double layer of shielding material between adjacent fuel containers  226 , similar to the illustrated embodiment of  FIGS. 2-6 . 
     Like the other embodiments, the rack assembly  220  may further include a plurality of outer plates (not shown) positioned around the outer perimeter of the assembly  220  for enhanced criticality properties. Moreover, the rack assembly  220  may have a layer of shielding material on the surface of the containers  226  that face outwardly from the container assembly. Therefore, the outer shielding assembly may be supported by either the container assembly or the frame assembly or both. 
     While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the disclosure.