Patent Number: 041773853
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1 and 2 are general arrangement view, wherein an egg crate structure is formed of stainless steel plates 10. These plates extending through the entire height of the rack must be generally equal to or greater in length than the active length of the fuel assemblies to be stored and must be coterminous therewith when the assemblies are stored within the rack. Support bars 12 pass through the lower portion of the rack for support of the fuel assemblies to be stored. This egg crate structure forms vertically extending rectangular openings 14 through 22. Fuel of low effective enrichment may be stored in each of these openings. Fuel of a higher enrichment (typically about 3.5 to 4.0 weight percent U 235) which is normally expected to be stored, may be safely stored in a checkerboard pattern using openings 15, 17, 19, 21 etc. With practical center to center spacing, this will normally provide safe storage of the highest enriched fuel expected to be stored. Since only one half the openings are used, the capacity of the rack with such a storage mode is limited to one half the possible capacity. As the capacity of the storage rack in this mode is approached, it may be desirable to increase the capacity. This is accomplished by adding inserts 24 of stainless steel or some other neutron absorbing material as illustrated in FIG. 3. These inserts are formed of two plates 26 and 28 which are parallel to adjacent sides of the opening and are spaced therefrom a predetermined distance. The appropriate distances and fuel enrichment capability are calculated by considering the total mass of the fuel to be stored, the plates 10 of the egg crate structure, the inserts 26, and the water gap 30 between the sides of the boxes and the plates. Each insert has a bent portion, or spacer 32 at each end and a spacer piece 33 at the intervening corner. These cooperate to maintain the insert properly spaced within the opening and to maintain the predetermined space of the water gap 30. As best seen in FIGS. 2 and 4 rectangular openings 34 are supplied in plates 10 of the egg crate structure. The inserts have an upwardly extending tab 36 which is bent so as to lock into the opening 34. Tab extensions 38 are welded to the tab and prevent the tab from passing through the opening 34. The insert is supported on plate 12 and the tabs function to maintain the insert in position and retain the insert in position when the stored fuel assemblies are withdrawn. When fuel of still higher enrichment must be stored there are several options available, each of which involves the use of a high neutron absorbing plate such as material containing boron 10 or hafnium. Referring to FIG. 5, poison plates 40 and 42 are installed in the water gap between the insert 24 and the side plates 10 of the opening. In the particular construction illustrated these plates may be inseted by springing back tab 38 and inserting the plate. They may be supported either from the tab itself or on an additional support member added to the egg crate structure at the bottom. The enrichment of fuel which may be stored at this time must be calculated taking into consideration the poison plate as well as the times discussed with respect to the flux trap principle arrangement. An alternate method of storing additionally enriched fuel is illustrated in FIG. 6 wherein the poison plates 40 and 42 of FIG. 5 are removed along with the insert 24. A rectangular storage box 50 of stainless steel or a higher neutron absorber poison material, which is commercially available, may be installed in the openings and supported on bars 12. The enrichment of fuel which may be stored in this mode must again be calculated by nuclear physics principles according to the physical structure which exists.