Spent fuel racks up26 October 2004
The Interlock Cell Matrix Rack has been developed to optimise use of storage pool space. By Luis Costas de la Peña and Enrique Gomez Poncela
The difficulties in finding a final repository for spent nuclear fuel elements are forcing nuclear plant operators to maximise the storage capacity of their spent fuel pools. Therefore solutions of high (also referred to as ultra-high) storage density are normally required in new or in re-racking spent fuel pool contracts. Maintaining sub-criticality of the fuel arrangement has required the use of specific neutron poison materials.
Two solutions are classically offered:
- The Boral (or similar aluminum-boron) solution. This is a classical solution for the USA (and countries influenced by its nuclear industry), consisting of structural plain stainless steel tubes, with the neutron poison panel attached using non-welded procedures. These tubes are welded in the corners to others to form a compact rack.
- The Japanese or European (and similar countries in terms of their nuclear industry) solution consisting of borated stainless steel (BSS) tubes that serve both as structural and neutron poison material, welded to other tubes to form a compact rack.
Equipos Nucleares (ENSA) has designed and manufactured both solutions. But in order to take into account the three current main requirements of optimisation of free space in pool, cost, and use of the borated stainless steel in a non-welded form, ENSA has developed an Interlock Cell Matrix.
This patented design optimises the cost per cell while maintaining a minimum pitch. It uses an updated rack manufacturing concept: the use of rectangular slotted grids similar to those used in egg crates or those forming the separators for glassware shipment. The slotted grid sheets are assembled in half steps to ensure that the racks, once assembled, will not have any slippage between the grid plates. The plates in one direction are installed at a different level than the transverse plates in such a way that the final assembly is a solid grid (see Figure 1). The plates are cut by laser beam, which provides very good dimensional tolerances that ensure a tight fit between plates. This avoids undesirable gaps in slot connection between plates.
The plates contacting the rack base, up to the height where the fuel element is active, and the upper level plates from the height where the fuel element is not active, are made from plain stainless steel. The rest of the plates are made of borated stainless steel. In some places, the lower borated stainless steel plate in one direction is extended down to the base plate.
The upper and lower stainless steel (non-borated) grids are welded structures and the lower structure is firmly welded to the base plate. The complete structure is then held together (top to bottom) with lateral stainless steel (non borated) strips which are welded to the upper stainless steel (non borated) plates and the lower stainless steel (non borated) plates grids, creating an integral structure capable of withstanding both hydraulic and seismic loading. This construction concept avoids any welding or bending of the borated steel that is used as structural steel.
The resulting structure has the potential of decreasing the pitch and, as a consequence, more positions may be inserted, although, logically, higher neutron poison capacity would then be required. In addition, this configuration, known as an optimised interlocking cell matrix, allows a simple and economical base plate design (see Figures 2 and 3).
At the end of the slots of a typical plate there is a construction device used to connect this free part of the plate between slots with its correspondent transverse plate. So the transverse movement of the free part is avoided and, as a result, the buckling length of the plates is reduced and therefore the structural behavior is improved. This device also allows improved alignment between vertical successive plates (see Figure 4).
This ‘Interlock Cell Matrix’ solution has several variations for different cases (Boral or BSS, single or double wall, and so on), but it is mainly oriented towards the single BSS wall solution because, in comparison with the classical ones, it has the following advantages:
- It uses BSS, a well-known uniform alloy that does not require any in service inspection.
- The pitch is minimised because, unlike Boral (or similar) solutions, both the neutron poison and the structural material are the same. The classically welded BSS tube cannot attain minimum pitch because it needs some corner shims.
- Use of BSS in a non-welded form (with a lower corrosion risk) that is accepted in the US market.
- The manpower and cost per position is reduced.
Luis Costas de la Peña and Enrique Gomez Poncela, Design & Analysis Department, Equipos Nucleares, S.A., Avda. Juan Carlos I, 8, 39600 Maliaño, Cantabria, Spain