Patent Number: 041359747
Section: summary

BACKGROUND OF THE INVENTION This invention pertains to structural support systems for the core of a nuclear reactor and more particularly to such support systems which are subject to both thermal perturbations and radiation induced swelling; as with liquid metal fast breeder reactors (LMFBR). The primary restraint on the design of the core support system for an LMFBR is that the system must accurately and predictably position the fuel assemblies while causing a negative overall power coefficient of reactivity. Such a design is particularly difficult to achieve since thermal perturbations and radiation induced swelling constantly change the positional relationship of the structural elements. The prior art basically discloses two contrasting approaches to this problem. The first approach features a relatively loose core, to wit, one in which the fuel assemblies are allowed to bow and other structural elements are allowed to freely change their positional relationship. The operating characteristics of the reactor are then predictable on the basis of the ultimate positional relationship of the elements at operating temperature. The EBR-II nuclear reactor is an example of this approach. The second approach features a tight core which restricts bowing, of which Fermi is an example. However, these present designs preceded the recognition of the degree to which radiation induced swelling effects the positional relationship of the structural elements of the core and its support system, and accordingly, the above examples made insufficient allowance for the resulting problems. SUMMARY OF THE INVENTION In accordance with this invention, a relatively restricted nuclear core is achieved at operating conditions while sufficient clearance between fuel assemblies at refueling temperature is obtained through the application of metals with different coefficients of thermal expansion, careful choice of fuel assembly dimensions, clearances, spacer pad locations and the application of a temperature compensated radial restraint system. The core of an LMFBR is generally cylindrical in shape and made-up of hexagonal fuel assemblies surrounded by similarly shaped blanket and reflector assemblies. Control rod assemblies are interspersed throughout the core. The core may be positioned between an upper and lower core support structure. The individual assemblies may also be fitted with springs to take up differential thermal expansion and keep the fuel assemblies positively positioned relative to the core support. A typical core assembly may consist of an array of individual fuel rods surrounded by a hexagonal can having a plurality of raised spacer pads distributed along its length. The fuel assemblies are fitted with nozzles at either end. These nozzles fit into receptacles in the upper and lower core support structures and a coil take-up spring may be used at the lower end of the fuel assemblies to compensate for differential thermal expansion. In accordance with this invention, a relatively restricted core is achieved which will allow adequate clearance upon refueling by providing a plurality of structural elements which maintain positional predictability under conditions of thermal perturbations and radiation induced swelling. The structural elements utilized include core plates of a different metal from that of the fuel assemblies such that refueling clearances are closed due to differential thermal expansion, a double lower core support plate structure which maintains the fuel assemblies in a vertical or nearly vertical position even when the upper core plate is removed, spacer pads positioned and displaced laterally from each other a predetermined amount such that the gaps are closed as the reactor approaches power, a radial restraint system utilizing relatively compliant springs with spring like back-up members strategically located outside of the active fuel zone and support columns for the upper and lower core plates which bend to allow movement of same.