Bowing and interaction of fast reactor subassemblies

Deformations of the subassembly structural components, in particular the bowing of the hexagonal wrapper which encloses the pin bundles, due to stainless steel swelling as a result of fast neutron irradiation give rise to operational and safety problems especially in large breeder reactors where the neutron flux is much larger than in smaller reactors. The restraint on bowing induces heavy restraint loads and high stresses in the wrapper, which tend to limit the target burn-up of the fast reactor fuels. Therefore, a realistic analysis has to include the phenomenon of creep to determine the extent to which the stresses in the wrapper would be relaxed due to both thermally induced and irradiation induced creep. Apart from this, determination of deformations of the subassemblies in the core due to the interaction among them is also necessary. (author)