Heterogeneous cores for fast breeder reactors

Considering thermohydraulics the heterogeneous core needs somewhat higher coolant flow rate and has got a much less uniform temperature distribution in the coolant and the fuel subassemblies than the homogeneous core. With respect to mechanical design this results in greater irregularities concerning element bowing, higher local thermal stresses in the subassemblies and more need for consideration of alternating thermal stresses in components above the core. All of these effects require more detailed analyses and increased use of three-dimensional computer codes not only in the fields of thermo-hydraulics and mechanics but also in preceding nuclear design. The codes required for this are being developed. Aside from these differences lying more in detail fundamental disadvantages of the heterogeneous core with respect to thermohydraulics and mechnics cannot be seen. (orig./RW)