A note on treatments of sodium void reactivity effects in a large fast reactor

Fine spectra in large fast reactors of various compositions have been calculated by exactly treating slowing down due to elastic scattering of light elements, and the resulting composition-dependent sets of the effective cross sections have been compared with currently typical standard sets. The following results have been obtained: (1) Effective cross sections depend largely on composition. The dependence is large for groups where predominant resonances exist. A composition-independent set underestimates sodium void effect compared with a composition-dependent set, when variations of fine spectrum are considered. Therefore a composition-independent set like YOM is not adequate, especially for a large reactor. (2) In the present standard set, the self-shielding factors of both elastic removal and elastic scattering cross section are treated under the same principle. This procedure is not valid, especially at the predominant resonances they vary inversely to each other with variations of composition. (3) The effective cross section in a blanket region, except close to the interface and boundary, are hardly affected by the buckling assumed, and the effects of buckling on effective multiplication factor as well as the sodium void reactivity effect are small enough to be neglected. (author)