Improvement of numerical analysis method for FBR core characteristics (IV)

Takeda, Toshikazu*; not registered; Kitada, Takanori*

Investigation was made on the following three themes as a part of the improvement of numerical analysis method for FBR core characteristics. [Part 1: Improvement of Reaction Rate Calculation Method in Blanket Region.] A method to calculate multiband parameters directly from the precise energy structure of a cross section was established. This method can treat the precise neutron balance equation including the inter-band scattering. The procedure to treat the multiband effect with the conventional Sn code by the use of direction-dependent microscopic cross sections is shown. This procedure was applied to reaction rate distribution analysis on MONJU. As the result, the reaction rates increased in the blanket regions: the maximum increase was 1O% for U-238 cap, 12% for Pu-239 fis, 12% for U-235 fis, and 1% for U-238 fis. It became clear that the effect of the use of direction-dependent microscopic cross sections is small; thus most of the effect can be attributed to the change of microscopic cross section itself. [Part 2: Improvement of Reactivity Calculation by the Use of Monte Carlo Perturbation Method.] Applicability of the two Monte Carlo perturbation methods; the correlated sampling method and the derivative operator sampling method, to the perturbation problems with large change in material densities was investigated through numerical calculations. The result shows the fact that the present derivative operator sampling method, which treats only the first order term, is insufficient and higher order terms should be considered for such a large perturbation. Investigation was made on the new method which can treat the effect of adjoint flux change in energy and space distribution due to the perturbation, and a new formulation based on the exactperturbation was derived. [Part 3: Investigation of Error Estimation of Neutron Spectra in FBR and Suggestions to Improve the Accuracy.] A comparison was made between the two methods to improve the accuracy ...