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In a fast reactor, we evaluated a new core concept that prevents severe recriticality after whole-scale molten formation in a severe accident. A core concept in which Duplex pellets including neutron absorber are loaded in the outer core has been proposed. Analysis by the continuous energy model Monte Carlo code MVP using the JENDL-4.0 nuclear data library revealed that this fast reactor core has large negative reactivity due to fuel melting at the time of a severe accident, so that the core prevents recriticality. Regarding the core nuclear and thermal characteristics, the loading of Duplex pellets including neutron absorber in the outer core caused no significant differences from the normal core without Duplex pellets.

A new homogenization method of control rod assemblies has been developed to accurately calculate control rod worth in fast reactors. In this method, the homogenized cross sections are defined so as to preserve the reaction rate ratio between a control rod region and a surrounding fuel region in a supercell model before and after homogenization. This new method has an advantage that the cross section of adjacent fuel assemblies is not necessary to change. From supercell calculation, this method produced k, values which agreed with the reference heterogeneous calculation within 0.2xdk. The flux weighting method underestimated k , up to 1.4%dk and the reaction rate preservation method did not produced a converged cross section.In the verification test of core calculation in RZ geometry, we confirmed that the reaction rate ratio preservation method is more accurate than the flux weighting method for control rod worth calculations.It was found from core calculations that the control rod worth obtained by the flux weighting method is overestimated compared to that by the reaction rate ratio preservation method by about 3%. This overestimation is remarkable for a safety rod with 90wt% enriched 1°B than for a shim rod with 40wt% enriched log.

A new homogenization method of control rod assemblies has been developed to accurately calculate control rod worth in fast reactors. In this method, the homogenized cross sections are defined so as to preserve the reaction rate ratio between a control rod region and a surrounding fuel region in a supercell model before and after homogenization. This new method has an advantage that the cross section of adjacent fuel assemblies is not necessary to change. From supercell calculation, this method produced k, values which agreed with the reference heterogeneous calculation within 0.2xdk. The flux weighting method underestimated k , up to 1.4%dk and the reaction rate preservation method did not produced a converged cross section.In the verification test of core calculation in RZ geometry, we confirmed that the reaction rate ratio preservation method is more accurate than the flux weighting method for control rod worth calculations.It was found from core calculations that the control rod worth obtained by the flux weighting method is overestimated compared to that by the reaction rate ratio preservation method by about 3%. This overestimation is remarkable for a safety rod with 90wt% enriched 1°B than for a shim rod with 40wt% enriched log.