Advances in methods of commerical FBR core characteristics analyses; Investigations of a treatment of the double-heterogeneity and a method to calculate homogenized control rod cross sections

Sugino, Kazuteru; Iwai, Takehiko*

A standard data base for FBR core nuclear design is under development in order to improve the accuracy of FBR design calculation. As a part of the development, we investigated an improved treatment of double-heterogeneity and a method to calculate homogenized control rod cross sections in a commercial reactor geometry, for the betterment of the analytical accuracy of commercial FBR core characteristics. As an improvement in the treatment of double-heterogeneity, we derived a new method (the direct method) and compared both this and existing methods with continuous energy Monte-Carlo calculations. In addition, we investigated the applicability of the reaction rate ratio preservation method as a advanced method to calculate homogenized control rod cross sections. The present studies gave the following information: (1)An improved treatment of double-heterogeneity: for criticality the existing method showed good agreement with Monte-Carlo result within one standard deviation; the direct method was consistent with existing one. Preliminary evaluation of effects in core characteristics other than criticality showed that the effect of sodium void reactivity (coolant reactivity) due to the double-heterogeneity was large. (2)An advanced method to calculate homogenized control rod cross sections: for control rod worths the reaction rate ratio preservation method agreed with those produced by the calculations with the control rod heterogeneity included in the core geometry; in Monju control rod worth analysis, the presented method overestimated control rod worths by 1 to 2%compared with the standard method, but these differences were caused by more accurate model in the presented method and it is considered that this method is more reliable than the standard one. These two methods investigated in this study can be directly applied to core characteristics other than criticality or control rod worth. Thus it is concluded that these methods will contribute to the ...