Transport criticality analysis of FBR MONJU initial critical core in whole core simulation by NSHEX and GMVP

The Japanese prototype fast breeder reactor MONJU development project is of prime importance in the establishment of the commercialized fast breeder reactor technology in Japan. This work presents new results obtained by the 3D hexagonal transport nodal code NSHEX, which for the first time has been applied to MONJU criticality analysis in whole core simulation and up to 70 energy groups. Significant energy-group approximation effect on keff caused by the collapsing into coarse energy-group structure has been found and then confirmed separately using the Monte Carlo analysis. On the basis of the presented applied results, a method of transport correction evaluation, based on 70 energy-group approximation can be recommended instead of the conventional few-group method that assumes inter-independency of energy-group approximation and transport effect. (author)