Development and application of the improved multicell method for the PWR fuel assemblies adjacent to reflector

The conventional “two-step” scheme based on assembly homogenization is widely applied in the modeling and simulation for PWRs. In the “two-step” scheme, the reflective boundary condition is applied in the two-dimensional calculation of fuel assembly, providing the few-group constants and ADFs (Assembly Discontinuity Factors) for the three-dimensional core calculation. However, the neutron-leakage phenomenon would occur to the fuel assemblies adjacent to the reflector, in which case the approximation of the reflective boundary condition applied to the single fuel assembly would no longer be suitable. In order to consider the neutron-leakage phenomenon, the multicell method originally developed for CANDU reactor has been improved and applied to PWRs for the fuel assemblies adjacent to reflector in this paper. Through the heterogeneous simulation of the multicell case, which are composed of fuel and reflector assemblies to characterize the neutron leakage from the fuel assembly, the multicell correction factors for the few-group constants and ADFs can be determined and applied to the core calculation. Applying the improved multicell method, our home-developed Bamboo-C code has been improved. For the method verifications, the baffle-reflector PWR and heavy-reflector PWR have been modeled with the improved Bamboo-C code, and the k_eff and power distributions are compared with reference values provided by the Monte-Carlo code. The numerical results show that the improved multicell method can reduce the power-distribution differences notably, which indicate the effectivity of this method for the fuel assemblies adjacent to both the baffle-reflector PWR and heavy-reflector PWR.