An improved submesh method to treat the environmental effect in PWR-core two-step analysis

The two-step procedure based on assembly homogenization is still widely employed in simulation of PWR cores. In this method, the few-group constants of fuel and reflector assemblies are generated with the single-assembly lattice calculation under the reflective boundary condition, which is different from their actual environments. It can introduce calculation error usually named as the environmental effect. In order to treat it, an improved submesh method is proposed over the legacy one. It corrects the few-group constants, including diffusion coefficients, cross-sections and the discontinuity factors, by using the submesh calculation results. As for the improved submesh method, various few-group constants have been corrected by different methods. Besides, the correction of the reflector has been improved by appending the single-assembly calculation of the reflector assembly. Both the original and new methods have been implemented in our self-developed code NECP-Bamboo. Multi-assembly problem, 2D mini-core problem and the 3D PWR-core problem have been simulated and analyzed. Numerical results demonstrate that both methods can effectively treat the environmental effects. For the original one, the error of k eff can be reduced by 50∼100pcm and the power distributions can be reduced by 1∼2 %. In contrast, the improved one can further reduce the error of k eff by about 50pcm and reduce the error of power distribution by ∼1 %. In other words, the improved submesh method can double the calculation accuracy and treat the environmental effect effectively. • An improved submesh method is proposed. • Different correction methods for various few-group constants. • A Single-Assembly calculation to match the fuel and reflector corrections. • It can also improve the computational accuracy over the original submesh method.