Boundary condition free homogenization and evaluation of its performance in fast reactor core analysis

This paper proposes a new homogenization method, 'Boundary Condition Free Homogenization (BCFH)'. The traditional homogenization method separates the core calculation and the cell (assembly) calculation by assuming a specific boundary condition or a peripheral region in the cell calculation. Nevertheless, there are ambiguities and approximations in these assumptions that cause a potential decrease in accuracy. BCFH aims to avoid these problems and improve the accuracy of the cell calculation involving homogenization. We imposed the condition that physical quantities in the cell related to reaction rate preservation be preserved for any incoming partial current during homogenization. That is, the response matrices of cell average (or total) flux and outgoing partial current should be the same form between the heterogeneous and homogeneous system. As a result, homogenized parameters such as cross-sections, super-homogenization factors, and discontinuity factors are no longer dependent on a specific boundary condition. The new homogenized parameters obtained in this way are extended from the conventional vector form to the matrix form in BCFH. To investigate the performance of BCFH, numerical tests were performed for the simplified models, which originated in a 750 MW-class sodium-cooled fast reactor core with MOX fuel in Japan. It was found that BCFH is particularly effective in evaluating the control rod reactivity worth and reaction rate distribution compared to the traditional method. We conclude that the BCFH is a promising homogenization concept for fast core neutronic analysis. (author)