Nodal Equivalence Theory Based on Functionalized Discontinuity Factors

In this study, we have proposed a new and unique method to improve accuracy of the nodal equivalence theory in reactor core analysis. The DFs are expressed as a function of node interface current-to-flux ratio. The evaluation of FDFs requires just a few more lattice calculations and they are updated during iterative calculation of whole-core calculation without significant change in computing time. It is clearly demonstrated that a quadratic FDFs can reduce the k_eff error more than 50% in both conventional UO₂ and UO₂-MOX problems. Also, we have shown that the assembly power distribution is improved by using the FDF method. For a more practical utilization of the FDF method, its feasibility study for two-dimensional practical fuel assembly problems is underway. The contemporary reactor core analysis for the Light Water Reactor (LWR) heavily replies on the fuel assembly homogenization based on the nodal equivalence theory, which is the idea of preserving the equivalency between an original heterogeneous assembly and a homogenized assembly in terms of reaction rates and node interface currents. In the nodal equivalence theory, the single assembly homogenization is most widely used to obtain homogenized parameters for the calculational efficiency. However, because reflective boundary conditions are used for the single assembly calculation, the accuracy of whole core analysis deteriorates noticeably when two very different assemblies are neighboring each other and the node interface current is not close to zero. In this study, we propose a new and unique approach to reduce the error which comes from the incomplete assembly homogenization in the lattice calculation