Feasibility of Nodal Equivalence in CANDU Analysis With the APEC Method

Abstract

CANDU6 reactor analysis relies on the Canadian AECL’s coarse-mesh FDM code, RFSP. However, the RFSP code is found to be subject to inconsistency issues, which is mainly due to the lack of nodal equivalence. Several attempts have been made to apply the simplified equivalence theory commonly used in PWR core analysis to CANDU core analysis, but they have not been very successful. In this study, the APEC (albedo-corrected parameterized equivalences constants) method is introduced to consider the nodal equivalence theory in the CANDU reactor core calculations. In the APEC method, two-group homogenized cross-sections (XSs) can be parameterized as a function of the assembly-wise current-to-flux ratio (CFR) and discontinuity factors (DFs) are expressed as a function of surface-wise CFR. It was shown that two-group XSs and DFs can be corrected very close to the reference value and the nodal accuracy was improved. The CANDU core analysis was performed based on the well-known two-step procedure. An in-house nodal expansion method (NEM) code in conjunction with a partial-current based CMFD (p-CMFD) acceleration was used for the full-core calculation. As a result of correcting both XSs and DFs, the nodal accuracy was significantly improved.