A Predictor-Corrector Scheme for the Microscopic Depletion Solver of the COCAGNE Core Code

EDF R&D is developing a new calculation scheme based on the transport-Simplified P_n (SP_n) approach. The lattice code used is the deterministic code APOLLO2, developed at CEA with the support of EDF and AREVA-NP. The core code is the code COCAGNE, developed at EDF R&D. The latter can take advantage of a microscopic depletion solver expected to improve the treatment of spectral history effects. However, the direct use of the microscopic depletion solver within COCAGNE is computationally very intensive because very small evolution steps (typically 100 MWd/t) are needed to reach a good accuracy, which is not always compatible with industrial applications. In order to reduce the calculation time associated with the use of the microscopic depletion solver, a predictor-corrector scheme has been implemented within COCAGNE. It enables the use of larger evolution steps, up to 1000 MWd/t. Two kinds of tests were performed to validate this predictor-corrector scheme. Firstly, direct comparisons with APOLLO2 results were made on COCAGNE depletion calculations of fuel assemblies. These calculations involved depleting the fuel from 0 GWd/t to 60 GWd/t while keeping fixed thermal-hydraulical conditions and boron concentration, so as to be consistent with the calculation conditions of APOLLO2. It is shown that isotopic concentrations and the multiplication factor (keff) obtained by COCAGNE are consistent with APOLLO2 results. This test was also used to perform a numerical analysis of the convergence order of the new scheme. Secondly, results of the predictor-corrector scheme were compared with those of the existing calculation procedure of COCAGNE on a realistic core calculation case. The impact on global indicators such as boron concentration and cycle length was also considered. This second test indicates that, for industrial applications, the predictor-corrector procedure also gives fairly accurate results while significantly reducing the calculation time related to microscopic depletion.