Feedback on 239Pu and 240Pu nuclear data and associated covariances through the CERES integral experiments

Abstract

Benchmark measurements of irradiated and un-irradiated fuel samples were performed in the framework of the CERES collaborative program between AEA (UK Atomic Energy Agency) and CEA (French Atomic Energy Commission). These experiments provide relevant data for the validation of fuel burn-up and criticality-safety calculations for the whole fuel cycle. As part of this program, pile-oscillation measurements were carried out on a range of mixed-oxide samples with plutonium of various mass and isotopic contents, both in the MINERVE and DIMPLE reactors. Four core configurations, two over-thermalized situations and two pressurized water reactor (PWR)-type situations, were constituted with different forward and adjoint flux spectra, emphasizing fission and/or capture contributions. The experiments were analyzed using reference TRIPOLI4 calculations with the JEFF-3.1.1 library, using exact three-dimensional models of the core configurations. In a first step, calculations of each DIMPLE configuration were performed and compared with the experiment, showing very good agreements with a maximum C-E of −230 pcm. In the second step, reactivity worth experiments were analyzed, using recently developed exact-perturbation capabilities in TRIPOLI4. A consistent assimilation of calculation over experiment discrepancies was performed with the CONRAD code, using the integral data assimilation method. Covariance matrix on multigroup neutron cross sections and multiplicities were generated and significant trends were identified, especially on the 239Pu and 240Pu capture cross sections in the thermal energy range (E < 0.1 eV). Further investigations should be required to confirm these conclusions, due to the strong dependence of these trends and of posterior covariances to prior covariances.