First Conclusions of the WPEC/Subgroup-22 Nuclear Data for Improved LEU-LWR Reactivity Predictions

Abstract

This paper is a summary of a collective work in the framework of the Working Party in International Nuclear Data Evaluation and Co‐operation (WPEC) to investigate the reasons for systematic reactivity underprediction of thermal LEU‐LWR (Low‐Enriched Uranium, Light‐Water Reactor). This keff underprediction (≈ −500 pcm) is observed with the most recent nuclear data libraries (ENDF/B‐VI.8, JENDL3.3 and JEFF3.0) This report reviews the evaluation work performed at several laboratories [Oak Ridge National Laboratory (ORNL), Los Alamos National Laboratory (LANL), Commissariat a l’énergie atomique de Bruyeres‐Le‐Chatel (CEA‐BRC), International Atomic Energy Agency (IAEA)] as well as the integral tests (mainly at LANL, Knoll Atomic Power Laboratory (KAPL), Bettis Atomic Power Laboratory (BAPL), Nuclear Research and Consultancy Group NRG‐Petten, CEA and IAEA) of the successive versions of the new evaluated files. The present status of the work can be summarized as follows:• Improved evaluations of 238U inelastic data proposed by LANL and CEA‐BRC were tested against integral benchmarks and partially improve the reactivity prediction.• The thermal capture cross‐section of 238U has been revised, and a new evaluation of 238U resonance parameters, up to 20 keV, is in progress at ORNL. Integral tests have ensured that the modifications of 238U capture cross‐section in the thermal and resolved range were still compatible with 238U integral measurements (238U capture rate ratios measured in critical facilities and 239Pu build‐up prediction in a depleted pressurized water reactor (PWR) assembly).It is demonstrated that the combination of the new inelastic data (LANL or BRC) with the preliminary ORNL resonance parameter set gives a good correction of the reactivity under‐estimation. The provisional conclusions of this collective work are expected to contribute toward the improvement of the future versions of nuclear data libraries.