Improved Gamma-Heating Calculational Methods for Fast Reactors and Their Validation for Plutonium-Burning Configurations

A new calculational scheme has been developed for the accurate assessment of gamma heating in fast reactors, its special feature being the determination of the gamma source distribution that is formulated in a near-to-exact manner. The improved methodology, which has been implemented into the ERANOS (European Reactor Analysis Optimized System) code package, is currently validated for Pu-burning configurations, for which gamma-heating target accuracies are particularly high. This has been accomplished through comparisons with new integral measurements conducted at the MASURCA facility, as well as with reevaluated earlier experiments. In the new measurements, absolute gamma-heating rates were determined in PuO2/UO2-fueled cores surrounded by a steel/sodium reflector, mainly using TLD-700 thermoluminescent dosimeters. Thereby, a considerable effort was undertaken to minimize systematic errors. The calculation/experiment values determined from the analysis of the critical experiments are 0.90 for the PuO2/UO2 core region, 0.84 for the steel/sodium reflector, and 0.89 for an internal steel/sodium diluent zone. The most plausible causes for the observed discrepancies have been identified to be data related, i.e., too low fission gamma energies and too low capture cross sections for the structural elements. The transferability of the current validation findings to a modified Superphénix configuration, in which the radial fertile blanket is replaced by a steel/sodium reflector, and to the 1500 MW(electric) Pu-burning CAPRA 4/94 reference design has been demonstrated.