Development and validation of gamma-heating calculational methods for plutonium-burning fast reactors

The need for accurate calculational tools for the determination of gamma heating in fast reactors has increased considerably in recent years following the planned modification of certain existing fast breeders into plutonium-burning configurations. The latter are characterized by a steel/sodium reflector (replacing the fertile blanket) and a large number of core diluting sodium/steel sub-assemblies, i.e. regions in which gammas account for about 90 % of total heating. In the current doctoral research, a new calculational scheme for the accurate determination of gamma heating in fast reactors has been implemented and its validation for Pu-burning configurations accomplished through comparisons with integral measurements in representative critical assemblies. The particularity of the new calculational methodology is that gamma production multiplicities for fission, capture and inelastic scattering are folded with the corresponding effective (self-shielded) neutron cross-sections and then summed up to yield the total gamma production matrices. This allows one to take advantage of the latest improvements in computing effective cross-sections at the cell level, in particular the consideration of spatially varying cross-sections in non-fuel regions such as reflectors. The new methodology requires gamma production multiplicities separately for fission, capture and inelastic scattering, and accordingly a special library containing these nuclear data was generated from the latest and most appropriate data evaluations, mainly JEF2.2 and ENDF/B-VI. Furthermore, the delayed emission through disintegration of fission and activation products was explicitly considered. In the course of creating the gamma production library, a careful check could be made on the quality of the basic data available. It was found that a major shortcoming in this context is the large uncertainty (  8 %) on the gamma production in fission, since total gamma fission emission values given by various authors differ significantly. The validation of the currently developed calculational tool was accomplished through comparisons with new gamma-heating measurements conducted in the framework of the CIRANO experimental programme at the MASURCA facility, as well as with reevaluated earlier measurements by Calamand et al. in the BALZAC1-DE1 configuration of the same facility. The latter had a steel/sodium (diluent) zone at the center of the core region. In the current CIRANO measurements, absolute gamma-heating rates were determined in PuO2/UO2 fueled cores surrounded by a steel/sodium reflector using TLD-700 thermoluminescent dosimeters. Thereby, a considerable effort was undertaken to minimize systematic errors in the measurements and to reduce the statistical uncertainty, in order to ensure a total experimental error smaller than the target accuracy for the gamma-heating calculations. To achieve this goal, a highly reproducible measuring procedure was established (statistical error < 2 %), individual TLD calibration was carried out in a consistent way with respect to the reactor measurements and the various correction factors (determined using the latest calculational methods and data) were investigated in detail. The correction most in doubt, viz. the cavity relation, was derived by applying both Burlin cavity theory and MCNP coupled photon-electron calculations, with TLD irradiations in various surroundings providing a check on the latter. The total experimental error (1 σ) on the TLD measurements has been estimated to be less than 6 %. The calculation/experiment (C/E) 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 the steel/sodium diluent zone. The most plausible causes for the observed differences have been identified to be data related, viz. too low fission gamma energies and too low capture cross-sections for the structural elements. Thereby, the data for Pu239 and Fe56 are the most suspect, since the former nuclide is the dominant contributor to the gamma production in the core while the latter is that for the reflector and diluent regions. The transferability of the current validation findings to the SUPER-PHENIX power plant (in its planned modified form as Pu-burner) and the 1500 MWe CAPRA 4/94 reference design has been demonstrated by comparing the experimental configurations with the full-scale power reactors in a quantitative manner with respect to gamma-heating characteristics. With the transferability established, a set of correction factors (fpower=E/C) can be defined for application to calculational results. This enables the prediction of gamma heating in the various regions of the considered power reactors to within the current target accuracy of   7.5 %.

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