Fluctuations of Mass Yield Distribution for the Epithermal Fission of ²³⁵U and Its Impact on HCLWR Calculations

The need for accurate nuclear data represents a permanent challenge to improve the calculation tools used in reactor physics. Uranium-235 is one of the most important isotopes, and its related nuclear data need to be known with a high degree of accuracy. In this context, many studies have been undertaken to improve the fission yields of ²³⁵U for the main fission products in spent fuels.

For a few years, an increasing interest has been observed for high-conversion light water reactors, for better use of the fuel and for nonproliferation considerations. These concepts are based on a low moderation ratio (˜0.9) and the use of highly enriched mixed oxide fuels (>8%). Because of a neutron flux much harder than in pressurized water reactors, calculations require a good knowledge of nuclear data in the epithermal range to accurately predict the fuel depletion with burnup. In particular, the energy dependence of the fission yields must be considered, because of the existence of fluctuations of the fragment fission yields in the resonances of ²³⁵U(n,f). Unfortunately, the fluctuations are incorrectly taken into account in calculation codes because of the rough energy description in the usual nuclear data libraries.

In this paper, integral experiments are presented for the identification and the quantification of this effect in an almost 1/E neutron spectrum. The experiments consist of the irradiation of ²³⁵U samples in the MINERVE reactor (CEA Cadarache), first in a mostly thermal neutron spectrum and second in a mostly epithermal one. Measurements of some abundant fission products are realized with gamma-ray spectrometry and show fluctuations of the ²³⁵U cumulative fission yields between the two experiments. Results are interpreted with the multimodal random neck rupture model as a variation of 5% of the weight ratio w1/w2 between the standard I and standard II fission modes, and compared with the differential experiments.