Orateur
Description
Nuclear applications require a continuous improvement of nuclear data, in particular for the fission yields involved in a large time-scale calculations of reactor physics observables. Then,this time-scale requires a consistency between independent and cumulative yields. The Laboratory of Physics Studies (LEPh) of CEA Cadarache has developed a new methodology for the evaluation of fission product yields, within the framework of the recent JEFF-4.0 nuclear data library. Statistical tests of experimental datasets and data ranking are used to estimate with the best accuracy the fission yield central values, uncertainties and associated correlation matrix. In order to properly evaluate the independent and cumulative yields, the analyses of mass and chain
yields represent two key elements to perform a complete and consistent evaluation. These two attributes are the goal of the new JEFF-4 library: it provides a complete description of the correlation matrix of each observable and ensures consistency with the sum rules for the main statistical moments - namely mean values, uncertainties and correlations. Results on mass yields of four fissionning systems will be presented and discussed: 233,235U(nth,f) and 239,241Pu(nth,f). Because statistical analysis is performed using only the experimental data, the results are free of models and can be compared independently. The large amount of data used (EXFOR database) allows us to obtain unmatched precision with relative uncertainties between 1.5% and 3% for the high yields depending on the fissioning system. This new precision brings the possibility to discuss the origin of the structures in the post-neutron mass yields and their consistency with both the pre-neutron mass yields coupled with the prompt neutron multiplicity per mass. The presentation will be focused on the selection of the experimental data used and the statistical method applied to propose a reinterpretation of experimental data uncertainties in order to build a consistent dataset. The new fission product evaluations will allow us to address the physical structure in light of recent improvements in microscopic calculations.
