Abstract
Inelastic (n,n') cross section is a key quantity to accurately simulate reactor cores, and its precision was shown to need significant improvements. To bypass the experimental difficulties to detect neutrons from (n,xn) reaction and to discriminate inelastically scattered neutrons from those following the fission process in case of fissile targets, an indirect but yet powerful method is used: the prompt γ-ray spectroscopy. Along this line, our collaboration has developed the GRAPhEME setup, optimized for actinides, at the GELINA facility to measure partial (n,xn γ) cross sections, from which the total (n,xn) cross section can be inferred. (n,xn γ) experiments with actinides are still particularly challenging, as their structure presents a high level density at low energy, and the competing neutron-induced fission reaction contaminates the γ-energy distribution. New precise measurements of the partial (n,xn γ) cross sections provide a stringent test to theoretical model and offer a way to improve them. This is a path to a better determination of the total inelastic scattering cross sections. In this contribution we discuss modeling aspects of the 238U and 182W (n,n' γ) reactions, also measured with GRAPhEME, using the three codes TALYS, EMPIRE and CoH. We will highlight the needed/expected improvements on reaction modeling and nuclear structure input.
Highlights
Inelastic scattering reactions are important processes in a reactor core as they contribute to the slowing down of the neutrons and have an impact on macroscopic core parameters as keff or radial power distribution
For specific transitions, differences between experimental data sets pose additional challenges. This is illustrated for two γ-rays in 238U in Fig. 3, which compares the (n,n’ γ) cross sections measured with GRAPhEME, and measurements from previous experiments, with three calculations performed with the codes TALYS [13], EMPIRE [14] and CoH [15]
To help quantifying the consequence of this, we have developed a Monte Carlo simulation based on TALYS 1.8 code which allows to estimate the sensitivity of a γ transition cross section to the uncertainty on Branching Ratio (BR) of other γ transitions
Summary
Inelastic scattering reactions are important processes in a reactor core as they contribute to the slowing down of the neutrons and have an impact on macroscopic core parameters as keff or radial power distribution. In 1999, the NEA WPEC-subgroup 4 [1] has already emphasized the large discrepancies between the evaluated 238U inelastic (and capture) cross sections and emphasized the need of new experimental and theoretical studies. This first report has motivated many experimental works. In 2008, the NEA WPEC-subgroup 26 has compiled, for several GEN-IV reactors, the cross sections accuracy needed to reach the target uncertainty for the reactor core parameters [2]. The recent studies performed by CEA/DEN [4] and PSI (Zürich) [5] have shown
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