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Use of integral data assimilation and differential measurements as a contribution to improve $^{235}$U and $^{235}$U cross sections evaluations in the fast and epithermal energy range

Abstract : Critical mass calculations of various HEU-fueled fast reactors result in large discrepancies in C/E values, depending on the nuclear data library used and the configuration modeled. Thus, it seems relevant to use integral experiments to try to reassess cross sections that might be responsible for such a dispersion in critical mass results. This work makes use of the Generalized Least Square method to solve Bayes equation, as implemented in the CONRAD code. Experimental database used includes ICSBEP Uranium based critical experiments and benefits from recent re-analyses of MASURCA and FCA-IX criticality experiments (with Monte-Carlo calculations) and of PROFIL irradiation experiments. These last ones provide very specific information on $^{235}$U and $^{238}$U capture cross sections. Due to high experimental uncertainties associated to fission spectra, we chose to consider either fitting these data or set them to JEFF-3.1.1 evaluations. The work focused on JEFF-3.1.1 $^{235}$U and $^{238}$U evaluations and results presented in this paper for $^{235}$U capture and $^{238}$U capture, and inelastic cross sections are compared to recent differential experiment or recent evaluations. Our integral experiment assimilation work notably suggests a 30% decrease for $^{235}$U capture around 1-2.25 keV, a 10% increase in the unresolved resonance range when using JEFF-3.1.1 as "a priori" data. These results are in agreement with recent microscopic measurements from Danon et al. [Nucl. Sci. Eng. 187, 291 (2017)] and Jandel et al. [Phys. Rev. Lett. 109, 202506 (2012)]. For $^{238}$U cross sections, results are highly dependent on fission spectra.
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Virginie Huy, Gilles Noguere, G. Rimpault. Use of integral data assimilation and differential measurements as a contribution to improve $^{235}$U and $^{235}$U cross sections evaluations in the fast and epithermal energy range. EPJ N - Nuclear Sciences & Technologies, EDP Sciences, 2018, 4, pp.41. ⟨10.1051/epjn/2018035⟩. ⟨cea-02305677⟩

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