Integral experiments in reactors or critical configurations claim to have very small experimental and technological uncertainties. Therefore these latter can be considered valuable experimental information in nuclear data evaluation. Because in the evaluation process the information is carried by model parameters, to perform a rigorous feedback on a nuclear model parameters p - for instance using a measured reactivity ρ-sensitivities S =∂ρ/ρ⁄∂p/p are needed. In usual integral feedbacks, sensitivity to multi-group cross sections are first obtained with deterministic code using perturbation theory. Then these multi-group cross section sensitivities are “convoluted” with parameter sensitivities in order to provide the sensitivity on nuclear model parameter. Recently stochastic approaches have been elaborated in order to obtain continuous cross-section sensitivities thus avoiding the multi-group discretization. In the present work we used the recent Iterated Fission Probability method of the TRIPOLI4 code [1] in order to obtain directly the sensitivity to nuclear physics parameters. We focus here on the sensitivity on resonance parameters and exemplified the method on the computation of sensitivities for 239Pu and 16O resonance parameters one the ICSBEP benchmark PST001. The underlying nuclear model describing resonant cross sections are based in the R-matrix formalism [2] that provides not only the interaction cross sections but also the angular distribution of the scattered neutrons i.e. differential cross sections. The method has thus been updated in order to compute parameter sensitives that include both contributions: cross section and angular distributions. This extension of the method was tested with exact perturbation of angular distribution and fission spectrum.

This article reviews two recently established methods to compute sensitivities of some core parameters to basic nuclear data. First, perturbation theory offers an efficient way to compute sensitivities to nuclear parameters in continuous energy transport simulations: making use of the Iterated Fission Probability method, and by coupling the Monte Carlo code TRIPOLI-4® to the nuclear evaluation code CONRAD, we were able to compute the sensitivity of core reactivity to nuclear parameters for simple ICSBEP benchmarks. Second, using a multipoint description of a nuclear system and deterministic transport calculations the sensitivity of the state eigenvector of the system to multigroup nuclear data is computed using simple and fast partial importance calculations.

Current assimilation of integral experiments often consists in adjusting multi-group cross sections with feedbacks from critical reference benchmarks. In order to maintain the constraints coming from nuclear models, we present here a method to achieve assimilation of integral experiment on nuclear parameters, from which nuclear data are evaluated. This method, based on Bayesian inference, uses continuous energy reactivity sensitivities to nuclear parameters, throughout all the nuclear data types (cross section, angular distribution, energy distribution, fission multiplicity and spectrum). This improvement was made possible by coupling a stochastic transport code and a nuclear data evaluation code. The study of a test case – the assimilation of Jezebel ICSBEP benchmark on a plutonium-239 toy evaluation – shows that angular and energy distributions have a non-negligible impact on the assimilation process and results.

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