Measurement of the neutron capture cross-section of $^{235}$U at the n TOF facility

The first crucial ingredient of reactor and fuel cycle analysis is nuclear data. When designing or assessing the safety of a reactor system, nuclear data for a wide range of reactions and materials has to be known. Designers and physicists must address many variants of nuclear plants and undertake extensive calculations to estimate the performance of a critical nuclear system. For accurate and reliable estimates, these studies should incorporate the most accurate and reliable nuclear data and neutron cross-sections, compiled in evaluated libraries such as ENDF/B-VII.1, JENDL-4.0a, or JEFF-3.2 . These evaluated libraries are tested against relevant benchmark experimental data, thus validating the current knowledge of nuclear cross-sections and nuclear data. These major evaluated libraries predict the measured criticality of nuclear systems extremely well (for many assemblies, although not for all). However, such good performance in integral testing creates a false sense of optimism due to compensating errors, calibration of some critical parameters, and discrepancies between libraries. Over the years, great efforts have been made to obtain reliable neutron-induced cross-sections of the $^{235}$U, which are the most important physical constants in nuclear energy applications. In particular, for the neutron capture cross-section, there are several measurements. The neutron-induced cross-sections for this isotope are very important, not only for major nuclear thermal reactors but for Fast Breeder Reactors (FBRs) because many critical experiments for FBRs have been performed at critical assemblies where UO2 fuels were used as driver fuels. The experimental data obtained at such critical assemblies has a great impact on design work for FBRs. Recent studies show that calculated sodium void reactivity values for BFS experiments underestimate the experimental results by 30-50%. These significant discrepancies not only exceed the target accuracy of 20% for an FBR design but also undermine the design accuracy estimated with the cross-section adjustment and bias factor techniques. To tackle the discrepancies in the neutron cross-section data of the major nuclides, the IAEA CIELO pilot project is re-evaluating the major nuclides important for the nuclear applications: $^{1}$H, $^{16}$U, $^{235}$U, $^{238}$U, and $^{239}$Pu. The main goal of this project is the production of improved and validated evaluated nuclear data files. The work presented in this manuscript focuses on the framework for improving the neutron capture cross-section at low neutron energy and improving current knowledge of the resonance parameters for the $^{235}$U isotope.