The CoRREx neutron spectrum filtering campaign at VENUS-F for calculation-to-experiment discrepancy interpretation

Research reactors and particularly zero power reactors serve as bridges between reactor design and deployment. The fast spectrum VENUS-F zero power reactor operated at SCK CEN has been conceived to support the development of fast heavy-metal-cooled reactor designs. Over time, several discrepancies between model results and experiments were observed, especially where the neutron spectrum is more epithermal. To identify their origin, the CoRREx experiment was conducted: B${}_4$C filters of different thickness were loaded in VENUS-F to investigate the relevance of the epithermal neutron spectrum to those discrepancies. We present measurements of spectral indices (fission rate ratios), ${}^{235}$U fission rate traverses and reactivity worth and compare them with the results of Serpent calculations using JEFF-3.3, ENDF/B-VIII.0, ENDF/B-VIII.1 and JENDL-4.0u. CoRREx shows that the Serpent model performance in reproducing the experiment results is rather similar in the active core center and periphery. Furthermore, using B${}_4$C neutron spectrum filters and conducting a thorough propagation of experimental uncertainty CoRREx provides insights on the minimal contribution of the epithermal neutron flux spectrum component to the observed discrepancies. CoRREx indicates that the observed spectral index calculation-to-experiment discrepancy is attributable to the fast neutron spectrum. The Serpent model shows good performance for ${}^{235}$U fission rate traverses and a slight underestimation of the filter reactivity worth.