Spectral indices measurements for neutron data validation in the lead - bismuth VENUS-F fast cores for MYRRHA

In order to support the construction and licensing of the MYRRHA ADS, a number of zero-power experiments have been fulfilled at the VENUS-F reactor in sub-critical and critical cores. The experiments in the sub-critical cores were devoted to on-line sub-criticality measurement methods while core safety aspects and nuclear data validation were investigated in the critical core experiments. The systematic issues with C/E results observed in several VENUS-F critical cores, for spectral indices as well as for reactivity effects (PHYSOR-2016, ND-2016, PHYSOR-2018) initiated the current work. New measurements were performed in different core conditions (i.e. neutron spectrum, surrounding materials) compared to the previous studies. The spectrum indices F49/F25, F37/F25 and F28/F25 were measured with fission chambers in specially prepared assemblies containing only one type of material (fuel rodlets, Al2O3, C, Pb) and also inside in-pile-sections mock-ups containing polyethylene or air. All experimental results are analyzed with the MCNP6.2 code and JEFF-3.3, ENDF/B-VIII and JENDL 4.0 data. The C/E dependence on the neutron spectrum and on the composition of the specific core materials are discussed.