Comparison of the neutron energy spectrum and neutron fluence rate in the LVR-15 research reactor with fuel with different enrichment

This contribution compares measured neutron energy spectra and neutron fluence rates in the LVR-15 reactor core fully equipped with IRT2M nuclear fuel (enrichment 36% of ²³⁵U in the form of UO₂) and then with a partially replaced core equipped with three IRT4M nuclear fuel assemblies (enrichment 19.7%). The measurements were performed in the LVR-15 reactor at Research Center Rez Ltd. in the Czech Republic, and were related to a planned transition to low-enriched nuclear fuel within the scope of the RERTR programme. An activation method was chosen for the neutron spectrum measurement. Iron, cobalt, nickel, copper, titanium, iridium and niobium foils were irradiated at four positions near the replaced fuel assemblies. Reaction rates for observed reaction channels were determined using gamma spectroscopy. Reaction rates along the height of the reactor core at the same positions were determined using iron, nickel, and cobalt foils. The SAND-II and STAYNL computer programs were used for neutron spectrum adjustment, and input approximation for both programs was calculated using MCNPX (v2.6). The results include a comparison of theoretical and measured data. Differences were found between thermal neutron fluence rates inside IRT2M fuel assemblies and IRT4M fuel. This difference was predicted by preliminary calculations, but it becomes less significant as distance from fuel assemblies increases.