Abstract
The Neutron Physics Department at Centro Atómico Bariloche developed new models for the interaction of thermal neutrons with water which have been validated against experimental data, including new thermal scattering experiments, and were adopted for the release of ENDF/B-VIII.0. Although the older models are, in general, good for most applications, some discrepancies had appeared in the case of heavy water, and this motivated new measurements that validated the new model. In the case of light water, the new model predicts a reduction of the total cross section around 0.025 eV when the temperature is increased from room temperature. This reduction, that is not predicted by the existing models, and potentially affects the calculation of temperature reactivity coefficients in nuclear reactors, has been traced to a shift in the vibrational frequency spectrum of liquid water. The only experimental data previously available is from an experiment performed at the Demokritos reactor in the ’60s at 293 K and 473 K, which validates the new model when the cross section ratios are computed. In order to verify this effect at a lower temperature range, a transmission experiment was carried out at the VESUVIO spectrometer in the ISIS facility in the UK in June 2018, measuring the total neutron cross section in the range from 283 K to 353 K. Here, we present this new experimental data and its comparison with the models.
Highlights
The nuclear data library ENDF/B-VIII.0 [1] incorporates new evaluations for the thermal neutron scattering libraries for light and heavy water
The new model correctly predicts the decrease in the cross section of 1.1 b in the energy range 0.025 eV < E < 0.030 eV when the temperature rises from 283 K to 353 K, an effect that was not observed in calculations performed with older models of the thermal neutron scattering on light water and with evaluations different from the ENDF/B-VIII.0
The results at room temperature are compatible with previous experiments, and the temperature variation shows changes in the total cross sections clearly observable with the achieved statistics
Summary
The nuclear data library ENDF/B-VIII.0 [1] incorporates new evaluations for the thermal neutron scattering libraries for light and heavy water These libraries were derived from models developed at Centro Atómico Bariloche, known as the CAB Models for Water [2], which are based on molecular dynamics results as well as on experimental data. The applications of these libraries to nuclear criticality benchmarks at room temperature help to reduce the bias previously found in the calculation of water moderated reactor benchmarks [1, 3]. Previous experiments on the same instrument [9] had shown this requirement to be achievable
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