Delayed neutron fraction and prompt decay constant measurement in the Minerve reactor using the PSI instrumentation

The critical decay constant (β/Λ), delayed neutron fraction (β) and generation time (Λ) of the Minerve reactor were measured by the Paul Scherrer Institut (PSI) and the Commissariat à l´Energie Atomique (CEA) in September 2014 using the Feynman-α and Power Spectral Density neutron noise measurement techniques. Three slightly subcritical configuration were measured using two 1-g 235U fission chambers. This paper reports on the results obtained by PSI in the near critical configuration (-2⊄). The most reliable and precise results were obtained with the Cross-Power Spectral Density technique: β = 708.4±9.2 pcm, β/Λ = 79.0±0.6 s-1 and Λ = 89.7±1.4 μs. Predictions of the same kinetic parameters were obtained with MCNP5-v1.6 and the JEFF-3.1 and ENDF/B-VII.1 nuclear data libraries. On average the predictions for β and β/Λ overestimate the experimental results by 5% and 11%, respectively. The discrepancy is suspected to come from either a corruption of the data or from the inadequacy of the point kinetic equations to interpret the measurements in the Minerve driven system.