Kinetic Parameter Measurements in the MINERVE Reactor

In the framework of an international collaboration, teams of the PSI and CEA research institutes measure the critical decay constant (α₀ = β/A), delayed neutron fraction (β) and generation time (A) of the Minerve reactor using the Feynman-α, Power Spectral Density and Rossi-α neutron noise measurement techniques. These measurements contribute to the experimental database of kinetic parameters used to improve nuclear data files and validate modern methods in Monte Carlo codes. Minerve is a zero-power pool reactor composed of a central experimental test lattice surrounded by a large aluminum buffer and four high-enriched driver regions. Measurements are performed in three slightly subcritical configurations (-2 cents to -30 cents) using two high-efficiency 235U fission chambers in the driver regions. Measurement of α₀ and β obtained by the two institutes and with the different techniques are consistent for the configurations envisaged. Slight increases of the β values are observed with the subcriticality level. Best estimate values are obtained with the Cross-Power Spectral Density technique at -2 cents, and are worth: β = 716.9±9.0 pcm, α₀ = 79.0±0.6 s^-1 and A = 90.7±1.4 μs. The kinetic parameters are predicted with MCNP5-v1.6 and TRIPOLI4.9 and the JEFF-3.1/3.1.1 and ENDF/B-VII.1 nuclear data libraries. The predictions for β and α₀ overestimate the experimental results by 3-5% and 10-12%, respectively; that for A underestimate the experimental result by 6-7%. The discrepancies are suspected to come from the driven system nature of Minerve and the location of the detectors in the driver regions, which prevent accounting for the full reactor.

I. Introduction

Measurement of delayed neutron and generation time in integral experiments, for instance in zero-power research reactors, is of relevance to the nuclear data community and for code validation. Procedures to calculate these kinetic parameters were recently included in Monte-Carlo codes, like MCNP5-v1.6 [1], [2], SERPENT2 [3] and TRIPOLI4.9 [4], and therefore, need to be validated for a wide range of spectral conditions. The International Handbook of Evaluated Criticality Safety Benchmark Experiments database [5] together with several independent experiments (e.g. [6]) are already available for that purpose. Nonetheless, new experiments continue to be performed in zero-power facilities around the world to enlarge the benchmark database and reduce the experimental uncertainties (e.g. [7], [8]). This paper reports on new measurements performed in the Minerve zero-power facility at CEA, Cadarache.