In-core dosimetry for the validation of neutron spectra in the CROCUS reactor

Ecole Polytechnique Fédérale de Lausanne (EPFL),

^* vincent.lamirand@epfl.ch
All authors are with the Laboratory for Reactor Physics and Systems behaviour, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland. Y.Jiang is the corresponding author, currently with the Experimental Physics Service, French Atomic Energy Commission, 13108 Saint- Paul-lez-Durance, France (e-mail: yifeng.jiang@cea.fr)
V.Lamirand is with the Laboratory for Reactor Physics and Thermal Hydraulics, Paul Scherrer Institut, CH-5232 Villigen, Switzerland
A.Pautz is with the Nuclear Energy and Safety Research Division, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.

Published online: 20 January 2020

Abstract

The present article describes the preliminary validation study of simulated in-core and reflector n eutron spectra in preparation of oncoming experimental programs in the zeropower reactor CROCUS at EPFL. For this purpose, a set of activation foils were irradiated at three characteristic positions in the CROCUS reactor, and the subsequent activities were analyzed via γ spectrometry. The experimental setup was then modeled with the Monte Carlo neutron transport code Serpent2 and associated with an analysis tool to include the effect of the reactor power history during experiments.

The comparison of calculated and measured reaction rates (C/E) indicates a general consistency (at 2σ) between calculated and measured spectra. However, offsets of C/E values were observed in (n, γ) reactions, up to 18% for ¹¹⁵In and 8% for ⁶³Cu dosimeters. This could be caused by an unexpected isotopic composition, uncertainties in nuclear data, or the spectrometry analysis.

In addition, a 100-groups spectrum unfolding was performed using the experimentally determined reaction rates and the Serpent2 spectra as the prior knowledge. The unfolded spectra were mainly adjusted in the thermal and fast ranges, while few modifications w ere m ade i n t he e pithermal r egion d ue t o the low contribution of epithermal neutrons in activation processes. Moreover, within energy groups where the capture reactions show resonant behavior, flux depletion (up to 38% as compared to the prior spectra) is observed due to the absence of self-shielding effect in the unfolding process. For this purpose, an unfolding method based on energy groups weighting is developed and tested.