Investigation of Noise Detection Systems in the CROCUS Reactor from SANDcaStLE

¹ Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, 48109 MI, USA
² Los Alamos National Laboratory, Los Alamos, 87545 NM, USA
³ Laboratory of Reactor Physics and Systems Behaviour, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
⁴ Nuclear Energy and Safety Division, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland

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Published online: 6 November 2025

Abstract

The need for accurate, non-invasive methods to monitor reactor kinetics motivated SANDcaStLE (Simultaneous Analysis of Noise Detectors and Spectroscopy of a Low-power Experiment) at the CROCUS zero-power reactor. This study compares various neutron and gamma-ray detection systems, including fission chambers, a novel 3D neutron detection array called SAFFRON, and inorganic scintillators, to estimate the prompt neutron decay constant (α) from the fission “noise” at 8.92 mW critical. Our results show that gamma-ray measurements are highly effective, achieving a relative uncertainty of less than 15% in just one minute, an improvement over the three minutes required by ex-core fission chambers. Additionally, the SAFFRON array could detect fission noise despite its low total efficiency. The SAFFRON array was distributed through the radius and height of the core, suggesting the importance of detector spatial distribution in noise analysis. The fission chamber and inorganic scintillator systems agreed with the Serpent 2 IFP simulation α within 10 minutes of measurement time.