The LEAF system and gamma detection applications in CROCUS

In a nuclear reactor, gamma-rays are ubiquitous and arise from fissions, decays following activation of fission products or from capture processes occurring in fuel or in reactor components. Gamma-rays have a major contribution to the heating of the reactor structures and to the surrounding dose rate. Accurately characterising the gamma field in nuclear reactors is therefore of strong interest for core operation and safety concerns. Zero power research reactors, such as the CROCUS reactor facility, offer the possibility to measure gamma rays in a pure neutronic environment, allowing for validation experiments of computed spectra, dose estimates, burn up calculations, prompt to delayed gamma ratios and reactor noise measurements. The CROCUS reactor is a two-zone, uranium-fueled light water moderated facility operated by the Laboratory for Reactor Physics and Systems Behaviour (LRS) at the Swiss Federal Institute of Technology Lausanne (EPFL). The work at hand introduces the LEAF system (The Large Energy-resolving detection Array for Fission Gammas) which consists in two large Bismuth Germanate (BGO) and two smaller Cerium Bromide (CeBr3) scintillators. All four scintillation detectors are calibrated in energy and efficiency before being used for several gamma detection applications. Measurements of in-core gamma spectra are performed, at different sub-critical and critical states of CROCUS, and compared to both MCNP6.2 simulations and to a prompt U-235 spectrum produced with the LLNL FREYA code. The comparison of the experiments to MCNP6.2 enables the identification of resolved peaks of delayed processes not modelled by codes, such as the fission products Cs-137 and La-140. In the scientific community, a recent effort has been the determination of delayed gamma contributions. A new method allowing the estimation of local prompt to delayed gamma ratios in the reactor core is developed. This technique estimates the delayed gamma fraction to be 0.30 ± 0.01 in the CROCUS control rod tubes, i.e. in the outer U-metal region of the core. Neutron noise analysis, as a non-invasive approach to determine the kinetic parameters of a reactor, is common practice. LEAF allows the first gamma and neutron-gamma correlations measurements in a reactor. CeBr3 scintillators enable a precise and rapid reproduction of the prompt decay constant mean values previously measured with He-3 neutron detectors, at different sub-critical and critical states in CROCUS, and predicted with Serpent 2 simulations. Beyond these promising results, BGO scintillators which were selected for their high efficiency, provide us with gamma noise measurements at distances up to six meters from the reactor core centre, at critical (power 20 mW). At such distances, the obtained prompt decay constant is still comparable to the one provided by the He-3 neutron measurement. We finally discuss the possible future applications of LEAF, regarding both spectroscopy and gamma noise investigations.