Comparison of ²³⁸U and ²³⁵U Fission Chambers for Monitoring the Reactivity of the VENUS-F Accelerator-Driven Subcritical Reactor Using the Current-to-Flux Method

In this article, we evaluate how the use of threshold fission chambers (FCs) affects the performance of the current-to-flux (CTF) technique being considered for accelerator-driven system (ADS) reactivity monitoring. These detectors are envisaged because of their longer service life in the lead-bismuth eutectic reactor of the MYRRHA ADS. For this purpose, we study an extract of the new CTF experiments carried out at the GUINEVERE facility, equipped with FCs featuring ²³⁵U or ²³⁸U deposits. The extract selected for its representativeness is about the measurement of the change in subcriticality of the VENUS-F reactor triggered by the removal of one of the control rods (CRs) from its core. The results show that raw CTF reactivity values depend on the composition of the FC used and on its relative position to the CR moved. The space-energy effects, which distort the reactivity values, are understood with the help of simulations computed with the Monte Carlo neutron transport code MCNP. Based on simulated fluxes and detector count rates at all assembly positions, we investigate the physics mechanisms of absorption and diffusion by the boron carbide filling the CRs, which wear away the flux around the CRs. The amplitude of the phenomenon depends on the neutron energy and the position relative to the CR and consequently affects differently the FCs with or without a fission threshold placed close to the neutron absorber.