Reactivity Relative Monitoring using Threshold Fission Chambers with the Current-to-Flux Method for the accelerator driven VENUS-F Subcritical Reactor

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 ADS reactivity monitoring. These detectors are envisaged because of their longer service life in the lead-bismuth eutectic reactor of MYRRHA ADS. For this purpose, we study an extract of the new CTF experiments carried out at the GUINEVERE facility, equipped with fission chambers (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 FCs 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 (B4C) filling the CRs, that wear away the flux around the CRs. The amplitude of the phenomenon depends on the carbide neutron energy and the position relative to the CR, and consequently affects differently the FCs with or without fission threshold placed close to the neutron absorber.