Clad failure detection system based on Delayed Neutron Detection without photoneutron noise for Sodium-cooled fast reactors

Abstract

In the context of failed fuel detection in Sodium-cooled Fast Reactors (SFR), Delayed Neutrons Detection (DND) systems are implemented. This study deals with the design optimization of such a system by Monte-Carlo particle transport simulation (MCNP). During a clad failure, neutron precursor fission products (mainly halogen ones: 137I, 87Br, 88Br) escape into the primary coolant. The detection of these precursors through their delayed neutrons ensures a diagnosis on clad failures events. In DND systems, 3He proportional counters are chosen as the best available technology for neutron detection purposes. Associated with 3He counters, polyethylene blankets are required in order to thermalize neutrons. Feedbacks from past SFR have shown an important noise coming from photoneutrons mitigating the signal-to-noise ratio. These photoneutrons originate from the 2D(γ,n) reaction in polyethylene (in natural abundance) and is due to the 24Na activity. Another material avoiding photoneutrons production is considered in this paper as moderator. Indeed, the graphite appears as a good candidate, sufficiently light for neutron thermalization and with a C(γ,n) reaction threshold higher than energies considered here. The first simulations run with MCNPX code provide satisfactory results for a low-noise DND system based on graphite thermalizing.