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

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:
¹³⁷
I,
⁸⁷
Br,
⁸⁸
Br) escape into the primary coolant. The detection of these precursors through their delayed neutrons ensures a diagnosis on clad failures events. In DND systems,
³
He proportional counters are chosen as the best available technology for neutron detection purposes. Associated with
³
He 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
²
D(γ,n) reaction in polyethylene (in natural abundance) and is due to the
²⁴
Na 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.