Probabilistic safety assessment and reliability engineering: Reactor safety and incomplete information

The protection-system of GEN III+ reactors partially relies on self-powered neutron detectors (SPNDs), in-core devices measuring the neutron flux. Those devices, also called collectrons, provide indirect measurements of the neutron flux assembly-wise averaged values, which are used, through a suitable algorithm, to reconstruct the core hot spot. For operation and safety reasons, it is worth quantifying the increase in the uncertainty affecting the reconstructed hot spot value, when one or more SPNDs fail. The goal of this paper is to estimate the robustness and reliability of those detectors for surveillance purposes, using two different and independent mathematical approaches. The results show a quite good convergence and the same trends. As expected, the sensitivity of the hot spot reconstruction to the SPND availability and position is confirmed. Nevertheless, the increase of the uncertainty in the worst case - loss of a full column of SPNDs due, e.g., to an electrical connection failure - does not seem too large, compared to its already quite high value when all the detectors are available. As expected, the sensitivity appears to be lower in 3D, mainly due to a smaller individual contribution of the detectors to the hot spot value reconstruction. Finally, more precise calculations accounting for the energy dependence of neutrons do not change the results significantly, except when the breakdown affects the collectrons which are located very close to the hot spot