Dependability of the fission chambers for the neutron flux monitoring system of the French GEN-IV SFR

The neutron flux monitoring system of the French sodium-cooled fast reactor will rely on fission chambers that permit both reactivity control and power level monitoring from startup to full power. They are installed inside the reactor vessel, putting severe constraints on the detector design to ensure its dependability. In this paper, we present the Photonis high-temperature fission chambers (HTFC) featuring wide-range flux monitoring capability and justify their specifications with the use of simulation and experimental results. We show that the HTFC dependability is enhanced thanks to a robust physical design. In order to satisfy the requirement of wide-range capability, we propose to estimate the count rate of a HTFC using the third-order cumulant of its signal. The use of this cumulant can be seen as an extension of the so-called Campbelling mode, based on the variance, hence the name high order Campbelling method (HOC). (author)