Regenerative fission chambers with low variation of sensitivity for high fluence incore measurements

Abstract

Fission chambers are widely used for online neutron flux monitoring in nuclear reactors. However, the depletion of the fissile deposit of the fission chamber with irradiation limits the life duration of the detector when used in a permanent incore location within the vessel of a power reactor. The addition of a fertile material has been considered, at the cost of yielding a non-monotonous evolution of the sensitivity which could be difficult to distinguish from a genuine flux evolution.In this paper, our goal is to find, for a given neutron flux, a feasible mixture of fissile and fertile isotopes to obtain the more stable sensitivity over a high fluence. We devise a methodology based on the DARWIN suite with no simplification of the evolution chains of isotopes and applicable to any neutron flux. All the isotopes that are routinely used in fission chambers are investigated. It is illustrated with a flux that is representative of the one of a Pressurized Water Reactor: we obtain a mixture of 235U−232Th the sensitivity of which is stable up to 3% over four years at full power corresponding to a fluence about 3⋅1022n/cm2. The practical realization of such a mixture for a fission chamber is described. We discuss the robustness of the stability performance with respect to nuclear data, initial isotopic composition and prior knowledge of the neutron flux. The spectral index, which quantifies the amount of thermalized neutrons, needs to be assessed with a precision about 8%. As a result, such a stable and long-living regenerative fission chamber has to be specifically conceived for a dedicated reactor and measurement location.