Optimum Strategy for Ex-Core Dosimeters/monitors in the IRIS Reactor

International Reactor Innovative and Secure (IRIS) is a medium-power (~300 MWe) advanced light water reactor that features an integral primary system configuration to enhance safety. Steam generators are located inside the pressure vessel above the core, forming a thick (~1.68 m) annular region, that extends into an equally thick downcomer surrounding the core. As a result, neutron fluence at the pressure vessel and in the cavity is reduced by 5-6 orders of magnitude relative to present loop-type Pressurized Water Reactors (PWRs). Reduction of the RPV fluence eliminates embrittlement concerns, but introduces new challenges for the ex-core flux monitors. This paper proposes using advanced flux monitors, such as SiC semiconductor neutron detectors, and examines their optimum placement in the downcomer region. Furthermore, the requirements on neutron dosimetry/monitors considered for the IRIS-reactor are common to Generation-IV Integral Primary System Reactors (IPSRs).