Development of in-vessel neutron monitor using micro-fission chambers for ITER

Abstract

A neutron monitor using micro-fission chambers to be installed inside the vacuum vessel has been designed for the International Thermonuclear Experimental Reactor (ITER). The monitoring system needs to be insensitive to the changes of the plasma position and the profile, and the locations behind upper and lower outboard blankets were selected as appropriate based on the neutron transport calculations with the Monte Carlo code for neutron and photon transport (MCNP). Employing both pulse counting and Campbelling modes in the electronics, the ITER requirement of 107 dynamic range with 1 ms temporal resolution will be accomplished. The system meets the 10% accuracy required for the fusion power monitor. A set of a U235 micro-fission chamber with 12 mg UO2 and a fissile-material-free “blank” detector to eliminate noise issues arising from γ rays, etc. were fabricated based on the design. The vacuum leak rate of the chamber with the mineral insulated (MI) cable, resistances between the central conductor and outer sheath, and mechanical strength up to 50 G acceleration were tested to meet the design criteria. The output signals for γ rays were measured with the Co60 γ-ray irradiation facility at Japan Atomic Energy Research Institute (JAERI)-Takasaki and the influence was estimated to be less than 0.1% of the signals for neutrons. Excellent linearities between count rates, square of Campbelling voltage, and neutron fluxes were confirmed in the temperature range from 20 °C (room) to 250 °C with the Fusion Neutronics Source (FNS) facility of JAERI. The influence of the surrounding material was studied with the shielding blanket mock-up, and it was verified that the chamber provides an effective response although the sensitivity was enhanced by slow-downed neutrons. As a result, it was concluded that the present micro-fission chamber is applicable to ITER power monitoring.