Engineering design of the ITER invessel neutron monitor using micro-fission chambers

An invessel neutron monitor using micro-fission chambers is one of the most important diagnostics to measure the fusion power in ITER. A single micro-fission chamber with 10 mg of 235U to be installed in a gap between a shielding blanket module and a vacuum vessel can cover the range of the fusion power from 100 kW to 1 GW. For the low fusion power operation including D-D operation, a micro-fission chamber bundle consisting of 13 micro-fission chamber units has been designed. Nuclear heating of the micro-fission chamber for high power operations and the micro-fission chamber bundle for low power operation were analyzed by neutron transport calculations using the MCNPX code. Heat transport analyzes for the micro-fission chambers were carried out with a general purpose Finite Element Method code ANSYS 8.1, which indicated that the detector temperature might be kept less than the operational temperature limit by the thermal conduction to the vacuum vessel with the heat transfer coefficient of 100 W/m2 K or larger