Shutdown dose-rate assessment during the replacement of in-vessel components for a fusion DEMO reactor

The shutdown dose rate at Japan's demonstration nuclear fusion (DEMO) plant was analyzed using MCNP-5 and DCHAIN-SP2001 to assess its significance in the development of a maintenance program. It was assumed that the blanket segments were integrated with a shielding plug (SP) and replaced through vertical upper ports, whereas divertor cassettes, also integrated with the SP, were replaced through bottom ports. To minimize the dose rate, remote handling equipment should approach from behind the SP and be fixed with an attachment to it. The estimated dose rates during manifold cutting and rewelding in the maintenance ports were as low as 0.01 Gy/h for the blanket segment and 0.1 Gy/h for the divertor cassette. When the outboard blanket segments and divertor cassettes were removed along with the SP, a spatial dose rate of 100 Gy/h was found at the maintenance ports. The spatial dose rate of the in-vacuum vessel at ITER during maintenance using remote handling equipment is limited to 250 Gy/h. The study confirmed that a maintenance scheme involving pipe cutting or rewelding would be applicable to ITER at the estimated spatial dose rates.