Shutdown dose rates in-cryostat outside the EU-DEMO vacuum vessel
Future fusion reactors using deuterium-tritium fuel will create high fluences of high-energy neutrons inside and around the reactor vessel. As well as causing material damage, fusion neutrons will activate materials, the decay of which leads to radiation fields in and around the reactor after shutdown. Gamma-ray emission from activated materials is a particular radiological hazard to personnel required inside the cryostat for maintenance or fault correction during periods of reactor shutdown. This must be accounted for in the design of the reactor shielding to reduce shutdown dose rates (SDDRs) to levels acceptable for personnel access.
Recent neutronics work has evaluated the SDDRs around the ports and throughout the cryostat for the EU DEMOnstration power plant (DEMO), incorporating prospective shielding improvements to the vessel and ports. These include changes to the vessel material and outboard thickness, updates to the equatorial port electron-cyclotron (EC) design, and proposed changes to the shielding design of the lower port, outboard midplane limiter and upper port.
Prior to the proposed shielding design improvements, calculations for the model including the helium-cooled pebble bed (HCPB) blanket showed that radiation leakage through the blanket and vacuum vessel leads to biological-equivalent SDDRs (following 12 days’ decay) above 1000 µSv/h throughout the cryostat, ignoring additional contribution from the ports. Inclusion of the proposed vessel changes reduces this dose rate to below 100 µSv/h, a more acceptable value for hands-on maintenance. The work finds an approximate order-of-magnitude reduction in SDDR throughout the cryostat when all proposed shielding improvements are applied. The work shows that to further reduce dose rates inside the cryostat, improving the shielding performance of the ports is required, with particular emphasis on the lower and the equatorial EC ports which currently dominate the dose rates.