Materials selection and shield design to improve ignitor operational safety

Neutron transport calculations in the zone of the diagnostic ports and of the cryostat of Ignitor show that streaming of neutrons through the port makes the neutron flux on the cryostat behind the diagnostic ports much higher than in those parts of the cryostat shielded by the magnets and c-clamp structures. Neutron-induced activation of cryostat materials (AISI316) in that zone turns out to be so high to prevent their direct handling soon after the beginning of DT operation at maximum power level. Scope of this study is to determine whether an additional shield for the cryostat put in the port zone could reduce activation of this component, down to permit direct handling of it. Different materials for the shield are examined, and its thickness according to the reduced activation goal is determined. It is recommended the adoption of a Borated Polythene additional shield for cryostat behind diagnostic ports, with a 32 cm thickness. This shield would permit to reduce the activation of the AISI316 steel in the cryostat behind diagnostic ports to about 100 μSv/h, with 3 days of cooling, after 1 year of operation at maximum DT power level.