Preliminary study on capsule material for ITER neutron activation system

A neutron activation system (NAS) measures neutron fluence at the first wall and the total neutron flux from plasma, providing the fusion power evaluation. A pneumatic transfer method is conventionally utilized to transfer encapsulated activation samples between the irradiation stations and counting station. The temperature of the irradiation station, near the first wall could reach too high for the conventional polymer-based materials, such as polyethylene, to be used as a capsule material for the ITER NAS. Considering the environment of the irradiation station of the ITER NAS, the candidate capsule materials are chosen as four materials: RAFM (reduced activation ferritic martensitic) steel, SiCf/SiC composite, tungsten, and CFC (carbon fiber-reinforced carbon). Preliminary investigation reveals that the CFC is the most promising capsule material for ITER NAS due to its good thermal and magnetic properties as well as low activation by neutron irradiation. Various kinds of mock-up capsules are fabricated using CFC with the consideration of the volume of inner space accommodating activation samples. Preliminary pneumatic transfer experiments carried out in the small-scale test-bed suggest that the transfer speed of capsule should be slower than 10 m/s and the wall thickness of the capsule should be thicker than 2 mm so as not to be broken by impact damage. The present study shows the feasibility of using CFC as a capsule material for the ITER NAS.