Recent progress in development of the microfission chamber for fusion power diagnostics on ITER

Ishikawa, Masao; Kondoh, Takashi; Nishitani, Takeo; Kusama, Yoshinori

The effects of the branch pipe and streaming neutrons to the responses of the MFCs are investigated by MCNP version 5. Results indicate the linear combination of total responses of MFCs at the lower and upper outboard positions is insensitive to the changes in the position of the plasma while the ratio of streaming neutrons at the upper outboard position is higher than that detected at the lower outboard position. It was found that the branch pipe reads to significant increase in the MFC response rate. The increase in the MFC response is caused by the slowing down of the neutrons due to the cooling water. One possible solution to reduce the effect is to cover the MFC with a material that absorbs thermal neutrons such as cadmium. The ways in which the absorbent material may affect MFC response is analyzed through neutron transport calculation. Results indicate that the increase in the MFC response can be reduced to 10 % through cadmium coating.