Effect of thermal neutrons on fusion power measurement using the microfission chamber in ITER

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

A Microfission Chamber (MFC) provides time-resolved measurements of global neutron source strength in ITER. Measurements of the neutron source strength could be affected by cooling water in branch pipes installed near the MFC. The effect of the branch pipes upon the MFC is assessed through neutron transport calculation. Results indicate a significant increase in the MFC response rate due to the branch pipe. The increase in the MFC response is caused by the slowing down of the neutrons due to the cooling water in the branch pipes. 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.