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

A microfission chamber (MFC) provides time-resolved measurements of global neutron source strength and fusion power in ITER. The MFC is a pencil-sized gas counter containing the fissile material, 235U and will be installed behind blanket modules at upper and lower outboard positions. 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 using MCNP 5. Results indicate a significant increase in the MFC sensitivity (up to approximately 40% higher) 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. The effect of thermal neutrons on MFC response is especially significant. 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.