Neutron Transport Analysis of the Processes Affecting the in situ Calibration of ITER In-Vessel Neutron Flux Monitors Equipped with a Micro-Fission Chamber System

Neutron transport analysis is used to evaluate the effects of neutron calibration source position, support structure, and water coolant on the in situ calibration of the in-vessel neutron flux monitor using the micro-fission chamber (MFC) system by applying a Monte Carlo code for neutron and photon transport (MCNP). Results indicate that changing the position of a neutron calibration source leads to a longer calibration time of the MFC detectors. When positioned below the source, the supporting rail significantly affects the detection efficiency of the lower MFC detectors. On the other hand, though it has smaller impact when positioned adjacent to the neutron source, the analyses results suggest that the position and the size of the rail need to be optimized because the detection efficiency is sensitive to scattered neutrons by in-vessel components. Furthermore, water coolant can significantly affect the detection efficiency. This result indicates that when the in situ calibration is performed, the cooling water should be filled in the blanket module in the same manner as the ITER operations.