Technical feasibility study on a fusion power monitor based on activation of water flow

As a part of development of nuclear instrumentation for the international thermonuclear experimental reactor (ITER), technical feasibility of a fusion power monitor based on activation of water flow was studied. The fusion power monitor determines D–T neutron yield, i.e., fusion power, by measuring 6.1 and 7.1 MeV γ rays accompanied by disintegration of $N16$ nuclei that are produced by the $O16(n,p) 16N$ reactions with 14 MeV neutrons in water. The fusion power monitor consists of a water pipe loop laid from the first wall to the outside of the bioshield and a γ-ray detector, e.g., a BGO scintillator. Fusion power can be measured in an absolute value by this monitor similar to a neutron activation method with a pneumatic tube system; in addition, this monitor has a feature that continuously measures the fusion power with time resolution faster than 1 s. This monitor also has other excellent features in terms of long-term stability, sensitivity only for D–T neutrons and being maintenance free. In order to confirm the technical feasibility of the fusion power monitor based on activation of water flow, experiments for dependence of $N16$ γ-ray yield on the velocity of water flow and for time response were carried out. It was found that the time response was described in a turbulent flow model, and time resolution of 50 ms that fully satisfied the time resolution of 100 ms required from plasma diagnostics was achieved. Furthermore, detection sensitivity of the fusion power monitor when it is applied to the ITER was estimated based on the experimental results; it was determined that the monitor had sufficiently high sensitivity.