Long-life in-core neutron detector for prototype heavy water reactor ''FUGEN''

A long-life in-core neutron detector was developed from 1973 through 1978 and since then, it has been applied to the prototype Heavy Water Reactor ''FUGEN'' as a local power monitor. More than 10 year's operation experience of the detector proved its performance as was expected. The detector is a fission chamber of regenerative type whose tube is made of Zircaloy-2 metal for neutron economy. In-core neutron detectors for the Local Power Monitor are placed in heavy water moderator of 500C. The in-core conditions bring a high level thermal neutron flux environment. It tends to hurt the linearity and shorten the life time of the detectors because of rapid burnup of the neutron conversion material (235U) in the detector compared to the light water reactors. These problems were solved by some improvements and it was demonstrated that the detector had the performance of high linearity and long-life characteristics