Development of non-destructive inspection method for on-site observation. Preliminary examination for internal visualization of HTTR

In the TEPCO's Fukushima Daiichi NPS, one of the important problems in controlling the accident is removing of fuel debris. As this preparation, investigation of the internal reactors is planned for searching the fuel debris. Because of the high dose around the reactors, development of remote sensing method is required to observe from the outside of the reactor. We focused on a non-destructive inspection method using cosmic-ray muons as a candidate method for observation of internal the reactor from the outside of a reactor building. In this study, the applicability of this method for the reactor investigation was confirmed by a preliminary examination with High Temperature Engineering Test Reactor (HTTR). From the results of this examination, it appears that high density structures, such as the core and concrete walls, were able to observe by using muon telescope with coincidence method from the outside of the pressure vessel. Furthermore, we proposed some improvements of this muon inspection system for on-site investigation at the Fukushima Daiichi NPS. (author)