On the Design of a Remotely-Deployed Detection System for Reactor Assessment at Fukushima Daiichi

The premise behind the research described in this record is the design of a system that will allow fuel debris characterisation at the Fukushima Daiichi nuclear power plant. The precise location of the debris is not known, for example as to whether it remains within the reactor pressure vessel or it has leaked through into the base of the pedestal below. Additionally, the state of the fuel is also in question as to whether this has evolved from within its cladding or if it is intact. The most likely scenario is a combination of all four of these situations. The flooding of the reactor floors immediately following the Fukushima accident adds an extra element of complexity for the detection system, for example requiring it to be submersible and to hold any detector system in water-tight confinement. The research carried out has involved extensive modifications to a previously-designed, low-cost, small-scale AVEXIS submersible inspection vehicle and the incorporation of a variety of radiation detectors. The latter has been designed to allow for mapping and determination of the situation that is present within the primary containment vessels. The challenges addressed with the detection system arise from the high dose rates that have been recorded around the reactor pressure vessels which can be as high as 1000 Gy/hr. In such a harsh environment not only will the radiation detectors struggle to operate but the components that make up the remote-operated vehicle are also likely to suffer radiation damage after only a relatively short period of time. The research presented here evaluates the components comprising the AVEXIS system in terms of their radiation tolerability as well as presenting the combination of detectors to be used in the remote probe for the investigation of the fuel debris.