Technology development to evaluate dose rate distribution in PCV and to search for fuel debris submerged in water

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The Great East Japan earthquake off the pacific coast of Tohoku was the most powerful recorded earthquake ever to hit Japan and triggered powerful tsunami waves. These inundated the coast including the area of the Fukushima Daiichi nuclear power plant. The flooding that followed disabled the auxiliary cooling systems at the power plant which, although being shut down, caused the reactors to overheat as a result of the effect of decay heat. This resulted in core damage to 3 of the 6 reactors on the Fukushima site. The damage is strongly suspected to have resulted in fragmentation of the nuclear fuel inside the reactors themselves and thus they are inoperable and need to be decommissioned. A key task is the removal of the nuclear fuel from the reactors. Once this is removed and stored safely elsewhere, radiation levels will fall significantly rendering the plant much safer than at present which will enable the remaining decommissioning requirements of the plant to continue more quickly, easily and with reduced cost.However, the fuel debris cannot be removed until we know how much there is and the form it has adopted after the accident i.e. is it a molten lump confined to the reactor or a mixture of damaged fuel elements with some egress beyond the primary containment? The reason we need to know this information is that it is essential that the likelihood of re-criticality is assessed (the chance that the nuclear fission reaction could start up inadvertently when the debris is disturbed) and also that we know of the extent of radioactivity and the form it is in in order to plan disposal routes. This information is very difficult to obtain because the type of reactor affected at Fukushima operated in what is known as a primary containment vessel or PCV. This is a large, thick-sided steel tank that is bolted shut very securely. A PCV has only a few, narrow access routes by which it is possible to get inside. There is no light inside and in order to set the reactor into a safe state and maintain its cooling, each reactor at Fukushima has been flooded with sea water. As a result of the radioactivity from the fuel there are very high levels of radiation exposure which prevent human beings from getting near to the PCV, whilst access to the PCV would result in serious injury to people and could damage untested instrumentation. The water filling the PCVs further complicates matters but, for the timebeing, cannot be removed as it acts to cool the fuel debris and to shield the surrounding area from radiation emitted by the reactor.In this project we combine two world-leading research activities in the United Kingdom associated with the portable detection of radioactivity (Lancaster University) and the development of small, submersible remote-operated vehicles (Manchester) in collaboration with the Japan Atomic Energy Agency, the National Maritime Research Institute of Japan, BeamSeiko Instrument Co. Ltd (Tokyo) and the Nagaoka University of Technology. The key objective of the research is to determine whether we can combine these capabilities to produce a remote-operated submersible vehicle with a radiation detection payload to detect neutrons and gamma rays. This device will be either mobile in the PCV, and thus able to inform us of the distribution of fuel debris in the reactor, or tethered in place in order to provide a continual indication of the core state; neither capability currently exists and clean-up of the reactors cannot continue without an assessment of this type. The distinction of the neutron and gamma-ray detection recommended for the payload on the submersible in this project is that the combination of the information provided by these two radiation types can tell us about the resident radioactivity, the risk of re-criticality and also provide a means for comparison with severe accident calculations to determine what happened to the reactor fuel in the accident.