Home Environment Reflections on the Fukushima Daiichi Nuclear Accident
Management of Nuclear Fuels in Nuclear Reactors and Spent Fuel Pool
Figure 15.3 illustrates an example of management of nuclear fuels in the reactors of Units No.1–3 of Fukushima Daiichi Nuclear Power Station and of spent nuclear fuels in the pools of Units Nos. 1–4.
The spent nuclear fuel in the pools will be classified into non-damaged spent nuclear fuel and damaged fuel. The former will be washed, loaded into canisters, and stored. (The policy on the nuclear fuel cycle in Japan after the Fukushima accident is not discussed in this chapter.) It is not yet clear how to manage the damaged spent fuel concretely and whether the non-damaged spent fuel will be reprocessed. In order to reduce the risk of spent fuel in the pools, the transfer of spent nuclear fuel in the pools to the common pool has started. In Unit No. 4, 726 fuel assemblies (704 spent and 22 new fuels) have been transferred to the common pool as of April 23, 2014, and the transfer of 1,533 fuel assemblies (1,331 spent and 202 new fuels) will be completed around the end of 2014. In Unit No. 3, the removal of large pieces of rubble from the pool is underway. In Unit No. 2, after progress is made in decontamination and shielding within the reactor building, formulation of a concrete plan will be discussed. In Unit No. 1, the construction of a yard to operate large and heavy machines is scheduled, and the demolition of the reactor building cover commenced in the first half of 2014.
Investigation has been performed on the amounts of fuel that were melted down and where the debris is distributed, and the decontamination and the fixation of
Fig. 15.3 Example of management of nuclear fuel in reactors No. 1–3 and spent nuclear fuel in the pools of No. 1–4
the pressure boundary of the primary containment vessel, as well as examination of physical and chemical characteristics of the debris. Furthermore, management of these nuclear fuels includes resolving the problems of where the nuclear fuels removed from the nuclear reactors are to be stored, who is to implement the final disposal, what the repository site selection procedure is, what the concept of disposal including the nuclear fuel cycle is, and how the legal system is prepared. This will entail the development of the method for processing and disposal of debris/melted nuclear fuel and development of technologies consistent with this method, the development of a specific container, and the development of remotecontrolled technologies to locate the leaks of water in reactors and to repair those, as the work of removing the nuclear fuel from the reactors is carried out under the condition in which the reactors are filled with water. In addition to these, it is critical to confirm the soundness of the pressure vessels and containment vessels for a long time (at shortest, until completion of removing all nuclear fuels) because sea water containing salt was boiled in the vessels.
Although some of these steps cannot be taken until the conditions of nuclear fuels are known, we must take steady and appropriate actions one by one.
Concept of Radioactive Waste Disposal
All nations that have been using nuclear energy have adopted the method of disposal into the underground environments where the radioactive waste is generated. This is because it is most appropriate that the radioactive waste generated in a country is disposed of in its own territory; because currently available technologies, knowledge, and skills can be used in disposal; because the long-term safety assessment of disposal is expected to be performed reasonably; and because the retrievability of radioactive wastes is not technically impossible if it is required.
In Japan, very low-level radioactive waste has been disposed of at approximately 5 m below the ground surface (“landfill disposal” or excavation disposal), and relatively low-level radioactive waste, which is generated in operation and maintenance of nuclear reactors, has been disposed of at approximately 10 m depth from the ground surface (“vault disposal”). For low-level radioactive waste whose level of radioactivity is relatively high, the disposal at depths of 50–100 m (“subsurface disposal”) has been considered and will be planned. High-level radioactive wastes are legally mandated to be disposed of at 300 m or more below the ground surface (“geological disposal”). This concept of disposal in Japan is illustrated in Fig. 15.4 [5, 6]. In vault disposal or subsurface disposal, radioactive waste is solidified using cementitious or other materials within a drum or a container specific to the solidification, and the drum or the container is covered by cement or concrete and further covered with material such as clay or mixture of clay and sand through which the groundwater is hard to penetrate and by which cations are easy to be trapped. The barrier of these artifacts is called an artificial barrier, and the safety of disposal is secured by the multi-barrier system which consists of the artificial barrier and the natural barrier which functions in the geosphere and the biosphere. In geological disposal, high-level radioactive liquid waste is mixed with borosilicate glass material and solidified in a stainless steel
Fig. 15.4 Four types of disposal in Japan [5, 6]
canister (vitrified waste), and inserted into an overpack (e.g., carbon steel). The overpack containing the vitrified waste is transferred to the underground repository, placed in the tunnel and covered with compacted clay materials.
What is described above was the fundamental concept of radioactive waste disposal in Japan before the Fukushima accident. This concept is natural, scientifically reasonable, and technically sound. We mine, for example, iron, copper, or uranium ore, and refine it to use as iron, copper, or uranium. This fact clearly indicates that the deep underground environment has the capability to retain and contain metals over a long period of time in a geological sense (some 10 million years to some 100 million years).
To be accommodated in the disposal system mentioned above, it is expected that the waste contaminated with radioactive materials and the damaged nuclear fuel generated by the Fukushima accident will also be adequately processed, sealed in suitable containers, stored for a certain period, and then disposed of in the repository built in the relevant geological environmental conditions, according to their radioactivity levels. The plan, design, and implementation of the processing of radioactive waste must be optimized for achieving the safety in the final disposal, because the processing may influence the feasibility of disposal options.
Not only the technical and regulatory compatibility with the current system, but also broad support from the public for final disposal of radioactive waste generated by the Fukushima accident must be achieved. It is imperative that the Japanese people share their opinions on this issue with each other. The discussion will inevitably extend to Japan's nuclear future, i.e., whether Japan continues to exploit nuclear energy or phases it out.
Many countries, not only Japan, which have been using nuclear energy and have plans to use nuclear energy were forced to rethink the ethical value of use of nuclear energy by Fukushima Daiichi Nuclear Power Plant accident. Considering the final disposal of all nuclear fuels generated by the Fukushima accident, we fully recognize that radioactive waste management is an enterprise which will not be completed within the 21st century. Simultaneously, it goes without saying that the safe and steady management of radioactive waste is the premise for the restoration and revival of communities and residents of Fukushima. In such a situation, the management of various radioactive wastes generated by the Fukushima accident will be expected to proceed steadily and safely under greater coordination among science and technology, politics, and the public and society.
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