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In the Three Mile Island Unit 2 reactor accident, a large amount of fuel debris was formed whose criticality condition is unknown, except the possible highest U/U enrichment. The fuel debris had to be cooled and shielded by water in which the minimum critical mass is much smaller than the total mass of fuel debris. To overcome this uncertain situation, the coolant water was borated with sufficient concentration to secure the subcritical condition. The situation is more severe in the damaged reactors of Fukushima Daiichi Nuclear Power Station, where the coolant water flow is practically “once through.” Boron must be endlessly added to the water to secure the subcritical condition of the fuel debris, which is not feasible. The water is not borated relying on the circumstantial evidence that the xenon gas monitoring in the containment vessels does not show a sign of criticality. The criticality condition of fuel debris may worsen with the gradual drop of its temperature, or the change of its geometry by aftershocks or the retrieval work, that may lead to criticality. To avoid criticality and its severe consequences, a certain principle of criticality control must be established. There may be options, such as prevention of criticality by coolant water boration or neutronic monitoring, prevention of the severe consequences by intervention measures against criticality, etc. Every option has merits and demerits that must be adequately evaluated toward selection of the best principle.

For the decommissioning of the Fukushima Daiichi Nuclear Power Stations, fuel debris involving molten structural materials should be retrieved from each reactor unit. The fuel debris, which is of uncertain chemical composition and physical state, needs to be treated with great care from the standpoint of criticality safety. For developing criticality control for the fuel debris, the Japan Atomic Energy Agency (JAEA) has been planning to modify the Static Experiment Critical Facility (STACY) and to pursue critical experiments on fuel debris. STACY, a facility using solution fuel, is to be converted into a thermal critical assembly using fuel rods and a light water moderator. A series of critical experiments will be conducted at the modified STACY using simulated fuel debris samples. The simulated fuel debris samples are to be manufactured by mixing uranium oxide and reactor structural materials with various chemical compositions. This report summarizes a facility development project for an experimental study on criticality control for fuel debris using the modified STACY and simulated fuel debris samples.

It is my great honor and pleasure to speak to you this morning on the occasion of the International Symposium on Nuclear Back-end Issues and the Role of Nuclear Transmutation Technology after the accident of TEPCO’s Fukushima Daiichi Nuclear Power Stations. I would like to thank the organizers, especially Professor Hirotake Moriyama and Professor Hajimu Yamana, for inviting me to this Symposium.

I believe that this Symposium is very important and well timed to solve urgent problems concerning nuclear back-end issues and to develop nuclear transmutation technology. I myself am a nuclear theoretical physicist and am ignorant of nuclear technology. However, I believe that nuclear energy is indispensable for the future of human beings and that nuclear engineering must be further developed.

My talk consists of the following four subjects:

Concerning the disposal of high-level radioactive waste (HLW) in Japan, the Nuclear Waste Management Organization of Japan (NUMO) has been making efforts toward beginning a literature survey, a first step of HLW disposal according to fundamental policies and final disposal plan based on the “Designated Radioactive Waste Final Disposal Act.” However, a difficult situation continues in which responses from municipalities, which are necessary for beginning a literature survey, are not being made.

In September 2010 the Science Council of Japan (SCJ) received a deliberation request from the Chairman of the Japan Atomic Energy Commission, and SCJ formed a Review Committee for Disposal of High-Level Radioactive Waste. The Review Committee made a Reply on Disposal of High-Level Radioactive Waste in September 2012, in which six proposals are made including safe temporal storage and management of the total amount of HLW. In this chapter, an outline of the current HLW disposal policy in Japan and the contents of the Reply are introduced.

Although nuclear power has become recognized as a social issue—one that concerns us all—there is still, in Japan, insufficient public debate on the problems posed by this form of energy. In particular, interest among the younger generation on this and many other issues is limited, a situation reflected in the low turnout of young people at elections. The disposal of high-level radioactive waste is an issue that cannot be simply solved by shutting down nuclear reactors. Yet, in spite of the need to urgently find a solution to the problem of nuclear waste, many young people appear to be apathetic. Part of the reason for this lack of interest is that students majoring in the so-called humanities do not feel confident approaching the issue. As a way to raise such students’ interest in the issue of nuclear waste disposal, debating courses were held in the social science departments of two universities located in Aichi Prefecture, Japan. This chapter reports on these courses, discusses the value and effectiveness of debate in raising awareness of social issues, and assesses potential problems with implementing debating in educational contexts.

It has been recognized that carbon-14 (C) is one of the dominant radionuclides affecting dose from transuranic (TRU) wastes. This radionuclide has a decay half-life of 5,730 years, and C organic materials have very low sorption properties to clay and rock in the environment, which raises some concerns about the releases of C to the biosphere from radioactive waste repositories. For the safety assessment of TRU waste disposal, we studied the behavior of C in rice paddy field soils. We also determined key parameters such as soil–soil solution distribution coefficients (s) and soil-to-rice plant transfer factors (TFs) of C in the field soils. The TFs were obtained in laboratory and field experiments. In our laboratory experiments, we used [1,2-C] sodium acetate as a source of C because it has been suggested that low molecular weight organic-C compounds are released from metallic TRU wastes. The results showed that C-bearing sodium acetate in irrigated paddy soils was rapidly decomposed by indigenous bacteria. Although some of the C was assimilated into the bacterial cells, most of the C was released into the air as gaseous compounds. The main chemical species of C gases was CO, and a part of the released CO gas was used by rice plants during photosynthesis. Only a negligible amount of C was absorbed through the roots. Therefore, the contamination of rice plants is mainly caused by gasification of C, and microorganisms are responsible for driving this process. The activity of microorganisms is a key issue in the behavior of C in paddy fields.

The separation conditions for iodine species were investigated to analyze I in contaminated water and tree samples generated from the Fukushima Daiichi Nuclear Power Station (FDNPS). Inorganic iodine species in the samples from FDNPS were thought to be iodide (I) and iodate (IO); therefore, the behaviors of these species during separation using a solid-phase extraction sorbent, Anion-SR, for water samples and combustion for tree sample were studied. When the amount of I was 1 μg and used within a few hours, I was extracted with the Anion-SR in 3 M NaOH and diluted HCl (pH 2) solutions, whereas IO was only slightly extracted in these solutions. In contrast, 15 ng I with a larger amount of IO (1 μg I) in the diluted HCl (pH 2) and allowed to stand for 1 day was only slightly recovered. It is possibly that I was changed to another species in a day in this condition. Iodate was successfully reduced to I with NaHSO in the diluted HCl solution and extracted with the Anion-SR. Consequently, the solution condition to analyze both I and IO using Anion-SR was observed to be the diluted HCl at pH 2 with a reductant. For the tree samples, a combustion method was applied and the rate of temperature increase was optimized to avoid anomalous combustion. Greater than 90 % recovery was obtained for both I and IO, and the chemical species in the trap solutions was observed to contain I.

The earthquake and tsunami on March 11, 2011, caused severe accidents at the several Fukushima Daiichi Nuclear Power Units, and a significant volume of highly contaminated water was generated from the accident. Several methods have been applied to decontaminate the water, including systems from AREVA S.A. and Kurion, Inc., in addition to the SARRY (Simplified Active Water Retrieval and Recovery System) and ALPS [Advanced Liquid Processing System; incorporated in the MRRS (Multi Radionuclide Removal System)] systems from Toshiba Corporation. After the decontamination treatments using these systems, various kinds of sludge and spent adsorbents were generated as secondary wastes. These wastes are now tentatively stored at the site, but further treatment shall be applied to produce appropriate waste forms for interim storage and final disposal in a repository.

Waste management—the treatment, storage, transportation, and disposal of these wastes—is believed to require several decades. The authors examined how to manage these wastes in consideration of the large volume of waste, the variety of waste types, and the long period required to carry out their treatment and disposal in a safe and efficient manner. The requirements for an inventory list and online waste management system; a development strategy for waste treatment, storage, transport, and disposal; formation of an R&D implementation and evaluation team; and long-term knowledge management are discussed in this chapter.

Municipal solid wastes (MSW) with elevated concentrations of radioactive cesium (rad-Cs hereafter) have been generated in some areas of Japan in the aftermath of the Fukushima Daiichi Nuclear Power Plant (F1 hereafter) accident. Both recycling and final disposal of the contaminated MSW have become a difficult problem in the affected areas, resulting in accumulation of treated residues in the treatment facilities.

The rad-Cs in MSW, especially fly ash, often showed a high leaching rate. Extraction of contaminated MSW with water or hot oxalic acid followed by selective removal of rad-Cs from the extract using ferrocyanide (Fer hereafter) coprecipitation technique could be an ultimate solution for waste volume reduction. The MSW extracts contain various metal components as well as chelating reagents like oxalic acid, and are often very saline. The composition of the extract varies widely depending on waste sources, applied treatment techniques, and rad-Cs extraction method etc. The applicability of the Fer coprecipitation technique had to be tested and validated before it could be applied for actual treatment.

In this work, we applied the Fer technique and observed removal of cesium (Cs) from water and oxalic acid extracts (all spiked with rad-Cs tracer or stable Cs) of various MSW samples collected from uncontaminated areas. Finally, the Fer technique was applied on site for removal of rad-Cs in the extracts of contaminated MSW. By modifying coprecipitation conditions according to solution matrix, Cs removal rates of higher than 95 % could be obtained.