Daisuke Sugiyama

Review of Characteristics of Post-Accident Waste Generated in Fukushima Daiichi Nuclear Power Plant Site and Issues to be Addressed in Processing and Disposal Stages

Review of Characteristics of Post-Accident Waste Generated in Fukushima Daiichi Nuclear Power Plant Site and Issues to be Addressed in Processing and Disposal Stages The accident at Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Station has produced and will continue to produce various types and large amounts of waste contaminated by radionuclides. The literature and published internet information on possible types of waste produced are reviewed from the viewpoint of their characteristics. Issues associated with the waste were selected and analyzed for each stage of the future waste management, considering the characteristics and properties of the waste obtained so far. The stages considered are current (temporary) storage, processing including decontamination and solidification/packaging, storage up to disposal, transportation, and disposal.

Diffusion Behavior of Organic Carbon and Iodine in Low-heat Portland Cement Containing Fly Ash

The diffusion of radionuclides in cementitious materials used as an engineered barrier is an important parameter in the performance assessment of the sub-surface repository system used for low-level radioactive waste disposal in Japan. In particular, organic carbon-14 and iodine-129 would provide large contributions to the dose evaluation, because of their low ability to be adsorbed on cementitious materials. In this study, the diffusion of acetate and iodide in hardened cement pastes was examined by through-diffusion experiments. Low-heat Portland cement containing 30 wt% fly ash (FAC), which is a candidate cement material for the construction of the sub-surface repository, was prepared for the diffusion experiments. The effective diffusion coefficients, D e , of the trace ions for hardened FAC cement pastes were estimated to be on the order of 10 -13 m 2 s -1 at the beginning of the diffusion experiments. Then, the rate of diffusion of the trace ions decreased over the experimental period of 1-15 months. This is probably due to the change in the microstructure of the FAC as the result of a pozzolanic reaction. After a few months, the values of D e were estimated to be on the order of 10 -14 m 2 s -1 . These results suggest that an engineered barrier made of FAC can act as an effective barrier inhibiting the diffusion of trace ions such as organic carbon and iodine.

RADIOLOGICAL PROTECTION FROM RADIOACTIVE WASTE MANAGEMENT IN EXISTING EXPOSURE SITUATIONS RESULTING FROM A NUCLEAR ACCIDENT

In environmental remediation after nuclear accidents, radioactive wastes have to be appropriately managed in existing exposure situations with contamination resulting from the emission of radionuclides by such accidents. In this paper, a framework of radiation protection from radioactive waste management in existing exposure situations for application to the practical and reasonable waste management in contaminated areas, referring to related ICRP recommendations was proposed. In the proposed concept, intermediate reference levels for waste management are adopted gradually according to the progress of the reduction in the existing ambient dose in the environment on the basis of the principles of justification and optimisation by taking into account the practicability of the management of radioactive waste and environmental remediation. It is essential to include the participation of relevant stakeholders living in existing exposure situations in the selection of reference levels for the existing ambient dose and waste management.

Laser ablation microprobe inductively coupled plasma mass spectrometry study on diffusion of uranium into cement materials

Abstract The diffusion of uranium (U(VI)) in solid cement monoliths of ordinary portland cement (OPC) and low-heat portland cement containing 30[wt.]% fly ash (FAC) was measured by an in-diffusion technique. Detailed sharp depth profiles of uranium in the solid cement matrices were successively and quantitatively measured using laser ablation microprobe inductively coupled plasma mass spectrometry (LAMP-ICP-MS), and the apparent (Da) and effective (De) diffusion coefficient of uranium in cement matrix were calculated as: Da=∼4×10−16 m2 s−1 and De=∼3×10−11 m2 s−1 for OPC, and Da=∼2×10−17 m2 s−1 and De=∼6×10−13 m2 s−1 for FAC.

Development of methodology of probabilistic safety assessment for radioactive waste disposal in consideration of epistemic uncertainty and aleatory uncertainty

ABSTRACT Probabilistic safety assessment has not been performed for radioactive waste disposal owing to the difficulty of dealing with the probability distributions of the parameters included in the long-term safety assessment of radioactive waste disposal. In this study, we develop a methodology of probabilistic safety assessment in consideration of both epistemic uncertainty and aleatory uncertainty, in which the probability density function (PDF) and cumulative distribution function (CDF) of the maximum annual dose can be calculated. We also propose an approach to demonstrating dose assessment results in compliance with the stepwise target annual doses of likely and less-likely scenarios according to the occurrence probability of the scenario without classifying the probabilities of parameters involved prior to the safety assessment. For the likely scenario, we can employ the larger of the modal value of the PDF and the 50th percentile of the CDF to meet the target annual dose (10 µSv y-1). For the less-likely scenario, we can adopt the 95th percentile of the CDF as the assessment result for comparison with the target annual dose (300 µSv y-1).

Integrating radiation protection criteria for radioactive waste management into remediation procedures in existing exposure situations after a nuclear accident

Experience after the accident at the Fukushima Daiichi nuclear power station has shown that there is a need to establish radiation protection criteria for radioactive waste management consistent with the criteria adopted for the remediation of existing exposure situations. A stepwise approach to setting such criteria is proposed. Initially, a reference level for the annual effective dose from waste management activities in the range 1-10 mSv should be set, with the reference level being less than the reference level for the ambient dose. Subsequently, the reference level for the annual effective dose from waste management activities should be reduced in one or more steps to achieve a final target value of 1 mSv. The dose criteria at each stage should be determined with relevant stakeholder involvement. Illustrative case studies show how this stepwise approach might be applied in practice.

What Opinions Do People Have Through the Understanding Concerning High-Level Radioactive Waste Disposal Project?

In Japan, the implementation of the high-level radioactive waste (HLW) disposal is one of urgent issues in the situation that Japan will continue the use of nuclear power. But, the lay people may not have the sufficient amount of information and knowledge about HLW disposal to hold their opinions about this issue. In this research, in order to clarify what opinions they will have with enough information and knowledge, we had the face-to-face dialogues about the HLW disposal with 2 or 3 lay persons. The dialogues were conducted 11 times with different lay persons’ groups. In these dialogues, after the lay participants had a certain amount of knowledge about HLW disposal, they became to talk about their opinions to the HLW disposal program in Japan. These opinions included the doubt against the open solicitation to select the siting area in the HLW disposal program of Japan, the emotion like NIMBY, the indication of lack of public relations about HLW disposal, and so on.Copyright

Dialogue Process Between Expert and Public Regarding HLW Disposal in Japan

Different people have different opinion on nuclear power use. As for HLW disposal, more complex issues such as uncertainty due to very long time-scale required for disposal would add to the difficulty of conducting smooth communication and building stable consensus between the scientific experts and the general public. We carried out Q&A dialogue experiments between an expert and public participants who were indifferent to HLW disposal. The dialogue process was divided into two periods. In the learning period, participants learned fundamental knowledge about HLW disposal. In the discussion period, participants discussed about one specific topic, such as “Resource and Energy”, “Geological Disposal”, or Safety Violation. These dialogue experiments can help the experts to communicate and conduct comprehensive activities with the public which would help implement the HLW disposal from the public point of view.Copyright

Modelling the Sorption of Actinides onto Cement: An Approach with the Surface Co-Precipitation Model

An approach, assuming that co-precipitation on the surface of the cement phase dominates ‘sorption’, is proposed and discussed to interpret the sorption behaviour of the cement-complex system. This model takes account of the mineralogical composition of Ordinary Portland Cement (OPC) and assumes that Calcium Silicate Hydrates (C-S-H) dominates the sorption (fixation) of actinide onto OPC and that a small amount of surface co-precipitation of actinides with calcium hydroxide in the structure of C-S-H is formed during the late period of the fixation process. Therefore, this will be referred to as the ‘Surface Co-Precipitation Model (SCPM)’. The modelling results show generally good agreement between the predicted and measured distribution ratios of plutonium, thorium and neptunium, of which the dominant species in high pH solution are neutral species, on OPC.