Recent activities in the field of nuclear waste management

The Journal of Nuclear Science and Technology covers a variety of subjects in the field of nuclear waste management, which includes radioactive waste treatment, radioactive waste disposal and environment, decommissioning and dismantling. This summary introduces activities presented in recent years.

Most of the research and development have been dedicated to follow the Fukushima Daiichi nuclear power plant accident. A removal system of radioactivity mainly Cs-134 and Cs-137 from highly contaminated water in the Fukushima Daiichi Nuclear Power Station was established after taking the experimental data on distribution of cesium on zeolite [1,2] and developing a simulation code for cesium migration on the system [3]. Based on the experimental results and computer simulations, 99.9% of radioactive cesium removal was achieved in the first three months after the accident [4]. For the safe storage of zeolite wastes generated by the Cs removal activity, fundamental properties of the zeolite adsorbent used in Fukushima Daiichi Nuclear Power Station were studied [5]. Furthermore, hydrogen production and corrosion of the spent zeolite vessels were investigated in the same paper. For the estimation of migration of radioactive Cs in the environment, the chemical state of fallout radioactive cesium in soils was investigated [6]. On the other hand, radioactivity of rubble, trees [7] and concrete structures [8] in the Fukushima Daiichi Nuclear Power Station were analyzed by radiochemical techniques to grasp the distribution of radioactivity on the site. Solution chemistry on strontium [9] and leaching behavior of radionuclides from UO₂/ZrO₂ solid solutions [10] were investigated for future treatment of contaminated water and melted nuclear fuel. A unique technique to assess the damage to reactors using cosmic-ray muon radiography of a UO₂ fuel assembly was performed for decommissioning of the reactors [11]. Research and development after the accident will be continuously important for decommissioning and dismantling of the Fukushima Daiichi Nuclear Power Station which will proceed for long time.

Radionuclide migration studies have been continuously active, especially on selenium because Se-79 is an important radionuclide for safety assessment of radioactive waste disposal. Due to the redox sensitivity of selenium from −II to VI, many studies have been performed with focusing on redox behavior of selenium with reducing agents, iron-containing minerals and microbes in the world. Redox potential and molar entropy between tetravalent and hexavalent selenium were determined using cyclic voltammetry [12,13]. Interaction between selenium and iron is also important because corrosion of overpack made of carbon steel releases iron(II) in the Japanese high-level radioactive waste (HLW) disposal system [14]; therefore, sorption behavior of selenium(II) onto iron-containing minerals were studied, and the selenium sorption modeling was performed [15]. Bioreduction of selenium(IV) under iron-containing conditions were also studied, and selenium(II) precipitate was observed under Fe(III)-citrate-containing conditions [16].

Migration studies other than selenium have also been conducted. The complexation mechanism of natural organic substances with radionuclide is still unclear; therefore the interaction of europium regarded as a chemical analogue of americium with natural organic substances isolated from an underground research laboratory was investigated [17]. A novel technique of in situ phase-shift interferometry was applied to faint dissolution of bulky buffer materials in alkaline solutions to observe molar dissolution flux of montmorillonite [18]. Reliable data (both experimental and computational) and models on radionuclide migration should be continuously obtained and developed, respectively. Systematic approaches for radionuclide migration have also been performed. Sorption and diffusion modeling of radionuclides in compacted bentonite, which is a major component of buffer materials in HLW disposal system, was applied to many elements including actinides [19,20]. Furthermore, an attempt to evaluate solubility of some key elements for the waste management using thermodynamic database was carried out with considering its uncertainty [21]. These activities will improve reliability of performance assessment of radioactive waste disposal.

Studies on low-level radioactive waste management have also been actively conducted. Dissolution behavior of sludge-like uranium-bearing wastes was investigated for recovering of pure uranium and effective decontamination of the wastes [22]. The leaching data of synthetic bituminized wastes were obtained to discuss the effect of salt content on leaching properties of radionuclides, which is very important for the safety assessment of the bituminized waste disposal [23]. A catalytic reduction method using a palladium–copper catalyst and hydrazine was applied to denitrate highly concentrated nitrate solution to reduce diffusion of pollutive nitrate into environment [24]. Removal of pure and pollutive materials is important for recycling useful materials and reducing pollutive effects into environment, respectively.

The candidate sites for HLW disposal have not been yet selected in Japan, while those have been selected and safety assessment reports have been published in Sweden [25] and Finland [26]. To support grouting techniques for construction of HLW repositories, a grout database containing many results of in situ works and laboratory-based tests was released [27]. The arguments relevant to building technical and social confidence in the Japanese research and development activities as well as international discussions were reviewed for the implementation of geological disposal in Japan [28]. To improve the reliability of safety assessment, a “hybrid” scenario development method by combining bottom-up and top-down approaches was proposed and applied for the case of geological disposal of HLW [29]. A new method for demonstrating the sufficiency of the safety assessment and safety margins of the geological disposal system has been developed [30]. These activities should be incorporated to a future safety assessment for implementation of geological disposal of HLW in Japan, which should be more acceptable to the public than the previous progress report [31] especially from social point of view.