Recent activities in the field of nuclear waste management

The Journal of Nuclear Science and Technology covers a variety of topics in the nuclear waste management including the radioactive wastes from decommissioning of the Fukushima Daiichi Nuclear Power Plant (FDNPP). This summary introduces activities presented in recent years.

As for the FDNPP, the waste management of the fuel debris and low-level wastes including spent ion exchange media were studied [1–3]. The various types of materials (stainless steel, zircaloy, sintered ceramic, cast-fused zirconia, metal + ceramic, melted inactive simulants, prototypic fuel debris, and irradiated fuel debris simulant) were studied for developing cutting and collecting techniques for the fuel debris. Laser cutting technique for the various simulants of the debris showed good performance, however, confirmatory experiments are still necessary with prototypic fuel debris, in particular with respect to the generation of airborne dusts and fumes [1]. The calculation methodology of radioactivity inventories in wastes at the FDNPP was developed by considering the distribution of radionuclide material balance and constructing calculation flowcharts of the transportation of radionuclide into waste for the reasonable and sustainable waste management for the decommissioning of FDNPP [2]. Furthermore, for safe storage or geological disposal of the spent ion exchange media with high concentration radioactive cesium, zeolitic structures were reported to be effective for containment of Cs [3].

For low-level wastes generating from uranium usage facilities, the treatment of solid and liquid uranium wastes was studied [4–6]. The uranium catalyst, which was used in several industrial fields, became problematic radioactive waste for the treatment and final disposal. Therefore, the dissolution and immobilization method of the spent uranium catalyst was studied and showed over 80% volume reduction and low leachability [4]. On the other hand, the basic treatment method based on the conversion of uranium catalyst of USb3O10 to U3O8 was studied [5]. As for liquid uranium wastes, uranium removal mechanism and performance using graphene oxides functionalized with diethylenetriaminepentaacetic phenylenediamine were shown to be highly efficient for the removal of uranium (VI) from aqueous solution [6].

Several studies for the waste management of the High-Level Waste (HLW) were presented, ranging from a verification technique, a disposal concept, and transportation of nuclear spent fuel transport casks [7–11]. As for the treatment of High-Level Liquid Waste (HLLW), an understanding of varied thermal decomposition behaviors of various nitrates in HLLW is necessary to elucidate a series of phenomena occurring within the cold cap. Gadolinium nitrate, which is one of main compositions of nitrates in HLLW, was chosen as an example, and its thermal decomposition was investigated to gain kinetic parameters to model its consecutive decomposition reactions [7]. The reprocessing technique of high burn-up UO2 and MOX fuels from light-water reactors (LWR) was studied with regard to the safety assessment of geological disposal of HLW [8]. Considering for effects of extended cooling period, it was reported that the waste loading in waste glasses from UO2 and MOX spent fuel could be increased with the current verification technology. In addition, 70% to 90% of Am separation was suggested to be effective in terms of thermal feasibility of buffer material in a geological repository [8]. An unique geological disposal system of wastes was proposed. The method of self-burial of radioactive waste into geological formations using direct heating rock by radiation was proposed [9]. In this concept, radioactive wastes are filled in spherical or cylindrical capsules and disposed into deep geological formation by melting the surrounding rock with irradiation energy of 60Co. Calculations made for granite and salt showed that an average surface 60Co activity of about 1017 Bq/m2 was to be required to achieve a descent velocity of 1 km per year [9].

The effect of gamma irradiation on association of Cu(II) with humic substances (HSs) was studied to understand radiation effects on complexation reactions of radionuclides with organic ligands in nature, which might be important for their migration through host rocks in the geological disposal of wastes [10]. Dose rate in the range 0–100 kGy had marginal impacts on the apparent complex formation constants of HSs with divalent copper ion. Furthermore, the degree of humification was studied for aqueous solution samples of humic acid and fulvic acid which were irradiated with gamma-rays in the dose range of 0–500 kGy at two dose rates, 0.1 and 1 kGy/h. Though the concentration of carboxylic groups decreased drastically with high radiation exposure, no significant change in apparent formation constants was found [10].

For the rational spent fuel transport cask, the two-way corrugated aluminum honeycomb, a new kind of filler material, was designed, based on the action mechanism of the impact limiter [11]. Quasi-static compression tests were used to compare the properties of the honeycomb and the traditional filler material, paulownia wood. The two-way corrugated aluminum honeycomb was shown as a filler material that has essential application value in impact limiters of spent fuel transport casks [11].