A safety assessment of the geological disposal of high-level radioactive waste was performed on a time scale of more than tens of thousands of years after disposal facility closure. Accordingly, it was necessary to establish techniques to evaluate the long-term variability in groundwater flow conditions such as groundwater velocity and groundwater travel time under the influence of long-term geological phenomena. This study focused on topographic changes associated with uplift and denudation, as well as climate perturbations. A method was then developed to assess the long-term variability of groundwater flow conditions using the coefficient of variation based on simulated steady-state groundwater flow conditions. The spatial distribution in areas with relatively long residence times, which are not significantly influenced by long-term topographic change or changes in the recharge rate over the past one million years, were estimated through a case study of the Tono area, Central Japan. By applying this evaluation method, it was possible to identify a local area with a low degree of quantitative and spatial variability in groundwater flow conditions associated with regional topographic changes and climate perturbations. Furthermore, based on the results of this case study, the use of the proposed extrapolation method to evaluate the long-term variability in future groundwater flow conditions is described, and the future application of the prediction method is discussed.

  Hardened cement pastes (HCPs) of water/cement ratio (W/C) 50% and 30% using fly ash (FA), blast furnace slag (BFS) and silica fume (SF) for 28 days were prepared. Apparent diffusion coefficients (D_a) of Cs and I in HCPs were obtained using electron probe microanalysis. For Cs, BFS and SF contributed to a reduction of D_a for W/C=50% and 30%, respectively. For I, BFS reduced D_a for W/C=50%, however a significant reduction in D_a for W/C=30% was not observed. Using SF enhanced sorption of Cs on HCP, and using BFS slightly improved sorption of Cs on HCP. It was also confirmed that the pore structures of HCP using SF and BFS were connected by fine pores. It was therefore considered that using SF and BFS contributes to the reduction of D_a in HCP.

  Compacted bentonite and bentonite-based materials with low hydraulic conductivity will be used as engineered barriers to inhibit the migration of radioactive nuclides. In this paper, the points of attention for conducting swelling pressure tests and hydraulic conductivity tests of compacted bentonite in the laboratory and for modelling the test results are described. It was found that the swelling pressure test results of compacted bentonite, which are affected by several factors, such as the stiffness of the test cell, the height of the specimen, and the initial degree of saturation, can be numerically simulated using the numerical model proposed previously. It was also found that evaluating the equilibrium swelling pressure by the diffusion double layer theory, it is necessary to pay attention to the setting of the interlamellar distance. As for the hydraulic conductivity test, measurement method of B-value in the rigid wall permeability testing apparatus was proposed because it is difficult to calculate saturation of samples with large fluctuations in soil particle density. Furthermore, in the case of using a flexible wall permeability test apparatus for undisturbed samples, an example of a new contrivance for preventing the specimen density and the cell pressure from becoming excessively large was shown.

  The spent control rod assemblies are to be processed as a waste for an underground disposal (depth≥70 m). The spent control rod assemblies (4 m in length) need to be cut into 4 sections in order to be stored in the storage containers (1.6 m x 1.6 m x 1.6 m). When the control rod is cut, the B₄C powder contained inside the rod is leaked into the water. Contaminating the surrounding water would increase the secondary treatment cost, and consequently the overall processing cost.

  In this study, we examined a cutting technology that combines press working and underwater plasma cutting, in order to seal the cut control rods and prevent the B₄C from being scattered into the water. We cut the control rods underwater in a vertical upright position, and confirmed the effects of B₄C scattering.

  After cutting, the water ingress of the neutron absorbers was 52.8 mg per control rod. The B₄C leaked during cutting was 3.6 g. The ratio diffuse of B₄C from per control rod was 0.05 %. In conclusion, the results showed that the cutting technology used in this study is effective, and the B₄C can be reduced considerably during processing.

  Japan Atomic Energy Agency has performed risk communication at Horonobe Underground Research Center, using the Public Information House and Underground Research Laboratory (URL), to promote understanding R&D of geological disposal technology and geological disposal to public. In this paper, we carried out the analysis of questionnaire investigation performing after visiting those facilities from FY2013 to FY2017. The results show that long-term safety would gain prominent attention as agita factor by growing understanding. The results also show that visiting to those facility would become valuable experience to understand geological disposal because, for example, respondents with visiting to URL positively evaluated necessity, appropriates and safety of geological disposal, compared with those without visiting URL.