[en] It is important to estimate the cost of decommissioning at various stages of planning to manage decommissioning projects efficiently. Uncertainties regarding radioactive waste management including the availability of disposal facilities are making the estimate challenging. Some of the uncertainties such as unit disposal cost and transportation cost directly add up to the total decommissioning cost, while other factors may also influence the overall cost by limiting how the components and structures are dismantled in order i.e. for the waste to be segregated properly and fit into designated containers. In this study, impacts of radioactive waste management on dismantling cost was examined in order to plan decommissioning effectively while dealing with these uncertainties. Assuming a case of dismantling a reactor building of a 1100 MW BWR facility, labor cost and the amount of waste in each waste category were estimated for several cases with different waste container sizes. Other factors that may affect the dismantling cost are also examined including the timing when disposal facilities become available, i.e. duration of the storage of the dismantled waste before disposed.

[en] Some fundamental experiments have been performed on the measurement of tritium in the environmental water by a low background liquid scintillation counter, LSC-LB III made by Aloka. Owing to the function of eliminating static electricity provided on LB III, the effect of static change was eliminated. Measured values with N₂ gas flow were lower than that without the gas. Owing to the function of checking chemiluminescence provided on LB III, it became clear that the presence of the effect from chemiluminescence was put off by about 1 and half days. The detection limit became 0.36 Bq/l and FOM values were 190 with LB III, from which it could be confirmed that the performance had been improved by about 2.5 times in comparison with LB I. (author)

[en] In this study, basic strategies for the decommissioning and site remediation of the Fukushima Daiichi Nuclear Power Station (FDNPS) were investigated. Six scenarios were formulated based on two of the three decommissioning strategies of nuclear power plants defined by the International Atomic Energy Agency (IAEA): immediate dismantling and deferred dismantling. A multicriteria decision analysis was performed to analyze the preferences of the options from the viewpoints of the timeframe to complete decommissioning, the resulting waste, the site usability, and the availability of the radioactive waste disposal route. The same six scenarios were applied to both the FDNPS and the nuclear power plants that ceased operation after a normal plant life cycle for comparison. For the FDNPS, the decommissioning project involved fuel debris retrieval, dismantling, and site remediation. The analysis results suggest that the balance between the amount of waste and the time to achieve the end state may be one of the most critical factors to consider when planning the decommissioning and site remediation of the FDNPS

[en] A subcommittee organized under the Investigation Committee on the Nuclear Accident at the Fukushima Dai-ichi NPP (1F) published an interim report “Waste management from international perspectives”. Because it will be a long-lasting process to manage waste from decommissioning and site remediation of 1F, it is important to define interim end states for major activities of decommissioning (de-contamination and dismantling) and site remediation, and to steadily make progress toward them, with an eye to the final end state. In addition, multiple “scenarios” to attain the final end state were hypothesized, and advantages and disadvantages of each scenario were identified in order to further the discussion on the final end state. This article presents a summary of the interim report. (author)

No abstract available

[en] In the safety assessment of radioactive waste disposal, it is critical to understand the porewater chemistry in compacted bentonite in order to predict long-term migration behavior of radionuclides in the engineered barrier. This study estimates the activity coefficients of dissolved ions in the porewater of compacted bentonite from the concentrations of ions at which CaCO₃ precipitation occurred. Solutions containing CaCl₂ and NaHCO₃ were introduced under electrical potential gradient from the opposite sides of the compacted Na-bentonite packed at the dry density of 1.0 kg/dm³. After the electromigration, the spatial distribution of ions along the compacted bentonite sample was determined. Sequential extraction method was developed to distinctly determine the concentrations of free ions in the porewater and in solid phase in bentonite. The results show that the exchangeable Na⁺ ions were progressively replaced by the incoming Ca²⁺ ions, and the compacted bentonite sample can be divided into three zones: Ca-, Ca-/Na-, and Na-bentonite zones. Precipitates of CaCO₃ were observed both in Ca- and Ca/Na-bentonite zones. The experimentally determined activity coefficients were at least smaller by a factor of 3 compared to the theoretical approximation calculated using PHREEQC code assuming dilute-solution conditions with no electrostatic interactions between ions and bentonite surface. (author)

[en] Studies on migration behaviors of colloidal materials through engineered and natural barriers are highly important for the safety assessment of radioactive waste repositories since the fate and transport of radionuclides may be significantly affected by interactions with colloids. Poly (ethylene glycol) coated gold nanoparticles (AuNPs) were used as a model colloid and the effects of the properties of AuNPs on their migration behaviors were investigated in batch and column experiments with silica sand. Deposition of AuNPs on silica sand and aggregation of AuNPs were observed more for larger AuNPs despite the unfavorable conditions of both AuNPs and silica sand having overall negative surface charges. Sloping plateaus were observed in the breakthrough curves, suggesting that a limited number of sites arising from charge heterogeneities on silica sand may be responsible for attractive forces between AuNPs and silica sand. However, stronger affinity toward silica sand and with each other was observed for larger AuNPs which have more significant negative surface charges. The results suggest that interactions other than electrostatic and van der Waals (vdW) are controlling the migration behavior of AuNPs, including steric effects arising from the surface coating, thus need to be considered when assessing fate and transport of nanoparticles. (author)

[en] In this study, sorption and diffusion coefficients of ¹³⁷Cs were experimentally determined for hardened cement paste (HCP) altered by leaching with different solutions. The migration behaviors of ¹³⁷Cs are discussed from the viewpoints of the activation energy for diffusion and the microstructure of altered HCP. Hardened cement paste was prepared by mixing ordinary portland cement with deionized water at the water-to-cement (w/c) ratio of 0.36, followed by curing in a cement-equilibrated water for 28 d at 323 K. The HCP specimen were leached in different solutions simulating different leaching conditions at FDNPS in de-ionized water, synthesized sea-water, and 6 mol L^-1 ammonium nitrate solution. Surface morphology and microstructure of altered HCP samples were analyzed with ²⁹Si-MAS NMR, XRD, MIP, and SEM-EDX. The batch-type sorption experiment was conducted for ground HCP samples under different temperature conditions (288-323 K). The mixture of ground HCP and the cement-equilibrated water spiked with ¹³⁷CsCl (solid/liquid ratio of 1/100) was periodically shaken for 28 d. Sorption coefficients (K_d) of ¹³⁷Cs were determined using the initial and final ¹³⁷Cs concentrations in liquid phase measured with NaI(Tl) scintillation counter. The one-dimensional non-steady diffusion experiment was performed under constant temperature conditions (288-323 K) using ¹³⁷Cs tracer for blocks of the HCP samples altered with the ammonium nitrate solution. The concentration profiles of ¹³⁷Cs in the HCP samples after diffusion were obtained with an imaging plate (BAS IP SR2025E, Fujifilm). The HCP samples were altered to different degrees of leaching. The EDX analysis confirmed the decreases in Ca/Si ratio, while NMR and XRD analyses showed the decline in portlandite and C-S-H in the altered HCP. The increase in porosity and a shift in pore size distribution towards larger pore sizes were also observed after leaching alteration. The K_d values of ¹³⁷Cs are shown. The ¹³⁷Cs sorption to unaltered HCP was negligibly small, whereas sorption of ¹³⁷Cs was observed to the altered HCP samples. The sorption enthalpy (ΔH) of ¹³⁷Cs was approximately -21±1.9 kJ mol^-1 and -34±3.5 kJ mol^-1 for the HCP samples altered in synthesized sea-water and in ammonium nitrate solution, respectively. We show the temperature dependence of the apparent diffusion coefficient (D_a) of ¹³⁷Cs in the HCP samples altered with the ammonium nitrate solution, together with the D_a for the unaltered HCP samples. The D_a values for the altered HCP samples were slightly lower than those for unaltered HCP samples. Although it can be speculated that the changes in pore structure due to leaching may have increased the diffusion coefficients of ¹³⁷Cs, the leaching treatment decreased the D_a value, suggesting a significant retardation of Cs by sorption in the altered HCP. The activation energy for diffusion (E_a) of ¹³⁷Cs was determined to be 52 ± 4.3 kJ mol^-1 for the altered HCP from the slope of the regression line (solid line). It was larger than the E_a value reported for unaltered HCP (36 ± 2.2 kJ mol^-1). Since the value of E_a was determined from the temperature dependence of D_a, the E_a value includes the effects of the temperature dependence of K_d (i.e. the effect of ΔH). Thus, the activation energy without the effects of sorption was determined from E_a and ΔH to be approximately 18 kJ mol^-1, which is almost the same as the activation energy for diffusion of Cs⁺ ions in free water (16.2 kJ mol^-1). These findings suggest that Cs in the altered HCP predominantly diffuse in relatively large pores filled with free water, simultaneously being affected by the retardation by sorption to the altered solid phases. The HCP samples with different degrees of alteration were obtained by leaching in different solutions. Diffusion and sorption coefficients as well as activation energy for diffusion and enthalpy of sorption were determined to suggest that Cs in the altered HCP predominantly diffuses in relatively large pores filled with free water, simultaneously being affected by the retardation by sorption to the altered solid phases

[en] Document available in extended abstract form only. Cementitious materials are to be used for the solidification materials, the low diffusion barrier, the structural materials, and plugging for the geological disposal of such radioactive wastes as TRU wastes in Japan. They play an important role in confinement of radionuclides often coupled with compacted clay. Since cementitious materials raise the pH of the water in contact by leaching of alkaline components, clay material in contact with pH water can be altered. It is important to understand the diffusion behavior of alkaline components Na⁺, K⁺, and Ca²⁺ ions together with typical fission products Cs and Sr in pathway in cementitious materials. The diffusion of the species in cementitious materials is often related to capillary pore characteristics, which is strongly dependent on water to cement (w/c) ratio. In the present work, the dependence of ²²Na⁺, ¹³⁷Cs⁺, and ⁸⁵Sr²⁺ radiotracer diffusion on w/c ratio in hardened cement paste (HCP) was studied by a non-steady diffusion method, and the diffusion behaviors were examined. HCP specimens were prepared by mixing ordinary Portland cement, which was purchased from Japan Cement Association, and deionized water in w/c ratio 0.36, 0.45, 0.50 and 0.60. Mixtures were remixed every 30 minutes until no bleeding water was observed, and were molded in polyethylene containers (24 mm in diameter and 55 mm in height). The solidified cement paste rod was taken from the molds and cured for 91 days at 323 K in supernatant of cement. After curing, two specimens (20 mm in height) were taken from the center of the HCP rod and water-saturated with supernatant of cement after evacuating gas. A small amount of the ²²Na, ¹³⁷Cs, or ⁸⁵Sr tracer solution (1.0x10^-5 dm³) was applied to the flat surface of each of them, and their surfaces were coupled for diffusion experiment. The temperature of specimens was kept at 303 K. After prescribed period of diffusion, the specimens were sectioned from the source surface by the following two methods: (1) 0.60 mm in thickness by diamond blade for ²²Na, and (2) 0.15 to 0.30 mm polishing using emery paper for ¹³⁷Cs and ⁸⁵Sr. The relative radioactivity concentration of ²²Na, ¹³⁷Cs, and ⁸⁵Sr was determined with a well-type NaI scintillation counter as a function of penetration depth for sectioned cement paste for case (1), and the powder remained on emery paper for case (2). Results and Discussion Typical examples of the concentration profiles of ²²Na⁺ and ¹³⁷Cs⁺ ions are given in Figure 1, and that of ⁸⁵Sr²⁺ ion in Figure 2. Solid line represents the regression line for concentration profiles using the solution to Fick's second law under non-steady diffusion from instantaneous source. The profiles of ²²Na⁺ and ¹³⁷Cs⁺ ions are well reproduced. This indicates that the migration of ²²Na⁺ and ¹³⁷Cs⁺ ions can be described based on a single diffusion process. For ⁸⁵Sr²⁺ ion, while the profiles in higher w/c can be described using a single diffusion coefficient, these in lower w/c can't be described. It is suggested that ⁸⁵Sr²⁺ ion at low w/c ratio involves two or more diffusion processes or. Major component of cement paste is calcium-silicate-hydrate (C-S-H), which contains gel pore that can be classified into low density (LD) C-S-H and high density (HD) C-S-H. Thus, there are two types in gel pores: small gel pore (in HD C-S-H) and large gel pore (in LD C-S-H). Cs⁺ and Sr²⁺ ions can be attached to C-S-H by ion exchange with Ca²⁺ ions within the structure of C-S-H gel, and the sites are grouped to three types in terms of tendency of ion exchange depending on Ca/Si ratio of C-S-H . Therefore, it is suggested that the difference of diffusion process among ²²Na⁺, ¹³⁷Cs⁺, and ⁸⁵Sr²⁺ ions is mainly caused by the pore type, the sorption property, and so on. (authors)

[en] Mass transports in a rock formation has been studied since it can work as natural barriers against the migration of radionuclides in geological disposal of high level radioactive waste. The rock formation can be categorized into crystalline rock as a fractured medium and sedimentary rock as a porous medium. However, the Wakkanai formation (siliceous mudstones) in Horonobe underground research laboratory (URL) in northern Hokkaido, Japan, has unique features having both porous and fractured medium. Therefore, matrix diffusion toward the crack-free porous media should be considered in addition to the advection-dispersion through fractures in the porous medium. In this study, in-situ dipole tracer migration tests were conducted at the G.L. -250 m gallery of the Horonobe URL. Laboratory experiments were also conducted to determine the apparent diffusivity (D_a) and sorption coefficient (K_d) of cesium and water (HTO) in the rock taken at the gallery to evaluate the performance of Wakkanai formation as a natural barrier. In the in-situ dipole tracer migration tests, a non-sorbing tracer (Uranine) and a sorbing tracer (cesium) were injected to a section in one borehole, and collected at a section in the other borehole to obtain the data on the mass transport between two parallel boreholes where a single fracture crosses. The D_a of cesium and water were determined from the laboratory tests by a non-steady, one-dimensional diffusion method. The K_d of cesium were also determined by a batch method. The breakthrough curves of non-sorbing tracer (Uranine) obtained in the in-situ dipole tracer migration tests were well described by a dual-channel model in which one-dimensional advection dispersion was taken into account. This suggests that the tracers migrate through at least two different pathways in the fracture. The breakthrough curves also indicate that the peak concentration of the sorbing tracer (cesium) was much smaller than that of the non-sorbing tracer (Uranine), suggesting that the Wakkanai Formation has a high sorptive and low diffusive properties for cesium. This specific property of the rock was confirmed at the laboratory experiments. The D_a value obtained for cesium was about 2.9×10"-"1"2 m"2/s, which is significantly smaller than that of water (3.4×10"-"1"0 m"2/s), and the K_d value of cesium was determined to be 488 ml/g. These new findings can be useful for understanding the mass transport in the fractured sedimentary rocks having unique features. (author)