Cation exchange selectivity coefficients of bentonite are crucial for estimating cation compositions of bentonite buffer in the geological disposal of radioactive wastes. For confirming the difference in selectivity coefficients of Ca(II), Mg(II), and K(I) for Na(I) between compacted and dispersed bentonite, column and batch experiments were performed under various ionic strengths. Further, as an applied study, a column experiment was performed using coastal groundwater (ionic strength 0.28 M), and the cation compositions of bentonite after the experiment were numerically simulated using selectivity coefficients for the compacted and dispersed states. The experimental results showed that the selectivity coefficients in the compacted state were higher than those in the dispersed state, and they increased slightly with increasing ionic strength in both states. The numerical simulations agree with the experimental results of cation composition after the column experiment when using the selectivity coefficients determined under the compacted state at the ionic strength of 0.25 M. These results suggest that the selectivity coefficients obtained under the compacted state are preferable for predicting the cation composition of compacted bentonite, especially in saline groundwater environments.

For elucidating the validity of accelerated Ca leaching tests, the chemical and physical changes of a hardened ordinary Portland cement induced by the leaching into deionized water were compared with those induced by the accelerate leachings with proton-type cation exchange resin for the continuous elimination of Ca2+, with an electrostatic electric field for the electrophoretic elimination of Ca2+, and with NH4NO3 solutions for the elimination of OH-. The use of the resin and the electrostatic field were found to be inappropriate because both the methods caused the formation of strong acid which severely attacked and etched the paste. The use of a NH4NO3 solution was an appropriate method for the accelerated test, since Ca2+ was homogeneously leached in the paste and the amount of the leaching was controllable by properly selecting the concentration and the volume of the NH4NO3 solution.

The structural model of 66.7PbO-33.3B2O3 glass was constructed using a reverse Monte Carlo (RMC) method, in which bond valence sum (BVS) was added as a constraint condition to suppress formation of unrealistic local structures. Based on the crystal structures, the optimal BVS calculating conditions were determined. As a result, BVS distributions with small deviation were successfully achieved without lowering the reproducibility of other experimental constraints. The geometric asymmetry of PbOn polyhedra was evaluated from the eccentric distance between Pb and gravity center of oxygen atoms. The average eccentric distance was shorter than that in the lead borate crystals, indicating less asymmetry of PbOn units in the RMC glass model. The connectivity between BOn and PbOn units was investigated. It was consequently concluded that the glass had a different network structure from the crystal with the same composition, which might be due to the different chemical bonding character between the lead borate glasses and crystals.

Calcium leaching from cementitious materials into bentonite is a key process for the long-term alteration of cement–clay interfaces of engineered barrier systems. Strong chemical gradients between cement and clay drive the precipitation of minerals such as calcium silicate hydrate (C–S–H) and calcite. To analyze the mineralogical and porosity evolution at the cement–clay interface, composite specimens consisting of cement paste and bentonite mixed with various amounts of sodium carbonate were subjected to immersion and chloride migrations tests and were investigated by electron probe micro-analysis (EPMA), thermogravimetry/differential thermal analysis (TG-DTA), and X-ray diffraction (XRD) after 4–20 months of immersion. The results show that adding sodium carbonate to the bentonite enhanced the formation of calcite in the form of a surface layer on the cement paste. This suggests pore clogging at the interface and implies the existence of a threshold amount of carbonate addition above which pore clogging occurs. This is the first of two papers; the accelerated evolution of the samples in the presence of an electrical field is discussed in the second paper.

Chemical conditions and mass transport properties of engineered barrier systems in TRU waste facilities would change with time due to the interaction of cement/bentonite materials. (‘TRU waste’ is one of categories of the radioactive wastes and contains a significant amount of alpha-emitting transuranic nuclides. In some countries, these wastes are classified into the Intermediate Level Waste (ILW).) Previous numerical model analyses to assess the long-term performance of engineered barrier systems in TRU waste repositories predicted to form Calcium Silicate Hydrate (C-S-H) species at the interface between the cementitious and bentonite materials. If C-S-H precipitates in the bentonite side of the boundary, mass transport in the bentonite buffer decreases and mineralogical alterations are expected to be restricted for a long period. The evidence of C-S-H precipitation in the bentonite side, however, still has not been identified in the former experimental studies. To improve the reliability of numerical analyses, immersion experiments were performed using contact samples of cementitious and bentonite materials, and X-ray absorption fine structure (XAFS) analysis was carried out to detect C-S-H precipitation at the contacting interface. Precipitation of C-S-H was confirmed from the obtained XAFS spectra. This result is one of the evidences to show the validity of the current numerical model analyses, which suggests that the bentonite buffer performance as an engineered barrier would be kept over a long period.

It is very important to predict alterations in the concrete used for fabricating disposal containers for radioactive waste. Therefore, it is necessary to understand the alteration of cementitious materials caused by calcium leaching when they are in contact with ground water in the long term. To evaluate the long-term transport characteristics of cementitious materials, the microstructural behavior of these materials should be considered. However, many predictive models of transport characteristics focus on the pore structure, while only few such models consider both, the spatial distribution of calcium silicate hydrate (C-S-H), portlandite, and the pore spaces. This study focused on the spatial distribution of these cement phases. The auto-correlation function of each phase of cementitious materials was calculated from two-dimensional backscattered electron imaging, and the three-dimensional spatial image of the cementitious material was produced using these auto-correlation functions. An attempt was made to estimate the diffusion coefficient of chloride from the three-dimensional spatial image. The estimated diffusion coefficient of the altered sample from the three-dimensional spatial image was found to be comparable to the measured value. This demonstrated that it is possible to predict the diffusion coefficient of the altered cement paste by using the proposed model.

Zircaloy cladding of the spent nuclear fuel, which is highly activated and contaminated, is expected to be disposed of in a deep underground geological repository, both in the once-through and recycling fuel cycles. Carbon 14 (C-14), mainly originated from the nitrogen impurity in Zircaloy cladding and formed through the N(n, p)C reaction in the reactor, is a typical activation product. In a preliminary performance assessment of the deep geological disposal in Japan, C-14 gave a significant dose impact, which is due to the relatively large inventory, relatively long half-life, i.e., 5730 years, higher release rate and the chemical speciation and consequent migration parameters. In a preliminary Japanese safety case, relatively high IRF (instant release fraction), 20%, was assumed regarding the oxide film formed on the metal surface as a source of the instant release. With respect to this source term issue, we have prepared an irradiated BWR fuel cladding (Zircaloy-2, average rod burnup of 41.6 GWd/t) which has an external oxide film of 25.3 μm thickness. Although the specific activity in oxide was approximately 3 times higher than the base metal activity, due to the additional C-14 generation by the O(n, α)C reaction, the C-14 abundance in the oxide was less than 10% of total inventory. A static leaching test using the cladding tube was carried out in an air-tight vessel filled with a deoxygenated dilute NaOH solution (pH of 12.5) at room temperature. After 6.5 years, C-14 was found in each leachate fraction of gas phase and dissolved organics and inorganics, the total of which was less than 0.01% of the entire C-14 inventory of the immersed cladding tube. Both the C-14 abundance and the low leaching rate suggests that C-14 in oxide does not have a significant impact on the IRF in the safety case.

This report gives details of the definition of the modelling approaches to be followed by each of the participants of Work Package 3, related to Modelling and Interpretation of the Experiments to be carried out in Work Package 1. The written descriptions reported here have been provided by the project partners, which are listed below as contributors. These contributions were reviewed and compiled by the WP3 leader, as editor of this deliverable.

The iodine release behavior from the iodine-immobilized cement was investigated. From the results of immersion tests using ion-exchanged water (IEW) and calculations, the solubility equilibrium model could describe the iodine release behavior. To assess the performance of cement in an actual environment, it is important to confirm that the solubility equilibrium model is applicable to the geological disposal conditions. From immersion tests using simulated ground water, the release of iodine from the cement occurred in a shorter period of time than in the case of IEW, and reactions with CO32− and Cl−, which were contained in the simulated ground water, had an influence on the iodine release behavior. As a result of calculations using the solubility equilibrium model, the liquid-solid ratio at which the iodine was completely released was mostly in agreement with the results of immersion tests. The results show that this model can be applied in a wide range of environments.

Iodine filters expended after nuclear fuel reprocessing contain radioactive iodine (I-129), almost all of which exists as silver iodide (AgI). The synthetic rock technique is a solidification treatment technique using hot isostatic press (HIP), in which the alumina adsorbent base material is synthesized to form a dense solidified material (synthetic rock), and I-129 is physically confined in the form of AgI in the alumina matrix. Thus, it is necessary to understand the matrix dissolution behavior to evaluate the iodine release behavior. Experiments involving the dissolution of the matrix were carried out under various temperatures (35–70 °C) and pH values (10–12.5) that reflect the disposal conditions. The results of the experiments showed that the dissolution rate of Al visibly increases with temperature and pH. The dissolution rate constant was calculated from the initial data assuming the dissolution of the matrix as a primary reaction. The logarithmic rate constant showed a good linear correlation with the pH and the reciprocal of temperature. The 27 Al-NMR analysis of the solutions of the dissolved matrix showed that the major chemical species present in the solutions was Al(OH) 4 - . This indicated that the dissolution of the matrix can be described by the following equation: Al 2 O 3 + 2OH - + 3H 2 O → 2Al(OH) 4 - . Subsequently, the empirical equation of the rate of dissolution of the matrix as a function of the temperature and pH was derived. It will be used to evaluate the iodine release behavior from the synthetic rock.

The porosity of hardened cement paste changes when it is exposed to the actual environments, and it may be expected that the chemical structure of the C-S-H undergoes changes. The C-S-H is the main component, comprising 60% or more by volume of hardened cement paste, and this study focuses on changes in the chemical structure of C-S-H. Deterioration induced by ammonium nitrate solution was accompanied by changes in the pore structure as well as structural changes in the C-S-H in the hardened cement paste. It was ascertained that when evaluating the decreases in the elastic modulus of the hardened cement paste, both the pore structure and the structure of the C-S-H must be considered.

The development of an iodine immobilization technique that can fix radioactive iodine in waste form for a long period and constrain its leaching into pore water is necessary in order to secure the long-term safety of geological disposal of transuranic (TRU) waste. Lead borate glass vitrified at a low temperature is regarded as a promising material for immobilizing the Iodine-129 that is recovered from spent AgI filters generated by reprocessing plants in Japan and which may have a significant effect on the long-term safety of geological disposal. Batch leaching tests were conducted to understand glass dissolution behavior in various solutions that account for geological disposal conditions. Boron dissolved at the highest rate in all types of solutions to be used as an index element for measuring the glass dissolution rate. On the other hand, lead dissolved in these solutions at a much lower rate. These results are consistent with an electron micro-probe analysis (EPMA) of the altered glass surfaces that indicated the depletion of boron and enrichment of lead near the surfaces. The altered glass surfaces were further examined by scanning and transmission electron microscopy (SEM/TEM) and X-ray diffraction (XRD). SEM/TEM observation showed formation of a porous altered layer consisting of fine crystallites on the pristine glass and euhedral crystals on the altered layer. XRD analysis indicated that the fine crystallites and euhedral crystals are hydrocerussite, Pb3 (CO3)2(OH) 2, which was predicted by geochemical calculation as the precipitate for the experimental system.

In order to evaluate the long-term behaviour of the engineered barriers in geological disposal sites for transuranic element-bearing (TRU) waste, an evaluation by numerical analysis is required. Although chemical and hydraulic/mechanical analyses have been conducted independently until now, essentially both type of phenomena occur simultaneously and produce synergistic effects. Therefore, we focused attention on the buffer (bentonite) engineered barrier and conducted a study of which involved incorporating hydraulic/mechanical phenomena into the chemical analysis of bentonite alteration. The simulations employed weakly-coupled chemical and hydraulic/mechanical effects to study the behaviour in one dimension. The results showed that the dissolution of the montmorillonite is suppressed in the buffer section nearest the cement material. Moreover, in order to achieve a fully coupled analysis in future, the present study also identifies issues that need to be resolved.

An iodine-immobilizing cement solidification process using calcium aluminate cement with gypsum additive was developed. Powdered cement solid was repeatedly immersed in ion-exchanged water with varying liquid-to-solid ratios (L/S) in accelerated dissolution tests simulating interaction with groundwater at waste disposal sites. The measured concentrations of iodine in the water were on the order of 10 −5 to 10 −3 mol⋅dm −3 in the entire L/S range. These concentration levels are extremely low compared with those in the case of ordinary Portland cement. Calculations with a solution equilibrium model for the cement immersed in ion-exchanged water showed that the observed iodine release profile versus integrated L/S ratio from the immersion test was explained by a dissolution model of minerals in the cement.

In this report, X-ray photoelectron spectroscopy (XPS) and 29Si-MAS-NMR was used for the evaluation of deteriorated hardened cement pastes. The deterioration by ammonium nitrate solution was accompanied by changes in the pore structure as well as by structural changes in the C–S–H in the hardened cement paste. The CaO/SiO2 ratio of the C–S–H decreased with the progress of deterioration, there was also polymerization of the silicate in the C–S–H. It was confirmed that the degree of polymerization of silicate of the C–S–H in hardened cement paste can be determined by XPS. It was also shown that the polymerization depends on the structure of the C–S–H.

High strength and ultra low permeability concrete (HSULPC) is thought to be an alternative material for a radioactive waste package containing transuranic radionuclides (TRU wastes) for geological disposal. Thus high confining ability is required for HSULPC. In addition, knowledge of time-dependent fracturing and change of permeability due to fracturing is important. For cementitious materials, self sealing of fracture due to the precipitation of calcium compounds can occur in water. This can affect long-term stability and decrease of permeability. In this study, the sealing behavior of macro-fractured HSULPC in water was investigated using micro-focus X-ray CT and image analyses. It was shown that the sealing of fracture by precipitation occurred only near the edge of the specimen. Using the technique of image registration and image subtraction considering the gap between X-ray CT images, the sealed regions in fracture were extracted successfully. The percentage of sealed volume in fracture increased with elapsed time. It is concluded that high confining ability of HSULPC can be maintained due to the self sealing even if fractures are included.

Introduction Generation of C-S-H at the boundary of the cementitious materials and bentonite due to their interaction is an important phenomenon to evaluate for more accurate prediction of long-term behaviors of the engineered barriers proposed for the geological disposal of radioactive waste. The C-S-H among secondary minerals had been difficult to analyze, however, the study to date demonstrated quantitative analysis of the C-S-H by using the XAFS measurement. In this study, generation of the C-S-H was analyzed for two types of bentonite aiming at identifying differences in its behavior depending on the difference of the type of the bentonite.