The as-welded microstructure of 308 stainless steel is usually a mixture of austenite and ferrite phases. The morphology of the duplex structure is a function of the composition of the weld. The resistance to sensitization is proportional to the amount and distribution of austenite-ferrite boundary area and the probability of precipitation of embrittling phases such as σ and α′ is proportional to the volume fraction of ferrite phase. Consequently, the properties of 308 stainless steel welds are strong functions of their microstructures. These, in turn, are primarily determined by the composition of the weld. Accordingly, to maximize the properties of welds of canisters of nuclear waste may require a restriction of the compositional limits of 308 stainless steel in order to preclude the formation of microstructures that are particularly susceptible to sensitization and/or mechanical embrittlement.

The state of knowledge concerning the internal constitution of cement systems is reviewed. It is concluded that many properties relevant to radwaste immobilization are strongly influenced by the presence of thermodynamically metastable phases. In order to ascertain cement properties at long ages, models have been developed which include compositional and time-dependant factors. These require a validated data base. Progress in developing models and creating the data base are reviewed. In the course of accumulating data, specific radwaste species -cement interactions can also be quantified.

Cement blends are likely to form a large fraction of the materials in the near field of radioactive repositories. Being chemically reactive and slightly soluble, their effect on near field chemistry will be very marked. Predictions of nuclide migration in this zone will therefore depend in part on accurate modelling of cement chemistry.

The models being developed require the establishment of correct phase relations in the Cao- SiO2- Al2O3- MgO- SO3-H2O system (the major cement components). More than 40 compatibility experiments were carried out to determine these relationships. Purified cement hydrates were shaken continuously in CO2- free water, for 10 weeks. The results from these experiments together with data from the literature, enabled us to construct a better validated phase relations scheme than has hitherto been possible.

Interpretational problems were encountered. Thus, mixtures containing Ca(OH)2, gave rise to the precipitation of metastable phases. This is due to special dissolution/ precipitation equilibria obtaining at early age, allowing ‘AFm type’ (4CaO.Al2O3.SO3.12H2O) phases to form, where C3AH6 (3CaO.Al2O3.6H2O) and AFt (6CaO.Al2O3.3SO3.32H2O) are thermodynamically favoured.

The chemical properties of the CaO-Al2O3-SiO2-H2O system are important for understanding and predicting the behaviour of Portland cements in cementitious wasteforms and radioactive waste repositories. Solids of known average composition in this system have been synthesised by the co-hydrolysis of mixed alkoxides. The solids have been equilibrated with water at 25°C and the composition of the aqueous phase characterised. A thermodynamic model for the system has been developed by extending an earlier model of solid solutions in amorphous gels to include aluminium compounds. The model accounts for most of the experimental data and predicts that the main influence of aluminium is to form a hydrogarnet solid solution of general composition C3AH6−2xSx The model can be used to predict the chemistry of hydrated cements and the interaction of cements with groundwaters.

The wealhering of concrete by carbonation causes the formation of calcite solid solutions that can significantly improve concrete as a barrier to the migration of divalent radionuclide cations. Example calculations for 90Sr and 60Co indicate that release rates from carbonated concrete are five orders of magnitude lower than for intact, unweathered concrete and that the thin carbonated zone is a significant sink for these radionuclides.

The geochemical modeling codes EQ3NR/EQ6 were used to model the interaction of cementitious materials with ground water from the Yucca Mountain proposed nuclear waste repository site in Nevada. This paper presents a preliminary estimate of the compositional changes caused by these interactions in the ground water and in the cement-based compounds proposed for use as sealing and shaft liner materials at the Yucca Mountain site.

Research on the longevity of cement-based grout materials for sealing a deep geological disposal vault is an important aspect of the Canadian and other nuclear waste management programs. These studies include assessments of the chemical durability of cement grouts, and the effects of leaching and phase transformation on the long-term hydraulic and diffusion characteristics of grouts.

This paper presents the results of laboratory studies carried out to assess the effects of leaching of cement phases on the pore structure of hardened grouts. Measurements of mercury intrusion and scanning electron microscopy with energy dispersive x-ray analysis, have been used to investigate the changes in pore structure of both a reference grout (90% Type 50 cement, 10% silica fume, water-to-cementitious materials ratio between 0.4 and 0.7) and ALOFIX-MC (a fine cement product of Japan), as a function of leaching time.

The work discussed here reveals that the porosity of hardened grout does change during leaching, but within limits that depend on grout composition and initial porosity. The results confirm that the materials have the potential to self-seal and maintain their performance for longer periods than those currently predicted by longevity models.

Our studies of the porosity and permeability of grouts suggest that pore size distribution rather than total porosity is the more important parameter in determining longevity.

Diffusivities of Cs, Sr and Co were measured in several kinds of water-saturated concretes. The measured penetration profile of every tracer was composed of two parts. Those were a steep slope near the surface and a gradual slope in the deeper part. This profile was successfully explained by considering two diffusion paths in concrete. One diffusion path was possibly fissure with a width of a few microns and another was network of submicron pores. The volume of submicron pores was approximately 90% of the total free space volume in every concrete. The orders of magnitude of apparent diffusivities for all tracers were 10−12–10−11 m2/s through the fissure and l0−16–10−15 m2/s through the network of pores. The difference between the diffusivities of the two paths is thought to be caused by small geometrical factor of the network of submicron pores.

In situ experiments at Atomic Energy of Canada Limited’s Underground Research Laboratory have demonstrated that a specially developed high-performance cement-based grout, called the reference grout, consisting of 90% sulphate-resistant cement (Canadian Type 50), 10% silica fume (SF) and superplasticizer can seal very fine fissures in granitic rock. Studies are now in progress to determine the longevity of these materials. This paper describes progress in studies of the hydraulic conductivity (i.e. coefficient of permeability), K, of the grout at water/cementitious materials ratios, w/c, between 0.4 and 0.8. The K was assessed with the grouts under compressive and tensile stress using specially designed radial flow permeameters. The results confirm that the K of the reference grout is two to three orders of magnitude lower than that typically observed for intact granite rock (10−12 m/s). Moreover, the data show that adding silica fume and reducing the w/c decreases K. The hydraulic conductivity data coupled with field work and other laboratory data strongly suggest that the reference grout can be successfully used to seal finely fractured rock in the vicinity of a nuclear fuel waste disposal vault.

A preliminary evaluation of the impact on waste-form performance characteristics due to variations in dry-solids-blend material source has been performed. The statistical treatment technique utilized for data evaluation, described herein, provides a technically sound method of comparing the effects of material source and can be used to assess the statistical significance of effects of other variables on waste-form characteristics.

Cement-based waste forms are one of the most widely used waste disposal options, yet definitive knowledge of the fate of the waste species inside the waste form is lacking. A fundamental understanding of the chemistry and microstructure of the waste forms would lead to a better understanding of the mass transfer of the waste species, more confidence in predicting and extrapolating waste-form performance, and improved design of waste forms. Better and cheaper leach tests would lead to quicker and more cost-effective screening of waste-form alternatives, cement-based and otherwise. In addition, assessment of durability may be important to predicting waste-form performance in the field over periods of decades and centuries.

Mechanical and hydraulic behavior of buffer material during water uptake in a repository is a major issue from the viewpoint of mechanical stability of engineered barriers and near-field conditions for performance assessment. This paper presents the results of hydraulic-mechanical modeling of buffer material and the simulations carried out on an engineered barrier system under water uptake.

Hydraulic behavior of compacted bentonite of buffer material was modeled as moisture diffusion. An elasto-plastic model was applied to the deformation behavior of compacted bentonite, of which swelling pressure was described by swelling coefficient under restraint condition. The hydraulic diffusivity and swelling coefficient were given based on the result of swelling tests of KUNIGEL-V1 bentonite which contains about 50 % montmorillonite. Being used this model, simulations on re-saturation behavior of an engineered barrier system were carried out for the cases of water uptake from the whole surface of both crystalline and sedimentary rock and from partial surface of opening. The results are : (1) The hydraulic and mechanical behavior of compacted bentonite can be described by a swelling-elasto-plastic model. (2) The distribution of the water content depends on the water uptake conditions. (3) The deformation of compacted bentonite and the displacement of the overpack under water uptake are negligibly small.

AECL Research is carrying out large-scale in situ experiments at its Underground Research Laboratory (URL). The Buffer/Container Experiment is designed principally to investigate the full scale, in situ performance of bentonite-based buffer material in a single emplacement borehole environment. In addition, the response of the rock to excavation and heating will be investigated. The experiment also allows for the development of the technologies needed to demonstrate some of the vault engineering activities proposed in the Canadian nuclear fuel waste disposal concept. These include excavation of large diameter boreholes for waste emplacement and in situ compaction of a bentonite/sand buffer mixture. Although these methodologies developed for the URL have not been optimized for the commercial, full-scale operations needed for a disposal vault, results show that the equipment and methodologies needed for vault operations are a reasonable extrapolation of existing technology.

A series of laboratory experiments was conducted to establish the heat-induced moisture movement in a bentonitic clay buffer. The buffer material is proposed as an engineered barrier to isolate a heat-emitting high-level nuclear fuel waste container from its emplacement borehole located in a deep rock repository. In the experimental simulation, the hygrothermal phenomena are initiated by a cylindrical heater placed within the compacted buffer material in a borehole centrally located in a granite block. The experimental results illustrate the time-dependent distribution of temperatures within the rock mass and the residual moisture distribution at the termination of the experiment.

Groundwater flow from the repository to the surface through the sealed access shaft may be induced by the heat generated from high-level radioactive waste(HLW). It is important to estimate the effect of this heat generation on groundwater flow.

This paper presents an evaluation of groundwater flow in the vicinity of an access shaft filled with a backfill material. The evaluation uses a coupled thermo-hydraulic model. According to the current design, the backfill material within the shaft will be tamped 5 to 15 times every 20 cm depth using a 60 kg vibrating compactor. The properties of the backfill material were determined from experimental results for this construction method.

A transient coupled thermo-hydraulic analysis was carried out using an axisymmetric FEM model. Two kinds of repository models were considered, one was a single layered repository located at 1000 m depth, and the other was a two layered repository located at 800 m and 1000 m depths. The access shaft of each model was located at the center of the repository.

The results show that the vertical hydraulic gradient of groundwater flow along the sealed shaft will be approximately 0.001 at a time 1000 years after closure of the repository.

To predict the mechanical performance of earthen backfill in a nuclear fuel waste disposal vault, the effects of temperature and moisture content on the backfill load-deformation characteristics must be known. Modified California Bearing Ratio (CBR) tests were used to obtain these data. The results show that the load-bearing capability of the backfill is not likely affected by water salinity. The load-bearing capability decreases with increasing temperature and water content or decreasing dry density. The effects of temperature, dry density, and moisture transport processes on load-bearing capability were found to be relatively small compared to those of moisture content. Under the expected Canadian vault environment, the backfill should retain sufficient bearing capacity to satisfy its mechanical design function, which is principally to limit expansion of swelling buffer material.

A one-dimensional analytical model has been developed to describe the compressive behaviour of clay-based shaft backfills under large hydraulic loads. The analytical model has been calibrated with results from a compression test on a partially saturated swelling clay backfill mix in the high-pressure compression cell constructed at Ontario Hydro.

A series of tests have been performed at the Geotechnical Research Centre on backfill material under imposed temperature gradients. It has been demonstrated that the water content distribution during the transient process does not affect the temperature distribution. The Trial Function Technique (TFT) has been applied to calculate the thermal conductivity of the backfill material. Using these values and the proposed method of solution, the temperature profiles have been calculated. The comparison between the theoretically predicted and the experimentally measured temperature profiles shows good agreement except in the vicinity of the heating boundary, where a thin thermal boundary layer exists. The vaporization process in this thin layer contributes to the turbulent effect that can be modeled by using a thermal shock condition.

Solubility and speciation data are important in understanding aqueous radionuclide transport through the geosphere. They define the source term for transport retardation processes such as sorption and colloid formation. Solubility and speciation data are useful in verifying the validity of geochemical codes that are part of predictive transport models. Results from solubility and speciation experiments of 237NpO2+, 239Pu4+, and 241Am3+/Nd3+ in J-13 groundwater (from the Yucca Mountain region, Nevada, which is being investigated as a candidate high-level nuclear waste disposal site) at three different temperatures (25°, 60°, and 90°C) and pH values (6, 7, and 8.5) are presented and compared with published modeling calculations. The comparison results indicate that there is a great need for experimental data on the solubility and speciation of transuranium elements under a wide range of conditions, for example, pH, Eh, temperature, and composition of groundwaters. Additionally, the influence of alpha radiation and the radiolysis of the secondary transuranium solids on solubility and speciation should be studied. Solubility studies and model calculations should be extended to other important long-lived nuclear waste radionuclides such as nickel, zirconium, cadmium, radium, and thorium.

Photoacoustic spectroscopy using pulsed laser excitation is being developed by a number of research groups as one of the most promising methods for studying speciation of actinides in solution at environmentally relevant concentrations. We present details of a number of hardware and software techniques we have implemented which, once fully developed, we believe will improve the sensitivity of the method. Our approach is based on more extensive waveform analysis. While most signal processing techniques extract the analytical signal from only a small portion of the acoustic waveform produced in the detector by an absorption event, we describe two methods that use more of the waveform. Other methods of minimizing noise sources using both hardware and software are also described.