The effect of radiation on the mechanical properties of Topopah Spring tuff was investigated by performing uniaxial compressive tests on irradiated and control samples of the tuff from the potential repository horizon at Yucca Mountain. Test results are presented, including stress-strain curves and peak strength and Young's modulus values. The results from this preliminary study show that for uncracked samples of Topopah Spring tuff, exposure to gamma radiation had no discernible effect on the unconfined compressive (peak) strength or the Young's modulus. However, results for samples that contained partially healed subvertical cracks indicate that exposure to radiation may reduce the strength and Young's modulus significantly. This is attributed to weakening of the cementing materials in the cracks and fractures of the samples that were irradiated. These results are preliminary, and additional studies are warranted to evaluate whether radiation weakens cementing materials in welded tuff.

The aeschynite structure-type (Ce,Nd,La,Th,U,Ca)(Nb,Ti)2O6, and the rare-earth silicate apatite structure-type with the formula (Ce,La,Nd,Ca,Th)10(SiO4,PO4)6(O,F,OH)2 are important rare-earth and actinide host phases for high-level nuclear waste. Natural phases of these structure-types have calculated alpha-decay doses up to ∼1017 α-events/mg which have accumulated over hundreds of millions of years. Transmission electron microscopy has been used to study the microstructure of α-decay damage in aeschynite and britholite. Electron diffraction analysis of natural aeschynite revealed that minerals originally crystalline gradually lost their crystallinity with increasing alpha-decay doses. Helium bubbles were found in the aeschynite which have accumulated up to ∼2×1016 α-events/mg. These bubbles may nucleate within collision cascades during a-decay damage. Electron irradiation has an enhanced rare-gas migration and the formation of larger bubbles. High-resolution electron microscopy (HRTEM) revealed that amorphization during accumulation of a-decay damage was from alpha-recoil nuclei collision cascades, in both the aeschynite and britholite.

Heavy-ion bombardment of a glass surface is a conventional laboratory technique for producing damage of interest for radioactive waste encapsulation. At energy of order 100 keV such a bombardment simulates the damage produced by α-recoil nuclei and fission fragments resulting from the nuclear decay. The damage region is 100–500 nm depending on conditions of the bombardment.

In the present work some results of EPR study of point defects formed in silicate, borate, borosilicate, phosphate and other oxide glasses irradiated with different charge particles (C, N, O, Ar. Mn, Cu, Pb) at energy E=150 keV and large total fluence of ions (up to 1017 cm-2) are reported. Electron paramagnetic resonance (EPR) is a very sensitive technique which gives an information on the structure of point defects and their content. It is shown that in some cases (for example, in borate glasses) the oxygen hole centers similar to ones observed in γ-irradiated glasses are formed after ion bombardment. However, in the majority of cases new defects which are not typical of γ-irradiated oxide glasses were found They were large molecular oxygen ions (O2-O3-O4-) located in the cavities formed under ion bombardment in the near surface layer of glass. It should be noted that the relative content of these defects is of the order of several tens per 1000 incident ions. This content decreases with increasing fluence and atomic mass of incident ions. It indicates indirectly that point defects are clustered when the damage of the near surface layer becomes strong. The formation of gaseous oxygen is possible in cavities of the damage surface layer.

It was found that some elements (for example C, N and transition metals) form chemical compounds with oxygen. The migration of alkali ions promotes the formation of such compounds since the chemical compounds were detected by means EPR in glasses rich in alkali oxides.

Wrought and cast low-carbon steel are candidate materials for the thick (e.g. 10 cm) outer barrier of nuclear waste packages being considered for use in the potential geological repository at Yucca Mountain. Dry oxidation is one potential degradation mode for these materials at the moderately elevated temperatures expected at the container surface, e.g. 323–533 K (50–260 °C). Therefore, numerical predictions of dry oxidation damage have been made based on experimental data for iron and low-carbon steel and the theory of parabolic oxidation. A numerical approach employing the Forward Euler method has been implemented to integrate the parabolic rate law for arbitrary, complex temperature histories. Assuming growth of a defect-free, adherent oxide, the surface penetration of a low-carbon steel barrier following 5000 years of exposure to a severe, but repository-relevant, temperature history is predicted to be only about 0.127 mm, less than 0.13% of the expected container thickness of 10 cm. Allowing the oxide to spall upon reaching a critical thickness increases the predicted metal penetration values, but degradation is still computed to be negligible. Based on these physically-based model calculations, dry oxidation is not expected to significantly degrade the performance of thick, corrosion allowance barriers constructed of low-carbon steel.

The generation of hydrogen gas from metallic waste in corrosive disposal environment is an important issue for the safety analysis of low-level radioactive waste disposal facilities in Japan. In particular iron and aluminum are the possibly important elements regarding the gas generation. However, the corrosion behavior of these metals has not been sufficiently investigated under the highly alkaline non-oxidizing disposal conditions yet.

We studied the corrosion behavior of iron and aluminum under simulated disposal environments. The quantity of hydrogen gas generated from iron was measured in a closed cell under highly alkaline non-oxidizing conditions. The observed corrosion rate of iron in the initial period of immersion was 4 nm/year at 15 °C, 20 nm/year at 30 °C, and 200 nm/year at 45 °C. The activation energy was found to be 100 kJ/mol from Arrhenius plotting of the above corrosion rates.

The corrosion behavior of aluminum was studied under an environment simulating conditions in which aluminum was solidified with mortar. In the initial period aluminum corroded rapidly with a corrosion rate of 20 mm/year. However, the corrosion rate decreased with time, and after 1,000 hours the rate reached 0.001 to 0.01 mm/year.

Thus we obtained data on hydrogen gas generation from iron and aluminum under the disposal environment relevant to the safety analysis of low-level radioactive disposal facilities in Japan.

Mathematical models to enable long-term prediction of the corrosion behaviour of carbon steel overpacks for radioactive waste have been developed. An existing model of the growth of pits, implemented in the CAMLE software, has been extended and used to investigate the sensitivity of the predictions to input parameters, including cathodic reaction kinetics and the relative position of the anode and cathode. Predictions have also been made of the aeration period of the repository, during which localised corrosion is possible.

Actinide solubilities in highly concentrated chloride solutions are about one order of magnitude higher than in similar inert electrolyte (NaClO4) solutions. This increased solubility is due to interactions between actinide and chloride ions. Contradictory results exist regarding the interaction mechanism between actinide and chloride ions. Specifically, both inner-sphere complex formation and ion pair association have been implicated in the interpretation of spectrophotometric and extraction data. To address this controversy, we investigated the interaction between actinide ions in the (III), (IV), (V) and (VI) oxidation states and chloride ions using a multi-method approach. Spectroscopie techniques (TRLFS, Raman, UV-Vis absorption, EXAFS) were used to distinguish between changes in the inner coordination sphere of the actinide ion and effects of ion pairing. X-ray absorption spectroscopy and single crystal X-ray diffraction were used to determine structural details of the actinide chloro complexes formed in solution and solid states.

Experiments suggest that dissolved organic ligands may primarily modify mineral dissolution rates by three mechanisms: (1) metal-ligand (M-L) complex formation in solution, which increases the degree of undersaturation, (2) formation of surface M-L complexes that attack the surface, and (3) formation of surface complexes which passivate or “protect” the surface. Mechanisms (1) and (2) increase the dissolution rate and the third decreases it compared with organic-free solutions. The types and importance of these mechanisms may be assessed from plots of dissolution rate versus degree of undersaturation.

To illustrate this technique, calcite, a common repository cementing and vein-filling mineral, was dissolved at pH 7.8 and 22°C in Na-Ca-HCO3-Cl solutions with low concentrations of three organic ligands. Low citrate concentrations (50 μm) increased the dissolution rate consistent with mechanism (1). Oxalate decreased the rate, consistent with mechanism (3). Low phthalate concentration (< 50 μM) decreased calcite dissolution rates; however, higher concentrations increased the dissolution rates, which became faster than in inorganic solutions. Thus, phthalate exhibits both mechanisms (2) and (3) at different concentrations. In such cases linear extrapolations of dissolution rates from high organic ligand concentrations may not be valid.

Natural analog, experimental, and thermodynamic studies indicate that the properties of secondary uranyl minerals are likely to control the source term for U and other radioéléments incorporated in these phases in the proposed Yucca Mountain repository. Thermodynamic calculations using data from the 1992 NEA data base indicate an increase in the equilibrium constant for schoepite dissolution from 103.1 to 104.8 with decreasing temperature from 100° to 25°C, i.e., retrograde solubility. Enthalpies for mineral transformation reactions that consume protons and release cations are typically negative, suggesting that solubilities of other uranyl phases such as uranophane increase more than that of schoepite with decreasing temperature. Solubilities of mineral phases associated with spent nuclear fuel will be initially relatively low under the elevated repository temperature regime. As the temperature of the repository decreases due to radioactive decay and heat dissipation, source term mineral solubilities increase. The rate of release of U and other species is controlled by a series of processes: transport of oxidants and flux of water; oxidative dissolution of spent fuel; uranyl mineral precipitation; uranyl mineral dissolution or transformation; and radionuclide transport. Decreasing diffusion and reaction rates and increasing uranyl mineral solubilities with decreasing temperature may lead to a change with time from solubility to transport or reaction rate as a source term controlling mechanism. Preservation of large quantities of uranyl minerals formed by oxidation of uraninite and radiometrie ages of secondary uranophane at the natural analog site at Peña Blanca indicate that oxidative alteration of uraninite was fast relative to transport of U away from the deposit. The successive formation of schoepite and uranophane in natural settings where uraninite has been oxidized may represent a paragenesis reflecting increasing temperature or increasing incorporation of environmental components. In contrast, diminishing temperature conditions in a repository source area could lead to the reverse sequence of mineral formation.

The influence of alkaline degradation or radiolytic degradation of asphalt on plutonium solubility has been investigated. Asphalt has been contacted with water, sodium hydroxide solution or concrete leachate at 80°C for periods of up to approximately 2 years. Sodium nitrate was also present in some of the experiments. Plutonium solubilities were measured at pH 12 in the leachates and found to be less than 10-8 mol dm-3 for most degradations. Relatively low levels of Total Organic Carbon were measured in the leachates. Alpha radiolysis of asphalt in the presence of concrete and water has also been studied. Samples of asphalt were encapsulated in concrete after coating with the 238PuO2, crushed and leached at room temperature. The solubility of plutonium was measured in samples of the leachates after approximately 90 days and 180 days had elapsed. The results showed that the solubility of plutonium in the α-radiolysis leachates remained low and was in the range 2 × 10-11 to 8 × 10-9 mol dm-3. A consideration of these results, and data published elsewhere, suggests that chemical and radiolytic attack on asphalt or bitumen under anaerobic, alkaline conditions typical of a deep cementitious repository is unlikely to generate complexants for plutonium which are effective at high pH. Any enhancement of plutonium solubility is likely to be less significant than that arising from the degradation of some other organic materials.

The chemical behavior of actinide elements in tank solutions, in soil, and in groundwater is dependent upon the chemical species that form when aqueous solutions come in contact with the actinide compounds. In particular the chemical speciation of the reduced actinide oxidation states (III and IV) are important, for example, to DOE waste tank processing and, more generally, to nuclear waste disposal issues. Predicting the solubility of the actinides in these solutions requires identification of the strong aqueous complexes, such as carbonates and organic chelating agents, that can form in aqueous solution.

Previous speciation work has often relied on indirect techniques such as potentiometric titrations or solubility measurements. Recent XAS experiments determine directly the speciation of the Th carbonato species of seven solutions under a range of carbonate concentrations and pH conditions. The presence of the pentacarbonato complex is confirmed and the complex's stability at low carbonate concentrations is determined. These experimental results support a proposed thermodynamic model that describes the solubility of Th(IV) hydrous oxide in the aqueous Na+-HCO3--CO32--OH--ClO4--H2O system extending to high concentrations at 25°C. This model is relatively simple in that only two aqueous species are included Th(OH)3CO3- and Th(CO3)56-.

The use of computer simulations in the performance assessment for a repository for spent nuclear fuel, are in many cases the only method to get information on how the rock-repository system will work. One important factor is the solubility of the elements released if the repository is breached. This solubility may be determined experimentally or simulated. Ifit is simulated, several factors such as thermodynamical uncertainties will affect the reliability of the results. If these uncertainties are assumed to be small, the composition of the water used in the calculations may play a major part in the uncertainties in solubility. The water composition, in tum, is either determined experimentally or calculated through water-rock interactions. Thus, if the mineral composition of the rock is known, it is possible to foresee the water composition. However, in most cases a determination of the rock composition is made from drilling cores and is thus quite uncertain. Therefore, if solubility calculations are to be based on water properties calculated from rock-water interactions another uncertainty is introduced. This paper is focused on uncertainty and sensitivity analysis of rock-water interaction simulations and the uncertainties thus obtained are propagated through a program making uncertainty and sensitivity analysis of the solubility calculations. In both cases the latin hypercube sampling technique have been used. The results show that the solubilities are in most cases log normal distributed while the different elements in the simulated groundwater in some cases diverge significantly from such a distribution. The numerical results are comforting in that the uncertainty intervals of the solubilities are rather small, i.e. up to 30%.

Environmental investigations show transuranic ions sorb to humic substances. The resulting species are often mobile and are expected to be important vectors in the migration of transuranic ions in natural systems. However, theses environmental studies yield no quantitative data useful for modeling. Laboratory complexation experiments with transuranic ions and humic substances generate mermodynamic data required for complexation modeling. The data presented in this work are based on the metal ion charge neutralization model, which is briefly described. When a consistent complexation model is used, similar results are obtained from different experimental conditions, techniques, and laboratories. Trivalent transuranic ions (CM(III), AM(III)) have been extensively studied with respect to pH, ionic strength, origin of humic acid, and mixed species formation. The complexation of Np(V) has been examined over a large pH and metal ion concentration range with different humic acids. Some data does exist on the complexation of plutonium with humic acid, however further work is needed. Calculations on the Gorleben aquifer system using the thermodynamic data are presented. Critical information lacking from the mermodynamic database is identified.

The solubility of Sn(IV) oxide was determined in a dilute NaClO4 solution with pH 2 through 12 at ambient temperature. Both oversaturation and undersaturation experiments were carried out in an inert gas glovebox where the concentration of the oxygen and carbon dioxide were less than 1 ppm. The solubility of Sn(IV) oxide was 3×10-8 mol/1 at neutral pH, and increased at pH >7.5. Equilibrium constants of soluble reactions were calculated from the experimental data, using curve fitting method.

The study suggests that the solubility of Sn(IV) oxide would be higher than that provisionally used in current safety assessments of HLW disposal sites.

One of the most complex problems concerning nuclear waste management and the restoration of plutonium production sites is the treatment and disposition of mixed and TRU wastes. Hydrothermal oxidation, which has been shown to be effective in oxidizing a wide variety of organic material to CO2, water, salts and other nonhazardous oxides, is a promising new technology for the treatment and volume reduction of actinide-containing waste. Information on the speciation and solubility of plutonium under process effluent conditions will facilitate the development of separation techniques for removing it from the treated solutions. Such a strongly oxidizing environment will generate plutonium(VT); and upon the destruction of organics, hydrothermal reactor solutions will contain carbonate. We are investigating the solubility and speciation of the plutonium(VI) carbonate system as a function of ionic strength (0.1 to 5.0 M). Formation constants for the tris- and biscarbonato complexes of plutonium(IV) were determined to be, log β130 = 17.7 and log β120 = 13.6, respectively, by spectrophotometry. These formation constants indicate that PuO2CO3(aq) is the plutonium (VI) carbonate solution species with the largest relevant stability range. We prepared and characterized the corresponding solid using XRD, EXAFS, and diffuse reflectance, and initiated solubility experiments in 0.1, 0.2, 0.5, 1,2, and 5 M NaCl at 22±1°C under 100% CO2. Data collected thus far yield the solubility products, log Ksp mol2/kg2 = -12.9 (0.1 m NaCl), -12.4 (0.2 m NaCl), -12.5 (0.5 m NaCl), -12.3 (1 m NaCl), -12.2 (2 m NaCl), -12.3 (5 m NaCl).

Radionuclide (RN) adsorption has long been recognized as important to assure the isolation of nuclear wastes in a geological repository [1]. Laboratory measured RN adsorption data have generally been expressed as distribution coefficient (Kd) values or adsorption isotherms. The proper application of these models is to site conditions nearly identical to those used in the laboratory adsorption experiments. This has required that multiple Kd's and isotherms be determined in a wide range of experiments designed to bracket expected repository conditions.

The surface complexation (SC) adsorption models were introduced in the late 1970's. The best known of these models incorporate electrical double layer (EDL) theory [2]. Their use requires that the water chemistry and surface properties of adsorbing rocks and minerals be fully characterized. Adsorption is then studied as reactions involving specific aqueous RN species (often complexes) and specific surface sites. Because the SC models are relatively mechanistic, they may allow extrapolation of adsorption results to repository conditions that lie outside the limited experimental range used to parameterize a given model. Turner [3] has shown that the diffuse layer model (the simplest SC model) fits a wide range of RN adsorption data as well as the more complex models. Others have suggested ways to generalize and estimate SC model parameters for a variety of minerals, rocks and engineered materials (cf. [4,5,6,7,8,9,10,11,12]. Degueldre and Werlni [12] and Degueldre et al. [13] have proposed a simplified SC model for RN adsorption that avoids EDL theory, in which the adsorption of RN species is estimated from linear free energy relationships.

It is appropriate to ask how accurately RN adsorption behavior must be known or understood for total system performance analysis (TSPA). In most geological settings now being considered for repository development globally, it may suffice to select bounding Kd values for the different rock types (cf. [14,15]). Use of the SC models to describe RN adsorption can provide us with increased confidence that minimum Kd's and the distribution of Kd values we might propose for TSPA are in fact conservative.

The sorption behavior of europium onto two iron-oxides, goethite and magnetite, focused on the effect of carbonate species concentration was investigated in 0.01 M NaClO4 solution by the batch method over a pH range of 4 to 11. It was found that sorption of europium was enhanced in the higher concentration solution of carbonate species for both materials. The zeta-potential of goethite in the solutions including sodium bicarbonate was also measured to estimate the effect of sorbed carbonate species. Our experimental results and model calculation suggest that the production of surface carboxylic group FeOCOO by the sorption of carbonate species lowered the zeta-potential and enhanced europium sorption.

The sorption of sodium (22Na), calcium (45Ca) and strontium (85Sr) was studied on mica gneiss, unaltered, moderately altered and strongly altered tonalite samples taken from hole SY-KR7 drilled in the Syyry area in Sievi, Western Finland. The crushed rock samples were sieved into six fractions from 71 μm to 1250 μm. A proportional mineral composition for the different fractions were estimated by X-ray diffraction. The specific fraction surface areas were determined by the BET nitrogen adsorption method. The fractal method was applied to characterize rocks and to describe quantitatively surface irregularity. The mass distribution ratio values for each fraction were determined using the static batch method. The sorption of tracers onto different minerals was observed using rock thin sections. Kd-values calculated from thin section Ka-values and Kd revalues obtained from batch experiments were in good agreement. Mass distribution ratios for different size fractions are given, and the effect of the specific surface area is discussed. Owing to larger specific surface areas considerably higher sorption on smaller fractions was found for altered tonalites.

The mechanism for the adsorption of uranyl onto alumina from aqueous solution was studied experimentally and the data were modeled using a triple layer surface complexation model. The experiments were carried out at low uranium concentrations (9×10-11 - 5×10-8M) in a CO2 free environment at varying electrolyte concentrations (0.01 – 1 M) and pH (4.5 – 12). The first and second acid dissociation constants, pKal and pKa2, of the alumina surface were determined from potentiometric titrations to be 7.2 ± 0.6 and 11.2 ± 0.4, respectively. The adsorption of uranium was found to be independent of the electrolyte concentration. We therefore conclude that the uranium binds as an inner sphere complex. The results were modeled using the code FITEQL. Two reactions of uranium with the surface were needed to fit the data, one forming a uranyl complex with a single surface hydroxyl and the other forming a bridged or bidentate complex reacting with two surface hydroxyls of the alumina. There was no evidence from these experiments of site heterogeneity. The constants used for the reactions were based in part on predictions made utilizing the Hard Soft Acid Base, HSAB, theory, relating the surface complexation constants to the hydrolysis of the sorbing metal ion and the acid dissociation constants of the mineral oxide surface.

A study of the sorption of the radioelements: technetium; uranium; neptunium; and curium onto geological materials has been carried out as part of the PNC programme to increase confidence in the performance assessment for a high-level radioactive waste repository in Japan. Batch sorption experiments have been performed in order to study the sorption of the radioelements onto bentonite, tuff and granodiorite from equilibrated de-ionised water under strongly-reducing conditions at both room temperature and at 60°C.

Mathematical modelling using the geochemical speciation program HARPHRQ in conjunction with the HATCHES database has been undertaken in order to interpret the experimental results.