Compacted bentonitic clay/sand mixtures are being considered for use as buffer materials in the Canadian concept for nuclear fuel waste disposal. This paper describes a laboratory study of the swelling pressures that develop in statically compacted, air-dry specimens of mixtures of sodium bentonite and silica sand as they are saturated with double-distilled, deionized water. The results are interpreted with the aid of scanning electron microscope observations of the soils' structures.

It is shown that the sand acts as an inert filler material, and swelling pressures are controlled by a parameter termed the effective clay dry density, γC, defined as the ratio of the mass of clay to the combined volume of the claq plus voids in the mixture. A threshold value of γC exists below which swelling pressures can be expected to be isotropic. Above the threshold value of γC, pressures parallel to the axis of compaction can be expected to be greatgr than those perpendicular to it. This is related to a change in soil fabric as γC is increased above the threshold value. For the Canadian disposal concept, γC would probably be below the limiting value and swelling pressures of 2.5 MPS or less are expected. The swelling pressures are likely to be isotropic within a saturated buffer mass.

This paper examines some previously reported experimental results on the migration behavior of selected radionuclides in compacted bentonite and demonstrates that a much better agreement between observed and theoretically predicted migration behavior can be obtained when the model considers nonlinear concentration effects. The radionuclide transport model used in this study employs a semianalytic, integral equation approach to the solution of the diffusion equation with nonlinear retardation mechanisms. The effect of radionuclide concentration on species mobility is modeled as a highly nonlinear Freundlich isotherm.

Several empirical correlations using the Freundlich isotherm form for retardation are presented and are shown to account for the deviation from linearity in the diffusion model for migration behavior. These correlations are then used in further numerical studies of long term backfill barrier performance for 3 and 4 member chain species after a hypothetical canister failure. The overall importance of nonlinear concentration effects to radionuclide migration estimation is discussed.

In support of the Nevada Nuclear Waste Storage Investigations Project experiments were conducted to study the hydrothermal interaction of rock and water representative of a potential repository in tuff. These experiments provided data relevant to near-field repository conditions that can be used to: assess the ability to use “accelerated” tests based on the SA/V (surface area/volume) parameter and temperature; allow the measurement of chemical changes in phases present in the tuff before reaction as well as the identification and chemical analysis of secondary phases resulting from hydrothermal reactions; and demonstrate the usefulness of geochemical modeling in a repository environment using the EQ3/6 thermodynamic/kinetic geochemical modeling code. Crushed tuff and polished wafers of tuff were reacted with a natural ground water in Dickson-type gold-cell rocking autoclaves which were periodically sampled under insitu conditions. Results were compared with predictions based on the EQ3/6 geochemical modeling code. Eight short-term experiments (2 to 3 months) at 150°C and 250°C have been completed using tuff from both drillcore and outcrop. Long-term experiments at 90°C and 150°C using drillcore polished wafers are in progress. This paper will focus on the results of the 150°C and 250°C experiments using drillcore polished wafers.

Welded tuff beneath Yucca Mountain adjacent to the Nevada Test Site (NTS) is being considered for development as a high-level radioactive waste repository by the Nevada Nuclear Waste Storage Investigations (NNWSI) Project. Because access into Yucca Mountain has been limited to borehole explorations, early geoengineering materials characterizations have been derived From laboratory tests on cores from Yucca Mountain and from laboratory and field tests on welded tuffs located in G-Tunnel on the NTS. G-Tunnel contains welded tuffs that have similar properties and stress states to those at Yucca Mountain and has been the location for in situ rock mechanics testing. The purpose of this paper is to summarize the geoengineering material property data obtained to date and to compare appropriate laboratory and field data from G-Tunnel to findings from Yucca Mountain.

Geomechanical and thermal data are provided and are augmented by limited geological and hydrological data. A comparison of results of laboratory measurements on tuffs from Yucca Mountain and G-Tunnel indicates good agreement between the bulk densities, saturations, moduli of elasticity, Poisson's ratios, and P-wave velocities. The G-Tunnel tuff has slightly lower thermal conductivity, tensile strength, compressive strength and slightly higher matrix permeability than does the welded tuff near the proposed repository horizon at Yucca Mountain. From a laboratory-to-field scaling perspective, the modulus of deformation shows the most sensitivity to field conditions because of the presence of joints found in the field.

The behavior of a group of nuclear waste elements (U, Th, Sr, Zr, Sb, Cs, Ba, and Sm.d)u ring hydrothermal alteration of glassy rhyolite is investigated through geochemaical analyses of whole rocks, glass and mineral separates, and thermal waters. Significant enrichments of U, Sr, Sb, Cs, and Ba are found in altered rock relative to unaltered rock. Excess Sr, Cs, and Ba are concentrated in zeolites in altered rock. Excess U is associated with titanomagnetite surfaces. Th, Zr, and Sm are relatively immobile during alteration, and are strongly concentrated in celadonite.

The Salton Sea Geothermal Field (SSGF), on the delta of the Colorado River in southern California, is being studied as a natural analog for the near-field environment of proposed nuclear waste repositories in salt. A combination of mineralogical and geochemical methods is being employed to develop a three dimensional picture of temperature, salinity, lithology, mineralogy, and chemistry of reactions between the reservoir rocks and the hot brines. Our aim is to obtain quantitative data on mineral stabilities and on mobilities of the naturally occurring radionuclides of concern in Commercial High-Level Waste (CHLW). These data will be used to validate the EQ3/6 geochemical code under development to model the salt near-field repository behavior.

Maximum temperatures encountered in wells in the SSGF equal or exceed peak temperatures expected in a salt repository. Brines produced from these wells have major element chemistry similar to brines from candidate salt sites. Relative to the rocks, these brines are enriched in Na, Mn, Zn, Sr, Ra and Po, depleted in Ba, Si, Mg, Ti, and Al, and strongly depleted in U and Th. However the unaltered rocks contain only about 2–3 ppm of U and 4–12 ppm of Th, largely in detrital epidotes and zircons. Samples of hydrothermally altered rocks from a wide range of temperature and salinity show rather similar uniform low concentrations of these elements, even when authigenic illite, chlorite, epidote and feldspar are present. These observations suggest that U and Th are relatively immobile in these hot brines. However Ra, Po, Cs and Sr are relatively mobile. Work is continuing to document naturally occurring radionuclide partitioning between SSGF minerals and brine over a range of temperature, salinity, and lithology.

The behavior of 90-Sr and 137-Cs in the near field of a HLW package can be investigated by use of Rb-Sr geochronologic information from igneous contact zones. The chemical behavior of 90-Sr is identical to other Sr isotopes, and Cs behavior is very similar to that of Rb. During igneous intrusion, the effect of heat from the igneous rock, coupled with fluid action, is to perturb Rb-Sr systematics of minerals in the intruded rock. Rb-Sr redistribution diagrams are useful not only for indirectly dating the time of intrusion, but also for determining the extent of closed versus open system conditions in the contact zones. Once open or closed system behavior is determined, the Rb-Sr systematics can be evaluated in terms of diffusion, fluid-induced metasomatism, selective dissolution, other, and coupled processes. All these can be discussed in terms of time-temperaturefluid compositional parameters. Rb migration is controlled predominantly by fluid flow, while diffusion models better explain some Sr isotopic behavior Cs, from geochemical arguments, should be less mobile than Rb. In most cases, even when temperature of the intrusive is 800–1200°C the cooling period on the order of 104 – 105 years, and in a highly convective system, Sr and Rb mobility is on the order of only microns to meters.

The long-term stability of nuclear waste form borosilicate glasses can be evaluated by understanding the processes that effect the long-term alteration of glass and by comparing laboratory alteration of synthetic basalt and borosilicate glasses with the observed stability of naturally occurring basaltic glasses in diverse geologic environments. This paper presents detailed electron microprobe analyses of naturally altered basaltic glasses (with maximum ages of 10,000 to 20 million years) from low-temperature environments. These results are compared to laboratory data on the corrosion of a synthetic basaltic glass in MCC-1 tests (90°C, a SA/V of 0.1 cm−1 and time periods up to 182 days), MCC-2 tests (190°C, a SA/V of 0.1 cm−1 and time periods up to 210 days) and hydration tests in saturated water vapor (240°C, an estimated SA/V of ∼ 106 cm−1 and time periods up to 63 days). Additionally, laboratory induced hydration alteration of synthetic basalt and borosilicate glasses is compared. These preliminary experiments provide evidence that the alteration processes observed for natural basalt glasses are relevant to understanding the alteration of nuclear waste glass, as both appear to react via similar processes.

Data are presented on the role of naturally occurring and synthetic organic compounds in the mobilization and bicassimilation of Th-232. The study is taking place in the state of Minas Gerais, Brazil, in and around an exposod and deeply weathered are body (the Morro do Ferro) estimated to contain 30,000 tonnes of Th and > 100,000 tonnes of the light rare earth elements. Indigenous plants collected from the are body surface are shown to contain high concentrations in the range of 20–1,000 μg/g ash of Th-232. Prcliminary estimates of Th-232 concentration factors range between 10−5 and 10−3 and are comparable with those reported for plutonium. Leaching studies using different chelating agents indicate that the release of Th-232 from. weathered soil and are as well as plant uptake are directly related to complex stability. Preliminary results of speciation studies using ultrafiltration techniques indicate that for water which has percolated through the are body, most of the dissolved Th-232 is associated with colloidal humic acids of molecular weight > 300,000.

Two types of simulated nuclear waste glasses were leached and simultaneously gamma-irradiated. The leachate pH was reduced through irradiation, which enhanced glass leaching at room temperature. In contrast, the pH was maintained almost neutral in the experiment at 90°C and the leach rates were about five times lower than those for the glass leached at 90°C but out of the radiation field. Most of the leach rates correlated with the final pH of the leachate. Ground water, granite and bentonite moderates the effect of gammairradiation, probably because of a buffering effect. Simultaneous weathering and irradiation could greatly change the surface morphology of the glass.

The growth of stresses in irradiated glasses is modeled, and the resulting effects on chemical durability are evaluated. It is shown that in actinide-doped glasses, maximum stresses are small when compared to those that act on a plane parallel to the ion tracks of an implanted glass, at equivalent fluences. The theory also offers an explanation for the apparent change in the dissolution enhancement factor with fluence in ion-implantation experiments.

Cantilever beam measurements of induced lateral stress have been made for He, Xe and Pb ion-implantation into the complex borosilicate radioactive waste glasses (PNL 76–68, SRP), a commercial borosilicate (Pyrex), and into fused Si02 (Suprasil 1). For the borosilicate glasses, implantation caused a buildup of stress to a saturation value (∼ 1 × 105 dynes/cm). The maximum occurred at ion fluences greater than those found for maximum stress in fused silica and without a stress relief mechanism. The separate family of stress curves for Pyrex and PNL 76–68 coincide when stress is plotted as a function of energy into electronic processes. Pyrex has 13 wgt.% B2O3 and 81 wgt.% SiO2, while PNL 76–68 and SRP have 10% B2O3, 40% SiO2 and N5 B2O3 and 58% Si0 2, respectively. Pyrex is known to be phase-separated, and Ghe waste glasses are susceptible to phase separation. It is suggested that the B2O3 phase may be more easily compacted than the SiO2 phase. The evidently greater effectiveness of the ionization component of the ion energy in creating damage in borosilicate glasses should be of considerable interest in waste glass studies because of the large amounts of ionization from α-decay (∼ 5 MeV He).

Gamma radiolysis of Permian Basin brine leads to equilibrium gas pressure of about 100 atm. at 75°C and about 40 atm. at 150°C, providing the gas space is very small and/or the total dose is very high. Dose rate dependence is being investigated but is not yet established. Alpha radiolysis of Permian Basin brine is still being evaluated, but it is clear that equilibrium gas pressures will be much higher than with gamma radiolysis. In addition, alpha radiolysis of brine results in a very high solution redox potential. Gas compositions in all cases have been about two parts H2 to one part O2. Efforts to simulate these results with computer models have been quite successful.

A generalized groundwater radiolysis model which is under development at Battelle-Columbus is described. This model uses a 55 reaction basis set for the radiolysis of the solvent. The basis set of reactions was chosen from the literature after comparing several descriptions against experimental data. This basis set has been augmented with 17 additional reactions to allow the description of the radiolysis of groundwater containing iron. The development of this mechanism is described, and comparisons with experimental data are shown.

The pyrochlore structure type, A1−2 B2 O6Y0−1 Fd3m Z=8, is a common constituent of polyphase, crystalline waste forms. Naturally occurring minerals with the same structure often occur in the radiation damaged, electron-diffraction amorphous, “metamict” state; and therefore, a principal concern with phases of this structure type is the effect of alpha-recoil damage associated with the presence of actinides. In this paper we outline crystal chemical constraints on the formation of novel actinide pyrochlores.

Significant amounts of the pyrochlore structure type, and derivative structures like zirconolite, have been reported to occur in crystalline, polyphase, nuclear waste forms [1,2,3,4]. Pyrochlore has abundant and chemically diverse natural analogues. Many of these contain U and Th and have been transformed to the electron-diffraction amorphous, “metamict” state by alpha-recoil nuclei. A variety of geochemical alteration processes may occur before, during, and after reaching the final saturation level for alpha-recoil damage (∼1026 alpha-events/m3 ). The purpose of this paper is to examine these alteration effects in radiation-damaged natural pyrochlores.

The pyrochlore structure type, A1−2 B2 O6 Y0−1, is a common phase in polycrystalline waste forms. A principal concern has been the effect of alpharecoil events on the longterm stability of these phases. Naturally occurring minerals of the pyrochlore group provide an example of the full range of radiation effects from fully crystalline to xray diffraction amorphous (the metamict state). Xray absorption spectroscopy is used to investigate the coordination geometry of Tisites in metamict and annealed (1,000°C) pyrochlores. The principal effects in the fullydamaged material (∼4026 alpha events/m3) compared to annealed samples include an increase in the asymmetry of the Tisite and a slight reduction in bond length and coordination number.

238Pu-substituted zirconolite (CaPuTi 2O7 ) was stored near ambient temnerature for 231 days, equivalent to an aipha decay dose of 3.1×1025 α/m3 or 3×105 years of storage time for SYNROC ceramic nuclear waste. Periodic indentation testing showed that hardness was decreased by alpha decay-induced conversion to the metamict state, while fracture toughness and resistance to cracking were increased, apparently as a consequence of the formation of a heterogeneous microstructure. These results imply improved stability of this nuclear waste phase as a result of self-irradiation damage.

Barium hollandite and nickel-iron spinel have been irradiated with alpha particles emitted from a PuO2 source in order to simulate the effects from alpha particles emitted in adjacent actinide-containing phases of the SYNROC assemblage. The unit cell of barium hollandite undergoes an apparent volume expansion of several percent and a weak transformation from a tetragonal to a monoclinic structure when irradiated with alpha particles to a fluence of 3 × 1020 alphas/m2 (∼0.05 dpa). The spinel structure, however, is stable with respect to alpha irradiation and the irradiation-induced volume expansion of the unit cell is less than 0.2%. The results are compared with available neutron-irradiation data, and the differences in observed behavior discussed.

Three crystalline ceramic materials, which occur as host phases for the long-lived actinides in many nuclear waste formulations, were doped with Cm-244, and the effects of self-radiation damage from alpha decay on microstructure and physical properties were investigated. The irradiation-induced microstructure consisted of individual amorphous tracks from both the alpha-recoil particles and the spontaneous fission fragments. The eventual overlap of the tracks at higher doses leads to a completely amorphous state. This radiation-induced amorphization process results in measured increases in volume, leachability, and stored energy. Thermal recovery of the radiation-induced swelling and amorphization occurs with full recrystallization to the initial structures.