Rock samples in the secondary ore deposit at Koongarra, Australia, were examined mineralogically to clarify the formation mechanism of saléeite (Mg(UO2)2(PO4)2.10H2O), a major secondary uranium mineral in the secondary ore deposit. Sklodowskite (MgSi2U2O11.7H2O) veinlets, present upstream of the ore deposit, are partially replaced by saléeite. Most grains of apatite (Ca5(P04)3F), an accessory mineral of the host rock, are also replaced by saléeite. Thermodynamic calculations by EQ3NR showed that the present Koongarra ground waters are undersaturated with respect to saléeite and also suggested that saléeite can be precipitated under the condition of higher U or P concentrations. Such conditions can be created at the reaction interfaces of dissolving sklodowskite, which releases U, or dissolving apatite, which releases P. The present study indicates that saléeite is formed by local microscale saturation upstream of the secondary ore deposit, which is different from the formation mechanism of saléeite downstream of the ore deposit, where saléeite microcrystals of 1 – 20 nm in size form by catalysis on iron minerals, the weathering products of the host rock.

A review of the natural analog study at the Akrotiri archaeological site is provided with regard to its use in support of long-term predictive modeling of chemical transport. Evidence for a plume of contaminants was collected using samples taken from the area under the location where artifacts were buried in silicic tuff for 3600 years. A transport model was developed using site characterization data, but no data for the plume. Qualitative data from the field support the model result that flux of metals from the artifacts was small. However, transient transport characteristics and the role of notable system heterogeneities and complexities were not fully represented by the model. The Akrotiri natural analog study indicates that long term releases and transport may be limited in an unsaturated repository site, but releases may be strongly affected by unknown processes or processes that are neglected in simplified models.

Uranium-series activity ratios for U(VI) minerals from the Shinkolobwe mine in southern Zaire indicate that these minerals have not experienced significant preferential loss of uranium since their formation more than 100,000 years ago. The minerals examined include rutherfordine, UO2CO3, schoepite, [(UO2)8O2(OH)12]·12H2O, becquerelite, Ca[(UO2)6O4(OH)6]·8H2O, and uranophane, Ca[(UO2)2(SiO3OH)2]·5H2O. No correlation between mineral species and mineral age was evident. The oxidative dissolution of primary uraninite (UO2+x) has maintained ground waters supersaturated with respect to all of the secondary U(VI) minerals, providing an inexhaustible source of dissolved U6+ for mineral formation and growth. As long as uraninite persists in an oxidizing environment, the assemblage of secondary U(VI) phases is determined by local ground water chemistry (including transitory changes), but not necessarily a unidirectional reaction path towards equilibrium with U(VI) minerals of lower solubility. Thus the Shinkolobwe mine displays a complex assemblage of U(VI) minerals that reflects variations in the availability of dissolved elements besides U. Similarly, for a geologic repository exposed to oxidizing waters, the assemblage of corrosion products that will form during the corrosion of spent UO2 fuel is likely to be as complex as mineral assemblages found in natural uranium deposits under similar conditions.

Stoichiometric and non-stoichiometric (defect) pyrochlores crystallize during the magmatic and late magmatic-hydrothermal phases of carbonatite emplacement (T > 450–550 °C, P < 2 kb). Defect pyrochlores can also form at low temperatures in laterite horizons during weathering. After crystallization, pyrochlore is subject to alteration by hydrothermal fluids (T ∼ 550-200°C) and ground water. Alteration occurs primarily by ion exchange of low valence A-site cations together with O, F, and OH ions. The high valence cations Th and U are generally immobile; however, we have documented one example of hydrothermal alteration involving loss of U together with cation exchange at the B-site in samples from Mountain Pass, California. During laterite accumulation, the cation exchange rate of pyrochlore greatly exceeds the rate of matrix dissolution. The exceptional durability of pyrochlore in natural environments is related to the stability of the B-site framework cations. In carbonatites, defect pyrochlores may contain significant amounts of Si (up to 7.6 wt% SiO2) which is negatively correlated with Nb.

A drill core sample penetrating at a natural fracture has been analysed by helium gas techniques and by α-autoradiography. Porosity and diffusivity profiles away from the fracture have been determined and compared with structural profiles. Model calculations are used to determine the effects of sample size on the measured porosities and diffusivities.

The distribution patterns of trace elements in magnetically separated fractions of a granite fracture filling material have been studied using instrumental neutron activation analysis. The distribution patterns of trace elements with similar ionic radii and the same charge, Fe-Co, Rb-Cs, Hf-Zr, U-Th, and light REE, were found to be similar. Whereas most of the elements are found in higher concentrations in the more clay rich fractions, Rb and Cs display a more homogeneous distribution.

A boulder sample was studied for its unusual U content. Analyses of U-series nuclides within the rock matrix perpendicular to an assumed fracture face show abrupt pulse-like concentration distributions with very low concentrations of U daughters. Both Th-230/U-234 and Pa-231/U-235 activity ratios are low, indicating recent U accumulation into the rock. Matrix diffusion is tested as a possible cause for the experimental observations. We assume that the diffusion process was triggered and controlled by rock expansion, strong mixing of different water types and rapid land uplift at the end phase of the last glaciation.

A source release model was developed to quantify time dependent liquid phase releases of radioactive material to the vadose zone from a disposal site. The model has been implemented to evaluate the source terms in the Performance Assessment for the Los Alamos National Laboratory (LANL) Low Level Radioactive Waste (LLRW) Disposal Facility at Area G, an analysis of the long term post-closure impact of disposal operations required by the USDOE orders.

Analytic solutions describe transport through the model compartments: the solid phase waste package, the liquid phase within each waste package and the liquid phase within the disposal unit. The model accounts for elemental solubility limits and retardation coefficients (R) separately in the waste package and in the disposal unit. Several parameters define the site specific aspects of the disposal unit. In our case for example, the disposal unit is waste buried with crushed volcanic tuff, and a small net infiltration rate is determined from independent numerical studies. The analytic solution allows efficient explorations of the sensitivity to the input parameters. Numerical solutions extend the model to account for decay product ingrowth which may have different transport properties than the parent nuclides.

Results quantify the decrease of source release rates with package or disposal unit Kd, and the effects of solubility limits. At the LANL LLRW disposal site, thorium and uranium are the only significant contributors -to dose which are solubility limited. Otherwise, a ‘rapid rinse’ waste package category dominates the peak disposal unit efflux. This occurs for the LANL disposal site at about 100 years for nuclides with Kd= 0 (R= 1) and for proportionately longer times as Kd or R increases. Additional simple analytic models or detailed numerical codes can be coupled to these results to predict groundwater concentrations.

Recently, we reported a value of ΔH°(TcO2(cr)) = -(458 ± 6) kJ·mol-1based on heat of solution measurements. The implications of this value on the database used in the Canadian Nuclear Fuel Waste Management Program for the evaluation of the technetium released by congruent dissolution of used UO2 fuel have now been assessed.

It is probable that the Tc(IV) oxides are more stable than previously predicted and, hence, they are less likely to be oxidized to TcO4(aq) under moderately reducing conditions. We have revised earlier calculations done to predict the solution concentrations of technetium species in a vault as a function of the oxidation conditions in model groundwaters.

Palladium solubility was measured in a dilute aqueous solution at room temperature in the pH range from 3 to 13 under anaerobic conditions. Crystalline Pd metal was clearly visible and the concentration of palladium in solution decreased gradually with aging time. The palladium concentrations in solution were less than 9.4×10-10M in the pH range from 4 to 10 and increased to 10-7M in the pH range greater than 10. This study suggests that palladium concentrations in certain high-level waste repository environments may be limited by Pd metal and may be less than 10-9M.

Computer simulations are commonly used in many areas to obtain information about a system. It is, however, important to remember that the indata to these models are encumbered with errors of varying magnitude. To investigate the effect of these errors, a sensitivityuncertainty (s/u) analysis is often made.

This paper focuses on how the solubility of a solid phase is affected by uncertainties in the enthalpy of reaction for the species present in the solvent. Usually, thermodynamic data are given for a temperature of 25°C, but calculations are not made at this temperature for many practical applications. It is interesting to know which enthalpies are important and how much given uncertainties in these enthalpies affect the calculated solubility of a solid phase.

The solubility calculations were performed by the thermodynamic equilibrium program PHREEQE and the statistical evaluations with a modified version of the SENVAR program.

The solubility of e.g. UO3-2H2O at 80°C was investigated. The conclusion was that the solubility ranged from lE-05 to 4.5E-06 when the enthalpies of reaction for the interesting species were changed within a four kcal/mole wide interval. The maximum solubility, based on an arbitrary distribution, was found to be no greater than 9E-05 with 95% confidence.

To develop capabilities for compliance determination, the Nuclear Regulatory Commission (NRC) conducts total system performance assessment (TSPA) for the proposed repository at Yucca Mountain (YM) in an iterative manner. Because the new Environmental Protection Agency (EPA) standard for YM may set a dose or risk limit, an auxiliary study was conducted to develop estimates of site-specific dose assessment parameters for future TSPAs. YM site-relevant data was obtained for irrigation, agriculture, resuspension, crop interception, and soil. A Monte Carlo based importance analysis was used to identify predominant parameters for the groundwater pathway. In this analysis, the GENII-S code generated individual annual total effective dose equivalents (TEDEs) for 20 nuclides and 43 sampled parameters based upon unit groundwater concentrations. Scatter plots and correlation results indicate the crop interception fraction, food transfer factors, consumption rates, and irrigation rate are correlated with TEDEs for specific nuclides. Influential parameter groups correspond to expected pathway behavior of specific nuclides. Results for nuclides that transfer more readily to plants, such as 99Tc, indicate crop ingestion pathway parameters are most highly correlated with the TEDE, and those that transfer to milk (59Ni) or beef (79Se, 129I, 135Cs, 137Cs) show predominant correlations with animal product ingestion pathway parameters. Such relationships provide useful insight to important parameters and exposure pathways applicable to doses from specific nuclides.

The proposed geologic repository at Yucca Mountain, Nevada, for spent nuclear fuel and other highly radioactive waste needs an official standard to protect the public from release of radioactivity. Standards proposed by the U.S. Congress, the nuclear industry (NEI), the Electric Power Research Institute (EPRI), the Department of Energy (DOE), and the TYMS Committee of the National Research Council (NRC) are reviewed. Each of these proposals would introduce a degree of leniency not heretofore experienced in radiation protection. No adequate scientific justification is presented. Some scientifically invalid proposals are said to be justified on the grounds of policy. Most leading industrial nations are designing geologic repositories to meet the traditional criteria for safety and for protecting public health, including quantitative calculations of doses for the periods when significant doses can occur, rather than stopping calculations at 10,000 years as many in the U.S. would have us do. There is no evidence that they are seriously seeking more lenient standards for public health protection. The U.S. has the resources and skills to protect future people from our waste with the same care that we now protect the public from radiation. We should assert our will to do so. The Yucca Mountain project will be seriously damaged if it is directed to depart from the traditional conservative criteria for determining safety and protection of public health, criteria that are adopted in other countries working on geologic disposal.

On March 31, 1995, tlhe IT. S. Department of Energy filed a draft application for certification of compliance with I the U. S. Environmental Protection Agency (USEPA) to show the Waste Isolation Pilot Plant's compliance with the USEPA's environmental standards, making the application the first of its kind. Demonstration of compliance is by performance assessment. This paper is an early review of the performance assessment in the draft application, by the Environmental Evaluation Group, an oversight group. The performance assessment in the draft application is incomplete. Not all relevant scenarios have been analyzed. The calculation of potential consequences often does not use experimental data but rather estimates by workers developing additional data. The final compliance application, scheduled for October 1996. needs to consider additional scenarios, and be fully based on experimental data.

Uncertainties in nuclide release calculations, based on the uncertainty in illitization rate for smectite backfill have been studied. This paper has three new features: (i) an analytical solution with temporal parameter changes, (ii) uncertainty study with respect to the rate of temporal changes, and (iii) using various data of illitization.

We show the following in this paper: (1) The uncertainty in nuclide releases is enhanced greatly in the case where the illitization period coincides with the transition time from solubility-limit source to exponentially decaying source; (2) The uncertainty in the retardation factor may have been overestimated in previous static analyses, leading to conclusions that the illitization problem is more vexing than it really is.