Geochemical Controls on Radionuclide Releases from a Nuclear

9 Geochemical Controls on Radionuclide Releases from a Nuclear Waste Repository in Basalt Estimated Solubilities for Selected Elements

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T. O. EARLY, G. K. JACOBS, and D. R. DREWES Rockwell International, Energy Systems Group, Richland, WA 99352

Two basaltflowswithin the Grande Ronde Basalt at the Hanford Site in southeastern Washington are candidates for a high-level nuclear waste repository. In order to determine the anticipated rate of release and migration of key radionuclides from the repository, solubility controls must be determined. Solubilities, solids controlling solubility, and aqueous speciation in groundwater have been determined from available thermodynamic data for a variety of actinides and fission products. Groundwater compositions used include all available analyses from the Grand Ronde formation determined by the Basalt Waste Isolation Project (BWIP). Over the range of conditions investigated, solids predicted to control solubility for the selected radionuclides include hydroxides and hydrous oxides (palladium, antimony, samarium, europium, lead, and americium), oxides (nickel, tin, thorium, neptunium, and plutonium), elements (selenium and palladium), and silicates (zirconium and uranium). Dominant soluble species include hydroxy complexes (zirconium, palladium, tin, antimony, samarium, europium, thorium, uranium, neptunium, and plutonium) and carbonate species (nickel, samarium, europium, lead, uranium, neptunium, plutonium, and americium). In addition to the limitations in completeness and accuracy of thermodynamic data, solubility estimates of the radionuclides are sensitive to the following: (1) Eh and the degree of redox equilibrium, (2) temperature, (3) formation of metastable solid phases, and (4) coprecipitation. The Eh effects have been evaluated for each radionuclide and are significant for selenium, palladium, and tin, and possibly uranium and neptunium. Solubility estimates also have been made at ambient temperature (~55 ± 5°C) for Grande Ronde

© 1984 Ameri^gtemifeâbPaf

Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

148


basalts for those nuclides for which sufficient data exist. Effects of metastability and coprecipitation cannot be treated quantitatively, but their contributions have been estimated in reference to available experimental data. The Basalt Waste Isolation Project (BWIP), under the direction of the U.S. Department of Energy, is investigating the feasibility of locating a nuclear waste repository in the deep basalts beneath the Hanford Site, Richland, Washington. The repository is designed to be constructed in one of four basalt flows (Rocky Coulee, Cohassett, McCoy Canyon, or Umtanum) in the Grande Ronde formation (Figure 1). The National Waste Terminal Storage Program has adopted the approach of utilizing a series of engineered barriers to complement the geologic site in providing for the safe isolation of nuclear waste. The engineered barriers include the waste package [e.g., waste form, canister(s), and backfill] and the repository seal system (e.g., shaft, borehole, room, and tunnel seals). Under current regulatory criteria (1-2), performance may be apportioned among the isolation components in order to demonstrate compliance with the proposed criteria and to ensure the safe isolation of the waste. Data from site characterization activities and information regarding the geochemical behavior of radionuclides may be used to determine the isolation capacity of the geologic site component for a range of expected conditions. From this information, the performance of the engineered components of the isolation system, required to complement the site for the safe isolation of nuclear waste, may be determined. From such an analysis, the required performance for the engineered components may be allocated among the waste package and repository seal systems so as to allow designs to be optimized for both performance and cost effectiveness, while at the same time providing for an adequate factor of safety in overall isolation capacity. In order to evaluate the rate and cumulative release of radionuclides from a repository in basalt to the accessible environment, a necessary first step is the identification of the important natural and engineered isolation parameters. They include the following: • • • • • •

Waste package containment time Radionuclide release rates from the waste package Site-controlled radionuclide solubilities Groundwater flow through the repository Groundwater travel time to the accessible environment Near- and far-field radionuclide sorption.


EARLY ET AL.

Geochemical Controls on Radionuclide Releases

DEPTH (m)

FORMATION NAME


HANFORD FORMATION RINGOLD FORMATION

SADDLE MOUNTAINS BASALT

WANAPUM BASALT

ROCKY COULEE COHASSETT CANDIDATE REPOSITORY HORIZONS

819 860 912 992 • GRANDE RONDE BASALT

McCOY CANYON

1 0 5 9

1099 UMTANUM 1170

Figure 1. Simplified Stratigraphy of Washington.

the

Pasco Basin,



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GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE

Of these parameters, radionuclide solubilities that are addressed in this paper represent an important control on the concentration of specific radionuclides in groundwater available for transport to the accessible environment. Radionuclide concentrations in groundwater outside the waste package will be controlled by the solid phases and aqueous species forming in the basalt geochemical environment. For a given radionuclide, one can predict via thermodynamic arguments the most stable solid phase which should form and thus control the concentration of the radionuclide in this system. Then incorporating the effect of complexing agents present in the groundwater, one may calculate the theoretical, maximum possible concentration of all dissolved radionuclide species in equilibrium with the stable solid phase. The solubilities, associated solid phases, and aqueous complexes, while thermodynamically predicted, may not actually occur in the natural basalt geochemical system. Deviations from these predictions could result from an incomplete thermodynamic data base, uncertainties in the thermodynamic data, kinetic inhibitions to the formation of the most stable phases, and/or the formation of colloidal particles. However, for the basalt site-specific case, it is useful to delineate baseline calculations from available thermodynamic data at this time, with the understanding that the limitations noted above may be very important in determining the final controls on the concentrations of radionuclides in the basalt geochemical system. Confirmation of these calculations undoubtedly requires careful laboratory tests addressiong solubility controls as well as kinetic and colloidal transport factors. Method Solubility Relationships. Estimates of the solubility of radionuclides in characteristic groundwaters for a nuclear waste repository in basalt (NWRB) were made by considering the thermodynamic data available for solid phases and aqueous species. For each radionuclide, published AG°f,298 values for individual solids and aqueous species in conjunction with log K values for appropriate chemical equilibria were used to derive a set of mathmetical expressions that relate the activity of all dissolved species in solution to the solid phase(s) calculated to be the most stable under a given set of environmental conditions (pH, Eh, groundwater chemistry). A detailed summary of the theoretical approach used can be found in standard texts dealing with chemical thermodynamics of heterogeneous geologic systems (3).

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9.

E A R L Y ET A L .



Assumptions and Limitations Thermodynamic Data. There are two key considerations in evaluating the effect of available thermodynamic data on calculated solubilities. The principal limitation is the lack of a complete thermodynamic data base containing all important solid and aqueous species pertinent to the basalt geochemical system. It is possible that thermodynamic data may not exist for solids that are more stable than those currently available, resulting in high solubility estimates. Alternatively, thermodynamic data may not be available for aqueous complexes of radionuclides that are more stable than those in the current data base, resulting in solubility estimates that are too low. Therefore, one must recognize the potential limitations of the thermodynamic data base in making solubility estimates. The reactions with associated solid and aqueous species and log K n values used in this study come from published compilations (4-6) and more recent experimental data (7-8). No attempt was made to critically evaluate the available data for accuracy, although comparison among different compilations was made as a check on typographical errors. Additionally, no attempt was made to evaluate the effect of data uncertainties on the resulting computations. These uncertainties will be addressed in future studies. e

Temperature. The estimated solubilities presented in this paper are strictly applicable only at 25°C. At elevated temperatures, thermodynamic data exist for only a few radionuclides and much of the data is estimated. This limitation should not significantly affect the applicability of present calculations to a repository in basalt because measured temperatures in the candidate horizons, the Rocky Coulee, Cohassett, McCoy Canyon, and Umtanum flows, are 49°C ± 1°C, 51°C ± 2°C, 56°C ± 1°C, and 58°C ± 2°C, respectively. From data provided in the literature (9-10), it is possible to estimate solubilities for uranium and plutonium at a temperature which closely approximates that of the candidate repository horizons. These estimates are discussed in a later section of this paper. Eh-pH Conditions. Measured pH values for Grande Ronde groundwater are found to be 9.5 ± 0.9. Hydrothermal experiments on the system basalt-groundwater (11) have shown that during the thermal period, pH could be depressed as much as 1 to 2pH units. However, since significantly elevated temperatures will not extend far into the host rock (12), this study treats only ambient conditions and is applicable to the far-field or the near-field after the thermal period.



152

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

Unlike pH, Eh in the basalt-groundwater system is difficult to determine quantitatively. Therefore, since some radionuclide solubilities are highly sensitive to Eh, results are presented for a range of Eh conditions encompassing values expected in the candidate horizons. This approach offers two advantages: (l)the sensitivity of radionuclide solubilities to Eh may be evaluated, thus defining the degree to which Eh must be known in the basalt geochemical system for establishing baseline solubilities and (2) key radionuclides and environmental conditions for experimental confirmation of calculated solubilities are defined. Measured Eh values for Grande Ronde groundwater, although having associated uncertainties, typically range from +0.35 to -0.2 V. Based upon an indirect approach utilizing mineralogical and aqueous speciation information (13), it is estimated that the Eh conditions in the basaltgroundwater system are -0.40 ± 0.05 V at Τ = 57°C ± 2°C and pH = 9.5 ± 0.05. A detailed analysis is now underway to evaluate ambient Eh conditions utilizing groundwater chemistry data, laboratory experiments at low and high temperatures, and geochemical modeling. Other Effects. In the very near-field, dissolution of the waste form may lead to local changes in pH and Eh. In addition, radiolysis effects in this region also may be important. The consequences of these factors are not addressed in this study because their effects are extremely localized and such a detailed analysis of solubility and speciation is beyond the scope of this paper. Sensitivity Analysis Two considerations: (1) kinetic limitations in aqueous sulfate/ sulfide species equilibration and (2) observed lateral and vertical differences in Grande Ronde groundwater chemistry, were evaluated for their effect on calculated solubilities and associated aqueous speciation. The presence of sulfate in Grande Ronde groundwater and the uncertainty of the actual Eh conditions require consideration of sulfate/sulfide equilibration kinetics in the calculation of solubilities and aqueous speciation of radionuclides. Under strongly reducing conditions, aqueous sulfide species can be very important complexing agents for some radionuclides, and the precipitation of very insoluble sulfides can control solubilities to very low values. To date, aqueous sulfide species have not been detected in Grande Ronde groundwaters (14). This suggests that either Eh conditions are not reducing enough to stabilize reduced aqueous sulfur species or that equilibrium has not been reached. It has been shown recently (15) that the kinetics of equilibration between aqueous sulfate and sulfide species are very slow



9. EARLY ET AL.


without the presence of a mediating component (e.g., sulfatereducing bacteria or reactive mineral surfaces) at the ambient temperature of the candidate repository horizons (~50°C to 60°C). For these reasons, it is useful to evaluate the effects of forming or not forming aqueous sulfide complexing agents and sulfide precipitates on radionuclide solubilities and aqueous speciation. Therefore, calculations were performed for key radionuclides for two cases: (1) reduction of sulfate to sulfide and bisulfide was allowed to occur as Eh was lowered and (2) sulfate reduction as a function of Eh was prohibited so that sulfate was the only sulfur complexing agent available at all Eh values. Most solubility computations were made using the results from 29 natural Grande Ronde groundwaters collected by BWIP hydrologists from five wells on the Hanford Reservation and chemically analyzed in the Basalt Materials Research Labora tory over the past 2yr. The rationale for using all available analytical data results from the fact that significant vertical and lateral variation in Grande Ronde water chemistry exists and using an average analysis probably is not justified. Sensitivity analyses for the effects of sulfate/sulfide equilibration kinetics in this paper were performed on a single groundwater composition (16) . This groundwater has been chosen by the BWIP as a standard reference Grande Ronde groundwater for use in experimental hydrothermal, sorption, and solubility studies. Consequently, its inclusion in this study will permit our conclu sions to be related to experimental studies now in progress. An analysis of the reference groundwater is listed in Table I. Results Figures 2 through 7 show the results of solubility calculations for a selected group of radionuclides considered in this study as a function of Eh. The bold, solid line in the figures delineates the total solubility of each element in the reference groundwater. In addition, a range of solubilities for each radionuclide is shown by the patterned zones. The range in solubilities results from calculations using the 29 Grande Ronde groundwater analyses. In all calculations the concentration of each aqueous species was computed from its respective activity coefficient. Activity coefficients were calculated by the Guntelberg approximation (17) which is useful for aqueous systems containing mixtures of electrolytes of unlike charge. Of the elements considered in this study (see Table Π), nickel, palladium, antimony, and lead are particularly sensitive to the presence of reduced sulfur species (S , HS") in the groundwater. For each of these radionuclides, if sulfur speciates under thermodynamic equilibrium conditions, solid sulfide phases will control their solubility at low Eh values. The implication of this fact is illustrated in Figure 1 by a bold, dashed line that corresponds to the solubility of nickel in the reference groundwater and a patterned zone representing the total range 2-



154

G E O C H E M I C A L BEHAVIOR O F RADIOACTIVE WASTE

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I

I

I

-0.5

-0.4

-0.3 Eh

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0.2

-0.1

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Figure 2. Solubility of Nickel as a Function of E h at 25°C. The horizontal lines i n the lower part of the figure show the range of dominance of solids controlling solubility and aqueous species as functions of E h . Figures 3 through 7 are interpreted similarly. See text for further explanation.)

Figure 3. Solubility of Selenium as a Function of E h at 25°C.



9.

EARLY ET A L .


Figure 4. Solubility of Tin as a Function of Eh at 25°C. [Sulfur speciation has only a minor effect on tin solubility (not shown in the figure) for Eh 10

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b

6a

7a

>10'

Solubility estimates from experiments and natural waters (mol/L)


9. EARLY ET A L .


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Geochemical Controls on Radionuclide Releases from a Nuclear

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