11 Preliminary Assessment of Oxygen Consumption and Redox Conditions in a Nuclear Waste Repository in Basalt D. L. LANE, T. Ε. JONES, and M . H. WEST
Downloaded by CORNELL UNIV on October 13, 2016 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch011
Rockwell International, Rockwell Hanford Operations, Richland, WA 99352
During construction of a nuclear waste repository in basalt (NWRB), Eh conditions in the repository horizon will be perturbed as a result of air-saturation of groundwater, temporarily leading to redox conditions more oxidizing than in the undisturbed system. Performance assessment of an NWRB requires information on redox conditions, since they will greatly affect the corrosion rate of canisters and the solubility and transport of certain radionuclides. Experiments were conducted to evaluate rates of oxygen consumption and redox conditions in the basalt-water system under conditions expected in an NWRB. Two methods were used to obtain these data: (1) the As(III)/As(V) redox couple and (2) the measurement of dissolved oxygen levels in solution as a function of time. These experiments have provided evidence that basalt is effective in removing dissolved oxygen and in rapidly imposing reducing conditions on solutions. At 300°C, calculations showed that an upper limit on E h of -400 ± 100 mV was attained in 11 days. The dissolved oxygen content of solutions from a 150°C experiment decreased from air-saturation (8.5-9 mg/L) to 0.4 mg/L after 8 days, while solutions maintained at 100°C for 130 days contained 1.8-1.9 mg/L dissolved oxygen. The basalt flows underlying the Hanford Reservation near Richland, Washington, are being evaluated as a possible repository site for long-term storage of high-level nuclear wastes. Characterization studies (1) and calculations based on redox-buffering reactions (2) suggest that the in situ conditions of groundwaters within the deep basalt formations are low redox potential (Eh), moderate pH, and low ionic strength. The longterm performance of a nuclear waste repository in basalt (NWRB) is based on the ability of the engineered barrier and host rock systems to provide initial containment and subsequent retardation of radionuclide transport by maintaining these and other in situ conditions. 0097-6156/ 84/ 0246-0181 $06.00/ 0 © 1984 American Chemical Society Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Downloaded by CORNELL UNIV on October 13, 2016 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch011
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GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE
An important part of the engineered barrier system is the waste package, consisting of the waste form, canister, and backfill. The waste package backfill has several performance requirements, one of which is to impose and maintain low E h conditions. These conditions will aid in minimizing canister corrosion and in limiting the dissolution rate, solubility, and transport of certain radionuclides (3). However, during repository construction and waste emplacement, the in situ low Eh conditions will be perturbed by entrapment of air, resulting in high dissolved oxygen contents (i. e., high Eh) of inflowing groundwater. Thus, it is necessary to evaluate the effectiveness of backfill components in re-establishing the low Eh conditions of the undisturbed basalt-groundwater system. Dissolved oxygen consumption will be the first step in this process. Since crushed basalt has been recommended as a major backfill component (1), experiments were completed to evaluate the rate of dissolved oxygen consumption and the redox conditions that develop in basalt-water systems under conditions similar to those expected in the near-field environment of a waste package. Two approaches to this problem were used in this study: (l)the As(IQ)/As(V) redox couple as an indirect method of monitoring Eh and (2) the measurement of dissolved oxygen levels in solutions from hydrothermal experiments as a function of time. The first approach involves oxidation state determinations on trace levels of arsenic in solution (4-5) and provides an estimate of redox conditions over restricted intervals of time, depending on reaction rates and sensitivities of the analyses. The arsenic oxidation state approach also provides data at conditions that are more reducing than in solutions with detectable levels of dissolved oxygen. An arsenic oxidation state experiment was conducted at 300°C and 300 bars pressure in the basalt-deionized water system, while dissolved oxygen experiments were performed at 100°C and 150°C and 300 bars in the basalt-synthetic Grande Ronde groundwater system. A control experiment consisting of synthetic Grande Ronde groundwater at 150°C and 300 bars was also conducted to evaluate dissolved oxygen levels in the absence of basalt. The synthetic groundwater composition was based on Hanford Site groundwater samples from the Grande Ronde Formation, Columbia River Basalt Group. Finally, this study does not address other processes in an NWRB which may affect redox conditions such as radiolysis of solutions. Experimental Materials. The basalt studied in these experiments was relatively unaltered tholeiite from the Umtanum flow entablature of the Columbia River Basalt Group, a repository host candidate for the Hanford Site (6). Phase and chemical characterization are discussed in Noonan et al. (7) and Palmer et al. (8).
Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
11.
LANE ET AL.
Oxygen Consumption and Redox Conditions in Basalt
The formulation of the synthetic Grande Ronde groundwater used as starting solution in the dissolved oxygen experiments is given in Jones (9). Table I provides an analysis of the starting solution. Table I. Analysis of Starting Solution for Dissolved Oxygen Experiments 8
Downloaded by CORNELL UNIV on October 13, 2016 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch011
Component
Concentration (mg/L)
Si
32
Na
340
Al
