Abstract
A novel CO2 sequestration project at Wallula, Washington, USA, makes ample use of the geoscientific data collection of the old nuclear waste repository project at the Hanford Site nearby. Both projects target the Columbia River Basalt (CRB). The new publicity for the old project comes at a time when the approach to high-level nuclear waste disposal has undergone fundamental changes. The emphasis now is on a technical barrier that is chemically compatible with the host rock. In the ideal case, the waste container is in thermodynamic equilibrium with the host-rock groundwater regime. The CRB groundwater has what it takes to represent the ideal case.
Résumé
Un nouveau projet de séquestration de CO2 à Wallula, Washington, Etats-Unis d’Amérique, utilise largement la collecte des données géoscientifiques de l’ancien projet de stockage de déchets nucléaires du site de Hanford à proximité. Les deux projets ont pour cible les Basaltes de la Columbia River (BCR). La nouvelle publicité pour l’ancien projet se produit à un moment où l’approche pour le stockage des déchets nucléaires de haut-niveau a subi des modifications fondamentales. L’accent maintenant est mis sur une barrière technique qui est chimiquement compatible avec la roche hôte. Dans le cas idéal, le conteneur de déchets est en équilibre thermodynamique avec le régime hydrogéologique de la roche hôte. Les eaux souterraines du BCR ont tout pour constituer le cas idéal.
Resumen
Un nuevo proyecto de secuestro de CO2 en Wallula, Washington, EEUU, hace un amplio uso de la recolección de datos geocientíficos del antiguo proyecto de repositorios de residuos nucleares en el sitio cercano de Hanford. Ambos proyectos apuntan al Columbia River Basalt (CRB). La nueva publicidad del antiguo proyecto se produce en un momento en que el enfoque de la eliminación de residuos nucleares de alto nivel ha sufrido cambios fundamentales. El énfasis ahora está en una barrera técnica que es químicamente compatible con la roca huésped. En el caso ideal, el contenedor de residuos está en equilibrio termodinámico con el régimen de agua subterránea de la roca huésped. El agua subterránea del CRB tiene lo que se necesita para representar el caso ideal.
摘要
美国华盛顿州Wallula新的CO2隔离项目充分利用位于Hanford老的核废料储库项目地质科学数据。两个项目都把哥伦比亚河玄武岩作为目标。当高水平的核废料处理方法经历了重大变化时,人们对老项目进行了新的审视。现在重点放在技术屏障上,即化学上和母岩兼容。在理想的情况下,废料容器与母岩地下水动态处于热力学上的平衡。哥伦比亚河玄武岩地下水具有具有代表理想状况下的各种条件。
Resumo
Um novo projeto de sequestro de carbono em Wallula, Washington, EUA, torna amplo o uso de coleções de dados geocientíficos de antigos projetos de disposição de resíduo nuclear nas proximidades de Hanford Site. Ambos projetos focam o Rio Basáltico Columbia (RBC). A recente publicidade para os antigos projetos vem por ocasião de mudanças fundamentais de alto nível em curso da abordagem sobre disposição de resíduos nucleares. A ênfase agora é em barreiras técnicas que sejam quimicamente compatíveis com a rocha recebedora. No caso ideal, o compartimento nuclear está em equilíbrio termodinâmico com o regime da rocha e água subterrânea que o recebe. As águas subterrâneas do RBC têm o que é necessário para representar um caso ideal.
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Thirty years ago, the Columbia River Basalt (CRB) in the western USA (mainly Washington State; Fig. 1) was a candidate for a high-level nuclear waste repository and possibly will be again, as a novel CO2 sequestration project has generated new interest in this attractive target for underground storage of natural gas, supercritical carbon dioxide and solid nuclear waste. On April 14, 2011, the US government decreed to stop funding the construction of the repository for spent nuclear fuel and other types of high-level nuclear waste in the Yucca Mountain, Nevada. Some days before that, Washington State granted a permit to inject 1,000 t of carbon dioxide into the CRB at Wallula, only a few kilometres from the Hanford Site, the former alternative to the Yucca Mountain repository. This comes at a time when the approach to nuclear waste disposal has undergone fundamental changes. The geologists no longer postulate that there exists a geological formation that can isolate the nuclear waste for a million years. Only a technical barrier in concert with the host rock is considered to be capable of completing this task. The waste container should be chemically compatible with the host rock. In the ideal case, the waste container is in thermodynamic equilibrium with the host-rock’s groundwater regime. The essay examines how this can be achieved.
Looking back, the CRB at Hanford was the only serious competitor left when eventually Yucca Mountain was designated by the Nuclear Waste Policy Act (US Department of Energy 2004) in 1987 to be the deep geological repository for high-level nuclear waste in the USA. As a consequence, the work on a deep repository in the CRB at Hanford was suspended; however, the CRB with its low-salinity groundwater regime offers what Yucca Mountain certainly has not: a geochemical environment in which metallic waste containers can survive without serious damage by corrosion.
The Yucca Mountain repository is unique because it is conceived for waste storage above the water table (Farmer et al. 2003). In other parts of the world, potential repositories are located below the water table (Schwartz 1996, 2008). The unsaturated zone precludes the protection of the metallic waste container by bentonite because the clay minerals would dehydrate and lose their isolating properties; thus, an entirely metallic barrier system has to be used. Evaporation of water from the metal barrier’s surface has been found to produce salinities up to 4 mol/L Cl in the water remaining on the barrier’s surface under the strongly oxidising conditions in the arid climate zone. Any kind of metallic barrier system is bound to fail in such an environment, long before the radioactive interior becomes harmless. In this respect, the Yucca Mountain project is similar to the salt-dome project at Gorleben, Germany (Schwartz 2012a), started in 1977 and ended in 2013 (German Federal Government 2013).
Worldwide, there are only two approved high-level waste repositories under construction—both the Forsmark project in Sweden (Schwartz 2012b) and the Olkiluoto project, Finland, are in a granite-dominated low-salinity environment and envisage the storage of copper-shielded waste containers. Compared with basalt, granite has lower concentrations of sulphur and higher Fe-III/Fe-II ratios, which is favourable when it comes to corrosion resistance of the copper shield, and low concentrations of copper (unfavourable with respect to resistance). Within a subaerially erupted basalt sequence (e.g. CRB), the top of an individual basalt flow has lower concentrations of sulphur and higher Fe-III/Fe-II ratios than the interior of the flow. Degassing at the top of the flow reduces sulphur concentrations and the reaction with atmospheric oxygen increases the iron-III/iron-II ratio.
The high-porosity tops of basalt flows are attractive targets for underground storage projects, not only for nuclear waste but also for natural gas (Reidel et al. 2002) and carbon dioxide (McGrail et al. 2011, 2014; McGrail and Schaef 2015; Matter et al. 2016). The reasons for the attractiveness, of course, are different. The top of a subaerially extruded sequence not only has low sulphur concentrations and high Fe-III/Fe-II ratios but also a high proportion of glassy phases containing calcium, magnesium and iron silicates that quickly react with wet CO2 to form stable carbonate minerals and amorphous SiO2; however, this is only one side of the story. Carbon-dioxide injection is like a large-scale hydrology experiment at its extreme. The huge volume of injected fluid causes high-speed groundwater movements, which are conveniently monitored at the surface by geophysical methods. The prediction of groundwater movement during the envisaged life of a nuclear waste repository, on the other hand, is one of the major tasks of a security analysis. Consider that a carbon-dioxide injection project in the CRB can be seen as a hydrology data collection event with a built-in time accelerator, which is convenient for projections concerning groundwater and nuclear waste disposal. The case of the deep CRB groundwater is especially interesting because it is very old: >30,000 years or even >100,000 years according to 14C or helium data, respectively (Reidel et al. 2002). Chemically, the old groundwaters are characterised by high pH as well as high sodium and fluoride concentrations, whereas calcium and magnesium concentrations are low. Both the Hanford Reference Repository and the Wallula CO2 injection zone (Table 1) are in typical deep CRB groundwater (Reidel et al. 2002; McGrail et al. 2009; Lavalleur 2012), where formation temperature ranges from 37 to 54 °C.
Among the various candidate materials for nuclear waste packages, copper has unique oxidation characteristics. The conversion from native metal (Cu0) to metal oxide (Cu2O) occurs in a mildly oxidising to mildly reducing environment (positive Eh for pH <7.7 at 25 °C; Fig. 2), whereas the conversion of steel, titanium or nickel-chromium alloy is not only possible under oxidising conditions but also under strongly reducing conditions, i.e., below the stability field of liquid water. The example of naturally occurring native copper in the Earth’s crust must be taken into account in the disposal site design. The largest native copper deposits are located on the Keweenaw Peninsula of Michigan (mid-west USA), where they have been mined to a depth of 2.2 km (Butler and Burbank 1929; Broderick et al. 1946; White 1968). These deposits formed 1 billion years ago at temperatures of 100–200 °C. The mineralising fluids were slightly less reducing but had much higher copper concentrations than commonly present-day groundwater.
In low-salinity groundwater (0.0005–0.05 mol/L Cl−), CuCl0 is the dominant aqueous copper species. Accordingly, the thermodynamically relevant parameter is the ΣCu/Cl activity ratio, which refers to the sum of activities of all dissolved copper species divided by the Cl− activity. Groundwater in the Deccan Basalt, India, has ΣCu/Cl ratios in the range from 10–3.5 to 10−5 (Muralidhar and Raju 1990). Similar conditions are achievable in the bentonite buffer around a copper container for non-basalt repositories if the bentonite contains artificial admixtures of fine-grained native copper. Thus thermodynamic equilibrium between the container and the external fluid on the high-Eh side of the native-copper stability field is implemented in the repository-container system as an analogue to the natural counterparts.
The low-Eh and low-pH side of the native-copper stability field is the copper-sulphide phase boundary (CuS and CuS2). The position of this boundary depends on the total activity of aqueous sulphur species (ΣS), which is ≤1.5 × 10−3 in CRB groundwater (Grande Ronde Basalt Formation; Reidel et al. 2002). Provided the bentonite backfill contains no sulphur-bearing minerals, the corrosion of the copper container is constrained by the diffusion of aqueous sulphide from the igneous rock aquifer through the bentonite to the container surface. This situation is quite different from corrosion of other package materials such as steel, titanium or nickel-chromium alloy, which all corrode by reaction with omnipresent water. It is not possible to control corrosion of these materials by a diffusion barrier around the container.
In the case of a sulphur-free buffer, general corrosion of the copper container can be determined by a simple calculation. The experimentally derived diffusion coefficient of aqueous sulphide in bentonite is approximately 7 × 10−8 cm2/s (King and Stroes-Gascoyne 1995; King et al. 2002). The CRB groundwater has an average sulphide concentration of 1.1 × 10−4 mol/L. Assuming a 35-cm-thick bentonite buffer, 2 × 10−16 mol per second and per square centimetre are transported through the buffer to the container surface. This is equivalent to a general corrosion rate of 10−7 cm/a, when Cu0 is transformed to Cu2S; thus, the corrosion depth is 0.1 cm within 1 million years and a service time for a 5–10-cm-thick copper shell beyond 1 million years is a realistic possibility.
The situation can be further improved if hematite is added to the bentonite buffer. Such a measure would expand the stability field of native copper to lower Eh in the presence of FeSO4(c) or pyrite by 0.07 V or 0.05 V, respectively. The average CRB groundwater with Eh = −0.3 V and pH = 9.4 would be in the stability field of native copper, and corrosion would be nil. The scenario replicates the natural process: the precipitation of native copper is linked to the dissolution of hematite in the Keweenaw deposits.
Paradoxically, the old CRB project (1968–1987) lives up to modern scientific standards. The more recent Yucca Mountain project (1978–2011) with its extremely high corrosion rates does not even come near those.
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Schwartz, M.O. The new Wallula CO2 project may revive the old Columbia River Basalt (western USA) nuclear-waste repository project. Hydrogeol J 26, 3–6 (2018). https://doi.org/10.1007/s10040-017-1632-y
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DOI: https://doi.org/10.1007/s10040-017-1632-y