Radioactive waste, the unavoidable by-product of nuclear energy operations and certain defense reprocessing programs, can take hundreds of thousands of years to decay.1 The safe long-term storage of such a material necessitates a scientific understanding that spans this supra-human time scale and incorporates multiple disciplines (nuclear science, geology, and materials science, to name a few). Furthermore, our scientific understanding must be robust enough to inspire widespread confidence in any proposed storage system.
One of the main hurdles of long-term nuclear waste storage, whether it is in deep bore holes or geological repositories, is the long-term degradation of the protective casing surrounding the spent fuel and the prospect of that spent fuel being distributed into the local environment. In a recent paper published in Nature Materials, researchers from Ohio State University elucidated the principles governing the corrosion of the container and the solidified nuclear waste composites stored within it.2
Corrosion of the stainless-steel container
Considerations for the long-term storage of radioactive waste must incorporate environmental changes over the entirety of the decay time of the spent fuel. As the radioactive waste slowly cools, a small amount of water will likely accumulate around the specialized stainless-steel containers.3 Groundwater has a low concentration of salts, but over long enough time scales the dissolved ions could attack the seemingly impenetrable stainless steel, in a process called “pitting corrosion,” boring into the container.4 Guo et al.2 showed what could happen inside that container, once that eventuality occurred.
Many flavors of corrosion
Corrosion is a pervasive phenomenon that plagues those relying on any given (usually metallic) material for an extended period. As a rule, most people are familiar with rust— one of many flavors of corrosion. A metal rusts when it reacts with oxygen in humid conditions to form layers of various types of metal-oxides. But corrosion is a more general term for describing the decomposition of a material to reach a more stable state. A metal rusts when it oxidizes (loses an electron), but many other materials can corrode, and they can corrode by being reduced (gaining an electron) as well. Stainless steel, the material used for nuclear waste storage containers, is an iron alloy designed specifically to prevent such corrosion.
Guo et al. found, however, that the stainless steel as well as the nuclear waste composite will corrode over time, and because they corrode together, the rate of degradation is (most of the time) even higher than it would be for each material individually. Prior to this study, models that predicted the breakdown of the enclosure system evaluated each component independently; Guo et al. showed that such an approach glosses over important interactions between the two materials that lead to even faster degradation of both materials.
Corrosion at the interface of nuclear fuel and stainless steel
Guo et al. carefully designed an experiment to replicate the geological conditions of Yucca Mountain—the long-standing and controversial potential site of long-term nuclear fuel storage. The study replicates the situation in which a small amount of saline water has entered the container and resides at the interface of the stainless steel and the nuclear waste composite. As a proxy for the solidified nuclear waste, they used a special type of glass—international standard glass (ISG)—or various ceramic materials. Here, both materials are called “nuclear material” for clarity, but note that no highly radioactive elements were used during the course of this research.
In the experiment, reactions occurring at the boundary of stainless steel and nuclear material were induced by placing the two materials together and submerging the aggregate in a concentrated NaCl solution heated to 90°C. They also conducted a parallel experiment, in which the stainless steel was replaced with a simple inert plastic, to distinguish the synergistic coupling between the stainless-steel and nuclear material.
They found that corrosion of the steel creates a “confined crevice space” in which positively charged metal ions from both the stainless steel and the nuclear material accumulate. The pH within this small crevice increases with the concentration of cations, creating a more basic (and caustic) solution that further degrades both the nuclear material and the stainless steel. This is positive feedback, a phenomenon that can be dangerous because of its inherently compounding nature (and it can be found in diverse phenomena ranging from climate change to population growth).
One upside of their findings is their discovery that the ISG (one type of nuclear material) secretes silica particles that form a gel passivation layer on the stainless steel, slowing down its corrosion, stalling the positive feedback. Ions secreted from certain types of ceramic, however, only increase the rate of corrosion of the stainless steel. And the rate of degradation for both the ISG and the ceramic is significantly increased.
Measuring material degradation
Evidence of the corrosion of stainless steel and nuclear material ceramic by a solution of NaCl over the course of only 30 days is not easily observable. Even if you could see such evidence with your eyes alone, it would be impossible to determine the mechanisms of corrosion, its microscopic patterning, and the ions that had the greatest impact. Instead, Guo et al. employed a series of advanced materials characterization techniques to probe every aspect of the corrosion. First, by using scanning electron microscopy, which can resolve features only a few nanometers large, they found that corrosion of the nuclear material occurs most dramatically at the boundaries of the interface—where water, glass, and stainless coexist (Fig. 1). Scanning electron microscopy is a suitable way of depicting material structure but is nearly useless on its own when it comes to elemental analysis—in some ways it’s just like a black-and-white photograph.
To determine the elemental composition of the corroded materials, Guo et al. employed X-ray photoelectron spectroscopy. In this technique (which is based on the famous photoelectric effect), X-rays interact with the sample and excite electrons that are recorded by a specialized detector. The energies of the detected electrons provide a unique fingerprint for each element present in the path of the X-ray beam. By compiling these fingerprints and matching them to specific locations in space, researchers showed that the gel-like layers accumulated on the stainless steel and glass contained elements from both materials (Fig. 2). This result proves that the two materials interact, and that any acceleration or retardation of corrosion depends on the ingredients composing the entire system—stainless steel and nuclear material combined.
More work to be done
This work has highlighted a crucial issue related to long-term storage of nuclear fuel; now that this issue is identified, engineers and scientists can work to design storage materials that avoid the pernicious positive feedback mechanism that Guo et al. discovered. One such design would preclude the presence of any water, but such a route may be insurmountably challenging for the millennial time scale. Another route would be designing materials and interfaces that can break the positive feedback loop, ensuring that even with unpredictable environmental conditions, the nuclear material will remain tightly trapped in its steel enclosure.
