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High-level nuclear waste storage vessels could degrade much faster than previously thought

04 Feb 2020
Nuclear warning
Warning signs: a recent study suggests that we need to rethink plans for the long-term storage of nuclear waste. (Courtesy: iStock)

An accelerated corrosion process in materials proposed for use in the long-term storage of high-level nuclear waste has been discovered by researchers in the US. Xialei Guo and Gerald Frankel at Ohio State University and colleagues made their discovery by doing a laboratory recreation of the conditions of nuclear waste storage under proposed guidelines. Their findings suggest the need for better storage systems.

High-level nuclear waste is created by weapons programmes and nuclear reactors. It must be stored securely for hundreds of thousands of years until it is rendered safe by radioactive decay. Most countries plan to store their high-level waste deep underground, but there are currently no operational facilities – although one is under construction in Finland.

The current plan is to immobilize high-level waste in glass or ceramic using a process called vitrification. The hot glass or ceramic would be cast into stainless steel cannisters that would be sealed and buried deep within underground repositories surrounded by highly impermeable rocks. Regular safety inspections would check the material for any corrosion both within the immobilizing material and the steel. With these precautions in place, researchers had hoped that waste could be safely stored for thousands of years.

Critical oversight

Guo and colleagues believe that they have spotted a critical oversight in this plan that relates to corrosion that occurs at glass-steel and ceramic-steel interfaces if water is able to penetrate the cannister – something that is likely to happen at some point in time.

They point out that after a steel cannister is cast, the glass or ceramic it contains will contract as it cools down, creating a thin gap between the vitrified material and the inside wall. The presence of water will encourage the interior steel surface to corrode, causing an increase in the acidity within the gap and creating a better environment for further corrosion. This will then cause the glass or ceramic to release corrosive ions, causing further corrosion to occur.

The result, they argue, is a feedback loop that accelerates the deterioration of both materials at their interface – leading to the release of radioactive material into the surrounding environment

To study the effect, Guo and colleagues recreated the conditions of nuclear waste storage in the lab by pressing a slab of stainless steel against samples of different glasses and ceramic materials. When placed in environments simulating geological repositories, they observed that interactions between the two materials accelerated corrosion at their interfaces.

Frankel’s team believe that this previously unconsidered effect could shorten significantly the useful lifetime of nuclear waste storage packages. If they are correct, this highlights a pressing need for policymakers to rethink how waste will be stored. The team also hopes that their results could help researchers develop more compatible materials to minimize interface corrosion, enabling them to design much better systems for waste storage.

The study is described in Nature Physics.

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