Interview: Small modular reactors get a reality check about their waste via Bulletin of Atomic Scientists

By François Diaz-Maurin | June 17, 2022

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Scientists have started working on independent reviews of those claims. The results showed that SMRs do not necessarily perform better than gigawatt-scale reactors on a variety of measures. A recent Stanford-led study published in the Proceedings of the National Academy of Sciences (PNAS) provides for the first time a comprehensive analysis of the nuclear waste generated by small modular reactors. The study concludes that most current SMR designs will actually significantly increase the volume and complexity of nuclear waste requiring management and disposal when compared to existing gigawatt-scale light water reactors.

Here, Bulletin associate editor François Diaz-Maurin talks with Lindsay Krall, the lead author of that study and a former MacArthur postdoctoral fellow at Stanford’s Center for International Security and Cooperation (CISAC) who is now based in Sweden.

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Diaz-Maurin: Great. Let’s turn to your research findings now. Most SMRs are said to adopt an “integral” design, in which the reactor core and auxiliary systems are all contained within a reactor vessel. Now, because of their smaller size and compact design, one can expect that SMRs will generate less waste than larger reactors that operate at the gigawatt scale. But you have reached the opposite conclusion in your study, that SMRs will produce more voluminous and chemically/physically reactive waste than light-water reactors. And this by factors of 2 to 30. How is that? It seems counterintuitive…

Krall: Well, one thing that’s clear from the analysis is that the waste output really differs depending on the type of coolant the reactor is using. If it’s using water, then we have processes to treat that water and decontaminate it and hold it so the water coolant itself does not become radioactive waste. However, for a sodium-cooled reactor, for instance, that sodium coolant is likely to become low-level waste at the end of the reactor’s lifetime, because it becomes contaminated and activated during reactor operation. So, the “up to 30 times more waste” that’s been driving the headlines, it’s mostly the sodium coolant. Another aspect is that things in a small reactor do not scale intuitively compared to other forms of energy. For instance, one thing I went into was “neutron leakage.”

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Diaz-Maurin: In the paper, you say that compared to large reactors, SMRs will increase the volume and complexity of those wastes. I get the volume part. But what is this complexity about?

Krall: It’s what I mean with “different characteristics” of the spent fuel, not least being this fissile isotope concentration. It also produces heat. It has a particular radionuclide composition, including fission products, which can be both short- and long-lived. And so, I employed four different metrics to measure the spent fuel. And then the long-lived low- and intermediate-level waste in the article is the activated waste. This waste is so close to the reactor core that it absorbs the neutrons that are being leaked and becomes activated. In current reactors, the activated waste is mostly steel from the structural components that keep the core intact. This steel will also become activated in SMRs and, as a result, it will contain short- and long-lived nuclides that need to be dealt with during decommissioning. Reactor decommissioning will require radiation shielding and that steel, the activated steel, will also need to be disposed of in a geologic repository.

Diaz-Maurin: What’s the difference between short-lived and long-lived waste from the perspective of waste management?

Krall: Long-lived waste should be disposed of in a permanent geologic repository—a passively safe, rock cavern with multiple engineered barriers—where the radioactive materials discharged from the reactors will be contained over long periods of time so that they can decay. Short-lived waste includes mostly the reactor structures that have come in contact with a primary coolant that was circulating around the reactor core and through the steam generators. This waste also should go to some sort of disposal site. Sweden, for instance, has a 50-meter-deep repository, whereas some countries just dispose of it in shallow landfills.

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