Easing the pressure on nuclear waste

Case study
Talking about
The Sludge Centre of Expertise

It may not have a snappy title, but the Sludge Centre of Expertise is doing critical work nonetheless. A partnership between the University of Leeds and Sellafield, the centre is helping to deal with the challenge of safely managing nuclear waste created by the UK’s nuclear power programme.

Nuclear power has been used to generate electricity in the UK since the 1950s, but many of the original power stations have now been decommissioned. The radioactive waste that resulted is mostly stored at the Sellafield nuclear decommissioning site in Cumbria, along with waste created through the reprocessing of nuclear fuel.

The Sludge Centre of Expertise focuses on the legacy nuclear waste held in storage ponds and silos at Sellafield. Among these are the Magnox Swarf Storage Silos, in which magnesium alloy casings from spent uranium fuel rods have been stored underwater since the 1970s. As the silos are now nearing the end of their lifespan, Sellafield has been developing plans to move the sludge created by the corroded fuel casings into new forms of storage: metal boxes that will safely hold the waste for up to a century above ground, and much longer when an underground repository can be found.

The cost of such an operation is significant, running to the tens of thousands of pounds for each box, with tens of thousands of them required. Anything that can minimise the number of boxes needed to store the waste while ensuring maximum safety will provide a huge saving to the UK taxpayer.

The problem of hydrogen

One of issues that has to be taken into account is hydrogen, which is created as the fuel casings corrode. Should the gas be easily released by the waste, it can then safely seep slowly from the box vents. But if gas gets trapped in the waste, it then risks being suddenly released, for example during transportation or minor earth tremors. This phenomenon must be fully understood and managed to ensure that hydrogen concentrations in and around the boxes do not reach flammable mixture conditions.

Martyn Barnes, Technology Manager at Sellafield and Lead for the Sludge Centre of Expertise, explains: “The easiest solution would be to limit the amount of sludge in each box, so it can cope with the maximum possible retention of hydrogen that might occur. But we’d probably need significantly more boxes to achieve that, so any improvement represents a significant cost saving.”

Instead, researchers at the University have been using x-ray tomography to look at the behaviour of hydrogen within sludges of different thicknesses, or yield stresses. Sludge with a low yield stress is the consistency of a soup, whereas sludge of a high yield stress is close to a solid.

In very low yield stress sludge, bubbles of gas can rise easily, so there is minimal hydrogen retention. In high yield stress sludge, cracks and fissures are formed which also allow the gas to escape. The uncertainty was around intermediate yield stress, where the consistency of the sludge does not enable either of those mechanisms to work. Using high resolution x-ray tomography, the researchers were able to see how tiny bubbles that were connected together in long strands created a network that enabled gas to diffuse between them and reach the surface.

Predicting retention and release

Dr Timothy Hunter, who leads the Nuclear Engineering group within the University’s School of Chemical and Process Engineering, says: “Understanding the different mechanisms by which hydrogen was escaping allowed us to predict the level of gas retention and release based on the yield stress levels of the sludge. This provided Sellafield with the means of managing the hydrogen levels within the new storage containers and highlighted which types of waste are likely to be at highest risk for hydrogen retention and so needed additional monitoring.”

The research findings have now been incorporated into Sellafield’s technical guidelines and best practice for dealing with this type of waste. The proposed work to transfer the waste into new storage, based on the research, is now being discussed with the Office for Nuclear Regulation. Two follow-on PhD projects are now planned with the University, applying these insights to other types of waste and storage facilities, and developing computational simulations and models to predict behaviour in more complex systems.

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