Researchers in the UK have devised a process that uses electricity to remove radioactive contaminants from irradiated nuclear graphite. The process could reduce the volume of waste from nuclear power plants that requires expensive and long-term storage.
Graphite is used to maintain the fission chain reaction in certain types of nuclear reactors. Over 300,000 tonnes of nuclear graphite waste are estimated to be awaiting storage or disposal worldwide. Due to a high density of legacy reactors, approximately a third of the world’s irradiated graphite waste is within the UK.
‘Most of the advanced nuclear reactor technologies being proposed for future low carbon energy production will also use nuclear graphite, so this waste burden is likely to increase for future generations unless novel solutions are examined to treat, reduce and recycle this waste form,’ explains Abbie Jones from the University of Manchester. ‘Technologies that can minimise this burden will not only massively reduce costs of managing legacy wastes but also improve the sustainability of future nuclear reactors and help achieve net zero targets.’
The UK’s current strategy, as outlined by the Nuclear Decommissioning Authority, is to store the waste graphite temporarily to allow short-lived isotopes to decay prior to final disposal. However, storing nuclear graphite waste is expensive, space-inefficient and runs the risk of contamination.
In response to these issues, Jones and colleagues set out to determine whether electrolysis in high-temperature molten salt media could be applied to decontaminate nuclear graphite. Team member Clint Sharrad says they chose to use molten salt due to its wide electrochemical window, allowing them to access electric potentials that could better force nuclear graphite contaminant removal.
As a proof of concept, the team performed tests using graphite samples from different reactor sites across the UK. After electrolysis, they assessed what corrosion and fission products the graphite released into the molten salt media. The group then adjusted the process parameters to optimise radioisotope transfer into the salt phase.
Post-treatment, the team evaluated the graphite and found a significant reduction in the radioactive content. The decrease in activity was large enough that the resulting graphite could be reclassified from intermediate-level waste to low-level waste.
Ken Czerwinski, a radiochemist at the University of Nevada, Las Vegas in the US, says the ‘key contribution of this work is the potential to concentrate radioactive waste and limit the amount of material destined for high-level radioactive waste disposal.’
Jones suggests that applying this method at an industrial scale ‘could save the UK over £1 billion’ of costs related to managing legacy nuclear graphite by ‘avoiding interim storage requirements and minimising waste volumes requiring managed disposal’.
Despite achieving high decontamination levels, the researchers also observed minimal graphite degradation. This suggests future reactors could reuse the graphite. Sharrad says this would result in ‘not only reducing the waste burden but also enhance the sustainability of nuclear reactor systems by providing a whole lifecycle approach for a main reactor core component.’
Next, the team plans to conduct follow-up research looking at a wider variety of graphite samples and explore whether treated nuclear graphite performs as effectively as virgin graphite in a reactor system. This will allow them to determine whether reusing nuclear graphite may be a feasible option in the future.
Article updated on 15 October 2021: A previous version of this article used a stock image of a reactor that does not use graphite so has been removed.
T Grebennikova, A N Jones and C A Sharrad, Energy Environ. Sci., 2021, DOI: 10.1039/d1ee00332a