A safe, cost effective way of immobilising five tons of contaminated plutonium residues, part of the UK’s 140 ton plutonium inventory has been demonstrated by engineers from the University of Sheffield.

– Engineers from the University of Sheffield have demonstrated a new way of immobilising plutonium wastes that could help to reduce the UK’s plutonium inventory – currently the largest in the world.

– Researchers show how heavily contaminated plutonium, arising from early research and development activities, could be transformed into a stable glass ceramic material.

– Current UK government policy is to re-use plutonium stock as fuel, but there is a lack of commercial interest in this from nuclear reactor operators – meaning a cost effective immobilisation method may be needed.

In the study, led by Professor Neil Hyatt from the University of Sheffield Energy Institute and Department of Materials Science and Engineering, the team demonstrates how a glass ceramic material could immobilise contaminated plutonium residues, arising from early research and development activities.

The UK holds the largest separated civil plutonium inventory in the world as a result of its legacy of being a pioneer of nuclear energy in the 1950s onwards.  The 140 ton inventory includes five tons of contaminated plutonium residues which could be unsuitable for reuse.

The plutonium residues are contaminated with chlorine, arising from degradation of PVC packaging material.  However, chlorine is known to have a low solubility in glasses, which could unduly limit the quantity of plutonium incorporated, to avoid the formation of water soluble plutonium chloride salts.

Using an extremely bright X-ray microscope at the European Synchrotron Radiation Facility, France, the Sheffield team developed an atomic scale model to understand how chlorine was bonded within the glass.  They were then able to determine the solubility threshold for chlorine in the silicate glass material and show that it exceeded the worst case expectation for the treatment of plutonium residues.  The important impact of this work is to show that these plutonium residues could be immobilised within the glass ceramic material, without needing any prior treatment to remove the chlorine contamination.

Current government policy is for the UK plutonium inventory to be reused as fuel in civil nuclear reactors, with any material unsuitable for reuse to be immobilised as a waste for geological disposal.

The government’s policy is challenged by a lack of commercial interest in reusing plutonium as fuel by reactor operators.

In the event that re-using the plutonium stockpile as fuel cannot be delivered, the UK will need to safely immobilise the material as waste.  It is estimated that managing the inventory to an end point, whether reuse or immobilisation, will cost at least £10 billion. 

We have shown that this glass ceramic material can take up the chlorine contaminant at a level greater than we would expect to be in the plutonium residues, in a worst case.  This means that the plutonium residues would not need to be heat treated to remove the chlorine contamination before being treated,” commented Professor Neil Hyatt, Energy Institute & Department of Materials Science and Engineering,University of Sheffield.

Avoiding the need for heat treatment to remove chlorine contamination before treatment would reduce the complexity and cost of the immobilisation process.  The glass ceramic product is expected to be suitable for disposal within the UK’s future geological disposal facility.

The development could help to de-risk the UK’s roadmap for addressing its plutonium inventory and reduce costs for nuclear plant operators. The work reduces the risk that contaminated plutonium residues will be orphaned without a suitable waste treatment process.

The Sheffield team is further developing the hot isostatic process used in this research, to immobilise the whole 140 ton inventory of plutonium, in the event that the current government policy for reuse cannot be implemented.

Reference
Thornber SM, Mottram LM, Mason AR, Thompson P, Stennett MC, Hyatt NC. Solubility, speciation and local environment of chlorine in zirconolite glass–ceramics for the immobilisation of plutonium residues. RSC Adv. 2020;10(54):32497-32510. doi:10.1039/D0RA04938G

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