17 March 2011
UK scientists have analysed the chemistry taking place in storage ponds at nuclear power sites, such as Sellafield, to come up with a way to remove radioactive waste as nuclear regulatory bodies are pressing on the nuclear industry to clean up the ponds.
Storage ponds are used to store spent Magnox rods, which are uranium fuel rods covered by a magnesium-aluminium alloy cladding. The rods contain large amounts of fission products, which are highly reactive. The ponds are maintained to minimise corrosion of the rods, but the cladding corrodes in water, creating fine particle sludge. 'The sludge in one of these ponds is estimated to contain tonnes of fuel debris including considerable quantities of plutonium,' says Stephen Parry from the University of Manchester.
Parry, together with his colleagues, made a model of Magnox storage pond liquor to study how plutonium interacts with the corroded Magnox sludge to find a way of removing the plutonium before the ponds are emptied.
Their pond consisted of plutonium, a sludge simulant, sodium carbonate, polyelectrolyte and silica to replicate real conditions. One component of the sludge is brucite (magnesium hydroxide), which sequesters plutonium, forming a colloid. This is soluble at neutral and acidic pH, meaning that the plutonium's mobility in the ponds is enhanced.
A magnified image of nuclear storage pond sludge showing brucite crystals, which sequester plutonium, making it difficult to remove from the mixture
'One potential problem is the risk that disturbing the sludge will release fine, plutonium-containing particles in the effluent from the ponds. Pond effluents are treated before discharge into the sea under authorisation, but we need to be sure that the treatment process will effectively remove plutonium from the effluents before we can start to empty them,' explains Parry.
The team found that a low carbonate concentration, high CMS concentration and high polyelectrolyte concentration resulted in almost all of the plutonium being filtered.
'The work we have done, supported by the site operators and the EPSRC, shows that it is possible to optimise effluent treatment and also which steps in the treatment process are the most important in ensuring efficient plutonium removal, helping to open the way to removal of the sludge,' concludes Parry.
Nick Evans, an expert in radiochemistry from the University of Loughborough, UK, says that the study contributes to the body of knowledge about how sludge should be treated. 'A lot of research is being done now and the nuclear decommissioning authority is being more proactive, as at Sellafield [nuclear decommissioning, reprocessing and waste management site], but there hasn't been much work on this in the past, particularly with regards to difficult materials like plutonium,' he says. 'The work should be useful for Sellafield for dealing with these problematic sludges.'
Parry and his team are working on immobilising the sludges so that they can be stored safely before disposal.