Nuclear waste separation

US researchers have discovered a compound that removes radioactive isotopes from liquid waste, aiding their disposal.

At present in the UK, high-level liquid effluent is immobilised by mixing it with glass-making ingredients, melting it in a furnace and pouring it into stainless steel containers. Intermediate level wastes are usually immobilised in cement and placed within steel drums.

The new method is claimed to be a simple way of separating the radioactive component from the waste.

The technique relies on the newly-identified compound, niobium heteropolyanions. hetero-polyanions (HPAs) were discovered in the 19th century in the form of oxides of vanadium, tungsten and molybdenum containing voraciously active, negatively-charged oxygen ions that bind to positively-charged large metal atoms,some radionuclides and certain viruses.

The compounds can be produced cheaply and at room temperature, but are only stable in acid environments, meaning they would deteriorate quickly and be unable to work in base environments such as nuclear waste. But the niobium HPAs are base stable, which, research claims, would also give them an important role in binding to and immobilising viruses including AIDS.

In the case of liquid nuclear waste, HPAs bind with the radionuclides to form a solid substance, said May Nyman who was working as lead researcher for the US Energy Department at the Sandia National Laboratories.

‘If the radioactive elements in liquid waste can be adsorbed on to a solid, the small amount of solid contains the radioactive component rather than a large amount of solution, making it much easier to handle and store,’ she said.

‘Separating the radioactive and non-radioactive parts is a necessary step in processing nuclear waste. But the challenge is finding a material that will adsorb or bind the radionuclides existing as a few parts per million while ignoring other materials such as sodium.’

Nyman believes her discovery will be of special interest to medical researchers, who have previously carried out exhaustive tests on the possible uses for HPAs.

‘It has been found that HPAs with small amounts of iron or niobium have a strong binding effect,’ she said. ‘Now we have HPAs that are completely niobium.’

According to Nyman, the compound’s discovery came about by accident after she was called to the Savannah River nuclear plant, South Carolina, to find the cause of clogging during attempts to extract caesium isotopes from liquid waste.

The waste was being passed through a column of microporous crystalline zeolites that normally separate out the radioactive matter by drawing it into its pores. Nyman discovered that the zeolites contained an impurity that had formed during their manufacture, causing clotting to occur.

Although the problem had been solved, her curiosity led her to recreate the foreign substance in her laboratory, and niobium HPAs were discovered.

David Wild of Nirex, the UK advisory body for the disposal of nuclear waste, said that adding the niobium compound to the waste might make the process longer and increase its complexity. ‘The question from Nirex would be how does it affect the long-term behaviour of the disposal concept,’ he said.