Green rust keeps radioactive waste in its place
A naturally occurring, gloopy substance called green rust excels at capturing radioactive waste and preventing it from leaking into the environment, new research shows. The findings, published in the journal Geochimica et Cosmochimica Acta, could inform the design of storage systems for radioactive waste, which need to remain intact for hundreds of thousands of years. The study was partly funded through the ACTINET-6 ('Network for actinides sciences') and FUNMIG ('Fundamental processes of radionuclide migration') projects, which received EUR 6.4 million and EUR 8 million respectively under the Euratom Sixth Framework Programme (FP6). Spent nuclear fuel contains a variety of radioactive substances, many of which remain hazardous for millennia. Currently, this waste is disposed of by burial deep underground. In Scandinavia, for example, radioactive waste is enclosed in copper-lined, cast-iron canisters and placed 500 metres under the surface. The waste can be further shielded from the environment by surrounding it with bentonite, a kind of clay that swells when it comes into contact with groundwater. A particularly worrying substance for those designing these waste repositories is neptunium; although it makes up just 0.05% of spent nuclear fuel, its lengthy half-life means it remains hazardous for an extremely long time and could pose a threat to human health if it leaked into the environment even 5 million years after deposition. In this study, the researchers investigated the ability of green rust to stop neptunium in its tracks. Green rust, a kind of clay, consists of iron that has only partially rusted; because it has not rusted fully, it has an electron deficit and so reacts very readily with other pollutants. This 'kind of green goop' (as the researchers describe green rust) occurs naturally in oxygen-poor water. Because it is so reactive, it quickly turns to ordinary red rust when it comes into contact with oxygen. However, green rust is very easy to make, forming when iron sulphate and caustic soda are added to water. According to the researchers, their tests showed that green rust is able to rapidly encapsulate and contain neptunium. The team hopes their findings will be applied to the design of future radioactive storages systems. 'Our study shows that even the safest encapsulation of radioactive waste could be made safer if radioactive waste canisters are buried in a place where green rust will form,' commented Dr Bo Christiansen. Looking to the future, the researchers note: 'The exact process of the incorporation reaction and the long-term behaviour of immobilised [neptunium] are, however, not yet clear and should be further investigated in detail.' For many years, green rust was seen as a problem, and researchers devoted a good deal of energy to working out how to block its formation in reinforced concrete. However, recent studies have shown that its reactive nature makes it a powerful tool in cleaning up many forms of pollution. 'Neptunium is a relatively exotic problem. Not a lot of people need to safeguard a radioactive waste depot. But green rust appears to be effective against nearly any kind of pollution,' said Dr Christiansen. 'A while ago we showed how green rust can react with carcinogenic chromium (6) and convert it to chromium (3). Besides being non-toxic, chromium (3) is one of the trace elements that the human body needs.' However, Dr Christiansen warns that green rust does not represent a 'quick fix' for pollution. 'Green rust is no quick fix to clean up after pollution that suddenly presents itself,' he states. 'But our experiments have shown the surprising result that nature can help to clean itself. Even when the pollution is with a substance as serious as neptunium.'For more information, please visit: University of Copenhagen:http://www.ku.dkGeochimica et Cosmochimica Acta:http://www.elsevier.com/locate/gca ACTINET project:http://www.actinet-i3.eu/ FUNMIG project:http://www.funmig.com/
Countries
Germany, Denmark