Certain microbes can use radionuclides such as uranium and neptunium in place of oxygen, studies have found. In doing so, they convert them from soluble to insoluble forms, making them less mobile.
This should give us more confidence in waste disposal plans, says Jonathan Lloyd, a geomicrobiologist at the University of Manchester, UK, who presented the research at the annual meeting of the Microbiology Society in Edinburgh.
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| Dinner is served Emory Kristof/National Geographic Creative |
The UK has accumulated around 4.5 million cubic metres of nuclear waste, enough to fill London’s Wembley stadium four times. Most of it is currently stored in ponds and silos at surface level at Sellafield in Cumbria. The government plans to dispose of the most highly active waste deep underground, in repositories encased in cement, but has yet to decide on a site. These plans take into account physical and chemical properties to stop radioactive material from escaping for hundreds of thousands of years – but not biological.
It had been thought that the presence of cement would result in conditions too alkaline for microbes to grow – it has a pH of around 11, similar to bleach. To see if this was so, Lloyd’s team studied a lime kiln site in the UK’s Peak District to see if microbes could be found growing in conditions similar to those that would be expected in a nuclear disposal site. “We went to see if there was biology there and there was,” says Lloyd. “We found they could grow at pH values you would probably find developing around these cementitious waste forms.”
“Radiation levels found at nuclear waste dumps don’t kill these bacteria, they stimulate them”
The radiation levels typically found at nuclear waste dumps don’t seem to pose a problem for bacteria either.
“It doesn’t kill them,” says Lloyd. “If anything, it actually stimulates the microbes.”
The study found that the way bacteria process waste products means hazardous material is less likely to seep into the environment. Some nuclear waste contains cellulose, which can break down to form isosaccharinic acid (ISA) under alkaline conditions. ISA can form a soluble complex with uranium, helping it to leak out of the waste repository. But bacteria seem to use ISA as a carbon source and degrade it, keeping radionuclides in solid form – which means they stay in place.
Microbes may also help prevent radioactive gases escaping. Hydrogen produced by reactions in the repositories could build up pressure and cause them to crack open or explode. But microbes can use hydrogen and keep the levels down. They can also grow in fractures in the rock, form biofilms and clog up pores.
“At the moment, they have safety case models that are built on chemistry and physical containment. If you start including the biology, it means that those models are actually overly conservative, which is a good thing,” says Lloyd.
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