Certain microbes can use radionuclides such as uranium and neptunium instead of oxygen. In the process, they convert the radionuclides from soluble forms to insoluble forms which makes them less mobile. This should improve our confidence in radioactive waste disposal plans according to Jonathan Lloyd. Lloyd is a geomicrobiologist at the University of Manchester in the U.K. He presented his research at the annual meeting of the Microbiology Society in Edinburgh.
The U.K. has accumulated about six million cubic yards of nuclear waste. This would be enough to fill London’s Wembley stadium four times. Most of it is currently stored in ponds and silos at ground level at Sellafield in Cumbria. The U.K. government plans to dispose of the most highly radioactive waste deep underground in repositories encased in cement. However, the government has yet to select a site for the repository. The plans take into account physical and chemical properties to stop radioactive material from escaping for hundreds of thousands of years. However, biological properties and processes are not included in the planning.
Cement has a pH of about eleven. It has been thought that this would result in alkaline conditions that would make growth of microbes impossible. In order to verify this theory, Lloyds team investigated a lime kiln site in the U.K.’s Peak District in order to see whether microbes could be found growing in conditions similar to those that would be found in a nuclear waste disposal site. He said, “We went to see if there was biology there and there was. 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 do not seem to bother bacteria either. “It does not kill them. If anything, it actually stimulates the microbes.
The study found that the way bacteria process nuclear waste products means that this hazardous material is less likely to leak into the environment. Some nuclear waste contains cellulose. This material can break down to form isosaccharinic acid (ISA) under alkaline conditions. ISA can form a soluble complex with uranium which helps it leak out of a nuclear waste repository. However, bacteria seem to use ISA as a carbon source which keeps radionuclides in a solid form. This means that they remain in the repository.
Microbes may also help prevent radioactive gases from escaping. Hydrogen produced by reactions in the waste repositories can build up pressure and cause them to crack open or explode. However, microbes can consume hydrogen and keep the levels of hydrogen down. They can also grow in fractures in the rock. This process results in the formation of biofilms which clog pores in the rock.
Lloyd said, “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.”