International scientists are using computer simulations to give insights into the long-term safety of nuclear waste in deep geological repositories.

Disposing of radiotoxic elements safely is a big challenge, with the concept of storing it in rooms excavated deep within stable, low-permeability rock being one possible solution.  Such a repository would need to be stable for up to a million years into the future, so understanding how the materials would interact with each other over that time is vital.

It is particularly important to retain actinides safely because they have such long radioactive half-lives.  One way this is done is by forming thermodynamically stable solid solutions, where the solids' cations can be substituted by the actinides preventing them from leaching out.  Victor Vinograd at the Institute of Geosciences, Frankfurt, Germany, Dirk Bosbach at the Institute of Nuclear Waste Disposal, Karlsruhe, Germany, and international collaborators have carried out computer simulations to try to understand the behaviour of one such solid solution based on powellite, a naturally occurring mineral, in which some calcium cations are substituted with sodium and europium.

'Europium has similar chemical behaviour to the actinides. Therefore, such simulations help us find the conditions under which radionuclides are stable in the repository system,' explains Vinograd. The team found that at temperatures typical for repository conditions, europium would regularly alternate with calcium and sodium in the powellite structure. Vinograd says that the results leave little doubt that at these temperatures, the formation of stable powellite solutions is possible only at specific calcium/lanthanide(actinide) ratios. The team hopes to move on to chemically more complex waste repository systems in the future, he says.

Madelaine Chapman