Clay – a radioactive waste solution?

Materials World magazine
,
8 May 2011
Two diffusion cells with built-in clay cylinders and vials with pore water containing neptunium and plutonium and a peristaltic pump. Image courtesy of the Institute of Nuclear Chemistry at the University of Mainz, Germany

A timely investigation into nuclear waste storage alternatives could see clay become a crucial commodity in Europe.

Researchers at the Institute of Nuclear Chemistry at the University of Mainz, Germany, are trialling deposits of Opalinus clay found in Switzerland and the southern part of Germany, to store radioactive plutonium (Pu) and neptunium (Np).

Argillaceous rock formations have recently been under consideration as potential host rock for high-level nuclear waste repositories in countries such as Switzerland, France, and Belgium.

Lead Researcher Professor Tobias Reich explains, ‘Argillaceous rocks have a low hydraulic conductivity and high sorption ability for actinides. The dominant process governing the transport of radionuclides in clay is diffusion. If a chemical element is strongly sorbed on the clay, its diffusion through a clay formation will be very slow.’

The average mineralogy of the clay being used has been 66% clay minerals (23% illite, 11% illite/smectite mixed layers, 10% chlorite, 22% kaolinite), 13% calcite, 14% quartz, three per cent siderite, one per cent pyrite and other minor components.

Reich continues, ‘In a geological repository, a multi-barrier system is foreseen to ensure the long-term containment of high-level nuclear waste. In the case of the direct disposal of spent nuclear fuel, the cladding containing the fuel pellets is the first engineered barrier. Special metal containers containing several fuel elements provide the second engineered barrier of the repository. After placing these containers into the tunnels of the repository, the tunnels are backfilled with bentonite (third engineered barrier). Then the final surrounding host rock [clay], together with overlaying formations, form the geological barrier.’

To test the suitability of the clay and conduct diffusion results over a reasonable time (months, instead of years), the team cut the bore cores into small round disks, with a thickness of 11mm. These clay disks were packed in diffusion cells and contacted with clay pore water containing radioactive Np or Pu.

‘We performed two types of experiments – batch experiments with clay powder suspended in pore water to determine the uptake of Np and Pu by the clay, and diffusion experiments with intact clay cylinders to determine the diffusion parameters of Np and Pu. We found that nearly all Np and Pu in the pore water is taken up by the clay. Due to the strong sorption of Np and Pu on the clay, the diffusion through intact clay is very slow,’ adds Reich.

The next step will involve comparative research into clays found in the northern and southerns parts of Germany to assess the difference in pore water composition. ‘The ionic strength of Opalinus clay pore water is about 0.4 mol/L, whereas pore water in contact with clay in northern Germany has a much higher salinity (up to four mol/L). The influence of the higher salinity on the sorption and diffusion properties of the clay is therefore an area for investigation,’ notes Reich.

Geochemist Fergus Gibbs at the University of Sheffield, UK, comments, ‘Clay-rich rocks are favoured as hosts for the disposal of nuclear waste, principally because of their low permeability to water and their potential to retard the migration of radionuclides through sorption onto the surfaces of the clay particles. Quantification of these effects is crucial to the safety case for repositories in such rocks, and the reported work looks encouraging, especially if it proves irreversible.’

 

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