Clay as sealing material in nuclear waste repositories

some agent. This agent is usually water, which can transport the radionuclides in dissolved form or in colloids or small particles. We can therefore establish that a basic characteristic of a safe repository is for it be impermeable, that is, water should not be able to penetrate it. At this stage it is appropriate to consider the physical form in which the waste will be taken to the repository and buried. The nuclear material will be condensed as a glass and introduced in metal canisters very resistive to corrosion. The amount of waste in the canister and the design of the canister will be such that the radioactive decay produces a maximum temperature on the outer surface of the canister of approximately 100 °C. The metal canister is the first protective barrier and is designed to last about 1000 years in favourable conditions of low fluid presence, mobility and corroding power. Thus, the required characteristics of the facility where the waste will be buried are as follows: 1. It should be able to adsorb the small amount of water that might be present in a ‘dry’ repository; 2. It should hinder any water flow to or from the repository; 3. It should be able to transmit the heat generated in the canisters to allow heat dispersion and avoid steep temperature gradients (these gradients are dangerous because they can physically or chemically destabilize the emplacement and trigger undesired processes); 4. It should have plastic behaviour that allows it to deform without the formation of fissures, in Javier Cuadros Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK. [email protected] Nuclear waste from thermal plants poses a lasting risk to the biosphere because of its long radioactive life. The planned definitive storage place for it is in deeply buried repositories. Such repositories would need to be both impermeable to water, and plastic during deformation, in order to avoid the formation of cracks that may allow water in. One of the clay minerals, smectite, has these two properties and is an ideal candidate as a sealing material or even host rock for nuclear waste repositories. The chemical stability of smectite in the repository environment is sufficient to maintain good sealing properties during the active life of the relevant radionuclides.