Radioactive waste is currently stored in containers that are expected to last only about 100 years. Longer-term storage will require containers that can resist radiation damage for thousands of years, and now an international collaboration believes it has identified a class of ceramics that fits the bill. Kurt Sickafus and his colleagues in the US, the UK and Japan describe their findings this week (2000 Science 289 748).
The so-called ‘complex oxides’ are a group of ceramics with a common chemical formula consisting of two pairs of metallic cations and seven oxygen atoms. Sickafus and co-workers at Los Alamos National Laboratory, Imperial College and the University of Osaka used computer simulations to predict that the relative size of the cation pairs would determine a material’s resistance to radiation damage. Radioactive emissions from nuclear waste knock atoms out of place in container walls. The simulation showed that different-sized pairs of cations tend to produce highly regular atomic structures, similar to the mineral pyrochlore, in which the cation pairs can’t change places easily. The displaced atoms therefore tend to build up, eventually causing the material to swell and fracture.
‘If a material wants to be highly ordered, and radiation defects are putting atoms where the material doesn’t want them, that raises energy in the structure. Ultimately, the material may have so much energy that it will suffer unwanted structural damage’, Sickafus explains.
In contrast, ceramics with similar-sized cation pairs have less regular structures, like the mineral fluorite. But the pairs can easily swap places and the material is therefore likely to be more resistant to radiation damage because the displaced atoms can dissipate, rather than building up to critical concentrations. Sickafus thinks this could be a basic rule that applies to materials beyond those in this study, but says there is more work to be done.
The team has also performed some preliminary experiments, irradiating crystals with pyrochlore- and fluorite-type structures. These initial results strongly support their theory, and they plan to extend their investigations to other ceramics. ‘We are continuing irradiation studies on fluorite/pyrochlore compounds’, Sickafus told PhysicsWeb, ‘and we have already synthesized some cerium-containing fluorite compounds which we have shown are highly tolerant to radiation. We also think that similarly structured oxides containing actinides like depleted uranium will be robust under irradiation’.
According to the group, these new findings will be invaluable for the development of chemically durable and radiation-tolerant containers for safe and reliable storage of radioactive waste.