A new technique tracks the structural damage nuclear waste inflicts on its storage material.
Scientists have announced a new way to assess the safety of storing nuclear waste. The technique, based on the familiar chemical principles of nuclear magnetic resonance (NMR), tracks the damage radioactive particles cause as they smash into atoms of surrounding material. Already, the method has shown that the ceramic mineral zircon (ZrSiO?4), a candidate for storing nuclear waste for over 250 000 years, would lose its ordered structure in a far shorter time.
As co-author Ian Farnan, from the University of Cambridge, UK, explained, scientists are still unsure of the best ways to store some long-lived radioactive elements, like plutonium and other actinides, that are created during nuclear fission. Plutonium-239, for example, enjoys a half-life of 24 110 years, and emits alpha particles by radioactive decay.
Although both Britain and the US plan to bury their nuclear waste underground, it is unclear whether surrounding rocks, copper canisters, and clay infill would provide enough protection from radioactive leakage. For extra safety, current thinking suggests, the radioactive elements should be combined with glass or minerals to form an immobile crystal, before being buried deep underground.
The crystal’s ordered atomic structure will gradually degrade as alpha particles shoot out of decaying elements, colliding with surrounding atoms. But measuring how long various crystal types can contain their radioactive stores has been a difficult task. Farnan’s team artificially doped zircon with radioactive plutonium, and used solid-state NMR to follow how many atoms were smashed out of alignment by each alpha particle decay. The highly radioactive samples had to be spun at more than 200 000 rotations a minute, and were triply sealed for safety.
The alpha particle damage was much greater than expected. The researchers suggest that zircon would start to swell and potentially crack within 210 years if plutonium was stuffed into the structure at the current guidelines of 10 per cent by weight. That sounds like bad news for zircon advocates. But, as Bruce Begg, of the Australian nuclear science and technology organisation, pointed out, a loss of ordered chemical structure doesn’t always mean that a material can’t contain nuclear waste. Borosilicate glasses, for instance, have no ordered crystalline structure at all, but can still lock up plutonium. And, Begg said, titanate zirconolites and pyrochlores (ceramics with a different crystal structure) seem to stay durable as their crystalline structure is destroyed.
The research highlights how little we understand about the long-term behaviour of nuclear storage materials, agree Ian Farnan and co-authors. ’Efforts should be made to produce a waste form which is tougher and has a durability we are confident of, before it is stored underground and before anyone tries to engineer around it,’ Farnan advised.
Richard Van Noorden