Problems Associated With the Storage of High-Level Nuclear Waste. (January 12, 2007)

    A letter in this weeks Nature by Ian Farnan, Herman Cho and William J. Weber describes the damage caused to a solid's structure by radio-active decay incorporated into the sort of ceramic proposed to be used for the storage of high-level nuclear waste.

The research done at the University of Cambridge, UK and the Pacific Northwest National Laboratory, Richland, Washington, USA is cogently summarised by the authors and ought to give pause to those designing any facility for the storage of spent high-level nuclear waste.

There are large amounts of heavy alpha-emitters in nuclear waste and nuclear materials inventories stored in various sites around the

The atomic order of a ceramic is     muddled into a glassy mess by     radiation.     Credit: Nature

 world. These include plutonium and minor actinides such as americium and curium. In preparation for geological disposal there is consensus that actinides that have been separated from spent nuclear fuel should be immobilized within mineral-based ceramics rather than glass because of their superior aqueous durability and lower risk of accidental criticality. However, in the long term, the alpha-decay taking place in these ceramics will severely disrupt their crystalline structure and reduce their durability. A fundamental property in predicting cumulative radiation damage is the number of atoms permanently displaced per alpha-decay. At present, this number is estimated to be 1,000–2,000 atoms/alpha in zircon. Here we report nuclear magnetic resonance, spin-counting experiments that measure close to 5,000 atoms/alpha in radiation-damaged natural zircons. New radiological nuclear magnetic resonance measurements on highly radioactive, ²³⁹Pu zircon show damage similar to that caused by ²³⁸U and ²³²Th in mineral zircons at the same dose, indicating no significant effect of half-life or loading levels (dose rate). On the basis of these measurements, the initially crystalline structure of a 10 weight per cent ²³⁹Pu zircon would be amorphous after only 1,400 years in a geological repository (desired immobilization timescales are of the order of 250,000 years). These measurements establish a basis for assessing the long-term structural durability of actinide-containing ceramics in terms of an atomistic understanding of the fundamental damage event.

 

Zircon (ZrSiO₄) has become the material of choice for many experiments modelling nuclear-waste storage.

The journal Science approached both Bruce Begg of the Australian Nuclear Science and Technology Organisation (ANSTO) and Linn Hobbs of the Massachusetts Institute of Technology Department of Nuclear Science and Engineering seeking their views of the work of Farnan, et al. and got opposing views.

Bruce Begg of the Australian Nuclear Science and Technology Organisation calls the Farnan team's work "very significant" but says it does not address the "key question": whether the alpha-induced transformation of ceramic to an amorphous state "has any detrimental impact on the ability of the waste form to lock up plutonium."

 

Many researchers believe it does. Linn Hobbs of the Massachusetts Institute of Technology Department of Nuclear Science and Engineering says that a form that becomes amorphous can change "the way that various elements are surrounding other elements." This could allow significant "dimensional changes" in the structure, according to Hobbs, which "may or may not have larger leach rates" into the surrounding environment.

In any case it's a good bet that the work will be cited by those opposing the building of nuclear power plants in Australia.

And the coal industry can continue to sleep untroubled, though it's unlikely it was ever very perturbed by the Prime Minister's public pronouncements on nuclear power while comforted in his continued de facto rejection of renewable energy technologies.