Abstract
There are large amounts of heavy α-emitters in nuclear waste and nuclear materials inventories stored in various sites around the world1. These include plutonium and minor actinides such as americium and curium. In preparation for geological disposal there is consensus2 that actinides that have been separated from spent nuclear fuel should be immobilized within mineral-based ceramics rather than glass2,3,4 because of their superior aqueous durability and lower risk of accidental criticality. However, in the long term, the α-decay taking place in these ceramics will severely disrupt their crystalline structure2,3,4 and reduce their durability5,6. A fundamental property in predicting cumulative radiation damage is the number of atoms permanently displaced per α-decay. At present, this number is estimated to be 1,000–2,000 atoms/α in zircon4. Here we report nuclear magnetic resonance, spin-counting experiments that measure close to 5,000 atoms/α 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.
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Acknowledgements
We thank N. R. Johnson, A. E. Kozelisky and R. D. Scheele for help with initial experimental development, sample handling and logistical support, and M. Zhang for help with the natural zircons. I.F. acknowledges funding from the UK EPSRC. A portion of the research described here was performed under a user programme at the Environmental Molecular Sciences Laboratory of the Pacific Northwest National Laboratory. PNNL staff and work were supported by the Environmental Management Science Program, Office of Biological and Environmental Research, US Department of Energy.
Author Contributions I.F. carried out the NMR work on natural zircons. I.F. conceived the radioactive MAS NMR experiments and I.F. and H.C. developed the technology and performed them in H.C.’s laboratory. W.J.W. provided the Pu zircon ceramics, supporting characterization data and radiological support. I.F. wrote the paper and all authors had a chance to contribute to and comment on the manuscript.
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Supplementary Figures
This file contains Supplementary Figures S1-S2. Figure S1 shows distribution of displaced atoms between crystalline and amorphous regions. Figure S2(a) shows saturation recovery T1 under static conditions and Figure S2(b) shows saturation recovery T1 under magic angle spinning. (PDF 553 kb)
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Farnan, I., Cho, H. & Weber, W. Quantification of actinide α-radiation damage in minerals and ceramics. Nature 445, 190–193 (2007). https://doi.org/10.1038/nature05425
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DOI: https://doi.org/10.1038/nature05425
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