Quantification of actinide α-radiation damage in minerals and ceramics

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 glass^2,3,4 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 structure^2,3,4 and reduce their durability^5,6. 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.