Advocates of nuclear power point to recent advances in waste storage materials that could allow the radioactive byproducts of the nuclear industry to be stored safely and indefinitely in ceramics rather than glass. However, a recent study using NMR suggests that storing high-level nuclear waste without leakage over thousands of years might not be possible with such materials.

Ian Farnan of the University of Cambridge together with Herman Cho and William Weber of Pacific Northwest National Laboratory, Richland, Washington studied the ability of synthetic zircons (zirconium silicates) to contain plutonium. Evidence from the study of zircons containing uranium and thorium show that they frequently endure extreme geological events over millions of years, hence the interest in their use as an alternative storage medium to glass.

The researchers used silicon-29 NMR spin counting experiments on samples with activities greater than 4 GBq. In this first application of radiological magic-angle spinning NMR, they demonstrated that as the plutonium decays, emitting an alpha-particle, the  associated recoil of the heavy nucleus knocks atoms in the ceramic out of kilter, which in turns makes the material more susceptible to break down. Such alpha-radiation damage is all but invisible at an atomistic level in the bulk to diffraction methods, whereas NMR can provide information about atoms whether they are in an amorphous or a crystalline bulk state.

They observed up to five times the number of atomic displacements per alpha-decay event than was previously estimated. This would suggest that ceramics destined for the storage of plutonium and minor actinides, such as americium and curium, could start leaking radioactivity after a mere 1400 years in a geological repository. This falls far short of the ideal immobilization timescale of 250,000 years.

In a commentary article accompanying the paper, Rodney Ewing of the University of Michigan, Ann Arbor, says that, "The increased damage observed by Farnan and colleagues in each alpha-decay event means that the amorphous state will occur in radioactively doped zircon sooner, rather than later. Such information is vital in assessing the durability of such materials, and in informing the development of strategies for the safe encapsulation of actinides." The findings do not preclude the use of zircon ceramics in the storage of radioactive waste but provides a stronger basis in long-term stability on which to make nuclear waste disposal decisions.

Related links:

• Nature, 2007, 445, 190-193
• Weber Page
• Cho Page

Article by David Bradley