Friday, January 19, 2007

Nuclear Waste - Less Forgettable than was Thought!

Consensus of opinion has been that high level nuclear waste can be encased in a suitable "inert" material and then buried underground for long enough that the radioactive elements decay to a harmless level - i.e hundreds to thousands of years. However, new measurements show that "inert" materials are changed by the radiation, potentially into forms that are less effective in locking-in radioactive elements such as plutonium over the long time periods required. Ian Farnan at the University of Cambridge has shown that one promising material - called zircon (zirconium silicate); also used for making synthetic gemstones - might be transformed from a hard, crystalline synthetic rock into a far less robust glass after just 1,400 years. Since many radioactive nuclei have half-lives much longer than this, and will persist in emitting radiation at dangerous levels for perhaps a quarter of a million years, this is not good news. Plutonium-239, for example, has a half life of 24,000 years (meaning that after 24,000 years, half the radioactivity will have decayed), and 10 half lives is the rule of thumb at which point the radioactivity has decayed to about 1/1000 th of its original intensity, i.e. 240,000 years (or near enough a quarter of a million years) so that the waste is fairly "harmless". I use that word advisedly, but meaning that by then the waste would be classified as "low-level" and less demanding in terms of the stringency that must be met to secure public protection from it.
Zircon, or any other material used to encapsulate nuclear waste, must hold the radioactive elements almost like a kind of scar-tissue, and prevent them from finding their way into the environment, e.g. into ground water. However, it is well known that radiation cases damage to most materials, modifying them chemically. Radioactive elements such as plutonium (also referred to as actinides, which are inescapable products of nuclear fission) emit alpha-particles when one of their atomic nuclei decays. Different kinds of radiation results in different types and levels of damage through a mechanism known as "linear energy transfer, LET": for example, a fast electron or a gamma-ray photon deposits its energy relatively slowly as it traverses a medium, and leaves a long radiation track behind it. I imagine such an entity as being something like a rifle bullet, whereas an alpha-particle is more like a cannon-ball and travels over a shorter distance but delivers most of its energy in one big punch, literally knocking hundreds of atoms out of its way. Further and worse damage is then caused by the fact that the nucleus that has "fired" the alpha-particle is knocked-backward, like the recoil of a cannon, and being much heavier travels a shorter distance but can knock thousands of atoms out of its way as it does so.
The effect is to destroy the initial stable crystalline arrangement of atoms in the tough zircon structure, and to turn them into an amorphous glassy material, which swells and becomes a less effective barrier with the environment. It is found that samples of zircon that have been damaged by radiation will dissolve in water hundreds of times faster than the original material, and so in the real situation of a nuclear waste depository, if the store should become wet the contamination could leak-out, with unforeseen but likely undesirable consequences.
Most previous evaluations of the effects of radiation on ceramics such as zircon, intended for use to encase nuclear waste, have been made mostly using calculations and computer models (rather like the vexed issue of climate change). These new conclusions arise from direct measurements made by Farnan at Cambridge using nuclear magnetic resonance (NMR), which are similar to MRI (magnetic resonance imaging) scans of the human body, to look for tumours and other conditions. The word "nuclear" was deliberately kept out of an acronym for the latter type of procedure as lay persons are understandably nervous about what that might mean in practical terms - perhaps thinking that it involves their being placed inside an atomic pile. Hence MRI is a euphemism for NMR. However, NMR is able to measure the relative proportions of crystalline and glassy regions in the zircon therefore allowing the influence of radiation in driving the phase transition to the glassy state to be seen directly. Other materials are known which may funnel-off the storms of radiation over long periods, but it is clear that not enough is known about the longer run durability of ceramic materials for storing high level nuclear waste (over the effectively geological periods necessary to run-down its radioactivity to safe levels), to conclude that the technology is in fact safe. Clearly, the issue of nuclear waste disposal is far from resolved, although the subject is often presented as if it no longer poses any real problem. At the dawn of the promised second nuclear age, apparently as part of the war against climate change, it remains a sobering matter for concern.

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