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Canned nuclear waste cooks its container

January 10, 2007 By Philip Ball This article courtesy of Nature News.

Estimates of radiation damage to materials have been too low.

Storing high-level nuclear waste without any leakage over thousands of years may be harder than experts have thought, research published in Nature today shows.

Ian Farnan of Cambridge University, UK, and his co-workers have found that the radiation emitted from such waste could transform one candidate storage material into less durable glass after just 1,400 years much more quickly than thought1.

Current plans for disposal of some of the most dangerous material generated in nuclear power plants, such as radioactive elements extracted from spent fuel rods, differ from one country to another. A common strategy being explored is to encase the waste in a hard, crystalline ceramic material a kind of synthetic rock and then put it in steel canisters and bury them in cavities excavated underground.

Because many radioactive substances continue emitting radiation for a very long time, the containment must persist for an awesome duration. Plutonium-239, one of the most deadly by-products of nuclear power, has a half-life of 24,000 years, meaning that only half of any initial batch has decayed over this time. Ideally it should stay put for about ten times as long: a quarter of a million years.

Candidate ceramic

Farnan and colleagues have investigated one candidate material hoped to do the job, called zircon (zirconium silicate). The plan is that this ceramic material will hold on fast to the radioactive atoms and stop them from finding their way into the environment for example by being dissolved and dispersed in ground water.

The problem is that the radioactive waste damages the matrix that contains it. Many of the waste substances, including plutonium-239, emit alpha radiation, which travels for only very short distances (barely a few hundredths of a millimetre) in the ceramic, but creates havoc along the way.

A fast-moving alpha particle knocks into hundreds of atoms in its path, scattering them like skittles. Worse still, the radioactive atom from which the particle comes is sent hurtling in the other direction by the recoil. Even though its path is even shorter than that of an alpha particle, the atom is much heavier, and can knock thousands of atoms out of place in the ceramic.

All this disrupts the crystalline structure of the ceramic matrix, jumbling it up and turning it into a glass. That can make the material swell and become a less secure trap. Farnan says that some zircons that have been heavily damaged in this way by radiation have been found to dissolve hundreds of times faster than undamaged ones. So if the ceramic gets wet, there could be trouble.

Hit and run

Previous estimates of the radiation damage to waste-storage ceramics have relied largely on calculations and computer simulations. Now Farnan and colleagues have measured it directly.

They used a technique called nuclear magnetic resonance spectroscopy similar to the method of magnetic resonance imaging (MRI) used in biomedicine to measure the relative amounts of crystalline and glassy material, both in artificial zircon containing plutonium and in naturally occurring mineral zircon, which commonly contains radioactive uranium. They estimate that each alpha-decay event of a radioactive atom displaces around 5,000 atoms in the zircon - between 2.5 and 5 times more than predicted previously.

"There's more damage than we thought," says Rod Ewing, a specialist in nuclear-waste disposal at the University of Michigan in Ann Arbor.

There are other materials that may fare better than zircon, including other zirconium minerals. But Farnan's work implies that we probably don't yet fully understand how well any of these materials might stand up to the battering of radiation. He thinks the findings should encourage engineers to think very carefully about the matrix encasing the radioactive waste, rather than focusing on the geological characteristics of the burial site. Ideally, the best material would be able to heal itself, with the atoms displaced by alpha decay moving back slowly into their crystalline positions.

Ewing notes that the technique used in this study could be used to investigate these alternative materials, hopefully to find a longer-lived candidate.

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  1. Farnan I., et al. Nature, 445. 190 - 193 (2007).


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