Radiation degrades nuclear waste-containing materials much faster than expected

In some European countries where law-makers have legislated in favor of phasing out nuclear power over the coming decades, the debate on the energy source has been reopened. Arguments around climate change and energy security are driving the discussions.

This is the case in Sweden and Belgium, two countries heavily dependent on nuclear energy, and to a lesser extent in Germany. Those in favor of keeping the reactors running argue they produce a clean and cheap form of energy that can help fight climate change. They also try to cast doubt over the potential of energy efficiency and renewables like solar, wind and biomass to replace the decommissioned nuclear power plants sufficiently fast. All careful phase-out plans however prove the contrary. Finally, their case was strengthened by the recent gas dispute between Belarus and Russia, which was seen by many as an omen symbolising Europe's increasingly fragile energy security. Nuclear power is much more reliable than dependency on foreign energy resources, so the argument goes.

On the European level, the EU's recently launched energy/climate plan has carefully avoided taking any clear position in the debate. The Commission keeps its hands off the issue and relegates decisions on nuclear back to the national level. France, the world's biggest exporter of both electricity as well as nuclear technology, has obviously had a strong hand in crafting this strategy.

Those against continuously point to the issue that has been haunting nuclear energy for decades, and that is of course the unavoidable, gloomy question of how to dispose of radioactive waste. The nuclear lobby has been avoiding the subject for as long as there have been reactors. In the 1960s it said it would solve the problem forever within two decades. Two decades later, it said the same. Today, we are still using the temporary 'solution' of storing the waste in glass, and locking it up in bunkers in some mountain or in the ocean, without really understanding how safe these techniques are over the ultra-long term.

As time passes by, scientists develop new models and methods that can help us forward in addressing the problem. But they also highlight our fundamental ignorance on the matter. Recent research shows where we stand: minerals that were intended to entrap nuclear waste for hundreds of thousands of years may be susceptible to structural breakdown within 1,400 years, much faster than expected, a team from the University of Cambridge and the Pacific Northwest National Laboratory reported in the journal Nature.

The new study used nuclear magnetic resonance, or NMR, to show that the effects of radiation from plutonium incorporated into the mineral zircon rapidly degrades the mineral's crystal structure.

This could lead to swelling, loss of physical strength and possible cracking of the mineral in as soon as 210 years, well before the radioactivity had decayed to safe levels, says lead author and Cambridge earth scientist Ian Farnan:
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According to current thinking, highly radioactive substances could be rendered less mobile by combining them, before disposal, with glass or with a synthetic mineral at a very high temperature to form a crystal.

However, the crystal structure can only hold the radioactive elements for so long. Inside the crystal radioactive decay occurs, and tiny atomic fragments called alpha particles shoot away from the decaying nucleus, which recoils like a rifle, with both types repeatedly blasting the structure until it breaks down.

This may increase the likelihood for radioactive materials to leak, although co-author William J. Weber, a fellow at the Department of Energy national laboratory in Richland, Wash., who made the samples used in the study, cautioned that this work did not address leakage, and researchers detected no cracking. Weber noted that the "amorphous" or structurally degraded, natural radiation-containing zircon can remain intact for millions of years and is one of the most durable materials on earth.

Some earth and materials scientists believe it is possible to create a structure that rebuilds itself after these "alpha events" so that it can contain the radioactive elements for much longer. The tests developed by the Cambridge and PNNL team would enable scientists to screen different mineral and synthetic forms for durability.

As well as making the storage of the waste safer, new storage methods guided by the NMR technique could offer significant savings for nations facing disposal of large amounts of radioactive material. Countries including the United States, Britain, France, Germany and Japan are all considering burying their nuclear waste stockpiles hundreds of meters beneath the earth's surface. Doing so necessitates selection of a site with sufficiently stringent geological features to withstand any potential leakage at a cost of billions of dollars. For example, there is an ongoing debate over the safety of the Yucca Mountain site in Nevada. A figure published in Science in 2005 put that project's cost at $57 billion.

"By working harder on the waste form before you started trying to engineer the repository or choose the site, you could make billions of dollars worth of savings and improve the overall safety," Farnan said.

"At the moment, we have very few methods of understanding how materials behave over the extremely long timescales we are talking about. Our new research is a step towards that.

"We would suggest that substantive efforts should be made to produce a waste form which is tougher and has a durability we are confident of, in a quantitative sense, before it is stored underground, and before anyone tried to engineer around it. This would have substantial benefits, particularly from a financial point of view."

PNNL senior scientist and nuclear magnetic resonance expert Herman Cho, who co-wrote the report, said: "When the samples were made in the 1980s, NMR was not in the thinking. NMR has enabled us to quantify and look at changes in the crystal structure as the radiation damage progresses.

"This method adds a valuable new perspective to research on radioactive waste forms. It has also raised the question: 'How adequate is our understanding of the long-term behavior of these materials?' Studies of other waste forms, such as glass, could benefit from this technique."

More information:
Ian Farnan, Herman Cho and William J. Weber, Quantification of actinide alpha-radiation damage in minerals and ceramics [abstract], Nature 445, 190-193 (11 January 2007).

Nature News: Canned nuclear waste cooks its container. Estimates of radiation damage to materials have been too low. January 10, 2007.

Eurekalert: Radiation degrades nuclear waste-containing materials faster than expected. January 10, 2007.