Tests promise more robust nuclear containers

High-level waste is stored initially in cooling tanks

Highly radioactive nuclear waste could be stored safely and for longer using a new type of ceramic container, say an international team of researchers.

If this work points the way towards finding the absolute best radiation-tolerant material and it is then used as an encapsulation material, then it is phenomenally important

Dr Robin Grimes

Currently, the waste, after spending years in cooling tanks, is encased in glass and sealed in metal drums.

But this type of storage will only work for a limited period - perhaps no more than 100 years at most - before the intense radiation coming from the waste products starts to damage the containment materials.

Now researchers from the US, UK, and Japan have pinpointed a group of materials they believe will withstand the extreme conditions - possibly for thousands of years.

Swell and crack

Typically, after a period of temporary storage, when the most radioactive products have had a chance to decay, high-level waste from spent nuclear fuel is locked into a borosilicate glass and sealed in stainless steel casing.

But this is not really a long-term solution because the radiation emitted by the waste will slowly attack the integrity of the container.

The emissions jostle the atoms out of their carefully ordered arrangement within the storage materials. Eventually, this could make the materials swell and crack allowing highly toxic substances to leak out.

"If a material wants to be highly ordered, and the defects are putting atoms where the material doesn't want them, that raises the energy in the structure," said researcher Dr Kurt Sickafus, of Los Alamos National Laboratory in New Mexico.

"Ultimately, the material may have so much energy that it will suffer unwanted structural change."

Crystalline structure

The team say materials with a more disordered structure, in which atoms can shift positions with ease, will tolerate better the minute defects caused by radiation.

Spent fuel rods would be processed to concentrate the most radioactive fission products

Computer simulations carried out by Dr Robin Grimes and Licia Minervini at Imperial College London together with laboratory testing on complex oxides at Los Alamos confirmed the researchers' ideas.

Two test compounds, erbium titanate and erbium zirconate, were exposed to xenon gas to check their resistance to radiation.

As expected, the zirconate performed well. Its crystalline structure has cations - positively charged atoms - that can trade places whilst keeping the overall structure intact.

'Phenomenally important'

The titanate, on the other hand, with its more ordered and less flexible crystalline arrangement, was seriously weakened.

Current casing technologies are only a short-term solution

The researchers suspect that other crystalline materials with relatively disordered structures may be resistant to radiation damage as well.

"We think this might be a basic rule that applies to other materials beyond those in this study, but we'll have to do more work to be sure," Dr Sickafus said.

The work has provided a way of predicting the likely performance of candidate storage materials. Dr Grimes said: "If this work points the way towards finding the absolute best radiation-tolerant material and it is then used as an encapsulation material, then it is phenomenally important."