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Long-term cement study seeks nuclear waste solution

UK scientists say they have produced a new mix of cement that should be much more effective at containing nuclear waste in a deep repository.

The material develops mineral phases that readily trap radioactive isotopes trying to pass through it.

Investigations at the atomic scale indicate the cement ought to retain this ability for at least 2,000 years.

The Sheffield University team believes the new mix is up to 50% better than previously proposed barrier solutions.

At some point the government will choose the location of an underground store for the hundreds of thousands of cubic metres of waste built up over more than 60 years of nuclear operations. A lot of this material will be immobilised and backfilled using cement (the binder in concrete).

This cement will need to block the passage of radioisotopes far into the future.

The Sheffield experiments have been performed at the Diamond Light Source in Oxfordshire.

This is the UK's big synchrotron, which shoots X-rays into samples to reveal their structure on the smallest scales.

Diamond now has a lab - or beamline, as it is called - that is specifically given over to long-duration studies.

It has allowed Dr Claire Corkhill and colleagues to probe the changing properties of different mixes of cement over the past 18 months.

"We've been able to gather some very high-resolution data, and this has allowed us to make some predictive models so that we can understand what phases are forming, and when, out to 2,000 years, which is exactly when we expect water to start interacting with a geological disposal facility," said the scientist from Sheffield's NucleUS research group.

Their optimum cement - known currently simply as No7 - contains blast-furnace slag.

The sulphides this introduces react with water to produce sulphate mineral phases that are exceptionally good at sorbing technetium-99.

"It's a high-yield fission product; it's only found in nuclear reactors; it's very mobile in the environment - but what we found is that our cement will actually lock tight this technetium-99 into its structure and prevent it being transported into the environment," explained Dr Corkhill.

The team's investigations show No7 to be a much better performer than the currently proposed cement barrier, called Nirex Reference Vault Backfill. But this is not the end of the story - further mixes are being investigated to find even more effective solutions.

The cement work has been discussed here at the annual meeting of the American Association for the Advancement of Science (AAAS) - as has the long-duration experiment facility at Diamond. It is the only one of its kind in the world, and was set up specifically to permit scientists to study the temporal behaviour of materials.

Researchers put their samples on a robotic bench and then leave the machine to it.

"It's a bit like a hotel for samples, or imagine a karaoke machine," said Prof Trevor Rayment, Diamond's director of physical sciences.

"Once a week, automatically and remotely, the sample is wheeled out across a table into the X-ray beam, and the data is collected.

"Then, that particular sample is withdrawn and somebody else's experiment is moved into the beam to gather their data. This could go on for two years.

"The scientists can stay in the comfort of their offices."

A lot of requests to use the facility have to do with battery technology - understanding how materials inside power cells change through countless charging cycles.

Britain leads race to make nuclear waste safe for 100,000 years

British scientists are designing a revolutionary cement that could withstand the impact of intense radiation for thousands of years. The project could prove vital in dealing with the challenges of Britain’s proposed expansion of its nuclear industry.

The government has announced plans to build several nuclear power stations over the next decade to provide power previously generated by coal, oil and gas stations. These are now likely to be phased out as part of the UK’s climate change commitments. However, a move towards more nuclear power will lead to the generation of extra nuclear waste.

It is estimated that about 300,000 cubic metres of highly radioactive intermediate waste – including old fuel rods and irradiated reactor components – will have accumulated in the UK by 2030 as a result of this expansion. At present, these swelling stocks are stored above ground, near reactors. However, their growing risk to the environment has forced the government to pledge to dispose of the material underground in a major depository.

However, before its location is agreed, planners will have to ensure that the waste will remain safe for at least 100,000 years, the time needed to allow their radioactivity to decay to a safe level. A key part of that effort will include the design of cement that can withstand intense radiation levels so that it can be used to cover nuclear waste once it has been placed in pits deep underground.

“To work out how materials – in this case cement – are going to behave for tens of thousands of years is quite mind-boggling, but that is exactly what we are now doing,” said the project’s leader, Claire Corkhill of Sheffield University. She is due to present details of the project at the annual meeting of the American Association for the Advancement of Science in Washington on Sunday.

The key to her team’s project is the UK’s Diamond Light Source, near Oxford. The facility accelerates electrons almost to the speed of light, so that they give off a light 10 billion times brighter than the sun. These bright beams are then directed off into laboratories, where they are used to study the properties of many different types of material: ice, viruses, cancer drugs – and cement.

“Many of the technological problems that affect society today are ones that take place at a very slow rate: melting ice in the Arctic or the decay of batteries,” said Professor Trevor Rayment, head of physical sciences at Diamond. “By using our machine to measure very accurately changes taking place in the materials we are studying, we can discover how those changes might affect them in hundreds or thousands of years.”

In the case of the cement being studied, Corkhill and her team have examined how it reacts with water over the long term. “That interaction between water and cement granules can go on for decades,” added Corkhill. “We are using Diamond to predict what cement will be like thousands of years in the future. No one has ever done that before.”

From this work, the group has designed a new form of cement which could then be used to cover nuclear waste inside underground stores. “That cement will be able to capture all of the radioactive elements that might be released from the waste over time,” added Corkhill. “Cements that are currently in use do not do this. Our cement will therefore make nuclear waste disposal even safer.”



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