text: 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.
  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.
  Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
target: 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.