Simulation of radioactive debris to aid clean-up at Fukushima site

Despite an 11-year-long clean-up operation at the site in Japan, the mixture of nuclear fuel and debris inside the three damaged reactors remains in place and is currently too dangerous to be dealt with.

To tackle this, a University of Sheffield team has worked with the Japanese authorities to develop a material that simulates the highly radioactive mixture at the centre of the plant. They hope this will help crack the method for the ultimate disposal of the ultra-dangerous debris.

South Yorkshire residents will be reassured that there is no similarly dangerous stock of simulated debris sitting in a laboratory somewhere in Sheffield as the project has involved replacing the most dangerous elements, including plutonium, with non-radioactive substitutes which replicate their properties.

“Being able to understand something about the material inside is crucial to developing the retrieval strategy,” said project lead Professor Claire Corkhill. “Until that fuel is retrieved, there’s always going to be a hazard at Fukushima.”

Corkhill, who is the chair in Nuclear Material Degradation in the university’s Department of Materials Science and Engineering, said the team are interested in understanding what the molten nuclear fuel inside the Fukushima reactors actually looks like.

“Because it is so radioactive, nobody has got any out – it’s too hazardous,” she explained. “But that’s really problematic because if you’re going to design a strategy to remove it, you need to know something about its chemical properties, its mechanical properties, how hard is it, what kind of robot should we use to cut it up.”

Corkhill said efforts were continuing to access the highly dangerous debris at the site and a robotic survey of the material in reactor one has now been launched by the Tokyo Electric Power Company.

She said the problem was getting more immediate because it takes 150m³ of water an hour to keep the reactors cool and storing this dangerous liquid is a “major headache”. She added the material inside is so toxic that if you were to sit on a container for a minute, there is a 50 per cent chance of death.

In collaboration with the Swiss Light Source, her team have gathered “ingredients” which would have been inside the reactor at the time of the 2011 disaster, including debris such as stainless steel flasks and specialist concrete and the fuel.

“We made a recipe essentially from what we know was inside the reactor and we’ve mixed it up and, a bit like a cake, we’ve baked it in an oven at the high temperatures of the accident – around 2,000°C – under the right conditions,” she said. “And we’ve refined that process until we’ve got something that we think looks like the material inside that reactor.”

Corkhill said the material can be handled and experts can perform experiments on them to get a better idea of what the properties of that material are. This will enable engineers to design robots or other technologies that would last inside the reactors.

According to Corkhill, the work has shown that essential information can be gleaned from a small amount of debris, which will be vital when samples of the actual material are eventually brought out of the reactor.