Dr. Robert Harrison is a Dalton Research Fellow at the Dalton Nuclear Institute in the University of Manchester in the U.K. His research has just been published in the journal of Corrosion Science. He says, “Since the 2011 Fukushima accident there has been an international effort to develop accident tolerant fuels (ATFs), which are uranium-based fuel materials that could better withstand the accident scenario than the current fuel assemblies.”
The current favorite nuclear reactor fuel consists of pellets of uranium oxide. One of the promising ATFs is a compound of uranium silicon (U3Si2). This compound is able to conduct heat much better than traditional uranium oxide fuel. This allows reactors cores to be operated at lower temperatures. In emergencies, this would permit operators more time bring the reactor core under control before it melts down.
Before a useful fuel can made from U3Si2, there are many questions that need to be answered about how this compound behaves in the core of a reactor. Harrison said, “One of these unknowns is how it will behave when exposed to high temperature steam or air, as may happen during manufacturing or a severe accident during reactor operation.”
In order to assess exactly how accident tolerant ATFs such as U3Si2 really are, Harrison and his team are conducting research with a compound of cerium and silicon (Ce3Si2) which has chemical and thermal properties that are similar to U3Si2 but is not radioactive. One of their experiments involves how Ce3Si2 behaves when it is exposed to high-temperature air.
Harrison’s team is making use of advanced electron microscopy techniques and instruments provided by The University of Manchester Electron Microscopy Center (EMC). They exposed samples of Ce3Si2 to air that had been heated to 750 degrees Celsius. When heated by the air, they found that the samples of Ce3Si2 tended to form nanometer size grains of silicon and silicon oxide as well as cerium oxide. There is concern that these nanograins might increase the corrosion of the fuel material or cause radioactive gases created by reactor operations to escape the core.
Harrison said, “Similarly, it would also allow for hazardous gaseous fission products produced during the splitting of uranium (such as xenon gas that would normally be trapped within the material) to diffuse out along these grain boundaries and be released, which would be potentially harmful to the environment.”
Harrison is reluctant to say that these ATFs are less safe during accidents than the nuclear fuel they are intended to replace. He does say that it has not been demonstrated that these ATFs are any safer than the current nuclear fuels. He concludes “However, with the new insight developed in this work it will be possible to develop and engineer ATF candidates to better withstand these accident conditions, perhaps by adding other elements, such as aluminum, or manufacturing composite materials to give higher protection of the fuel material”.
The title of the article detailing the work of Harrison and his team is “Atomistic Level Study of Ce3Si2 Oxidation as an Accident Tolerant Nuclear Fuel Surrogate.”