Physicists from the Moscow Institute of Physics and Technology (MIPT) and the Joint Institute for High Temperatures (JIHT) of the Russian Academy of Sciences have identified the mobility of line defects, or dislocations, in uranium dioxide. They believe this will enable future predictions of nuclear fuel behaviour under operating conditions. In a paper, published in the International Journal of Plasticity, Artem Lunev, Alexey Kuksin and Sergey provide data of a simulation of dislocation behaviour in uranium dioxide, widely used as nuclear fuel. They say it is the first time that dislocation mobility in uranium dioxide at high temperatures and under stress has been studied in detail.
They have produced a model that can be used to calculate dislocation velocity based on known temperature and stress parameters, which could be used to simulate more complex systems and study the macroscopic processes occurring in fuel pellets under operating conditions. This could help with understanding processes such as as nuclear fuel swelling and embrittlement during operation by means of computer simulations alone. The authors are looking for international collaboration to speed up the potential application of their work in the commercial and regulatory nuclear spheres.
There are several well-known predictive fuel performance codes used around the world, such as Frapcon in the USA and Femaxi in Japan. Lunev told World Nuclear News, Lunev that the "most consistent" Russian fuel performance and safety code is SFPR, which was developed by the Nuclear Safety Institute of the Russian Academy of Sciences in collaboration with France's Institute for Radiological Protection and Nuclear Safety, Germany's Forschungszentrum Karlsruhe, the Institute for Energy at the Joint Research Centre of the European Commission, and the US Nuclear Regulatory Commission.
The SFPR code was designed for mechanistic modelling of single fuel rod behaviour under various regimes of light water reactor operation and is being extended to fast reactors. It also serves as a prototype for a new mechanistic fuel performance code BERKUT. “There are also other codes, e.g. RTOP and START, but they have been used less extensively and have some serious issues," Lunev said.
Lunev told WNN: "Dislocation motion is not explicitly accounted for in any of the existing fuel performance codes, so the expression for the dislocation velocity presented in the paper cannot be plugged in there right now. However, most properties, which the performance codes try to describe, such as creep, swelling, crack formation, etc. do depend on how dislocations move and interact. Our next goal is to draw a bridge between these different levels of description, and we are currently working hard on developing a meso-scale dislocation dynamics model, which uses atomistic input for example, the mobilities of individual dislocations, dislocation-void interaction, etc.
He added: “Perhaps, we will achieve some progress in the next couple of years. This work could be hastened if we had more international collaborators, and we are basically open to any discussion."
The MIPT/JIHT study notes that when a nuclear reactor is in operation, the fuel in the pellets undergoes extremely complex transformations caused by both temperature and radiation. "Because the underlying mechanisms of these transformations are not yet fully understood, we are still unable to realize the complete potential of nuclear fuel and reduce the risk of accidents to a minimum," the authors said. Starikov, who is an associate professor at MIPT and a senior researcher at JIHT, said the new model is a "major advance toward being able to describe processes as complex as nuclear fuel swelling and embrittlement during operation by means of computer simulations alone".