Russian scientists claim breakthrough in nuclear fuel research

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 described the mobility of line defects, or dislocations, in uranium dioxide. This, they announced this week, will enable future predictions of nuclear fuel behaviour under operating conditions.

Their research findings have been published in the International Journal of Plasticity and they are looking for international collaboration to speed up the potential application of their work in the commercial and regulatory nuclear spheres.

In their paper, the scientists - Artem Lunev, Alexey Kuksin and Sergey Starikov - provide data of a simulation of dislocation behaviour in uranium dioxide, which is one of the most widespread compounds used as nuclear fuel in power plants. They say it is the first time that dislocation mobility in uranium dioxide at high temperatures and under stress has been studied in detail.

Dislocation dynamics work to determine fuel properties relevant to nuclear engineering, including plasticity and fission fragments diffusion. The scientists used computational methods to develop a model of an isolated dislocation in a perfect uranium dioxide crystal. They calculated the varying dislocation velocity as a function of temperature and the external forces affecting the crystal.

Specifically, they have produced a model that can be used to calculate dislocation velocity based on known temperature and stress parameters. This could be used to simulate more complex systems and study the macroscopic processes occurring in fuel pellets under operating conditions. They suggest this 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.

Performance codes

There are several well-known predictive fuel performance codes used in different parts of the world - for example, Frapcon in the USA and Femaxi in Japan.

In an interview with World Nuclear News, Lunev said the "most consistent" Russian fuel performance and safety code is SFPR, which was developed 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 - the sub-modules of which are MFPR and SVECHA - 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.

"We try to keep in touch with one of SFPR's key authors, Dr Mikhail Veshchunov, and he provides us with some interesting suggestions on the future development. There are also other codes, e.g. RTOP and START, but they have been used less extensively and have some serious issues," Lunev said.

Asked how the new development is linked to Russia's commercial fuel performance code improvement, Lunev said Starikov and Kuksin had contributed to the development of MFPR by calculating some atomic-scale properties, which are used in the constitutive equations, such as for predicting fission gas release. However, the research described in the new paper was not directly related to the maintenance and improvement of existing fuel performance codes, but rather was intended to be incorporated in an emerging multi-scale plasticity model in oxide nuclear fuel.

He added: "These activities follow the bottom-up modelling concept (as opposed to the top-down, mechanistic nature of the SFPR/MFPR codes) and are probably ideologically closer to the approach proposed by Michael Tonks in the BISON-MARMOT codes."

Tonks, assistant professor of mechanical and nuclear engineering at Penn State, was one of two recipients of the 2015 American Nuclear Society Materials Science & Technology Division's Special Achievement Award. The other recipient was Richard Williamson, a researcher at Idaho National Laboratory who, along with Tonks, developed the BISON-MARMOT fuel performance codes.

Fuel performance codes are seen as critical for predicting how reactor fuel behaves throughout its time in the reactor, since they predict both how the heat is conducted through the reactor and how it mechanically deforms, which Tonks says is important for "knowing if there will be a breach in the fuel cladding". The goal for gathering this data is to prevent these adverse things from happening and, at the same time, ensure that the reactor is efficient, Tonks told ANS.

Asked how far the new predictive capability of the Russian study is from being incorporated into the code, 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.

"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."

Immense potential

Nuclear fuel has immense potential as one of the most energy dense resources available - a single uranium dioxide fuel pellet weighing no more than a few grams releases the same amount of energy within a reactor core that is produced by burning several hundred kilograms of anthracite coal or oil.

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," they said.

To be used as nuclear fuel, uranium dioxide is formed into ceramic pellets that are sintered at a high temperature. This material has a very high melting point, is resistant to radiation-induced growth, and does not experience phase transitions within a broad temperature range. Theoretically, a solid body has a regular, ordered structure (crystalline structure), and there is a certain designated position for each atom to be at.

"In reality, perfect crystals do not exist, because some atoms or groups of atoms are always out of place, altering the ideal arrangement," they said. "In other words, there are defects (imperfections) in an actual crystal. They come in several types, viz. point defects, line defects (dislocations), planar defects, and bulk defects. Defects can move within the crystal, and the nature of their motion depends on external factors. Dislocation dynamics are known to determine fuel properties relevant to nuclear engineering (plasticity, fission fragments diffusion)."

Computer modelling enables scientists to trace individual fuel atoms and calculate their velocities and forces affecting them, along with other parameters. This allows systems of various complex configurations to be simulated and studied. Computer modelling is widely used in situations where performing an experiment is rather problematic. Research into nuclear fuel behaviour is one of those areas. Such large-scale calculations rely on modern supercomputers, as massive computing power is required to find the forces affecting individual atoms at each moment in time.

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".

Researched and written
by World Nuclear News