Scientists from the University of Texas have developed a new generation of reactors that will be safer than existing ones and will not allow a repetition of disasters such as at Fukushima. Nuclear Technology magazine writes about it.
Dr. Jean Raguz and Dr. Maurizio Eduardo Tano Retamales from the Department of Nuclear Engineering at the University of Texas conducted a comprehensive safety analysis of the new, fourth generation reactors. These pebble reactors use spherical, pebble-like fuel cells and a coolant (usually gas).
“You can think of a reactor as a big bucket with 40,000 tennis balls inside,” notes Raguz.
During an accident, when the gas in the reactor core begins to heat up, cold air begins to rise, a process known as natural convection cooling. In addition, the fuel balls are made of pyrolytic carbon and tri-structural isotropic particles, making them resistant to temperatures up to 1,600 degrees Celsius. As a very high temperature reactor (VHTR), pebble reactors can be cooled by passive natural circulation, making it theoretically impossible for an accident like Fukushima to occur.
However, during normal operation, the pebbles are cooled by a high-speed flow. This flow creates movement around and between the fuel pebbles, much like a gust of wind changes the trajectory of a tennis ball. The developers studied the process of friction between stones and the effect of this friction during the cooling process.
“We have located these ‘tennis balls’ using the discrete element method, in which we account for the movement and friction between all tennis balls caused by the flow,” Tano said. “The coupled model is then validated against the thermal measurements in the SANA experiment.”
The SANA experiment was carried out in the early 1990s and measured how the mechanisms in the reactor change places as heat is transferred from the center of the cylinder to the outside. This experiment allowed Tano and Ragusa to establish a standard against which they could test their models.
As a result, the teams developed a combined CFD model and discrete element methods to study the flow over a layer of pebbles. This model can now be applied to all high temperature pebble reactors and is the first computational model of its kind to do so. These very high precision tools enable suppliers to design more advanced reactors.