Scientists have proven that a new generation of reactors is not capable of repeating the Fukushima accident

Dr. Jean Raguz and Dr. Mauricio Eduardo Tano Retamales of the Department of Nuclear Engineering at Texas A&M University

The University conducted a full analysissafety of the new, fourth generation of reactors. These pebble-bed reactors use spherical fuel elements that resemble pebbles and a liquid 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 corethe reactor 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 cooledhigh-speed stream. 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 tennisballs ”using the discrete element method, in which we take into account the movement and friction between all tennis balls caused by the flow,” said Tano. "The coupled model is then validated against the thermal measurements in the SANA experiment."

The SANA experiment was conducted in the early 1990s andmeasured how the mechanisms in the reactor change places when 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 jointmodel of computational fluid dynamics and discrete element methods for studying 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.

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