Computational researchers develop advanced model, the pebble bed reactor
From: Texas A&M College of
Engineering
By Laura Simmons
May 20, 2021 -- When one of the largest
modern earthquakes struck Japan on March 11, 2011, the nuclear reactors at
Fukushima-Daiichi automatically shut down, as designed. The emergency systems,
which would have helped maintain the necessary cooling of the core, were destroyed
by the subsequent tsunami. Because the reactor could no longer cool itself, the
core overheated, resulting in a severe nuclear meltdown, the likes of which
haven’t been seen since the Chernobyl disaster in 1986.
Since then, reactors have improved
exponentially in terms of safety, sustainability and efficiency. Unlike the
light-water reactors at Fukushima, which had liquid coolant and uranium fuel,
the current generation of reactors has a variety of coolant options, including
molten-salt mixtures, supercritical water and even gases like helium.
Dr. Jean Ragusa and Dr. Mauricio Eduardo
Tano Retamales from the Department of Nuclear Engineering at Texas A&M
University have been studying a new fourth-generation reactor, pebble bed
reactors. Pebble-bed reactors use spherical fuel elements (known as pebbles)
and a fluid coolant (usually a gas).
“There are about 40,000 fuel pebbles in
such a reactor,” said Ragusa. “Think of the reactor as a really big bucket with
40,000 tennis balls inside.”
During an accident, as the gas in the
reactor core begins to heat up, the cold air from below begins to rise, a
process known as natural convection cooling. Additionally, the fuel pebbles are
made from pyrolytic carbon and tristructural-isotropic particles, making them
resistant to temperatures as high as 3,000 degrees Fahrenheit. As a
very-high-temperature reactor (VHTR), pebble-bed reactors can be cooled down by
passive natural circulation, making it theoretically impossible for an accident
like Fukushima to occur.
However, during normal operation, a
high-speed flow cools the pebbles. This flow creates movement around and
between the fuel pebbles, similar to the way a gust of wind changes the
trajectory of a tennis ball. How do you account for the friction between the
pebbles and the influence of that friction in the cooling process?
This is the question that Ragusa and
Tano aimed to answer in their most recent publication in the
journal Nuclear Technology titled “Coupled
Computational Fluid Dynamics–Discrete Element Method Study of Bypass Flows in a
Pebble-Bed Reactor.”
“We solved for the location of these
‘tennis balls’ using the Discrete Element Method, where we account for the
flow-induced motion and friction between all the tennis balls,” said Tano. “The
coupled model is then tested against thermal measurements in the SANA
experiment.”
The SANA experiment was conducted in the
early 1990s and measured how the mechanisms in a reactor interchange when
transmitting heat from the center of the cylinder to the outer part. This
experiment allowed Tano and Ragusa to have a standard to which they could
validate their models.
As a result, their teams developed a
coupled Computational Fluid Dynamics-Discrete Element Methods model for
studying the flow over a pebble bed. This model can now be applied to all
high-temperature pebble-bed reactors and is the first computational model of
its kind to do so. It’s very-high-accuracy tools such as this that allow
vendors to develop better reactors.
“The computational models we create help
us more accurately assess different physical phenomena in the reactor,” said
Tano. “As a result, reactors can operate at a higher margin, theoretically
producing more power while increasing the safety of the reactor. We do the same
thing with our models for molten-salt reactors for the Department of
Energy.”
As artificial intelligence continues to
advance, its applications to computational modeling and simulation grow. “We’re
in a very exciting time for the field,” said Ragusa. “And we encourage any
prospective students who are interested in computational modeling to reach out,
because this field will hopefully be around for a long time.”
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