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Breakthrough Model of Xenon Could Boost Molten Salt Reactors

Ondrej Chvala and Terry Price.

Left to right: Ondrej Chvala and Terry Price

NE postdoctoral research scientist Terry Price, along with Research Assistant Professor Ondrej Chvala, an international expert on Molten Salt Reactors (MSRs), and University of Ontario Institute of Technology Professor George Bereznai, a fifty-year nuclear industry veteran, have developed a dynamic model of xenon gas behavior in MSRs, which was published in the Annals of Nuclear Energy. This publication is the ninth in a series by Price and Chvala about xenon gas in MSRs.

Studying xenon gas behavior is critical to making MSRs a widely viable form of energy production. Xenon is a gas generated by nuclear fission that “poisons” the nuclear reaction by absorbing neutrons. Balancing the reaction rate is different for solid fuel reactors than for liquid fuel reactors like MSRs.

In a liquid fuel, xenon will be continuously removed from the liquid fuel through off-gassing, which can be aided by a sparging process, where the fuel is injected with an inert gas such as helium. While the fuel is re-circulated from the reactor’s inner core, bubbles of xenon rise to the surface and escape into off-gassing system for capture and sequestration.

“We have created the first model that appears to be descriptive of various transients in the 1960’s Molten Salt Reactor Experiment with the single parameter set,” said Chvala. “It would make sense to apply it to currently relevant designs.”

What separates xenon analysis in MSRs from xenon analysis in solid fuel reactors are the migrational processes. Xenon behavior is very complex because it takes many pathways, forms bubbles, and can migrate into graphite.

According to Price there had been some prior work done back in the 1960s and 1970s, at Oak Ridge National Laboratory, but their success in fitting their model to the experimental data was very limited.

“We built off of their work and other research work done in the past fifty years,” he said. “The breakthrough came when we realized that the xenon stripper, due to foaming action in the pump bowl, wasn’t as efficient as we initially thought at removing xenon.”

While there requires still more study of the effect of these xenon gas bubbles, the model the team developed provides reasonably accurate account of the poison fraction of the xenon compared with the experimental data.

The team believes that its model is the first one capable of fitting both startup and shutdown xenon behavior of MSR experiments using a single set of parameters, giving critical insight into their functionality.