Source: MIT News | Fatima Husain | July 10, 2018

Galvanized by the Fukushima Daiichi nuclear disasters, PhD student Xingang Zhao envisions a future with safe, efficient nuclear power.

“I was really passionate about the complexity of the nuclear systems. I couldn’t imagine how you could generate such a huge amount of power out of [a small reactor],” Zhao says. Image: Jared Charney

In March 2011, as the disastrous accidents were unfolding at the Fukushima Daiichi nuclear power plant in Japan, Xingang Zhao was contemplating his next steps after completing a combined bachelor’s/master’s degree in energy and environmental engineering at the National Institute of Applied Sciences in Lyon, France.

The Nanjing, China, native had moved to France to study engineering with a focus on climate change. There, he zeroed in on low-carbon energy systems, driven by his interest in their global technological, economic, and social impacts. He learned that nuclear energy supplied a significant proportion of France’s electricity, and he became captivated by the sheer power generation at nuclear power plants.

“I was really passionate about the complexity of the nuclear systems. I couldn’t imagine how you could generate such a huge amount of power out of [a small reactor],” Zhao says. “[Nuclear energy] is really amazing, and it’s clean energy. In a low-carbon world, it should be a really useful energy source.”

But he wasn’t committed to pursuing graduate studies in nuclear engineering until the Fukushima Daiichi nuclear disasters.

“I realized how important safety was not only for nuclear [systems], but also for human beings. Ten days after that disaster happened, I submitted my application” to a nuclear engineering program, Zhao recalls. “I said to myself, ‘Why don’t you study nuclear and try to make a contribution, to make nuclear reactors safer, to try to mitigate people’s concerns or fears?’”

Zhao went on to study nuclear engineering at the National Institute for Nuclear Science and Technology in France, where he tuned into the details of nuclear technology and the societal implications of nuclear energy development.

Now, Zhao is a fourth-year graduate student in MIT’s Department of Nuclear Science and Engineering, and he is on a quest to revamp nuclear energy safety measures to make nuclear technologies safer and more efficient.

Reassessing limits

During his early graduate studies at MIT, Zhao spent his summers interning at the Oak Ridge National Laboratory in Oak Ridge, Tennessee. There, he was invited to work on modeling and simulating the flow physics and heat transfer of nuclear reactors for applications in safe operating guidelines, through the Consortium for Advanced Simulation of Light Water Reactors (CASL), the U.S. Department of Energy’s first innovation hub bridging basic research, engineering, and industry.

Zhao’s research is aimed at preventing a boiling crisis — an accident scenario in which nuclear fuel rods can no longer be effectively cooled during the heavy heat-producing energy generation process. Boiling crises can result in “a set of cascading failures,” and Zhao hopes his model will improve the methods used to predict them. “As engineers, we’re here to make sure this will never happen,” Zhao says.

However, he says, pressurized water reactors, which generate the majority of the electricity derived from nuclear power worldwide, are operating at parameters that are too conservative, compromising efficiency and energy output.

Currently, commercial nuclear reactors are required to operate at lower than three-fourths of their critical heat flux — the threshold at which a boiling crisis is triggered. “In nuclear, we like everything as conservative as possible,” Zhao says. But that cautious approach could come with a hidden cost: inefficiency. Zhao suspects that under some scenarios traditional models that calculate critical heat flux place it too low, meaning that the current nuclear reactor fleets are operating well under capacity.

To find out just how low, Zhao refers to the principles of physics.

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