Making sure materials behave in nuclear reactors

Energy consumption is increasing, a large fraction of it powered by fossil fuels. But fossil fuel resources are finite and pose the threat of global warming from greenhouse gas emissions. Nuclear energy is an important way to reduce our dependence on fossil fuels and resolve global warming issues. However, even though the safety and efficiency of water-cooled reactors — so-called GEN-II and GEN-III reactors — have improved significantly, nuclear energy still is not playing a global role, due to concerns about issues like accidents, nuclear waste and non-proliferation.

The last commercial GEN-I reactor shut down in 2015. GEN-II reactors make up the majority of present-day power plants in the United States. GEN-III reactors began operating in 1996 to provide simpler, standardized designs to reduce cost, extend operating life, provide more safety assurances, and burn fuel more efficiently, according to the International Atomic Energy Agency (IAEA).

New reactors that surpass even water-cooled reactors in safety and economics are on the horizon. These GEN-IV reactors, currently in the design and testing phase, show promise to be safer than the conventional water-cooled type of reactors because their coolants operate under lower pressure, with less material degradation and consequent potential for failure.

Molten salt reactors (MSRs) are one example of this next generation. Molten salt is an excellent coolant as it retains thermal energy very efficiently. But these reactors run at more elevated temperatures than water reactors and face higher radiation levels and corrosion concerns. New structural materials and fuels that can resist this severe environment are needed to realize the potential of MSRs as a cost-effective, safe and sustainable nuclear option for GEN-IV commercial power plants.

Molten salts are complex materials whose properties challenge scientific understanding and prediction; the radiation environment adds further complexities. Studies are needed to characterize radiation-driven speciation — the formation of new variants of the substance — and to predict molten salt behavior and possible degradation of the reactor’s structural materials.

Ongoing research that provides a fuller understanding of the behavior of structural materials is critical for the next generation of nuclear energy applications and a more sustainable future.

Structural material degradation is crucial in nuclear applications. The structures include the core, reactor containment and coolant system, and structures used in the fuel cycling system. For example, I am conducting research to develop a method to predict stress corrosion cracking propagation in nuclear-fuel welded dry storage canisters, which are used as repositories for spent nuclear fuel. These canisters are strong enough to resist thermal, mechanical and radiation effects, but they can corrode and rupture due to stress corrosion cracking.

In one research effort, I fabricated a stress corrosion cracking system that can control water chemistry, temperature, pressure, and applied stress to measure crack growth rates during testing. These investigations revealed the fundamental mechanism of structural material degradation and established an experimental database for further studies.

In molten salt reactors and sodium-cooled fast reactors, we need to understand the chemical interactions of fission products with structures and their corrosion effects. I’m exploring the multidimensional characterization of the corroded alloys — a field in which research findings are rare. In addition, I’m developing novel computational methods and machine learning algorithms to predict material failure.

Yi Xie

Assistant Professor

School of Nuclear Engineering

College of Engineering, Purdue University

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