Designing a new breed of nuclear reactors

To most scientists, climate change is real. The challenge is to find more and better energy sources to generate electricity that do not emit greenhouse gases into the atmosphere.

Nuclear energy has been an attractive option, and accounts for about 20 percent of the U.S. electricity supply. The rate is a little more than worldwide - about 14%. In France, it’s as high as 75 percent.

Yet the current U.S fleet of nuclear reactors are aging. Engineers built most of them in the 70s and the 80s. Some have reached the end of their licensed life and their owners face either closing those down or working towards extending their licenses.

Scientists are developing radical, next-generation technologies to produce more efficient, cheaper, and longer lasting nuclear power plants. Designs will include passive safety, where reactors will shut themselves down automatically in emergency instances.

Adriean Couet, Ph.D., an Assistant Professor at the University of Wisconsin-Madison’s Department of Engineering Physics is working on these advances.

His job is to understand the aging of nuclear materials and how that can affect the design of these advanced nuclear reactors for longer lifetime and efficiencies.

“I do work to find materials that can sustain the environmental conditions of this new generation of nuclear reactors,” he said. “We want to go to harsher environments such as higher temperatures.”

Water coolant, the traditional cooling method, won’t do in the next generation of nuclear reactors with higher design temperatures.

A traditional nuclear reactor works by juxtaposing nuclear fuel rods in close proximity, immersed in water. Nuclear fission – where atoms split and release energy in the form of heat - raises the temperature of the water. That water converts to steam and drives turbines that generate electricity.

One way of achieving higher temperatures without pressurizing, which ultimately represents a safety concern, is to using alternative cooling media, like gas, molten salts and liquid metals that can transfer heat. That’s part of the advanced nuclear technology.

“For the next generation of nuclear reactors, since we need to increase temperature to increase plant efficiency, water is not suitable anymore,” Couet said.

The environmental conditions in a reactor are harsh. It exposes materials to high temperatures, irradiation damage and corrosive media.

“When you build a structural material for a nuclear power plant, this material is in contact with the coolant. That is where it is transferring the heat or contain the coolant media. It can be quite corrosive,” he said.

Take molten salts for instance. Leave your car next to the ocean for a year, and it's going to corrode. That's because the ocean air is saltier and salt is going to stick to your car. Salt is very corrosive. It's basically the same thing in an advanced nuclear reactor if you use molten salt, since molten salts can be much more corrosive than regular water.

“Corrosion is a big problem for advanced nuclear reactors in terms of materials, because, the liquid that is going to be used is more corrosive in general,” Couet said

The expectation for advanced nuclear plants is in finding materials that can sustain the conditions of these next generation reactors. Engineers use nickel alloy, steel and other high-temperature alloys, for instance, in the aerospace industry as well as other high temperature applications. Researchers test new alloys for ones that can withstand this environment with less corrosion, and Couet is at the forefront of that effort.

The materials used in the reactor must withstand temperature, corrosion and irradiation for decades. Those three conditions are going to degrade the material property over time.

“It’s very challenging to reproduce those conditions in the lab,” Couet said. “You need the irradiation, the temperature, maybe molten salts or liquid metals. So it's a very challenging, and that's one thing my group is trying to reproduce.”

He tries to replicate the conditions of a nuclear reactor and test different alloys for degradation. To do this, he uses large furnaces that can raise the temperature of a liter sample to hundreds of degrees Celsius for thousands of hours.

“We try to understand materials degradation and maybe model this degradation in the future,” he said. “If we can simulate or model degradation, then we can predict, for longer exposure times, how the materials are going to behave.”

His team exposes each alloy and cooling medium samples to 1,000 hours or more under these conditions and then characterizes the samples. To test the samples, Couet uses multiple materials characterization techniques, including electron microscopes. He also uses Glow Discharge Optical Emission Spectrometry (GD-OES) using a HORIBA GD-Profiler 2™. With it, he bores into the post-corroded sample, which may be a new alloy, and is able to measure the element concentrations in the alloy at nanometer depths. That helps him determine what effects the harsh, artificially created environment had on his sample.

“We want to know how deep the environment has penetrated into a sample or how deep the materials chemistry has degraded,” he said. “We are still using materials that have been used for 60 years, 70 years in nuclear power plants. They work well, but we still don't really understand the fundamentals of their corrosion. So even though the material works perfectly well and it's been used for decades, it's still interesting to try to understand the fundamental science, because then you can apply that to other advanced systems  you do not have as much experience with.”

How far away are we from a new age of nuclear technology?

“Not very far, to be honest,” Couet said.

Countries across the world have built several test reactors with these new technologies. That includes the U.S., France, Russia, Japan and China. In the U.S., the private sector has been investing in these efforts over the past decade. That money is providing a funding stream, alongside money from the Department of Energy, for this new set of technology.

“There are a lot of investors, actually startups and new companies that have emerged,” Couet said. “Sometimes it's about developing a newer reactor, sometimes it's about a service to a nuclear sector. I think it is the first time since the birth of civil nuclear energy that there is some kind of a synergistic effect between public and private funding in the history of nuclear energy.”

Couet said that back in the days when students came to the university to do graduate level research, they wanted to work in a national lab or large utility. Now more and more students want to work in a startup, face new challenges and take risks.

It might be another 10 years before this new technology begins generating electricity, according to Couet. Moreover, it could be 2040 before it contributes to the electric grid.

Yet despite the need for better materials for reactors, and the demand for renewable energy and clean energy sources, Couet is still a pure scientist at heart.

“Sometimes you do the science for the sake of science,” he said. “Even though it can be a bit controversial, doing science for the sake of better understanding our universe and environment is also something that is beautiful.”

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