The Future of Fusion: Q&A with University of Tennessee Nuclear Researchers Ane Lasa and Sophie Blondel

Research Scientist Ane Lasa and Research Assistant Professor Sophie Blondel, both from the University of Tennessee, Knoxville’s Department of Nuclear Engineering, work together to explore the interactions between plasma inside fusion systems and the materials that make up the walls of the reactors. Finding materials that work well in these extreme environments is key to making fusion economically viable and can boost public confidence in nuclear energy as a safe and affordable solution to US energy independence. UT spoke with Lasa and Blondel about their work following the recent publication of three of their papers.

Can you summarize how plasma interacts with other materials in nuclear reactors?

Lasa: Most of the plasma in the reactor is confined by a magnetic field, which is where the fusion reactions happen, generating energy. This confined plasma core doesn’t touch the walls around it. However, the edge of the plasma is not confined, and it interacts with the wall materials. We study the interaction of the plasma edge with the reactor wall to better understand and predict the performance and useful lifetimes of fusion reactor walls.

What kind of questions are you trying to answer with this research, and what are you learning?

Blondel: The plasma is composed of ionized atoms and neutrons produced in the fusion reaction. When these ions and neutrons leave the confined region of the plasma and interact with the walls around them, the material in the reactor wall gets degraded and its properties – mechanical, heat, etc.—deteriorate. The deterioration of the reactor wall shortens the reactor’s useful life. Ultimately, to make nuclear fusion financially viable as a source of plentiful clean energy, we need to design reactors that last long enough to be cost-effective. We’re modeling the impact of the process and giving industries and the government confidence that nuclear fusion energy production is viable. It’s an engineering challenge.

Lasa: The wall materials slowly erode over time because of contact with the plasma. It’s a bit like wind and rain slowly eroding a mountain. We want to make sure the wall of the reactor is structurally sound and not thinning out too much. You can’t build it arbitrarily thick for a few reasons: one, cost; two, temperature control of the materials to avoid melting of the surface; and three, harnessing the fusion neutrons, which is done by materials placed behind the plasma-facing surface. Therefore, we are looking to see if and how certain materials perform.

Blondel: We’re modeling instead of experimenting because we don’t have experimental facilities that can reproduce the conditions we expect to have in a fusion energy system. We’re trying to reproduce observations from existing fusion machines so we can say, “OK, we understand the physics, and so we believe we can now extrapolate to future fusion devices.”

Why is this kind of research important?

Blondel: In the long term, it will allow us to predict how fusion systems will behave so the general public can trust them. I like contributing to fusion and being able to say, “It’s going to be safe, and we know how long we can expect our nuclear reactors to perform at acceptable levels.” That is the big picture.

Lasa: There is also an opportunity to better define the portfolio of clean energies out there. With fusion, there are certain areas that have been worked on for decades—like understanding the plasma physics, which allows us to optimize the plasma—and then there are areas that generally fall into technology or engineering and largely involve reactor materials that have to perform under very specific and challenging conditions. Materials that work well in these extreme environments—they are key to making fusion happen in an economically viable way. It’s not just about extracting energy out of fusion but also making it worth the effort. That’s a large motivation for our field.

How will this research be used moving forward?

Lasa: Just to give you a small example, one thing we learned is it’s more important than we thought to measure the temperature of the plasma-exposed material and the impurity content when exposing tungsten surfaces to a helium plasma. Helium causes very particular effects in tungsten, and so people are interested in exposing tungsten to helium plasmas and in running helium plasmas in general. Adding the ability to measure the temperature of the sample will allow us to improve our modeling and improve the understanding of the evolution of the material. So we are informing the next set of experiments. Can you add a diagnostic to measure the tungsten temperature and plasma impurity concentration? Generally, this kind of work to understand the physics of fusion materials helps us optimize future machines.

Blondel: We should note that all the work here is being focused on tungsten because tungsten and tungsten-based alloys are currently the leading material of choice to build the walls of the reactor, due to high melting point and other favorable properties. But in the longer term we might need to add elements to the tungsten to make it more resilient.

Lasa: As Sophie said, this is also about creating public confidence in nuclear energy as a safe and affordable solution to US energy independence Modeling allows us to predict how materials will perform in nuclear reactors, helping us focus in on the most promising materials and scenarios to study experimentally. We are starting to work a bit with private companies to model their future machines. Or they might have two options for design, so our modeling helps them make the best choice. We can help private industry and government minimize expensive experimentation. It takes an army to run one of these machines, plus the energy that goes into creating the plasma and the wear and tear on the machine. So simulations can also help you optimize whatever run time you have and get the most out of it, and that’s better for everybody.

About the Researchers

Lasa serves on the US Department of Energy Fusion Sciences Advisory Committee. The committee helps advise the Office of Science director on issues surrounding the development and oversight of nuclear fusion, which has the potential to be the ultimate clean energy solution. She was tapped for the appointment because of her technical knowledge in the subfield of plasma-facing materials.

Blondel’s research focuses on developing continuum scale models for irradiated materials and implementing them for high-performance computing applications. She is the lead developer for Xolotl, an open-source spatially dependent cluster dynamics simulation code predicting the evolution of the divertor material under irradiation in a Tokamak device.