Understanding TerraPower’s Natrium Reactor Design and Demonstration Project Progress

In the proverbial shadow of the Naughton Power Plant, a station in Kemmerer, Wyoming, that will stop burning coal at the end of this year, TerraPower is constructing what it calls “the only advanced, non-light-water reactor in the Western Hemisphere being built today.” The project represents more than just a new power source—it’s a symbolic passing of the torch from fossil fuels to next-generation nuclear technology.

“We call it the Natrium reactor because it is in a class of reactors we call sodium fast reactors,” Eric Williams, Chief Operating Officer for TerraPower, said as a guest on The POWER Podcast. Incidentally, the word natrium is Latin for sodium. In fact, it’s where sodium’s elemental symbol—Na on the periodic table—comes from.

Benefits of the Natrium Plant Design

The Natrium design is a Generation IV reactor type, which is the most advanced class of reactors being developed today. “These designs have a greatly increased level of safety, performance, and economics,” Williams explained.

Williams said the use of liquid metal coolant enhances safety. “Liquid metals are so excellent at transferring heat away from the reactor, both to exchange that heat into other systems to go generate the electricity or to remove the heat in an emergency situation,” he said. “For the Natrium reactor, we can do that heat removal directly to air if we want to, so that provides a very robust safety case for the reactor.”

The design is also safer because it can run at low pressure. “The primary system is at atmospheric pressure; whereas, current pressurized water reactors have to pressurize the system to keep the liquid from boiling—to keep it in a liquid state,” Williams explained. “Liquid metal sodium doesn’t boil until about 800 to 900 degrees Celsius, and the reactor operates down at 500 degrees Celsius, so that can remain a liquid and still be at a very high temperature without having to pressurize it.”

The liquid metal coolant also provides performance benefits. “One of those is the ability to store the energy in the form of molten salt heat coming out of the nuclear island,” said Williams. “That is really giving us the ability to provide basically a grid-scale energy storage solution, and it really matches up well with the current needs of the modern electricity grid.”

In practice, Natrium’s nuclear island will operate with a constant heat output. The energy will then be stored in the molten salt. The stored energy will be used to drive a steam turbine and generate electricity as needed, allowing the plant to load-follow easily without cycling the reactor up and down. In other words, the reactor can produce heat at a constant 345-MW electric equivalent, while the plant supplies power of up to 500 MW when needed and at a very rapid rate of change. “It’s very competitive with combined cycle plants and coal plants that are used today for load following,” said Williams.

Meanwhile, the energy storage aspect also allows decoupling the electricity generation side of the plant—the energy island—from the reactor side of the plant, that is, the nuclear island. That allows the energy island to be classified as “non-safety-related” in the eyes of the U.S. Nuclear Regulatory Commission (NRC). “That side of the plant has nothing to do with keeping the reactor safe, and that means the NRC oversight doesn’t have to apply to the energy island side of the plant, so all of that equipment can be built to lower cost and different codes and standards,” Williams explained.

Notably, this also permits the grid operator to dispatch electricity without changing anything on the nuclear island. “That allows a different kind of integrating with the grid for a nuclear plant that hasn’t been achieved yet in the U.S.,” Williams said. “We’re very excited about that—the safety, the performance, and economics—and it really gives us the ability to have a predictable schedule, and construction will be complete in 2030.”

Bringing the Project to Fruition

TerraPower was founded in 2006 by Bill Gates and a group of like-minded visionaries, initially pursuing the ambitious traveling wave reactor concept before evolving into the current Natrium design. The company also has a molten chloride fast reactor under development, which Williams said shows great promise to be even more economic than the Natrium reactor in areas that are hard to decarbonize, such as maritime applications and high-temperature industrial applications.

After its founding, TerraPower spent more than a decade developing its technology and building partnerships, including early collaborations with some international firms including Toshiba. The breakthrough came in 2020 when the U.S. Department of Energy selected TerraPower to receive $80 million in initial funding under the Advanced Reactor Demonstration Program (ARDP), providing up to $2 billion in authorized funding through a 50/50 cost-sharing arrangement.

In June 2021, TerraPower partnered with PacifiCorp to build the Natrium reactor, ultimately selecting Kemmerer, Wyoming—a town of about 2,800 people—as the site for the demonstration project. The project faced some delays when TerraPower pushed back the launch date by a couple of years to 2030 due to fuel supply challenges, as Russia had been the only supplier of the required high-assay low-enriched uranium (HALEU). Despite this setback, the company submitted its construction permit application to the NRC in March 2024.

“We had our construction permit application submitted to the Nuclear Regulatory Commission last year, and it’s really ahead of schedule,” said Williams. Expectations were for the construction permit application to be approved by the end of 2026, but Williams said that will probably happen much sooner. “So, that’s pretty exciting,” he said. “That construction permit application allows us to begin construction on the nuclear side of the plant, but we can really start construction on the non-nuclear side of the plant anytime.”

Work in Progress

In fact, ground has already been broken at the Kemmerer site (Figure 1). “We actually started construction a year ago,” said Williams. “We started working on what we call the test and fill facility. It’s a dual-use facility that is really right next to the nuclear site, and it’s a test facility that allows us to do full-size prototype testing for some of the major equipment that’s going to go into the Kemmerer Unit 1 plant. It also is a facility that holds the liquid sodium that is used to fill up the reactor initially before it starts up.”

Williams said the foundation work was completed on the test and fill facility this month. Next, building erection will begin, which is expected to be completed by the end of the year. The second building to be constructed onsite will be the Kemmerer Training Center (KTC). Mobilization for that will start this month, according to Williams. The KTC will include a full-scope simulator for the Natrium reactor. It will also have classrooms for training, a couple of laboratories for operations staff to utilize, and it will also be a visitor center with an auditorium for meetings and other gatherings. “Those two buildings will both be under construction this year,” Williams said. “We’re going to move into construction on the non-nuclear side of the plant early next year or at the end of this year,” he added.

Meanwhile, the engineering phase of the project is in full swing. “We completed our preliminary design milestone in March of this year, so that means we’re in the detailed design—the final design stage,” said Willliams. “This is our last iteration through the design. It’s really about getting all of the components into fabrication and the construction is really going to be starting to mobilize here soon,” he said. There are more than 1,000 people working on the project, including TerraPower and its major partners, Bechtel and GE Hitachi Nuclear Energy, as well as various engineering services firms that are also involved. “It’s really the peak for engineering,” Williams said.

Concerning plant equipment, Williams said TerraPower has been procuring components since last year. “We’re basically under procurement now for the nuclear island, and we’re getting close to starting the fabrication phase on a lot of that equipment,” he said. “In fact, in a couple of weeks here, we’re going to be ordering long lead material for the reactor vessel head, and that’ll be a real big milestone for us in terms of starting that process and then doing it on other components as we move forward. And, then, those pieces of equipment will gradually start to move into fabrication.”

While there is clearly a lot that needs to be done, and first-of-a-kind projects rarely go off without a hitch, Williams seemed pleased with how the project was progressing. “We’re really excited to be working in the state of Wyoming. It is just an outstanding state for developing any kind of energy project, including nuclear energy. The people in the community are really welcoming to us. The state legislators are always looking for ways to remove any obstacles and just explain to us how to get the permits we need and everything. So, the project has been going really well from that standpoint,” he said. In the end, Williams appeared confident that TerraPower would hit its current target for completion in 2030.

To hear the full interview with Williams, which contains more about the Natrium design, molten salt management practices, HALEU fuel and other supply chain challenges, licensing and project timelines, and more, listen to The POWER Podcast. Click on the SoundCloud player below to listen in your browser now or use the following links to reach the show page on your favorite podcast platform:

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—Aaron Larson is POWER’s executive editor (@AaronL_Power, @POWERmagazine).