RETURN OF THE PEBBLE-BED REACTOR
The helium-cooled reactor fueled with billiard ball-size pods was once touted as the future of nuclear power. The design has been resurrected by a company hoping to use it to produce small modular reactors.
Written by Michael Abrams
Pebble-bed reactors pack their fuel into thousands of graphite spheres. Image: X-energy
WHEN INVESTORS ARE ASKED to make billion-dollar bets, most want to put their money on only the surest of sure things. That’s made innovation in the nuclear industry difficult. Even the plain-vanilla reactors similar to those that have been built for decades cost tens of billions of dollars. If the reactors we know how to build are that expensive, how much are ones that we haven’t even tried?
It’s for that reason that most companies working to develop the first small modular reactors, which are rated at 300 MW and below, start from the basic template of the first reactors built more than 60 years ago: Rods filled with enriched uranium sit in a bath of water; when the uranium heats up due to the fissioning of its nuclei, the water carries the energy away to a turbine that converts that energy to power. Start with something known to work and then iterate.
One company that has broken with that template is X-energy, a startup in Rockville, Md. Its vice president of reactor development, Martin Van Staden, contends that the company is, in fact, not going out on a limb with its design. “We try to steer away from any totally new design applications,” he said. Pointing to reactors that used the template X-energy is following, Van Staden said, “We’ve tried to take lessons from these operating reactors and get to a really low-risk design.”
The design X-energy is developing is called a pebble-bed reactor. It breaks from conventional nuclear reactors in several ways. The uranium fuel is packed into graphite-coated spheres, each one small enough to fit in the palm of a hand. The heat generated by the fuel is carried away by helium gas to a separate steam generator. And the reactor runs much hotter than conventional reactors, some 1,400 °F, enabling the system to operate at higher thermodynamic efficiencies than conventional light-water reactors.
Due to their ability to produce very high temperatures, these reactors may find applications beyond electric generation. Dow Chemical is partnering with X-energy to site four small modular reactors (SMRs) at a manufacturing plant in Seadrift, Texas, with the goal of using not just the electricity but the heat to drive industrial processes. X-energy has designed its reactors to be small enough to fill these sorts of industrial niches.
There’s never been a pebble-bed reactor in operation in the United States, but as Van Staden said, it isn’t a new concept. The concept was patented in 1945 by a chemist who had worked on the Manhattan Project, and the first prototype pebble-bed reactor began construction in 1961, only four years after the Shippingport Atomic Power Station started operation as a commercial nuclear power plant. Over the decades, the pebble-bed concept has intrigued advocates as a cleaner, safer, cheaper alternative to fossil fuels and conventional nuclear plants alike. What no one has done yet is find a way to bring the concept into the mainstream.
Each pebble is packed with 18,000 carbon-encased uranium fuel grains. Image: X-energy
“Think of a bunch of poppy seeds, about a millimeter in diameter. There are about 19,000 in each fuel pebble.”
– Ben Reinke
Pebbles have long been used as a means to heat gases. In the 1920s, scientists working to extract nitrogen from air found that river gravel placed in a furnace could heat air more effectively than other means. One of the scientists who worked on that research, Farrington Daniels, went to work for the Chicago Metallurgical Laboratory during World War II and combined those two experiences to develop the pebble-bed reactor. The pebbles, filled with uranium and coated with carbon to act as a moderator, would generate heat; an inert gas flowing through the bed would remove that heat for use in a turbine.
Daniels began work on a prototype reactor at Oak Ridge National Laboratory in the late 1940s, but the project was canceled so the scientists could work on a water-cooled reactor for the first nuclear submarine.
Light-water reactors derived from those built for the nuclear Navy have dominated the power industry, gas-cooled designs never went away entirely. An inert gas such as helium has many advantages over water. For instance, helium won’t absorb neutrons and become radioactive, and it can operate at higher temperatures. As such, the hot gas is useful for either running through a turbine to produce electricity with high efficiency or for direct process heat applications.
While some high-temperature designs rely on conventional fuel rods, the pebbles of the pebble-bed reactor are unique. Each one encloses thousands of tristructural Isotropic (TRISO) uranium fuel particles that embed the fissionable material in a matrix of carbon and ceramic, with the whole sphere coated in additional layers of graphite.
“Think of a bunch of poppy seeds, about a millimeter in diameter. There are about 19,000 in each fuel pebble,” said Ben Reinke, vice president for business development at X-energy. “The pebble has a rind of graphite and the internal structure is also graphite.” Each pebble is almost entirely carbon by weight, with only a few grams of uranium in each.
Within the reactor, the spheres are loaded from the top and as each one is removed from the bottom— “Similar to a big gum-ball machine,” is how Van Standen puts it—gravity pulls the pebbles through the core. Once removed, each pebble is inspected and, if it is approved for reuse, reloaded from the top. This continuous cycle means that used pebbles continue to react until they’re spent.
In spite of the concept being invented in America at the start of the Atomic Era, the first prototype pebble-bed reactor was built in Germany, with operations beginning in 1967 after six years of construction. A commercial-scale reactor (fueled with a combination of uranium and thorium) began operation in Germany in 1986, after some 15 years of construction. Both were decommissioned in 1988 after the Chernobyl accident turned German citizens against nuclear power.
The German experience, while not without its problems, stoked interest in pebble-bed reactors. During the 1990s and 2000s, a South African company developed plans to build a 165 MW pebble-bed reactor using German designs near Cape Town, with the hopes that it could be a prototype to launch a SMR industry. While that effort ultimately collapsed due to lack of investor interest, China licensed the German technology to build a 10 MW pebble-bed test reactor that started operations in 2003 and a 210 MW twin-reactor power plant that started commercial operations in December 2023.
A pebble like this contains 7 grams of uranium, enough to power a typical household for more than five years. Image: X-energy
The project with Dow points out the advantage of keeping the reactor size small and focusing on applications beyond electric generation. To keep the footprint of the plant small enough to tuck into an existing chemical plant, for instance, X-energy is relying on the TRISO fuel embedded in the pebbles to do a lot of the work down by reactor buildings and other structures.
“The Department of Energy has claimed TRISO fuel is the most robust fuel on Earth,” Reinke said. “But the graphite in the fuel pebble acts as an extra layer of retention. And then it’s inside of a pressure vessel. And so radioactive isotopes would have to escape the pebble and escape the reactor pressure vessel to get into the environment.”
In essence, the fuel is its own containment vessel. “We don’t have a containment building. We have what we call a functional containment philosophy, which has been approved by the regulator as well,” Van Staden said.
And while most reactors have a negative temperature coefficient, meaning they react less as they heat up, that effect is much stronger in the X-energy reactor, Reinke said.
“As the temperature heats up in the core, the reaction physics makes the reactor desperately want to turn itself off,” Reinke said. “And that temperature coefficient is negative across all temperature ranges.”
The extremely negative temperature coefficient means that, in an emergency, the passive safety system doesn’t just give operators seven days of respite before they can find a way turn on the electricity or bring a fire truck. The company claims the reactions shut down until it cools completely. “We can walk away indefinitely,” Van Staden said.
While X-energy calls its reactor “inherently safe,” that is clearly a claim that is hard to verify for every contingency.
“The smartest way to do it would be to say, ‘OK, let me build one of these, see how that goes,’” said M.V. Ramana, professor of public policy at the University of British Columbia and an expert on nuclear policy and proliferation. “Operate it for a few years and see whether it’s reliable, it has any safety challenges, it suffers shutdowns, it suffers leaks, things of that sort. Then, based on that, decide whether you’re going to go forward or not.”
The Department of Energy has been promoting advanced nuclear designs and understands the importance of their operating safely. “It is really important that new reactors make sure that the limit of their potential danger to the public is at the fence line,” said Kathryn Huff, assistant secretary for the Office of Nuclear Energy. “These designs have strived to reach that technical accomplishment, and NRC approval of such emergency planning zones is confirmation that they have succeeded.”
Fuel pebbles work their way through the reactor (at left), heating helium that goes to a steam generator (right). The steam powers a turbine to make electricity. Image: X-energy
Van Staden said the plan was to complete the design in the next several months. “By the end of 2025, we aim to have a complete detailed design package that will be handed over to start construction,” he said.
Of late, financing for even more tried-and-true nuclear technology has grown scarce. Portland-based NuScale, which is developing a light-water SMR, cancelled its planned demonstration reactor in Idaho late last year after escalating costs made its utility partner back out. And the pipeline for conventional, gigawatt-scale reactors in the United States is, at the moment, empty.
Van Staden, however, is undeterred. He was part of the team in South Africa that worked on that pebble-bed reactor project (he recently got his American citizenship) and is devoted to making the concept work.
“I’ve committed 25 years of my life to this technology,” he said. “I’m really convinced of it. And really, really determined to see it through and get it built.”
Michael Abrams is a technology writer in Westfield, N.J.
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