Flibe Energy’s approach to sustainable nuclear energy is a liquid-fluoride reactor that runs on the thorium fuel cycle. Our design builds upon the technologies that were demonstrated and proven during the early days of nuclear development. To better understand the validity of our approach, we will look at the history of these early nuclear programs and some of the issues they encountered. We will specifically look at the issue of uranium-232 contamination and how it affected the development of nuclear reactors, but more importantly, why it makes Flibe Energy’s design approach the ideal path forward for nuclear energy.

We will begin with the research done by one of the most important pioneers in nuclear chemistry. Dr. Glenn Seaborg was a brilliant chemist who had access to a high-powered experimental device called a cyclotron, and he used it to create elements and isotopes that had never before existed. He used neutrons from the cyclotron to transform natural uranium into tiny amounts of a new element he called plutonium. Later, he also transformed thorium into tiny amounts of a new isotope called protactinium-233, which steadily decayed into another new isotope, uranium-233. Plutonium is chemically distinct from uranium and that allowed Seaborg to separate and purify it using novel chemical techniques. Protactinium and uranium are also chemically distinct from thorium, and again, Seaborg was able to separate and purify them.

He was excited to discover that plutonium and uranium-233 are both fissile, which meant they could potentially be used to create nuclear energy. But at this same time, the United States had been attacked and had entered World War 2. The Manhattan Project was underway, and any fissile material would be meant for nuclear weapons rather than for nuclear energy. Soon Seaborg joined the Manhattan Project and producing plutonium became one of its highest priorities. He wondered if uranium-233 could also be used in nuclear weapons. The next year he was able to make larger amounts of plutonium and uranium-233, using one of the first nuclear reactors. He discovered that uranium-233 was very special. As it was consumed in fission, it released enough neutrons to replace itself from abundant thorium. This meant that thorium could be an unlimited source of energy. But he also discovered that its production was always contaminated by another isotope of uranium.

Uranium-232 is one mass unit lighter than uranium-233 but thousands of times more radioactive. It decays into isotopes of lead and bismuth that emit penetrating gamma radiations. Seaborg knew that heavy shielding would be required to prevent gamma radiation from damaging electronics and materials in any nuclear weapon they might build. This meant that weapons made from plutonium or uranium-235 would be lighter, simpler, and far more practical than weapons made from uranium-233. This is because plutonium and uranium-235 decay from alpha emission and are easily shielded by as little as a piece of paper. In addition, uranium-232 and uranium-233 are the same element, so they’re virtually impossible to chemically separate from one another. This meant that uranium-233 would be permanently contaminated with uranium-232.

Once this was realized, Seaborg recommended to General Leslie Groves (who was the director of the Manhattan Project) that they stop considering the use of uranium-233 in weapons. Ultimately, plutonium was used in the first successful test of a nuclear weapon at Trinity, and again a few weeks later at Nagasaki. After the war, the government plowed enormous resources into producing highly enriched uranium and plutonium for more weapons.

Unfortunately, nuclear fission had been introduced to the public in the most horrible way. But the government also tried to tell the public that atoms could be used for peace. Soon they felt public pressure to show how nuclear fission could power cities instead of destroying them. Scientists and engineers considered concepts for both solid and liquid nuclear fuels. The potential advantages of a thorium reactor were compelling. Scientists like Alvin Weinberg and Eugene Wigner believed in and advocated for thorium reactors. These reactors would transform common thorium into a nuclear fuel by way of the thorium fuel cycle—allowing them to continually run on thorium alone.

However, they did need another fuel for the initial startup. Using uranium-233 as the starting fuel was a potential option, but making solid fuels from uranium-233 was challenging and expensive because of the penetrating gamma radiation from U-232 decay. This would require a tremendous amount of shielding and protection during the entire fabrication process. It was actually much easier to make solid fuels from uranium-235 instead, since it had almost no radioactivity.

So, when they began to build a solid-fueled thorium reactor in 1955 in New York, they chose uranium-235 in the form of highly enriched uranium, as the starting fuel. By not using uranium-233, they avoided the challenge of gamma radiation from U-232 decay, at least initially. But operation of the reactor created U-232 contamination so significant that the spent fuel could not be chemically processed for reuse, creating excessive nuclear waste.

At Oak Ridge National Laboratory, liquid-fueled thorium reactors based on fluoride molten salts showed more promise. Thorium and uranium would be dissolved in the liquid fuel, which could be used and processed directly. This avoided the problem of fuel fabrication completely, and molten salts were impervious to damage from gamma radiation. Thus, the first nuclear reactor to operate on uranium-233 fuel was a molten-salt reactor at Oak Ridge in 1968.

Around the same time, Admiral Rickover, who oversaw the building of the first civilian reactor in 1957 in Pennsylvania, was now imagining how they could build a thorium reactor. They wanted to test whether a thorium breeder reactor was possible. And since the breeder reactor required U-233 at startup, they had to tackle the challenge of U-232 contamination.

They knew that there were three main ways that it can form in a nuclear reactor. This first way is when uranium-233 is struck by a fast neutron rather than a slow neutron. When uranium-233 is hit by a neutron that has been slowed down though moderation, then it is more likely to cause a fission. But sometimes a neutron hasn’t slowed down significantly enough though moderation. These are called fast neutrons, and while they are unlikely to cause reactions, sometimes they do.

A fast neutron sometimes strikes uranium-233 and instead of causing a fission reaction, its high energy actually knocks a neutron off the nucleus. This forms uranium-232. Another way U-232 can form is when a fast neutron strikes thorium-232. Once again, this can knock a neutron off the nucleus, which in this case forms thorium-231. Thorium-231 then decays to protactinium-231 in about a day. Protactinium-231 decays very slowly, but it has a huge appetite for absorbing neutrons. When it does, it forms protactinium-232, which then decays to uranium-232. Finally, the third way uranium-232 can form comes through thorium itself. When thorium is mined, it contains trace amounts of thorium-230, which comes from the decay of natural uranium-234. The amount of thorium-230 present in the thorium depends on how much uranium it was around when it was extracted. Thorium-230 also has a huge appetite for absorbing neutrons, and when it does it forms thorium-231. It then follows the same pathway as previously described, once again leading to the formation of uranium-232.

These three pathways all lead to the formation of uranium-232, and Rickover’s team sought to minimize them. They selected thorium ores that were isolated from uranium. Then they placed this thorium in the periphery of a special production reactor. This avoided fast neutron effects by giving plenty of room for fast neutrons to slow down. The reactor successfully created uranium-233 from thorium, with little contamination.

Rickover’s experiment was started in December 1977, with a command from the White House by President Jimmy Carter. It successfully generated millions of megawatt-hours of electricity for consumers in Pennsylvania, all from thorium. In the end, there was more uranium-233 in the reactor when it was shut down in 1982 than when it started. It remains the only thorium breeder reactor that has operated to date. However, the technique they’d used to limit U-232 contamination only applied to the first batch of fuel. Inside the reactor, all of the fuel had been exposed to fast neutrons, which led to the formation of significant amounts of U-232. And the ceramic fuel they had used was very difficult to chemically process, nearly impossible, and so all the fuel was ultimately disposed of as waste, rather than recycled into new nuclear fuel for another reactor.

Despite Rickover’s success, it was really Weinberg’s molten-salt reactor that held the most promise for using thorium efficiently. Even though it had not been demonstrated, it would be much easier to chemically process molten-salt fuels. This would allow for the removal of the fission products that are created when uranium fissions. These fission products contaminate the fuel, but molten salts allow for their removal and repurposing, drastically reducing waste. This is not possible with solid fuels. Liquid-fluoride reactors would also allow for fresh thorium to be continually added to maintain the thorium fuel cycle, creating an endless supply of energy from thorium.

But by the 1980s government support for both molten-salt reactors and the light-water breeder reactor approach had completely dried up. Funding went almost entirely to fast reactors that bred plutonium—plutonium that would be suitable for nuclear weapons. Flibe Energy was formed because we recognize the potential of liquid fluoride reactors to implement the thorium fuel cycle. Since our reactors don’t use solid fuels, they are immune to the challenges posed by the buildup of uranium-232. This is just one of many reasons why liquid fluoride reactors and the thorium fuel cycle go hand in hand with one another, to move us towards a future of energy abundance.

Note: the video link in this post now points to the copy of the video on Gordon McDowell’s “Thorium Remix” site rather than the original video upload on Flibe Energy’s YouTube page. This is because there was a minor labeling error in the original upload that was corrected in the upload to Thorium Remix.