The Idaho National Laboratory (INL) has cleared a major hurdle in making a Generation IV nuclear reactor practical. Using a new process, a team has developed a new way of processing fuel efficiently for cutting-edge molten salt reactors.
One of the big pushes in the effort to spark a 21st century renaissance in nuclear power is the development of new reactor designs that only a couple of decades ago were experimental with only a limited prospect of ever becoming practical.
One class of these so-called Generation IV or Gen IV power plants is molten salt reactors, which replace enriched uranium or plutonium fuel rods and water moderator/coolant with a mixture of nuclear fuel and molten salt. It's a concept that seems a bit odd at first, but it's one that provides all sorts of advantages over the common pressurized water reactors in use today.
There are several different kinds of molten salt reactors, but they have a number of features in common.
For one thing, they operate at higher temperatures than conventional reactors and at atmospheric pressure. This makes them much more efficient and reduces mechanical stresses, while also eliminating the threat of a runaway meltdown because the nuclear reaction is self-limiting. Also, dangerous or damaging gases like hydrogen and xenon are easily vented off by a simple chemical process.
Because they operate at temperatures of about 600 °C (1,112 °F), molten salt reactors have a 50% greater efficiency. They can continuously recycle their fuel, which reduces nuclear waste – and new fuel can be added and the waste removed by what is essentially plumbing.
They're also quite flexible, capable of handling a variety of fuels, which helps not only with the economics but also with reducing the proliferation of nuclear weapons. Not to mention that the reactor designs can be modular and readily adapted for small-scale plants that can be used for a variety of industrial applications, including petroleum production, hydrogen generation, desalination, floating power plants, and ship propulsion.
This sounds all well and good, but why haven't molten salt reactors been built before? The answer is that such reactors have been used since the very dawn of the nuclear age. In fact, one of the first reactor designs drawn up for the Allied Manhattan Project to build the first atomic bomb would have used a slurry of salt and uranium. However, that one didn't last long because there wasn't enough uranium fuel available and the molten salt design wasn't any good for making plutonium, so Oppenheimer et al went for a graphite reactor instead.
Since then, there have been a number of molten salt projects, including one for submarines and another for (Lord help us) powering aircraft, but they never really caught on. That's because nuclear reactors aren't anywhere near as simple as the illustrations in school textbooks might lead one to believe.
Despite their advantages, molten salt reactors do have their drawbacks. They're prone to corrosion problems as well as thermal and neutron stress. In addition, salts strip away the protective oxide layers from metal components. Then there's the problem of fuel reprocessing, which is mechanically simple, but becomes more complex when you remember that the fuel is radioactive.
On top of all this, basing nuclear reactions on flowing mixtures of hot liquids involves some areas of nuclear physics that are, in technical terms, a bit iffy. Not only is there a lack of standardized computational tools for reactor physics simulations, there's also a limited understanding of how the structural materials are affected by prolonged operation.
And if that wasn't enough to give a nuclear engineer a haunted look, there's the problem of fabricating the fuel for the reactor. You can't use metallic uranium like in conventional fuel rods. It has to be in a form that will dissolve in chloride salts. That means some form of uranium chloride, like uranium trichloride (UCl₃) or uranium tetrachloride (UCl₄), which present challenges including fabrication complexity, chemical stability and reactivity, additional chemical processing steps, and corrosion issues.
This is the problem that INL's Molten Chloride Reactor Experiment (MCRE) is dealing with. Since 2020, the technical team under Bill Phillips has been trying to come up with the right uranium compound and ways of manufacturing it in bulk with 90% efficiency.
MCRE, in cooperation with Southern Company and TerraPower, aims to build the world’s first critical fast-spectrum molten salt reactor, with the goal of having a demonstration reactor by 2028 and a commercial version by 2035.
The sticking point is that in 2020, INL could only make two or three ounces (57 to 85 g) of the fuel at a time. Unfortunately, the final reactor needs three and a half tonnes to reach criticality. So, INL has been working with denatured uranium, which is much cheaper than, but chemically identical to, fissionable uranium, to create more of the fuel per batch.
With a lot of trial and error, combined with a custom prototype furnace and specialized equipment, the team found how to combine the precise conditions, ingredients, and methods to produce 18 kg (39 lb) at a time.
According to INL, the next step is to produce five more batches by October 2025 to demonstrate the potential for full-scale production of the enriched nuclear fuel and to charge the MCRE for its first reactor experiments. These are aimed at studying the behavior of neutrons in the reactor, verifying the theoretical models for fast-spectrum chloride reactors, measuring fuel stability, making an assessment of corrosion resistance of structural materials in chloride salts, and studying radiation damage to containment materials.
"We started out wasting too much of the uranium metal we have access to, and we would not be able to make enough fuel salt for the reactor to go critical," said Nick Smith, MCRE project director. "After years of experimentation and revision, we finally found the right process to reach the perfect yield. “It takes a special kind of perseverance to keep working the problem when there is no guarantee that you will find a solution."
Source: INL
