
Nuclear energy is astonishing. It has one of the lowest carbon emissions per unit of energy produced of any energy source, and is simultaneously one of the safest forms of energy. To top it off, unlike renewables, it is on-demand, so there is no need for expensive gargantuan grid-level batteries. But nuclear isn’t perfect. It’s expensive, takes far too long to deploy and can be way more efficient and cleaner than it currently is. Luckily, there is a wave of upcoming next-gen nuclear technology promising to solve these issues, such as SMRs and fast reactors. Sadly, though, a new peer-reviewed paper has found that the fuel that powers these reactors, known as HALEU, can be used to make a nuclear weapon with no modifications and, as such, poses a national security risk! So, does this mean the future of nuclear power is a dead end?
Let’s start with what HALEU fuel is. Nuclear fuel is made of uranium, but only certain isotopes of uranium can actually undergo fission, the nuclear chain reaction these reactors use to produce power. Uranium 235 (U235) is to go to fissionable uranium isotope. However, natural uranium is only 0.7% U235, far too low to be used in a reactor, meaning it needs to be “enriched.” This involves removing other isotopes until the ratio of U235 is high enough to be used in a reactor. Typical nuclear fuel is enriched to the point where 5% is U235. HALEU fuel, which stands for high-assay, low-enriched uranium, is instead enriched to 20% U235!
Russia used to be the only mass producer of HALEU fuel, but Biden’s Inflation Reduction Act provided $700 million for a HALEU program to produce the fuel in the US.
Why? Well, many next-gen reactor designs, such as SMRs (small modular reactors) and fast reactors, require HALEU to function.
SMRs are designed to make nuclear power plants cheaper and quicker to build. They do this by shrinking the reactor design so they can be built off-site in a controlled factory and then ship it to location. New nuclear power plants can simply order however many they need for their power plant (hence the modular part of their name). Not only does this make the construction of the plant quicker and, therefore, cheaper, but it also uses economies of scale to make the reactors cheaper, as they can be mass-produced in a factory.
On the other hand, fast reactors use a higher-energy reaction to turn non-fissionable uranium 238 (U238), which makes up the vast majority of the non-U235 part of nuclear fuel, into fissionable fuel. That is a complex way of saying fast reactors can extract energy from almost all the nuclear fuel, not just the small portion made of U235. Not only does this make them more efficient and cheaper to run, but as the U238 produces less waste per unit of energy produced, and said waste is less toxic and less radioactive, it is also significantly cheaper to manage fast reactor nuclear waste than typical nuclear reactors.
Lots of SMR designs require HALEU fuel for many reasons, but one of the big reasons is that they would need to be refuelled far too often if they used regular nuclear fuel. Similarly, many fast reactor designs required the increased amount of U235 in HALEU fuel to get the higher energy reactions necessary to turn U238 into fissionable fuel.
But, a few nuclear scientists recently looked at the risks of HALEU and published a rather worrying paper that found it can be used to make nuclear bombs! You see, regular nuclear fuel has too little U235 to be used to make a bomb. Typical weapons-grade uranium is enriched to 90%, but even at its far lower 20% enrichment, HALEU has just enough U235 to be used as fuel for a nuclear bomb. Scott Kemp, one of five authors of the peer-reviewed paper, said, “This material [HALEU] is directly usable for making nuclear weapons without any further enrichment or reprocessing.” Not only that, but they found that it would only take 1,000 kg of HALEU fuel to create an atomic bomb with the same power as the one the U.S. dropped on Hiroshima, Japan, in 1945. For context, a typical SMR plant, or fast reactor, would need between 10,000 and 30,000 kg of HALEU fuel a year! The authors recognised that building such a device and using it in aggression would be challenging. But, the fact that it is feasible is, at the very least, a national security threat for whichever country adopts HALEU-based nuclear technology.
So, does that mean these next-gen nuclear reactors are doomed? Not necessarily. There are SMR and fast reactor designs that can use regular nuclear fuel; these designs just tend to be more complex and expensive. However, the authors did point out that as long as HALEU is limited to 12% enrichment, it can’t be used to create nuclear bombs. So there is a potential middle ground where SMR and fast reactor designs are optimised for fuel enriched twice as much as regular nuclear fuel, mitigating the atomic bomb threat whilst ensuring costs don’t get out of control. But, this might push up the price of SMRs and fast reactors, and, along with other factors pushing the price up, make them unviable. Sadly, I don’t have a crystal ball, so I can’t see how this will play out. But at the very least, this revelation on how unsafe HALEU fuel can be, has made the future of next-gen nuclear power less bright.
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Sources: Reuters, Science.org, World Nuclear, Will Lockett, Planet Earth & Beyond, Orano, NRC