Fusion Energy's Has A Secret Achilles Heel
The Tritium problem.
In theory, fusion energy is the perfect form of energy. By utilising the same process that powers the Sun, we can fuse hydrogen atoms into helium and release a truly gargantuan amount of energy on demand, with no carbon emissions and no nuclear waste. For decades, though, this technology has been just out of reach, as our reactors used more energy to fuse hydrogen than they can get from the reactions. However, recent advances are pushing us past this limit, and in the coming decades or so, we are set to have viable fusion energy. Or, at least, we are meant to. You see, fusion has a buried secret that could render it utterly useless, even if we crack fusion reactor technology. Let me explain.
As always, when discussing fusion, let’s quickly recap what it actually is. As I said, nuclear fusion is the process that powers stars. The temperatures and pressures in their core are so high that hydrogen atoms have enough kinetic energy to overcome the repulsive force that keeps atoms separate. So, when two hydrogen atoms collide in this super-hot, super-dense plasma, they fuse to form helium. But this helium is slightly lighter than the two atoms of hydrogen it came from. This spare mass is transformed into energy and released. Einstein’s famous E=MC² states that a little bit of mass is equal to a vast amount of energy. This means that if you fuse a tiny amount of hydrogen, a truly immense amount of energy is released! To give you a sense of scale here, if you fused 17 tonnes of hydrogen, it would produce enough energy to power the entire US for a year!
Our fusion reactors replicate this by heating and squeezing hydrogen plasma using magnets, lasers or pure kinetic energy.
But, different isotopes (forms of the same element with different numbers of neutrons) of hydrogen fuse more easily than others. By far, the best pairing, which fuses with the lowest amount of kinetic energy and yet releases enough energy from fusing, is deuterium and tritium. As such, almost all of our fusion reactors use a deuterium-tritium reaction.
Deuterium is a common stable isotope of hydrogen with a single neutron in its nuclei and can be easily extracted from seawater. On the other hand, tritium has two neutrons in its nuclei, making it unstable and radioactive, with a half-life of 12.5 years. As such, it is basically non-existent in nature and must be painstakingly artificially made. The most common and, so far, most productive way we have found to produce tritium is to irradiate an isotope of lithium known as lithium-6 in nuclear reactors. This creates a decay chain that eventually produces a tiny amount of tritium gas, which can be carefully extracted and refined from the nuclear reactor. This process is so slow, arduous and complex that tritium is actually one of the most expensive materials ever made at roughly $30,000 per gram!
This is Fusion’s Achilles heel. Even if we finally create reactors that can produce a net gain of energy large enough to make them a viable energy source, sourcing enough tritium to power them could render them financially unfeasible. Not only that, but we might not even be able to source enough to power just a single reactor, and just running a single large fusion reactor like ITER could easily use up the entire world’s supply of tritium. Let’s also not forget that many nations won’t let fusion reactors exhaust the world’s supply of tritium, as they need them to refuel their hydrogen fusion bomb arsenal, which also uses the deuterium-tritium reaction and, as such, needs refuelling every few years thanks to tritium’s half-life.
So, can we solve this problem? Or is fusion energy doomed?
Well, fusion reactors can create their own tritium. Deuterium-tritium reactions produce a huge amount of neutron radiation; this is the type of radiation that causes lithium-6 to transmute and decay into tritium. As such, many scientists suggest using blankets of lithium enriched to have a higher level of lithium-6 than normal inside fusion reactors. This, in theory, would let these reactors be self-sufficient, at least in terms of tritium.
However, much of the energy released by deuterium-tritium reactions is in the form of neutron radiation. So, syphoning off a significant amount of this radiation to tritium generation robs away a sizeable amount of the energy that can be extracted from the fusion reaction. As such, it means these reactors would need to produce a far larger net-gain in energy to be a viable energy source. This effectively means the reactors need to be even more efficient, which is exceedingly difficult! As such, solving the tritium problem in this could take decades of R&D and billions upon billions of dollars.
The other option is to use different, more readily available isotopes. Helion is one of the few projects using this method. Their reactor fuses deuterium with other deuterium atoms to form helion, an isotope of helium with a single neutron. It doesn’t extract energy from this reaction, but instead retains the helion and fuses it with leftover deuterium. This deuterium-helion reaction fuses almost as easily as deuterium-tritium, but it releases pretty much the entirety of its energy in the electromagnetic spectrum, which is easier to collect and convert into electricity than the neutron energy of deuterium-tritium.
However, Helion has its own issues to overcome. It has yet to get close to producing a net gain in energy, and as it effectively has to go through two rounds of fusion before it can produce energy, this challenge is equally as vast as using lithium blankets inside the reactor.
Either way, thanks to the tritium problem, fusion energy is still far from being realised.
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Sources: Physics World, Will Lockett, Helion, Will Lockett