In my world, there’s a highly energetic sediment called Teaterium that is found in seawater in extremely trace amounts (~7,500 molecules per ~200 milliliters of water). That said, despite its usefulness in achieving nuclear fusion at power plants, Teaterium is only as radioactive as the least radioactive element.
So, what properties could I give Teaterium that would explain how it’s used in nuclear fusion?
One approach would be to use Muon-catalyzed fusion. A muon is a unstable variant of an electron that is very good at catalyzing fusion, even at room temperature and pressure. Unfortunately, muons decay very quickly, so they are essentially impossible to find in nature, even though they are naturally generated on a regular basis. All known ways of making them use more energy than one would get back via fusion.
You could have Teaterium be an atom that contains a muon, and is either stable or nearly-stable. It would be slowly naturally generated over long periods of time. You could then extract it from seawater, and use it as a source of muons, where the muons are extracted from the Teaterium at the moment when they are needed in the fusion process.
So, i have several thoughts on this.
A concentration of ~40000 molecules per Liter of water is absurdly low. To the point where there is absolutly no scenario in which it would be viable to filter out. 1 Liter of water has this many molecules in it, 3300000000000000000000000. For Comparison.
Then there is a conceptual problem. We know all elements up to 118. And we know Elements beyond Lead tend to not be stable. And generally, the higher the Atomic number the more radioactive elements become. Even if you argue Teaterium is "only" Element 119, optimistically the half-life is measure in nanoseconds. So the notion of Teaterium not being screamingly radioactive does not hold.
Then it is also questionable why this would be good for Fusion. Fusion is all about overcoming the repulsion forces which make sure Atomic nuclei do not just merge. The bigger the nucleus is, the harder it is to fuse and at some point Fusion takes more energy than it releases. That cutoff is at Iron. There are certain situations during which the fusion of any element is possible, but like we are talking about Neutron Star collisions type forces at that point.
To address these concerns, i would advice to like raise the concentration of this stuff a lot, and make it an Isotope. So some variation of an Element with extra Neutrons. A lot of Isotopes are sort of stable, for instance my favored element Lithium has like at least 13. Some, like Lithium 6 (Because Irony) and Lithium 7 are stable. Your whole setup would make more sense if it was a Isotope of something common. Like Lithium, idk Helium or Magnesium.
Even then, Fusion has another issue that makes it pretty hard to get the "Perfect" Fuel. The very nature of Fusion is to ram particles at high speeds, under some pressure and absurd temperatures together until they stick. That process will never be without Radiation. Even if you Isotope / Element is Aneutronic (So the fusion itself does not generate free Neutrons), other effects like Bremsstrahlung will generate Radiation regardless. Bremsstrahlung btw is the radiation particles emit when their momentum is changed. Essentially, the harder of a U Turn a particle makes, the more radiation it emits.
We know, the moment you leave the Hydrogen domain of Fusion fuels, the reactivity goes down. So you need higher pressures and or temperatures to regain efficiency.
This graphs shows quiet well why for Instance He3 + He3 fusion is not going to happen. Helium 3 Fusion, even with Deuterium, is so endlessly less reactive than Deuterium + Tritium that it is not a contest. The types of reactors that can get a net energy out from DT Fusion are lightyears from achieving any net gain with D-He3. If your Element is anything other than another Hydrogen Isotope, it physically wont we able to generate any better Fusion than what we can already do. Which is not great.
So the best properties you can give it be about as good as Deuterium or Tritium. You will note, Deuterium and Tritium are literally the second and third isotopes of Hydrogen. As far as i know, the only empty spot for you would be Hydrogen 8 and upwards. But the trendline is that those isotopes have a short half life.
So what to do ? After all, so far my answer has been pretty down beat. Honestly, i think you should just not worry to much about it. Say its an Isotope, say it is found in nature, say it has good Fusion properties. Like that it has a high reactivity at "low" temperatures, the products it creates are not Radioactive themselves and its Aneutronic. That will pass for most people.
The idea of some new, naturally occurring element being out there that can be used as a fusion fuel is quite ridiculous, BUT there could be unrealized properties that a substance could have to make an unknown compound VERY useful in making a better fusion reactor. Scientists are constantly discovering better compounds for making better magnets; so, we can say with a high degree of certainty that we've yet to invent the best possible magnet yet.
If Teaterium was a specific isotope or ion of some generally known element that could be alloyed to make a super powerful rare earth magnet or a high temperature super-conductor magnet, then it could be used to help contain fusion plasma in a Tokamak reactor. Currently, the problem with fusion is that the only way to make a magnetic field strong enough to contain the plasma is with super-conductor electromagnets that consume more power than you get out of the system, but if Teaterium were a key ingredient in making a low power magnetic system strong enough to control the fusion plasma, then it would make economically viable fusion a possibility.
So what is Teaterium exactly? Other than being a rare substance, the exact answer to that question is a guarded secrete that you as a author have no reason to need to explain further. If other nations/corporations knew exactly what Teaterium is, then they could make their own fusion reactors; so, the name Teaterium was invented specifically to hide exactly what this rare substance is. It may not even be the stuff that these "Teaterium Refineries" are actually collecting. The super rare stuff found in salt water could just be a cover for the fact that the real solution is doable with mundane materials; so, the whole thing is just a misdirect sending rival powers on a wild goose chase to redivert their research efforts to identify and refine something that does not actually exist.
It contains lithium, boron, or both, both of which are fusion fuels which can dissolve directly in seawater or occur in solid crystals, and aren't just isotopes of hydrogen.
If you want a rare thing to help with fusion, I think of muon-catalyzed fusion. In our world muons are very like electrons, but about 207 times heavier. They can replace electrons in hydrogen atoms and make the atoms smaller, making fusion happen much more easily, even at room temperature. Unfortunately they are unstable and decay in about 2.2 microseconds. If they don't decay they can go from the atom they caused to fuse to another and cause another fusion. When they get picked up by one of the product helium atoms they are gone, but helium is still rare compared to hydrogen.
How you use this depends on what you want to change about our physics. I don't believe there is a theoretical prediction of the muon mass. The decay rate goes as the fifth power of the mass, so you could posit a lower muon mass to make it more stable. It wouldn't be as effective at catalyzing fusion, but you might find a sweet spot where it is both effective enough and long-lived enough for your purposes. There is also the square of Fermi's constant, which is determined from the muon lifetime. The standard model has a calculation in terms of the W and Z masses, but that is pretty esoteric, so you could just decree a longer lived muon. Muons rain down on earth from interactions of cosmic rays with the atmosphere, so a nearby supernova could give a much larger flux for a little while. That could give the stratum you desire.
@Haydentech is correct in his real-world analogy: the Tritium-Deuterium fusion reactor would be the holy grail of fusion power. However, Avatar invented Unobtanium; Star Trek invented so many things we can't list them all. You want your Teaterium; then so be it.
Teaterium must be a very light atom. Essentially an isotope of either Lithium, Helium, or Hydrogen would do best. If it is too big and heavy no one will believe its fused product has such a high energy of formation. It should have a high neutron emission rate which allows a sustained chain reaction. Perhaps each fusion releases three or four neutrons, that increase the probability of forming another unstable Teaterium atom.
It will be hard to explain how a specific isotope of a known substance doesn't decay normally, so avoid that explanation. A government secret, or such. But essentially you have found an isotope which normally decays quickly but in your seas, the reaction is slowed, making it available only n your seas.
Teatrium can form an allotype which is incredibly dense somehow, facilitating atomic fusion at lower energy levels. Fusion is accomplished by physically mashing the atoms together against their enormous electrostatic force driving them apart. Somehow the crystallization of Teatrium under the right conditions form a dense material like a diamond which can relatively easily be excited to the point of fusion. The "why" for this doesn't need any in-story explanation, because, while crystals themselves form because of the topology on their bonds, there really isn't any "why" each element bonds the way it does; the electron shells just sort of "want" to be filled or emptied, so for most of our crystal structures there isn't an ultimate "why" for any given crystal. Atoms just do what atoms do. However, it is possible to create your element that contradicts known chemistry, so avoid too many details. Stick to the story.
