Why Tritium?

You may have heard about some people working on a new energy source which claims to be the future of humanity: nuclear fusion. But, did you know that fusion reactions can be of many types? Why people working in this field talk about the deuterium-tritium reaction only?

To build a small sun on Earth has been a dream for scientists for some time now. Being capable of controlling the energy that feeds our lives and, at the same time, solve the geopolitical and poverty problems of the world, is an aspirational goal.

In general, heavy elements (like uranium) can be fissionable, while light elements (like hydrogen) can fuse with others. If you could add up the mass quantities of two nuclei of hydrogen, say deuterium (3H) and tritium (2H) before and after they fuse, you would see that the product (a nucleus of helium) does not weight the same. Mass is lesser after the fusion reaction.

But. . . where is that missing mass? If you can recall some school lesson where the teacher heavily stressed that mass cannot be created or destroyer, this can be disturbing. What happened here is that a very small amount of mass was transformed into energy, obeying the famous Einstein’s equation:

   

So, being proportional to the speed of light c, the energy released can be huge even for really small quantities of our fuel.

But, returning to the question, what can be used as a fuel for fusion? can you fuse anything? The answer is, as you can foresee, no. So, “why have I heard about one of this one only?” I can hear you say. The most known fusion reaction is deuterium-tritium (D-T) and is the nuclear reaction to be tested by the big experiment ITER. The use of this reaction has some drawbacks, though.

Drawbacks of tritium

• Tritium is radioactive. With fission power plants being decommissioned around the world, needing to deal with a radioactive fuel is a headache for security and licensing.
• The D-T reaction release neutrons, which activate structural materials and can make them even more hazardous than tritium (you can imagine the extra headaches coming out of this point).
• Tritium can permeate materials. Yes, as small as it is —a cousin of ordinary hydrogen—, this folk can go across walls in certain conditions.
• There is virtually no natural tritium existing on Earth. But it can be “manufactured” and there are some reserves from some traditional nuclear power plants (and from military purposes, these more difficult to quantify).

Then, why on Earth would anyone want to use tritium for anything if it is quite hazardous and you can’t get it from anywhere?

Well, the thing is that the D-T reaction has a special quality by which it needs relatively little amount of energy to activate. Other reactions —as deuterium, lithium or boron-based— need so much energy to get their nucleus so close for fusion to take place that the returned energy does not pay for the spent energy.

Let there be hope

Is there no other hope? In fact, one of the possible fusion reactions releases almost as much net energy as the deuterium-tritium one. It involves helium and, again, deuterium:

   

Here we find another obstacle towards a nice alternative fusion reaction. Helium-3 or is a weird isotope of helium (with one less neutron than ordinary helium) that cannot be collected on Earth. Shame. Nevertheless, there are ambitious projects willing to mine this new fuel from the surface of the moon, which happens to be spread with it.

Mining in places other than Earth seems to be far in time. But let’s dream about it by listening to a great band highly inspired by science, space and technology.

Bibliography:

• T. Tanabe, Tritium: Fuel of Fusion Reactors. Tokyo: Springer Japan, 2017.