Question: Why do most nuclear fusion models use deuterium as the fuel instead of protium or tritium.

Ondrej Kovanda answered on 18 Mar 2020:

Actually, previously I’ve worked for a fusion experiment 🙂 So the best thing would be to fuel the reactor with deuterium and tritium mixture (DT), since the D-T reaction has both the largest probability of all the reactions to happen and the largest energy yield. As Eleanor had pointed out, tritium is extremely rare and also radioactive, making it very expensive to obtain and store from both technical and legislative points of view. Some reactor designs do, however, count on an on-site tritium production – as the fusion reactions produce neutrons, these can be captured in a blanket of lithium, by turning the actual lithium nuclei into tritium. Currently, however, D-D (deuterium deuterium) reaction is a good balance between affordability and fusion performance. For protium: (I’m guessing a bit here) protons can’t fuse directly, but they’d have to combine to deuterium/tritium first, and then the deuterium/tritium would have to fuse. These processes combined are rather unlikely to happen, and thus not feasible for a fusion reactor (no problem in stars, where you have incomparably larger number of protons present).