The post by cryptonector is correct: gram for gram, tritium for weapons is even more expensive to manufacture than plutonium for weapons. It takes one neutron

The post by cryptonector is correct: gram for gram, tritium for weapons is even more expensive to manufacture than plutonium for weapons. It takes one neutron to convert an atom of uranium 238 into an atom of plutonium 239. It also takes one neutron to convert an atom of lithium 6 into an atom of tritium and an atom of helium [1]. However, the plutonium 239 atom has a mass nearly 80 times as great as the tritium atom, so the manufacturing cost per gram is much higher. The US has always produced tritium for weapons by irradiation of lithium 6; the excess neutrons above and beyond that needed to support reactor criticality could be used to produce either tritium or plutonium. The competing uses for excess neutrons were a major issue early in the American development of thermonuclear weapons, when it looked like a thermonuclear weapon might need a lot of tritium (and therefore curtail the production of a lot more plutonium). Heavy water reactors like Canada's CANDU produce some tritium "for free" as a natural byproduct of neutron capture by deuterium. This gets removed [2] periodically from the heavy water moderator to reduce its radioactivity, and some of the surplus tritium gets sold. But by law Canadian tritium cannot be sold for weapons applications. [1] https://en.wikipedia.org/wiki/Tritium#Lithium https://en.wikipedia.org/wiki/Tritium#Lithium [2] https://www.opg.com/powering-ontario/our-generation/nuclear/darlington-nuclear/ https://www.opg.com/powering-ontario/our-generation/nuclear/...