At the risk of pushing this even further from cryptography, I should say that tritium is used in the "boosting" of *fission* weapons. A mixture of tritium and deuterium is injected into the exploding fission core to increase the "alpha" (neutron multiplication "gain") of the system. The D-T thermonuclear reactions themselves contribute relatively little energy, but the increase in fission efficiency can be dramatic. Thermonuclear boosting was the second major improvement made to US fission weapons after WWII. The first was the "levitated pit", a gap between the conventional explosive/tamper assembly and the fissile pit to allow the former to gain significant momentum before slamming into the latter. Both techniques result in considerably more efficient use of fissile material, but are not absolutely necessary to make a usable weapon (as shown at Hiroshima and Nagasaki). I believe the simple uranium gun used at Hiroshima only fissioned a few percent of its U-235. Fat Man did better, but not that much. Phil