Before detonation, a few grams of tritium–deuterium gas are injected into the hollow "pit" of fissile plutonium or uranium. The early stages of the fission chain reaction supply enough heat and compression to start deuterium–tritium fusion; then both fission and fusion proceed in parallel, the fission assisting the fusion by continuing heating and compression, and the fusion assisting the fission with highly energetic (14.1 MeV) neutrons. As the fission fuel depletes and also explodes outward, it falls below the density needed to stay critical by itself, but the fusion neutrons make the fission process progress faster and continue longer than it would without boosting. Increased yield comes overwhelmingly from the increase in fission. The energy released by the fusion itself is much smaller because the amount of fusion fuel is so much smaller. The effects of boosting include:
* increased yield (for the same amount of fission fuel, compared to detonation without boosting)
* the possibility of variable yield by varying the amount of fusion fuel
* allowing the bomb to require a smaller amount of the very expensive fissile material – and also eliminating the risk of predetonation by nearby nuclear explosions
* not so stringent requirements on the implosion setup, allowing for a smaller and lighter amount of high-explosives to be used
The tritium in a warhead is continually undergoing radioactive decay, hence becoming unavailable for fusion. Furthermore, its decay product, helium-3, absorbs neutrons if exposed to the ones emitted by nuclear fission. This potentially offsets or reverses the intended effect of the tritium, which was to generate many free neutrons, if too much helium-3 has accumulated from the decay of tritium. Therefore, it is necessary to replenish tritium in boosted bombs periodically. The estimated quantity needed is  per warhead. To maintain constant levels of tritium, about  per warhead per year must be supplied to the bomb.
One mole of deuterium-tritium gas would contain about  of tritium and  of deuterium. In comparison, the 20 moles of plutonium in a nuclear bomb consists of about  of plutonium-239.