These small nuclear weapons that use tritium in their cores and have a dialable yield are based upon "boosted fission". The nuclear reaction in a conventional fission bomb ends within nanoseconds after reaching critical mass because the bomb is blown apart. Consequently with a conventional fission bomb only a tiny fraction of the original U235/plutonium is actually consumed in the nuclear reaction. This was the situation before they invented boosted fission. With boosted fission one puts a little dab of deuterium/tritium within the bomb pit of the weapon. The amount of this little dab of deuterium/tritium is what determines the weapon's yield. When the U235/putonium components of the bomb assemble to critical mass, nuclear fission occurs causing enormous temperature and pressures to briefly occur within the bomb pit. This causes the little dab of deuterium/tritium to undergo nuclear fusion. Again the bomb immediately self destructs so relatively little energy is liberated by the nuclear fusion reaction (this is not a true thermonuclear weapon). However a significant amount of fast neutrons are generated by this short lived fusion reaction (much more than from the fission reaction). This abundance of neutrons allows the fission reaction to continue even after the bomb blows itself apart. Consequently almost all of the U235/plutonium undergoes nuclear fission thus significantly enhancing the weapon's yield. The important point to note is the boosted fission weapon still works as a conventional fission weapon if there is no tritium. It would essentialy be at it's lowest setting.

Gary