The stable isotope of hydrogen used in the thermonuclear bomb is deuterium. Deuterium is an isotope of hydrogen that has one proton and one neutron in its nucleus, while the more common isotope of hydrogen, protium, has only one proton. Deuterium is used in thermonuclear bombs because it can fuse with other isotopes of hydrogen, such as tritium, to release large amounts of energy. This fusion reaction is the same process that powers the sun and stars.

Deuterium is also an important component of the hydrogen bomb's trigger, which is a small nuclear fission bomb that is used to start the thermonuclear reaction. The trigger is made of uranium or plutonium, and when it is detonated, it releases neutrons that are then absorbed by the deuterium and tritium atoms in the thermonuclear fuel. This reaction causes the deuterium and tritium atoms to fuse, releasing large amounts of energy and creating the thermonuclear explosion.

Deuterium is a relatively common isotope of hydrogen, and it can be extracted from water. However, the process of extracting deuterium from water is expensive and time-consuming, which is why deuterium is a valuable and strategic material.