A thermonuclear weapon in which the detonation of a primary fission device (an atomic bomb) compresses a secondary structure consisting of a fuel made from isotopes of hydrogen that encloses a sparkplug of fissile material and that is surrounded by a tamper made from uranium. The strong compression activates the sparkplug, causing atoms in the hydrogen fuel to fuse with a reduction in mass that is converted to and released as energy. Hydrogen bombs vary in yield, but all exceed 210 terajoules (TJ), equal to 50,000 tons of TNT. The first hydrogen bomb was detonated in 1952 by the USA.