Invented by Edward Teller. One of the most powerful forms of nuclear weapon. Uses a series of booster reactions. Chemical explosives compress uranium-235 or plutonium into a critical mass. The critical mass undergoes a fission reaction, liberating a great deal of energy (the original atomic bomb stopped at this point). The heat and energy from the fission reaction then causes tritium to undergo a fusion reaction, which liberates even more energy than fission of plutonium or uranium. Most modern thermonuclear weapons are hydrogen bombs, sometimes with the addition of an isotope of Cobalt to enhance the length of time the fallout lingers.