Nuclear energy can be released by fusion of two light elements (elements with low atomic numbers). The power that fuels the sun and the stars is nuclear fusion. In a hydrogen bomb, two isotopes of hydrogen, deuterium and tritium are fused to form a nucleus of helium and a neutron. This fusion releases 17.6 MeV of energy. Unlike nuclear fission, there is no limit on the amount of the fusion that can occur.
If light nuclei are forced together, they will fuse with a yield of energy because the mass of the combination will be less than the sum of the masses of the individual nuclei. If the combined nuclear mass is less than that of iron at the peak of the binding energy curve, then the nuclear particles will be more tightly bound than they were in the lighter nuclei, and that decrease in mass comes off in the form of energy according to the Einstein relationship. For elements heavier than iron, fission will yield energy.
The energy from the Sun – both heat and light energy – originates from a nuclear fusion process that is occurring inside the core of the Sun. The specific type of fusion that occurs inside of the Sun is known as proton-proton fusion.
Inside the Sun, this process begins with protons (which is simply a lone hydrogen nucleus) and through a series of steps, these protons fuse together and are turned into helium. This fusion process occurs inside the core of the Sun, and the transformation results in a release of energy that keeps the sun hot. The resulting energy is radiated out from the core of the Sun and moves across the solar system. The core is the only part of the Sun that produces any significant amount of heat through fusion (it contributes about 99%). The rest of the Sun is heated by energy transferred outward from the core.
In the 1930’s scientists, particularly Hans Bethe, discovered that nuclear fusion was possible and that it was the energy source for the sun. Beginning in the 1940’s researchers began to look for ways to initiate and control fusion reactions to produce useful energy on earth. From the start, the task was difficult, because fusion reactions required temperatures of hundreds of millions of degrees, too hot to be contained by any solid chamber. Instead, physicists sought to contain the hot plasma with magnetic fields, using, for example, the pinch effect where electric currents moving in the same direction attract each other through their magnetic fields. This approach was called “magnetic confinement”.
Initially this work in the U.S., UK and USSR was secret. However, by the mid-1950’s administrators and scientists alike were convinced that controlled fusion research had no military applications, and in particular had nothing to do with the development of thermonuclear weapons. The first thermonuclear weapons had been detonated in the early 1950’s. In an H-bomb, or thermonuclear weapon, the tremendous energy of a fission-based nuclear weapon is used to heat up a large amount—tens or hundreds of kilograms—of a fusion fuel to release fusion energy in an explosion.
By contrast, in controlled fusion research—and in a future fusion generator—not even one gram of fuel would be heated to high temperature at any one time. This tiny amount of highly heated fuel is far too small to serve as a “spark” for the kilograms of fuel needed for a weapon. In fact, any contact between the tiny amount of hot fuel plasma and a larger object, such as the fuel for a bomb, would immediately douse the fusion reaction by lowering the plasma’s temperature. (Thus the conversion of a fusion generator into bomb, sometimes portrayed in science fiction, is impossible.)
Since fusion research had no military applications, it was declassified by the major participating nations, and cooperation in fusion began between the U.S. and the USSR.