» Subsections: Enrichment
Fission is the process of splitting the nucleus of an atom into two smaller fragments. Uranium-235 and Plutonium-239 are the most commonly used fuels in nuclear fission reactions. Elements such as these can undergo induced fission, which means if a neutron collides with the nucleus, the atom will absorb the neutron, become unstable, and split apart immediately. Before these substances can be used, however, they must be enriched.
For example, in U-235 (shown below), as soon as the nucleus captures the neutron, it splits apart into two atoms and ejects 2-3 new neutrons. The resultant atoms emit gamma radiation. The entire process happens in only a matter of picoseconds (one trillionth of a second).
The chance of a U-235 atom capturing a passing neutron is relatively high. In a properly designed nuclear device, more than one neutron ejected from each
U-235 fission causes another fission. This can be described as a self-sustaining chain reaction of
U-235 fission. When this happens, scientists describe the condition as supercriticality.
The two resultant atoms and the neutrons together weigh less than the original atom. So where did the extra mass go? The answer can be found in Albert Einstein's famous equation E=MC2, which means energy can be converted to mass and vice versa. Since c is the value of the speed of light, it is evident that a small amount of mass can be converted to extraordinary amounts of energy. In this case, an incredible amount of energy is released in the form of heat and gamma radiation. A pound of highly enriched uranium (HEU) is roughly equivalent to an amount on the order of a million gallons of gasoline or more energy than the burning of 3 million pounds of coal. In other words, a baseball-sized amount of HEU can produce as much energy as a cube of gasoline with 50 feet on each side.
"In fission reactions, there are two kinds of neutrons that come out. There are prompt neutrons that come out as the process of the fission reaction itself and there are delayed neutrons that come out hundreds of milliseconds later. If you get the reaction with enough fissile material with enough enrichment, you don't need the delayed neutrons and then the energy can grow exponentially in a matter of hundreds of microseconds..." - listen
Dr. Rajesh Maingi, Senior Research Scientist, Princeton Plasma Physics Laboratory
If a stock of nuclear fuel has critical mass, then it has enough fissionable material to start a self-sustaining fission reaction and go supercritical. In a bomb, the fuel must be kept in separate, smaller subcritical masses to prevent premature detonation.
Two or more subcritical masses must be brought together to form a supercritical mass. Free neutrons must then be fired into the mass to start the fission reaction. Finally, as much of the fuel as possible must be fissioned before the explosion blows the bomb to shreds.
For pure uranium-238, the critical mass is 50kg (110lb). No U-235 is ever pure, so in reality more mass is needed. Plutonium is more easily fissionable than U-235. Its critical mass is only 16kg (35.2lb) for pure Pu-239.
There are two techniques in use for bringing subcritical masses together: gun-type and implosion.
"The key with fission is there is something that is known as a critical mass... If you have enough fissile material and you put it together and its a critical mass, then essentially all you need is a single neutron that comes in or a cosmic ray or some energy source that comes in that starts a chain reaction and the reaction will grow uncontrollably." - listen
Dr. Rajesh Maingi, Senior Research Scientist, Princeton Plasma Physics Laboratory