What is nuclear fission: some physics
Atoms are made up of a central nucleus, and "orbiting" electrons. The nucleus is made of protons and neutrons. Protons and electrons are charged: protons are positively charged, and electrons are negatively charged. The magnitude of their charge is the same, but the sign is opposite.
An "atom" is not charged overall, by definition (a charged particle is called an "ion"), and therefore has the same number of protons and neutrons (so that the charges balance).
The element is determined by the number of protons and electrons. For example, one proton is called Hydrogen, two protons is called Helium, three is Lithium and so on.
Protons and neutrons are held together in the nucleus by the nuclear force. In a physical sense, when you apply a force to something, say pushing a box, you are transferring energy from yourself to the box. The opposite it also true.
In simple terms, the force "field" is a store of energy, and when this field breaks down, it releases the energy it stores.
In nuclear fission, heavy atoms are encouraged to decay, to break apart into smaller atoms. Because less energy is required to hold two smaller nuclei together than one large one, there is a net release of energy.
How much energy is released during nuclear fission?
Let's assume a reaction. Atom X decays by fission into atoms Y and Z. What we find is that the mass of atom X is greater than the mass of Y and Z combined. There is a loss of mass during the decay, known as a mass deficit.
Now we apply one of Physics' best known equations: E=mc2. E stands for "energy", m stands for "mass" and c is the speed of light. m is the mass deficit in this case. The value of c2, that is c x c, is huge. The value of c is 300000000 metres per second. This means that even a tiny mass deficit can translate into a substantial amount of energy.
In fact, and I believe this is the third time this quote appears on this site,
At the atomic level, the thermal energy released in a fission event is 200MeV, compared with only a few electron-volts produced each time a hydrocarbon molecule is broken down by burning carbon-based fuels. As a result, a single nuclear-reactor fuel pellet just 1cm long can produce the same amount of electricity of 1.5 tonnes of coal.
Source: P. Norman, A. Worrall, K. Hesketh, Physics World, Vol 20, No 7, July 2007, page 25
Quite a lot, then!What is the fuel for nuclear fission?
The primary fuel used for nuclear fission is Uranium 235. We use U-235 because it is one of the easiest of the fissile materials to use for controlled fission, but others can be used as well, for example, Thorium.
It is also a result of the fact that nuclear power research was developed alongside nuclear weapons research, so that the one complemented the other, and Uranium can be used in nuclear fission bombs.
How does nuclear fission work?
There are many kinds of nuclear reactor in existence and in development which make apply this process. I won't discuss them all here, however. The idea, though, is to develop a controlled chain reaction.
Uranium does decay on its own but not in the way we force it to in nuclear reactors. The process must be started by us, by firing a neutron at the Uranium. One nucleus (parent) will capture it, which will cause it to become even more unstable, causing it to decay by fission into two almost equally sized products (daughters).
The important thing is that it also emits a few free neutrons in the process, which then do the same thing again with other nearby Uranium nuclei. In a bomb, this process is uncontrolled and so more and more nuclei decay until.... well, yes.
In a reactor, however, this process is controlled such that the number of neutrons flying around can be adjusted. This way, a steady number of decays can be achieved and a steady amount of power can be released.