I presume that by controlled you mean its for nuclear electricity generation applications. As opposed to an uncontrolled reaction found in an atomic weapon.
Usually in contemporary nuclear power plants, control rods which reflect or adsorb (in simple terms) neutrons are lowered into the reactor core.
If you read up on fission, you will know neutrons are what keep the fission reaction going assuming the fissile material is avaliable in abundance in the reaction. By limiting the supply of 'free' active neutrons, one can limit or 'throttle' the extent of the nuclear fission reaction.
Fissile material in nuclear power plants is usually moderately eniched Uranium, which can be a specific 'blend' of U-253 and U-258 (of course, with many trace elements).
EDIT: I forgot to mention that with some newer fourth-generation (experiment) nuclear power plants, the process and fuel are entirely different. Some use Thorium, and have radically different designs from 'traditional' nuclear power plant designs.How is the nuclear fission process controlled and what is fissionable material?
Nuclear fission the type of nuclear reactions occurring when a large atom is split into two smaller atoms. The sum to the masses of the two smaller atoms will be less than the mass the to larger atom that was split. This decrease in mass is converted into energy through E = mc squared which explains how fission reactions produce so much power.
To split an atom (e.g Uranium 235) a neutron is fired at the atom. The atom splits into Barium and Krypton as well as 3 more neutrons. These 3 neutron then split 3 more Uranium 235 atoms. If left uncontrolled, a nuclear reaction can actually sustain itself, with the number of atoms splitting increasing exponentially. This uncontrolled type of reactions is used in nuclear weapons, in fact the entire chain reaction in a nuclear explosion goes to completion in a split second!
To control a nuclear reaction, such as that in nuclear power stations, control rods are used, rods of fissionable material can be pulled out or placed in the reactor to control to reaction. The reactor also contains material which absorb neutrons, reducing the rate of reaction. Lastly heavy water (water made up of isotopes of hydrogen) are used in earlier nuclear reactors to improve power production.
fissionable material include Uranium 235 (an isotope of uranium) and Plutoium.
wikipedia is your best friend in this situation. I learned how a black hole works in and out in alf a day using it so I doupt there will be any problem you finding the awnsere there.
It is controlled by using control rods which absorb the excess electrons. Fissionable material are those materials when bombarded with electrons will deteriorate into materials by emmitting radiations in the form of protons nutrons or electrons.
The answer may not be exact but is on those lines.
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