Splitting large nuclei

1. Some very large nuclei, such as uranium-235 and plutonium-239, are unstable and can split into two smaller nuclei.
2. This splitting of a nucleus is called nuclear fission.
3. For fission to happen, the unstable nucleus usually has to absorb a neutron first.
4. A neutron has no charge, so it is not repelled by the positive nucleus and can get close enough to be absorbed.
5. When uranium-235 absorbs a slow-moving neutron it becomes uranium-236, which is so unstable that it splits almost immediately.
6. The two smaller nuclei produced are called daughter nuclei, and they are usually of unequal size, such as barium-141 and krypton-92.

Radiation from fission

1. Each fission event releases two or three neutrons as well as the two daughter nuclei.
2. Fission also releases gamma rays, because the newly formed nuclei are left with extra energy.
3. The daughter nuclei have too many neutrons to be stable, so they go on to decay and emit beta and gamma radiation.
4. This is why the spent fuel removed from a nuclear reactor is highly radioactive waste.
5. The total number of protons and the total number of neutrons are the same before and after a fission event, which you can check with a nuclear equation.

Energy transfer in fission

1. Each fission event releases a very large amount of energy, millions of times more than a chemical reaction involving a single atom.
2. Most of this energy is transferred to the kinetic energy stores of the daughter nuclei and the neutrons, which fly apart at very high speeds.
3. Some of the energy is carried away by the gamma rays.
4. The fast-moving particles collide with the surrounding atoms, so the energy is transferred to the thermal energy store of the material and it heats up.
5. In a nuclear power station, this thermal energy is used to boil water into steam, which drives turbines to generate electricity.

Chain reactions

1. The two or three neutrons released by one fission can each be absorbed by another uranium-235 nucleus.
2. Those nuclei then split and release more neutrons, which can cause still more nuclei to split.
3. This self-sustaining process is called a chain reaction.
4. If every fission causes more than one further fission, the number of fission events grows extremely quickly, from one to three to nine to twenty-seven and onwards.
5. In a nuclear reactor, the chain reaction is controlled, so that on average exactly one neutron from each fission goes on to cause another fission.
6. Control rods, made of a material such as boron that absorbs neutrons, are lowered into the reactor to slow the reaction and raised to speed it up.
7. In a nuclear weapon, the chain reaction is uncontrolled, so a huge amount of energy is released in a fraction of a second as an explosion.