The strong force in the nucleus acts over a very short distance; so, as a nucleus gets larger, it will reach a point where it is too big for the strong force to hold all the protons and neutrons together. Thus, very heavy nuclei are not very stable, and nuclei of intermediate mass are much more stable.
Fission is the process by which a very heavy nucleus (mass greater than 200 amu) splits into smaller nuclei of intermediate mass. This releases a large amount of energy.
An example of this type of process is that of Uranium-235 splitting when it is hit with a neutron. This causes the nucleus to become unstable and it will split into barium-141 and krypton-92 as shown by the following equation.
\(_{92}^{235}{\text{U}\ +_{0}^{1}{\text{n} \rightarrow _{36}^{92}{\text{Kr} +_{56}^{141}{\text{Ba} + 3_{0}^{1}\text{n}}}}}\)
Nuclear power plants use nuclear fission to produce power. The fuel used is uranium-235, which is hit with a slow-moving neutron to start the reaction. The neutron is absorbed, and the uranium becomes unstable. The nucleus then splits into two medium-mass nuclei of krypton-92 and barium-141 and emits three more neutrons.
The three neutrons that are given off can then hit more uranium nuclei which will also split, and a chain reaction will occur. As long as uranium is being split, then more uranium can be split. The process will stop when the neutrons escape without hitting any uranium.
A large amount of energy is stored within the nucleus. When the nucleus is broken apart, this energy is released. When the nucleus of uranium-235 is split, energy is given off in the form of heat. One kilogram of uranium-235 can release 2.5 million times as much energy as burning one kg of coal.
In a nuclear power plant, a controlled reaction is used to produce heat that will be used to generate electrical energy. The uranium-235 is packaged into fuel rods, which are in the core of the reactor. Control rods are placed between the fuel rods to absorb some of the neutrons so the reaction will not happen too quickly. The core of the reactor is surrounded by a containment structure that absorbs radiation. Circulating water absorbs the heat produced from the reaction and this heat is released in a steam generator which turns a turbine and generates electricity.
Question
What would happen if there were not enough control rods?
The reaction would happen too quickly and too much heat would be produced possibly leading to a nuclear meltdown.
Reflection
Why is uranium-235 used as a fuel for nuclear power plants?
