Nuclear Fission and Nuclear Fusion

Nuclear fission is the splitting of an atomic nucleus. It's used in nuclear reactors to release energy to make electricity.

The two fissionable substances in common use in nuclear reactors are uranium-235 and plutonium-239.

For fission to occur the uranium-235 or plutonium-239 nucleus must first absorb a neutron. The nucleus then becomes unstable and splits into two smaller nuclei, releasing two or three more neutrons and a lot of energy.

The neutrons released in fission may themselves go on to collide with othe uranium-235 or plutonium-239 nuclei, producing further neutrons and energy in a process called a chain reaction. Nuclear reactors control the rate of this chain reaction to release the energy required.

The new neutrons produced by nuclear fission can each cause a new fission. This is a chain reaction. It carries on and on and on.

The energy is released and heats the surroundings. Each fission reaction only releases a tiny amount of energy, but there are billions and billions of reactions every second.

Nuclear fusion is the joining together of two or more atomic nuclei to form a larger atomic nucleus. To achieve nuclear fusion a lot of enrgy is required.

A nuclear fusion reaction (when started) will release more enrgy than it uses. This makes it self-sustaining, i.e some of the energy produced is usedto drive further fusion reactions.

An example is the fusion of two isotopes of hydrogen,called deuterium and tritium. When they are forced together under high pressure, the deuterium and tritium nuclei fuse together to form a helium atom and a nuetron and a lot of energy.

Nuclear fusion is the process by whcih energy is released in stars. In the core f our nearest star,he Sun, hydrogen is being continuously converted into helium through nuclear fusion. This process provides the energy to keep the Sun hot and to allow life on Earth.

Stars, like our Sun, form when enough dust and gas from space are pulled together by gravitational forces, which always attract each other. This forms a nebula where a prostar is then formed.

Forcing material together increases the temperature and density, and nuclear fusion reactions start releasing huge amounts of energy. Eventually the attractive gravitational forces balance with the repulsive forces produced by radiation to make a star stable.

The newly formed star becomes a main sequence star. It will remain like this for many millions or billions of year until its supply of hydrogen runs out,

During the formation process smaller masses within the prostar may be attracted by the dominant larger mass to bemome planets. During its time on the main sequence, a star will produce all ofthe naturally occuring elements through the fusion process up to iron.

Eventually the hydrogen within a star runs out. What happens next is determined by the size of the star.

Stars about the size of the Sun.

• Star leaves main sequence and becomes a red giant
• It continues to cool before collapsing under its own gravity to become a white dwarf.
• It continues to cool and loses its brigtness to become a black dwarf.

Stars Bigger than the Sun

• Star leaves the main sequence and become a red super giant
• It cools but shrinks very rapidly and explodes as a supernova. The explosion releases massive amounts of energy, dust and gas into space, and forms elements heavier than iron.
• Depening on the precise mass of the remnants eitehr a neutron star or a black hole is formed.
• The dust and gas form new stars.