Fission and Fusion
Energy can be released by either breaking apart a big nucleus into two smaller ones (nuclear fission) or by joining together two smaller nuclei into one larger one (nuclear fusion). Energy is released in both processes, but we can only harness the energy released from nuclear fission because fusion requires ridiculously high pressures and temperatures which are tricky to replicate on Earth.
Nuclear Fission
Mushroom cloud from detonation of an atomic bomb over Hiroshima, Japan during World War II, 1945.
Nuclear fission is the process of splitting a nucleus to release energy. When a uranium-235 nucleus absorbs a neutron, this alters the ratio of protons to neutrons in the nucleus making it unstable. The unstable nucleus splits into two daughter nuclei and two or three neutrons. These neutrons can be absorbed by another nucleus of uranium, which will in turn split into two nuclei and more neutrons in a chain reaction. During each fission event, energy is released in the form of kinetic energy.
The large amount of energy released during the nuclear fission chain reaction is responsible for atomic bomb explosions. Atomic bombs usually contain radioactive isotopes of uranium or plutonium and are capable of destroying whole cities.
Nuclear Reactors
Fission is used in nuclear reactors to produce energy. Fission only works if the neutron is moving slowly enough to be absorbed by a nucleus so nuclear reactors contain something called a moderator, usually made of graphite, which reduces the speed of the neutrons, making them more likely to be absorbed by a nucleus. When a neutron is absorbed by a nucleus, it undergoes a fission event, breaking apart into two nuclei and two or three neutrons. These neutrons hit other nuclei in a chain reaction. It is important that the chain reaction doesn’t get out of hand, otherwise the reactor could explode. To stop this happening, the reactor contains control rods, usually made of boron, which absorb some of the extra neutrons so that only a single neutrons is released per fission event. The reactor is covered with a thick concrete shield to ensure that no radiation is able to penetrate through and escape the reactor.
The kinetic energy of the neutrons is harnessed by the nuclear reactor and used to heat water. The water evaporates into steam which turns a turbine, which in turn drives a generator to produce electricity.
Advantages and disadvantages of producing energy using nuclear fission
Disadvantages
The waste is radioactive so it needs to be carefully disposed of
Accidents can be catastrophic e.g. Chernobyl
Lots of hot water is produced which can harm marine life when disposed of
Cannot respond quickly to changes in electricity demand
Advantages
Does not produce polluting gases such as carbon dioxide so does not contribute to global warming
Low fuel costs
Only small amounts of fuel are needed - this reduces the effects of mining and transportation on the environment
Power stations have a long lifetime
Stars generate energy through nuclear fusion reactions, which requires temperatures of around ten million degrees Celsius.
Nuclear Fusion
Nuclear fusion is where two small, light nuclei collide at high speed and join together to make one heavy nucleus. Fusion reactions occur in stars where two hydrogen nuclei fuse together to form a helium nucleus. During this process, some mass is converted into energy, which radiates out from the star. Nuclear fusion requires the joining together of two positive nuclei which under normal conditions will repel each other due to their identical charges so it doesn’t happen at low temperatures and pressure. In order to overcome the repulsion of the positive nuclei, fusion requires such extreme temperatures and pressures that it is pretty much impossible for scientists to recreate these conditions on Earth to generate energy. It would be so expensive that generating electricity using this process would be extremely unsustainable.