Nuclear Fusion uses the splitting of atomic nuclei to release energy, most often used for generating electricity in a thermal power station.
In nature, many elements are stable or are radioisotopes with very long half lives. However, some of the elements are fissionable, i.e. if they absorb another neutron in the nucleas, the nucleas becomes unstable and splits into 2 or more smaller nuclei, releasing energy and neutrons.
The most commonly used nuclear fuel elements are uranium, 235U, and plutonium, 239Pu.
However, we can't predict which element the fissionable material will split into; some of the resulting material is highly unstable and radioactive, hence the problem of nuclear waste.
The diagram shows a single fission event. The neutrons released can go on to cause more fission events. If the neutrons are not controlled we can get a chain reaction.
To make this reaction useful we need to control it by limiting the number of neutrons so the reaction proceeds much more slowly, releasing energy in a steady and controllable amount.
The Nuclear Reactor
1. Absorbs heat energy from reaction.
2. Slows down the neutron to make them more likely to be absorbed by the U235 in the fuel and make fission more likely (this is called 'moderation').
In fission an atom splits into two or more 'daughter' nuclei and several neutrons.
The total number of protons and neutrons still needs to balance on either side of the equation.
Nuclear Fusion is the process of 'fusing' small atomic nuclei into larger nuclei. This process releases energy as long as the resulting nucleus is smaller than a mass of 56, i.e. Iron.
The smaller the nucleus, the more energy is released as it is created. This is the opposite to nuclear fission where energy is released when nuclei larger than iron split.
For fusion to happen the nuclei need to be forced together with enough energy to overcome the electrostatic repulsion.
This normally needs very high temperatures and pressures.
These kinds of environments rarely exist in nature, except in the heart of stars.
However, there are attempts to create suitable conditions for fusion on earth.
This is because:
- Fusion releases more energy per event than fission
- The waste product is Helium, an inert gas
The Life Cycle of Stars
All stars, regardless of size, are fuelled by fission during the main part of their lives. This fusion generates energy in all parts of the electromagnetic spectrum, not just visible light.
- Stars form from clouds of dust which spiral together due to gravitational attraction.
- The gravity compresses the matter so much that intense heat develops and sets off nuclear fusion reactions and the star begins emitting light and other radiation.
- At the same time that the star is forming, other lumps may develop in the spiralling dust clouds and these eventually gather together and form planets which orbit around the star.
- The Sun is one of many millions of stars which form the Milky Way Galaxy.
- The distance between neighbouring stars is usually millions of times greater then the distance between planets in our Solar System.
- Gravity is of course the force which keeps the stars together in a galaxy and, like most things in the Universe, the galaxies all rotate, kind of like a catherine wheel only much slower.
- Our sun is out towards the end of one of the spiral arms of the Milky Way galaxy.
- Galaxies themselves are often millions of times further apart than the stars are within a galaxy.
- So this means that the Universe is mostly empty space and is really, really big.
- The gravity on neutron stars, white dwarfs and black dwarfs is so strong that it crushes atom.
- The stuff in the stars gets squashed up so much that they're millions of times denser than anything on Earth.
- If enough matter is left behind after a supernova explosion, it is so dense that nothing can escape the powerful gravitational field. Not even electromagnetic waves. The dead star is then called a black hole. Black holes aren't visible because any light being emitted is sucked right back in.
- Astronomers can detect black holes in other ways - they can observe X-Rays emitted by hot gases from other stars as they spiral into the black hole.
The Life Cycle of Stars
1. Stars initially form from clouds of dust and gas.
2. The force of gravity makes the dust particles come spiraling in together. As they do, gravitational energy is converted into heat energy and the temperature rises.
3. When the temperature gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei and give out massive amounts of heat and light. A star is born. It immediatley enters a long stable period where the heat created by nuclear fusion provides an outward pressure to balance the force of gravity pulling everything inwards. In this stable period it's called a main sequence star and it lasts about 10 billion years.
4. Eventually the hydrogen begins to run out and the star then swells into a red giant. It becomes red because the surface cools.
5. A small star like our sun will then begin to cool and contract into a white dwarf and then finally, as the light fades completely, it becomes a black dwarf.
6. Big stars however, start to glow brightly again as they undergo more fusion and expand and contract several times forming heavier elements in various nuclear reactions. Eventually they will explode into a supernova.
The Life Cycle of Stars
7. The exploding supernova throws the outer layers of dust and gas into space leading a very dense core called a neutron star. If the star is big enough this will become a black hole.
8. The dust and gas thrown off by the supernova will form into second generation stars like our Sun. The heavier elements are only made in the final stages of a big star just before the final supernova, so the presence of heavier elements in the sun and the inner planets is clear evidence that our planet has all formed from a supernova.
9. The matter from which neutron stars and white dwarfs and black dwarfs are made is millions of times denser than any matter on Earth because the gravity is so strong it even crushes the atoms.