P2.7.1 Nuclear Fission 1
- This is the process of splitting a nucleus into fragment nuclei, and releasing 2/3 fission neutrons which go on to cause other fissionable nuclei to split-a chain reaction.
- Energy in the form of radiation, as well as kinetic energy of the fragment nuclei and fission neutrons is released.
- Most reators are designed to use enriched uranium as the fuel, which contains mostly non-fissionable isotope U-238 and about 2-3% fissionable U-235. Natural uranium is over 99% U-238, which don't undergo fission but change into other heavy nuclei, including Pu-239 (plutonium) which is used in a different type of nuclear fission reactor.
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P2.7.1 Nuclear Fission 2
- A nuclear reactor has uranium fuel rods evenly spaced out in the pressurised water reactor (PWR).
- Water: Acts as a moderator: It slows down fission neutrons by having them collide with atoms of water molecules, as fast neutrons can't cause more fission. Coolant: Molecules gain kinetic energy from neutrons and fuel rods, and then are pumped through pipes to a heat exchanger, and the water transfers energy for heating to the heat exchanger from the core.
- Control rods: Absorb surplus neutrons to keep the chain reaction under control. The depth of rods in the reactor is adjusted to maintain a steady chain reaction.
- Reactor core: made of thick steel to withstand the high temperature and pressure in the core, which is enclosed by thick concrete walls which absorb radiation through the walls of the vessel.
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P2.7.2 Nuclear Fusion 1
- This is fusing small nuclei like H, to form larger nuclei; stars release energy by doing so.
- Water had a lot of H atoms- a glass of water could provide the same amount of energy as a tanker full of petrol, but this type of reactor is out of reach at the moment.
- Fusion only releases energy if the relative mass of the nucleus is no more than about 55 (iron) and energy has to be supplied to create bigger nuclei.
- The sun is made of 75% H, 25% He-the core is so hot that it consists of a plasma of nuclei with no electrons, which move about and fuse when they collide, releasing energy.
- When 2 proptons (i.e. H nuclei) fuse, they form a heavy H nucleus, an isotope of H with a mass of 2 and other particles are created and emitted at the same time. 2 more protons collide seperately with 2 'heavy' H nuclei, turning them into heavier nuclei, forming the helium nucleus- H-4.
- Energy released at each stage is carried away as kinetic energy of the product nucleus and other particles emitted.
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P2.7.2 Nuclear Fusion 2
- There's a lot of problems with nuclear fusion:
- A plasma of light nuclei has to be heated very high for fusion to occur, as 2 nuclei approaching each other would repel due to positive charges but if they move fast enough, these forces of repulsion can be overcome.
- In a reactor, the plasma is heated by passing a huge current thorugh it. The plasma is contained by a magnetic field and so doesn't touch the reactor, or else it could go cold and stop.
- At the moment, scientists are working on experimental fusion reactors, and can fuse heavy H nuclei to make He nuclei, but only for a few minutes.
- If releasing energy by nuclear fusion becomes possible, the fuel for fusion reactors is readily available; heavy hydrogen is naturally present in sea water.
- The product, helium, is harmless and the energy released could be used to generate electricity.
- Fission- mostly uranium is used for fission, which is only found in certain parts of the world, and it makes nuclear waste that has to be stored securely for years. However, fission reactors have been in operation for over 50 years.
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