1. 1 NCEA Physics Nuclear Fission

2. 2 Nuclear fission Aims: To understand that a nucleus of U-235 can be split (fission) by collision with a neutron and that this process releases energy in the form of kinetic energy of the fission products. To know that the fission of U-235 produces two daughter nuclei and a small number of neutrons. To understand that a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei.

3. 3 Nuclear fission Aims: To describe in outline how the fission process can be used as an energy source to generate electricity. To understand that the products of nuclear fission are radioactive and the implications this has for their safe storage over long periods.

4. 4 Uranium 238 Uranium 238 is an unstable isotope that decays into thorium 234. As we would expect from a high proton number this decay involves alpha particle emission. Uranium 238 is radioactive but it is not used in nuclear power stations.

5. 5 Fission animation

6. 6 Uranium 235 In nuclear power stations we use uranium 235 to produce energy. Unfortunately uranium 235 has a long half life and is therefore of little use. However when an extra neutron is added uranium 236 is created. This isotope soon decays producing the energy we need.

7. 7 Neutron absorption

8. 8 More neutrons In this Uranium 236 fission two neutrons are released, each one could be absorbed by another uranium 235 atom to form a chain reaction.

9. 9 When uranium 236 decays two daughter nuclei are created. In addition to the daughter products two or three neutrons are also produced during the disintegration. Energy is released through the kinetic energy of the decay products. Parent and daughter

10. 10 Chain reaction The decay of uranium 236 produces three neutrons. Each of these neutrons can start another decay if they strike a uranium 235 nucleus. This is called a chain reaction. A single reaction leads to 3, then 9, 27, 81, 243 and so on.

11. 11

12. 12 Moderator rods The neutrons that are fired at the uranium 235 will ‘bounce off’ if they are travelling too fast. Moderator rods in a nuclear reactor slow down these neutrons so that they can be absorbed by the uranium.

13. 13 Controlling the reaction Control rods made of boron are used to absorb some of the neutrons given out when the uranium 236 decays. The control rods can decrease the number of neutrons that it turn cause further decays.

14. 14 Nuclear reactors

15. 15 BWR Boiling water reactor Steam at high pressure drives a turbine. The turbine then drives a generator which supplies electricity to the national grid.

16. 16 PWR Pressurised water reactor PWRs keep water under pressure so that it heats, but does not boil. Water from the reactor and the water in the steam generator that is turned into steam never mix. In this way, most of the radioactivity stays in the reactor area.

17. 17 Cooling Towers Cooling towers do exactly what their title suggests – they cool down steam and turn it into water. They do not give off dangerous smoke. In some power stations this steam is used to provide hot water for nearby residents.

18. 18 Nuclear power station Nuclear reactors Reactor cooling ponds Turbine generator building Cooling towers

19. 19 Nuclear power worldwide

20. 20 Nuclear waste The waste products and contaminated materials produced by nuclear power plants are hazardous. Great quantities of waste material can be generated especially when a nuclear power station is decommissioned. These materials can have very long half-lives and therefore require careful storage. A range of storage facilities are used often utilising disused mines and geologically stable areas. Special precautions must be made to make sure that radioactive materials do not contaminate any part of the food supply or local water table. The storage of nuclear waste is one of the main problems with this method of power generation.

21. 21 High level waste disposal

22. 22 Summary - Fission Uranium 235 absorbs a neutron and becomes unstable. It breaks up (fission) and releases 2/3 more neutrons and lots of kinetic energy. The extra neutrons can cause more fission during a chain reaction The heat given off is used to boil water, the steam drives turbines that drive generators and create electricity. The left over fission products are still radioactive. They must be safely stored for a very long time to stop contamination of land, water and the food chain.