Nuclear Fission and Fusion

• Fission

  • the splitting of a large unstable nucleus (uranium-235 or plutonium-239) into 2 fragments (smaller nuclei) releasing energy and 2-3 neutrons
  • so fission neutrons can collide with more uranium and cause a chain reaction:

  
• chain reaction

  • reaction where one reaction causes further reactions which in turn causes further reactions, etc.
• • a chain reaction is controlled in a nuclear reactor to control energy released.
  • this is done by removing some neutrons to keep chain reactions at a constant rate
  • explosions caused by nuclear bombs is caused by an uncontrollable chain reaction- extra neutrons aren't removed so chain reaction gets faster
• nuclear reactor

  • where fission takes place in order to boil water to produce steam - drives turbines which turns generator to create electricity
• Fusion

  • joining of two light nuclei to form a heavier nucleus, eg hydrogen can fuse to form helium
  • Deutrium + Titrium > Helium + neutron + energy
  • some of the mass of the nucleus is converted to energy- this produces thermal energy and also light energy: E = mc^2
• Fusion conditions

  • very high temperatures (10,000,000 C) and pressures
  • this is needed to force the nuclei together at very high speeds as they are both positively charged and will repel each other
  • fusion takes place in stars and experimental fusion reactors on earth
  • the sun (star) has a higher pressure so you can use a smaller temperature there
• fusion advantages

  • the fuel is readily available (isotopes of hydrogen called 'heavy' hydrogen is in sea water)
  • the product is helium which is a non-radioactive gas and is harmless
• fusion problems

  • nuclei need to be heated to high temperatures
  • this is because the electrons need to be stripped from the atoms to form plasma and the nuclei must be moving at very high speeds to collide
• Temperatures

  • 100,000,000 C on earth
  • 10,000,000 C on the Sun
  • this temperature is achieved with lasers or huge electric currents
  • the whole reaction has to be contained by a magnetic field to stop the charged plasma from touching the walls of the container (the reaction would go cold and would end)
• Safety measures

  • remote handling- used fuel rods are handled with remote control robotic arms
  • when fuel rods are removed from a nuclear reactor they're very hot and are stored in cold water for up to a year to cool down before they're moved
  • uranium and plutonium are chemically removed from the rods by special machines and then sealed in new containers remotely
  • the Uranium and plutonium can stay radioactive and give out heat for 1000s of years later so they need to be stored securely in concrete containers which are buried deep underground