Nuclear Fission

• The process by which a large unstable nucleus splits into two smaller nuclei which are more stable than the original nucleus
• The binding energy per nucleon increases in this process
• Usually, for fission to occur a large already unstable nucleus must first absorb a neutron, a process called induced fission
• When a Uranium-235 nuclei is bombarded with thermal neutrons it absorbs them. The neutrons will split producing:

  • two smaller nuclei
  • two or three fission neutrons
  • energy
• The fuel used most commonly as a fissionable nucleus is Uranium-235. Polonium-239 is the only other isotope that is fissionable
• Each fission neutron that is released can go on to cause fission in other nuclei which creates fission in more nuclei causing a chain reaction. The chain reaction, releases a vast amount of energy
• Why does a fission reaction release energy?

  • Because the fragments nuclei created are more stable (have more binding energy) than the original Uranium-235
  • The energy released is equal to the change in binding energy
  • When energy is released the fragment nuclei repel each other (being both positively charged) with sufficient force to overcome the strong nuclear force trying to hold them together
  • The fragment nuclei and the fission neutrons therefore gain kinetic energy