nuclear fission and fusion

• what is nuclear fission?

  Splitting of an unstable nucleus which has absorbed a neutron into two smaller nuclei this them gives of 2-3 neutrons
• what causes a chain reaction in nuclear fission?

  is any on the neutrons emitted are moving slow enough they can be absorbed by a nucleus and causes more nuclear fission
• what is nuclear fusion?

  Combining atomic nuclei to form a heavier nucleus, releasing a large amount of energy in the process.