P2.6 Nuclear Fission And Nuclear Fusion

P2.6 Nuclear Fission And Nuclear Fusion

1. P2.6.1 Nuclear Fission

   • Nuclear power reactors use a reaction called nuclear fission. Two isotopes in common use as nuclear fuels are uranium-235 and plutonium-239. However, uranium-235 is used in most nuclear reactors. The process of splitting a nucleus is called nuclear fission. Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors because their nuclei are relatively large and easy to split.
     • For fission to happen, the uranium-235 or plutonium-239 nucleus must first absorb a neutron. The nucleus splits into two smaller nuclei. Two or three neutrons are released. Some energy is released. The additional neutrons released may be absorbed by other uranium or plutonium nuclei, causing them to split. Even more neutrons are then released, which in turn can split more nuclei. This is called a chain reaction. The chain reaction in nuclear reactors is controlled to stop it going too fast.

2. P2.6.2 Nuclear Fusion

   • Nuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens. The Sun and other stars use nuclear fusion to release energy.
     • Stars form when enough dust and gas from space is pulled together by gravitational attraction. Smaller masses may also form and be attracted by a larger mass to become planets.
       • As the gas falls together, it gets hot. A star forms when it is hot enough for nuclear reactions to start. This releases energy, and keeps the star hot. This is the first stage; the protostar.
         • During its 'main sequence' period of its life cycle, a star is stable because the forces in it are balanced. The outward pressure from the expanding hot gases is balanced by the force of the star’s gravity. Our Sun is at this stable phase in its life.
           • A star goes through a life cycle. This life cycle is determined by the size of the star.
             • Stars have enough hydrogen to maintain their energy output for millions of years. As the star runs out of hydrogen, other fusion reactions take place forming the nuclei of other elements. Heavier elements than hydrogen and helium (up to iron) are formed. Elements heavier than iron are formed in supernovas.
               • Heavy elements are found in the Sun and planets of the solar system. This suggests that the solar system was formed from the remains of earlier stars that exploded as supernovas.