1.1 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.
1.1.1 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
2.1 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.
2.1.1 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
220.127.116.11 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.
18.104.22.168.1 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.
22.214.171.124.1.1 A star goes through a life cycle. This life cycle
is determined by the size of the star.
126.96.36.199.1.1.1 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
188.8.131.52.184.108.40.206 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.