GCSE P2 Chapter 7

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  • Created by: emma998
  • Created on: 13-03-14 17:39

Nuclear fission

  • Nuclear fission is the process of splitting an atom using a neutron to release energy
  • There are two known fissionable isotopes in common use in nuclear fission reactors
  • Uranium235 and plutonium239
  • Naturally occuring uranium is uranium238 which is non-fissionable
  • So most fission reactors use enriched uranium which contains about 2-3% of uranium235
  • In a nuclear fission reactor a neutron is fired at a fissionable nucleus
  • The nucleus absorbs neutron
  • This causes the nucleus to split into two smaller nuclei
  • Two or three neutrons are emited
  • This releases lots of energy - much more than chamical reactions such as burning 
  • A chain reaction is when one fission event goes on to produce another fission event
  • In nulear reactors control rods are used to control the reaction and prevent a huge chain reaction explosion. 
  • The control rods ensure that only one fission neutron on average relased per fission event go on to produce further fission events
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Nuclear fusion

  • Nuclear fusion is the process of forcing two nuclei close enough together so that they form a single larger nucleus
  • This can be done by making two light nuclei collide at very high speeds
  • However this is very difficult to do because:
    • Normally the nuclei will repel each other due to their positive charge
    • Therefore very high tempertaures are needed to give them enough energy to overcome the repulsion and fuse
    • Because of the high temperatures the reaction cannot be done in a normal "container"
    • It must be contained by a magnetic field
  • Fusion is the process by which energy is released in stars
  • The process releases much more energy than chemical reactions such as combustion
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Nuclear issues

Sources of background radiation:

  • Radon gas, seeps through ground from radiactive substances in rocks deep underground
  • Food and drink and air travel
  • Cosmic rays from outer space - CMBR
  • Nuclear weapons testing
  • Medical devices such as Xray machines - they have an ionising effect

Uranium and plutonium are chemically removed from used fuel rods in nuclear reactors as these substances can be used again

The radioactive waste must be stored safely for many years

Workers with radiation should:

  • Spend as little time possible exposed
  • Keep as far from the sources of radiation as possible
  • Wear protective clothing and shield themselves behind lead and concrete
  • Wear film badges to monitor radiation level intake
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The early universe

  • Most scientists believe universe was created about 13.7 billion years ago by the big bang
  • At first the universe was a hot glowing ball of radiation
  • In the first few minutes the nuclei of the lightest elements formed
  • Over millions of years the universe expanded and began to cool
  • Uncharged atoms were formed
  • Before galaxies and stars formed universe was a patchy cloud of hydrogen and helium
  • Eventually the dust and gas was pulled together by gravitational attraction to form stars
  • The resulting intense heat started off nuclear fusion reactions in the stars
  • They began to emit visible light and other radiation
  • Galaxies are billions of stars held together by thier own gravity
  • There are billions of galaxies in the universe with vast empty spaces between them
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The life history of a star

  • Gravitational attraction pulls dust+gas together to form protostar which becomes denser
  • The hydrogen atoms other light elements start to fuse together
  • This creates large amounts of energy causing the protostar to become hotter+brighter
  • It is then a main sequence star
  • The star is stable - inward force of gravity is balanced by outward force of fusion from core
  • This stage can continue for billions of years until the star runs out of hydrogen nuclei
  • Swell, cool down and turn red

Then, a star similar to the size of the sun will:

  • It is then called a red giant
  • Fusion of light elements such as helium will continue to form heavier elements
  • When the star runs out of light elements fusion will stop + contract to form a white dwarf
  • When no more light is emitted it becomes a black dwarf

A star much larer than the sun will:

  • become a red super giant
  • Eventually it explodes in a supernova throwing outer layers into space leaving its core
  • The core is left as neutron star if has enough mass will form into a black hole
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How the chemical elements formed

  • Chemical elements are formed by fusion processes in stars
  • The nuclei of light elements fuse to form the nuclei of hevier elements
  • This process releases large amounts of energy
  • Chemical elements heavier than iron are formed in a supernova because this requires input energy
  • The presence of elements heavier than iron in the sun and inner planets is evidence that they were created by the debris scattered by a supernova
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