International Nuclear Event Scale, 2011 Japan Nuclear Disaster

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International Nuclear Event Scale

  • Introduced in 1990 by IAEA
  • Enables prompt communication of safety significance in case of nuclear accidents
  • Logarithmic
  • There are 7 levels: 3 incident levels and 4 accident levels
  • Level 7 – Major accident: Chernobyl, 1986
  • Level 6 – Kyshtym disaster (Mayak, Soviet Union) , 1957
  • Level 5 – Three Mile Island US, 1979; Wind scale Fire (UK) , 1957
International Nuclear Event Scale

Chernobyl Disaster

Three Mile Island Disaster

2011 Japan Nuclear Disaster

  • At the three reactor units at the Fukushima Daiichi nuclear power station
  • Located in the town of Okuma in the Futaba District of Fukushima Prefecture, Japan. Combined capacity of 4.7 GW.
  • What caused it?
    • The earthquake caused a power failure
    • The subsequent tsunami knocked out the generators that produced the power
    • Lack of power in turn caused the cooling systems of the reactors to fail
    • This led to a partial meltdown in the three reactors
  • Background
    • Fukushima 1 reactor went critical in 1971
    • All the three are Boiling Water Reactors (BWR) and use demineralised water for cooling nuclear fuel
  • Methods that can be adopted to avoid a meltdown
    • Pump sea water to reduce the heat
    • Boric acid is an excellent neutron absorber. Using it would reduce the chances of nuclear reactions restarting even if the fuel is found loose inside the reactor core.

Nuclear Safety

  • The disaster has re-ignited the debate on the safety of nuclear power
  • AERB should be made an independent regulator


  • Ring of fire in the pacific ocean
  • Largest Earthquakes by Magnitude
    • Valdivia, Chile, 1960: 9.5
    • Prince Sound William, Alaska, 1964: 9.2
    • Sumatra, Indonesia, 2004: 9.1
    • Kamchatka, USSR, 1952: 9
    • Sendai, Japan, 2011: 9


  • Intensity measured by the Soloviev-Imamura tsunami intensity scale.
  • Major Tsunamis
    • 2011: Japan
    • 2004: Indian Ocean
    • 1908: Messina, Italy

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