This is a good explanation of how radiation disasters are rated based on the potential effects of the radiation. Very interesting to note now that the level of the Fukushima disaster has been raised to a level 7…
Graphic: International Atomic Energy Agency
Earthquakes have the Richter Scale; nuclear mishaps have the INES – International Nuclear and Radiological Event Scale.
The purpose of INES is to provide a means of “communicating to the public in a consistent way the safety significance of nuclear and radiological events.” There are seven levels to the scale which are applied to three “areas of impact”:
People and the Environment considers the radiation doses to people close to the location of the event and the widespread, unplanned release of radioactive material from an installation.Radiological Barriers and Control covers events without any direct impact on people or the environment and only applies inside major facilities. It covers unplanned high radiation levels and spread of significant quantities of radioactive materials confined within the installation.Defence-in-Depth also covers events without any direct impact on people or the environment, but for which the range of measures put in place to prevent accidents did not function as intended.
The seven levels are defined such that each level is ten times more severe than the previous level. Unlike the Richter Scale, where intensity of an earthquake is determined by a mathematical formula, the INES is based on a series of definitions. For example, under People and the Environment, Level 2 is defined as “exposure of a member of the public in excess of 10 millisieverts or exposure of a worker in excess of the statutory annual limits.” Level 3 is defined as “exposure in excess of 10 times the statutory annual limit for workers and non-lethal deterministic health effects from radiation (e.g. burns).”
Similarly, a Level 6 event is defined as a significant release of radioactive material likely to require implementation of planned countermeasures, whereas a Level 7 event is defined as a major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.
Fukushima Dai-ichi is currently considered a Level 6 event while Chernobyl is considered a Level 7 event.
While this may seem somewhat subjective, there is a very comprehensive, 218-page INES Users Manual developed by the International Atomic Energy Agency in cooperation with the Organization for Economic Co-operation and Development and the Nuclear Energy Agency. The manual removes a lot of ambiguity. Media reporting on a nuclear accident should consult the manual.
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