From anomalies to accidents
In response to accidents in nuclear power plants that attracted world media attention in the 1980s — notably the accidents at Three Mile Island in the US in 1979 and at Chernobyl in 1986 — , nuclear specialists started to work on developing a scale for simple and fast communication of the severity of a nuclear or radiological accident based on common criteria. France and Japan already had experience with such scales, similar to international scales comparing the severity of earthquakes, and the international scale was modeled on that in use in France.
The creation of the INES scale
The International Nuclear Event Scale (INES) was developed in 1990 by international experts convened by the IAEA and the OECD Nuclear Energy Agency (OECD/NEA) with the aim of communicating the safety significance of events at nuclear installations. The INES scale was refined and extended from 1992 to add radiological events and transport of radioactive material. In 2008, the International Nuclear and Radiological Event Scale User’s Manual was officially published.
The INES scale has 7 levels from “anomaly” to “major accident”. (See Figure of the INES pyramid)
Levels 1 to 3 are designated “incidents”, and levels 4 to 7 are “accidents”. A Level 0, or below scale level, was introduced a few years after the scale was created in order to accommodate events considered deviations from normal operation but without any safety significance.
These levels consider three areas of impact: people and the environment, radiological barriers and controls, defense in depth. There are precise criteria to help national authorities and operators responsible for rating events to choose the right level. For example, Level 7, the most serious, applies to accidents involving a “Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.”
The scale is designed logarithmically, so that the severity of an event is about 10 times greater for each increase in level on the scale.
Events that have no radiological or nuclear safety relevance are not classified on the scale (a fire in a non-nuclear building for example, or the collapse of a roof of a conventional building on a nuclear site).
For more information on INES scale:
But considering the fact that Fukushima combined several severe events (including several different criteria such as loss of coolant, core melt, hydrogen explosion, release of radioactivity) on the same site at the same time, some people consider that it could be rated further up on the ladder if the scale could go further up! Some argued that it should have been reported as three Level 7 events. However, INES ratings are not an exact science and are first and foremost considered a quick communication tool to indicate the severity of an event. The case of multiple severe accidents with the same root cause was probably not taken into consideration in designing the original scale.