The INES scale

The International Nuclear and Radiological Event Scale was created in 1990 by the IAEA and the OECD Nuclear Energy Agency, after Chernobyl, to communicate the severity of an event to the public with a single number, like the magnitude scale for earthquakes. It runs from 0 to 7. Levels 1 to 3 are incidents: anomaly, incident and serious incident. Levels 4 to 7 are accidents: with local consequences, with wider consequences, serious accident and major accident. Level 0, or deviation, has no safety significance. It is logarithmic: each step represents a severity about ten times greater.

An event is rated on three criteria and assigned the highest of the three. The first is the impact on people and the environment: doses received and the amount of material released. The second is the radiological barriers and controls inside the facility: contamination, core damage, worker exposure. The third is the degradation of defense in depth: how many layers of safety were lost even if nothing happened. That is why an event with no release at all can be level 2 or 3 if it left the plant with a single system between normality and an accident.

The examples fix the scale. Level 7, major accident: Chernobyl in 1986 and Fukushima Daiichi in 2011, both with massive releases and long-term evacuations, although Fukushima's release was around a tenth of Chernobyl's and there were no radiation deaths. Level 6: the explosion of the waste tank at Kyshtym, in the Soviet Union, in 1957. Level 5: the Windscale fire in the United Kingdom in 1957, Three Mile Island in 1979, with half the core melted but almost no release, and Goiânia in 1987, an abandoned cesium-137 source in Brazil that killed four people and shows the scale is not only for reactors. Level 4: the Tokaimura criticality accident in Japan in 1999, and Saint-Laurent in France in 1980.

INES has limits its authors acknowledge. It is a communication tool, not a safety assessment: it is not meant to compare the performance of plants or countries. It is discrete, so two very different events share the same number. And it saturates at 7: Chernobyl and Fukushima have the same rating despite an order-of-magnitude difference in what was released. The final rating is made by the country where the event occurs, sometimes years later: Fukushima went from 4 to 5 and from 5 to 7 within the first month.