The Japanese nuclear safety agency has rated the problems at the nuclear power plant at Fukushima at a four on a scale of one to seven nuclear incidents.
The International Nuclear and Radiological Event Scale (INES) -- designed by the International Atomic Energy Agency and the Nuclear Energy Agency of the Organization for Economic Cooperation and Development -- is used in over 60 countries to classify nuclear issues on seven increasingly serious levels.
* Levels one to three are labeled as "incidents," while levels four to seven are termed "accidents." Those without safety significance are termed "deviations" and are classified below scale at level zero.
The 1986 reactor meltdown at the Chernobyl nuclear power plant in Ukraine is the only level seven event to date. The large majority of events are classified at level two.
In order of increasing severity from levels one to seven, events are called: an anomaly, incident, serious incident, accident with local consequences, accident with wider consequences, serious accident and major accident.
* The scale covers a wide range of practices including industrial uses such as radiography, uses of radiation sources in hospitals, operations at nuclear facilities and transport of radioactive material.
* INES is meant to be applicable to all events, the vast majority of which relate to failures in equipment or procedures, and cover not only actual consequences but potential ones.
* Chernobyl is rated at level seven since it had a widespread impact on people and the environment. A key consideration in creating criteria for the rating was to ensure that the significance level of less severe and more localized events were clear demarcated from the severe Chernobyl accident.
Therefore, the 1979 accident at the Three Mile Island nuclear power plant in the United States is rated at level five.
Sources: IAEA, OECD.
(Reporting by Daniel Fineren, editing by Jane Baird)