Japanese authorities have informed the IAEA that new INES ratings have been issued for some of the events relating to the nuclear emergency at the Fukushima Daiichi and Daini nuclear power plants.
Japanese authorities have assessed that the core damage at the Fukushima Daiichi 1 reactor unit caused by the loss of all cooling function has been rated as 5 on the INES scale.
This is an upgrade from a previous rating of 12 March as 4 on the INES scale, which was based on an abnormal rise of radioactive dose rate at the site boundary.
Japanese authorities have assessed that the core damage at the Fukushima Daiichi 2 and 3 reactor units caused by loss of all cooling function has been rated as 5 on the INES scale.
The INES scale is a measure of 1 to 7. Events are classified at seven levels: Levels 1–3 are “incidents” and Levels 4–7 “accidents”. These levels consider three areas of impact: people and the environment, radiological barriers and control, and defence in depth. The scale is designed so that the severity of an event is about ten times greater for each increase in level on the scale. Events without safety significance are called “deviations” and are classified Below Scale/Level 0.
Japanese authorities have assessed that the loss of cooling and water supplying functions in the spent fuel pool of the unit 4 reactor has been rated as 3.
Japanese authorities have assessed that the loss of cooling functions in the reactor units 1, 2 and 4 of the Fukushima Daini nuclear power plant has also been rated as 3. All reactor units at Fukushima Daini nuclear power plant are now in a cold shut down condition.
Japanese authorities have informed the IAEA that, prior to the earthquake of 12 March, the entire fuel core of reactor unit 4 of the Fukushima Daiichi nuclear power plant had been unloaded from the reactor and placed in the spent fuel pond located in the reactor’s building.