18 Mar (NucNet): The reactor core damage and loss of all cooling function at units 3 and 4 of the Fukushima-Daiichi nuclear power plant, and the abnormal rise of radiation dose rates at the plant site boundary due to releases by unit 1 following an earthquake and tsunami on 11 March 2011, have been provisionally rated at Level 5 on the International Atomic Energy Agency’s International Nuclear and Radiological Event Scale (INES).*
Level 5 on the scale means an “accident with wider consequences”.
The event at unit 4, where there was loss of coolant to the spent fuel pool, has provisionally been rated as Level 3, which means “serious incident”.
Incidents at units 1, 2 and 4 of the Fukushima-Daini nuclear plant have provisionally been rated as Level 3.
In a report to the IAEA on units 1, 2 and 3 at Fukushima-Daiichi, Japan’s Nuclear and Industrial Safety Agency (NISA) said electricity rooms and seawater pump rooms were flooded when the tsunami struck, and the only cooling function available was from a turbine driven pump.
The operation of this pump increased the pressure in the reactor’s suppression chamber and the pump had to be stopped. This resulted in the loss of all cooling function and the declaration of a state of emergency, the report says.
In an effort to cool units 1, 2, and 3, seawater was injected into the reactor vessels using fire pumps. The gas in the containment vessels was also vented. When venting the containments, the radiation dose rate at the boundary level of the site exceeded the limit of 0.5 millisievert per hour, and for this reason the initial Level 4 classification for unit 1 was revised upwards.
The report says the explosions at the reactors on 12 to 14 March were believed to have been caused by hydrogen gas.
It adds that the behaviour of the pressure of the reactor vessel and the containment vessel, and the behaviour of the rector vessel’s water level, were “complicated” and some measurements could not be taken because of failures of measurement equipment.
* The International Nuclear and Radiological Event Scale
The International Nuclear and Radiological Event Scale (INES) was developed by the IAEA and the OECD in 1990 to better communicate and standardise the reporting of nuclear incidents or accidents to the public.
Events are classified on the scale at seven levels: Levels 1–3 are called “incidents” and Levels 4–7 “accidents”. The scale is designed so that the severity of an event is about 10 times greater for each increase in level on the scale. Events without safety significance are called “deviations” and are classified Below Scale / Level 0.
Chernobyl rated as 7 (Major Accident) on the scale and Three Mile Island rated 5 (Accident with Wider Consequences).
7 Major Accident
6 Serious Accident
5 Accident With Wider Consequences
4 Accident with Local Consequences
3 Serious Incident
2 Incident
1 Anomaly
0 Below Scale/No Safety Significance
>>Related reports in the NucNet database (available to subscribers)
Authorities Order Evacuation Around Fukushima-Daiichi Nuclear Plant (News in Brief No. 52, 11 March 2011)
EC Calls For Information On Back-Up Power Systems At European NPPs (News In Brief No. 66, 15 March 2011)
Japan Releases Radiation Data From More Than 50 Monitoring Posts (News in Brief No. 71, 17 March 2011)
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