Left:
The IAEA found that loss of coolant pumping capacity was a major factor in the Fukusima Daiichi disaster
Immediately after the earthquake, offsite power was lost
to all three units in operation at the power station, leading to automatic start-up of emergency diesel generators. Approximately 40 minutes later the tsunami struck. Waves measuring in excess of 14m high hit the site and flooded the power station. This caused the shutdown of emergency diesel generators at 15.41, leading to station black-out (SBO). Despite emergency diesel generators and battery
backups, the cooling supply was lost to Unit 1 within several hours of the earthquake-tsunami event. Similarly, Unit 2 lost cooling after 71 hours and Unit 3 lost cooling after 36 hours, leading to hydrogen build-up and subsequent explosions in all three units. This represented an unprecedented nuclear event and was rated as a Level 7 “Major Accident” on the International Nuclear and Radiological Event Scale (INES) on 12 April 2011.
Learning lessons Emergency preparedness and response management, as well as procedures for planning for the management of a severe accident, were completely updated as an outcome of the Fukushima incident. Weaknesses in plant design were addressed, particularly the assumption that there would
never be an interruption in electrical power at a nuclear power plant for more than a short period. The possibility of several reactors at the same facility suffering a crisis at the same time now had to be considered, together with the possibility of a nuclear accident occurring at the same time as a major natural disaster. Many of the conclusions were by no means unique to
Japan. Consequently, other countries responded to the accident with measures that included carrying out ‘stress tests’ to reassess the design of nuclear power plants against site-specific extreme natural hazards, installing additional backup sources of electrical power and supplies of water, and strengthening the protection of plants against extreme external events, including terrorist attack. From an engineering perspective, the loss of AC power
(due to earthquake) and DC power (due to tsunami) was a primary concern because it rendered the High Pressure Core Injection (HPCI) systems and the Reactor Core Isolation Cooling (RCIC) systems to fail in less than 48 hours. All the reactors at Fukushima stopped operation automatically when the earthquake motion was detected. This in turn led to the stoppage of the main feed water pumps. Failure of the water injection system over these extended
Above: Climate change and natural disasters could make pump reliability even more important
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