7 Apr (NucNet): Serious accidents were avoided at three nuclear power stations in northern Japan following the March 2011 earthquake and tsunami because there were no station blackouts and immediate accident management measures were successful, a report released by the Japan Atomic Industrial Forum (Jaif) says.
The report says that as a result of damage to off-site power facilities and transmission towers caused by the earthquake, off-site power was lost at all six units at Fukushima-Daiichi.
All emergency diesel generators at Fukushima-Daiichi – with the exception of one air-cooled diesel generator at Unit 6 – lost function because they were covered with water by the tsunami, the report says.
However, severe accidents did not occur at the Fukushima Daini, Onagawa and Tokai nuclear power stations, which were also affected by the earthquake and the tsunami.
This was because offsite power was not entirely lost. At Fukushima Daini one of four off-site power lines survived. At Onagawa, one of five lines survived and at Tokai, although all three off-site power lines were lost, two of three emergency diesel generators remained operational.
The report, based on analysis prepared by the Nuclear Safety Division of the Atomic Energy Society of Japan, also confirms that operators at Fukushima-Daiichi had prepared for a possible maximum tsunami height of less than half the height of the tsunami that hit the facility on 11 March 2011.
The report says there had traditionally been “no clear regulatory standards” for tsunamis and each plant operator had “renewed its expectations” for the potential height of a tsunami whenever there was new knowledge or experience.
The actual tsunami height at Fukushima-Daiichi was 13.1 metres, more than double the assumed maximum tsunami height of 6.1 metres, the report says. This led to damage to safety-critical equipment.
The country’s new nuclear regulator, the Nuclear Regulation Authority, now demands strict tsunami measures in its new regulatory standards, the report says.
At Fukushima Daini, the maximum tsunami height was around eight metres and the assumed maximum was 5.2 metres. At Onagawa, the maximum was 13 metres against an assumed 13.6 meters and at Tokai it was 5.5 metres against an assumed 6.61 metres.
The report notes that the seismic building at Fukushima Daini had been newly constructed based on findings from a previous earthquake and it had begun operation in July 2010. It served as the base for recovery efforts, playing a key role in achieving states of cold shutdown, the report says.
The Jaif report is online:
www.jaif.or.jp/english/news_images/pdf/ENGNEWS02_1396588415P.pdf
