Chubu Electric Power Company has announced its plans to increase tsunami and flooding protection for the Hamaoka nuclear power plant, which was closed by an extraordinary order from the Japanese prime minister.
The company has been required to take special action in addition to nationwide measures like the orders from the Nuclear and Industrial Safety Agency (NISA), and the upcoming stress tests for nuclear power plants.
NISA's instructions came in March, within days of the Fukushima accident, while the procedure for stress tests is to be announced shortly. However, in May prime minister Naoto Kan personally decided that the plant would have to shut until it had bolstered sea defences. He said this was out of respect for public opinion in the context of Hamaoka's position on the southern Tokai coast in one of the regions of highest seismic activity, where a large undersea earthquake could be expected within the next 30 years.
The company today detailed the steps it would take, allowing until December 2012 to complete the work for units 3, 4 and 5 at the site. Units 1 and 2 are already permanently shut down.
Behind a 60 metre row of sand dunes measuring between ten and 15 metres high above sea level, the company is to erect a new breakwater wall to reach 18 metres above sea level. This compares to the 15 metre wave that flooded the Fukushima Daiichi site.
Analysis of a potential triple-earthquake event under the sea in the Tokai region indicated an eight metre tsunami should be expected. For a magnitude 9.0 event, the same in size as the earthquake of 11 March, a ten metre tsunami could occur. The data and methods supporting these results have been submitted to NISA and the Ministry of Economy, Trade and Industry.
On the main plant site, measures will be taken to mitigate general serious flooding as well as for the possibility that a tsunami could overwhelm the breakwater. They include the waterproofing of diesel generator rooms and seawater pumps, as well as the installation of pumps in the building basements. Grid connections are to be doubled up, with another set of diesel generators complete with long term fuel supply installed on ground behind the main plant buildings about 25 metres above sea level. Spare parts for seawater pumps will be kept in a hardened building and heavy earthmoving capability will be maintained.
In addition to physical measures, Chubu is working to develop effective ways to cooperate with local authorities and agencies in the case of a wider disaster.
Researched and written
by World Nuclear News