Tuesday, December 6, 2011

Fukushima Tsunami Notes

@@Construction Design

Fukushima hill was shaved down from 35 to 10m, subject to 15m tsunami. They thought it was a good idea at the time, now not so much. Onagawa was set at 15m, so it survived a 13m flood.

Tuesday, July 12, 2011
Fukushima plant site originally was a hill safe from tsunami
Staff writers
...Katsumi Naganuma, 70, a former worker at Tokyo Electric Power Co., feels particular guilt because he knows that a 35-meter-high, tsunami-safe, bluff overlooking the sea was shaved down to build the Fukushima plant closer to sea level more than 40 years ago.
Tepco, assuming tsunami 3.1 meters or higher would never hit the coast, cut down the bluff by some 25 meters and erected the plant on artificially prepared ground only 10 meters above sea level.
"When I see the situation now, I feel it was wrong to clear that much of the hill away," said Naganuma, who worked at Tepco's local office preparing for the construction in the late 1960s.
"If they did not dig the ground down that much, we would not have faced this situation. (The nuclear disaster) would not have happened," said Naganuma.
Tepco further dug below the surface 14 meters more to create underground floors, including those for the turbine buildings, where the emergency diesel generators were installed.
The tsunami easily flooded the premises and knocked out the power systems, including those for running the critical reactor core coolant equipment. The meltdowns became inevitable.

..."In the past 700 years, Fukushima suffered almost no noteworthy damage from earthquakes except in the Aizu area (in current Fukushima Prefecture)," concluded a 1966 report attached to the application form Tepco submitted to the government to seek approval for building reactor 1 at the plant.
"Thus the site can be described as an area of low-seismicity, even compared with all the other areas of the country," the paper concluded, with little mention of the tsunami threat.

...In 2002, however, following recommendations from quake experts at the Japan Society of Civil Engineering, Tepco modified the facilities, including beefing up the breakwater, so they could withstand tsunami of up to 5.7 meters. This was also approved by the regulators.
During a recent interview with The Japan Times, Masatoshi Toyota, 88, former senior vice president of Tepco, said one of the reasons the utility lowered the bluff was to build the base of the reactors directly on solid bedrock to mitigate any earthquake threat.

...Toyoda also cited two other reasons for Tepco clearing away the bluff — seawater pumps used to provide coolant water needed to be set up on the ground up to 10 meters from the sea, and extremely heavy equipment, including the 500-ton reator pressure vessels, were expected to be brought in by boat.

...If the seawater pumps were placed on high ground, their operating costs would be accordingly higher.
"We decided to build the plant at ground level after comparing the ground construction costs and operating costs of the circulation water pumps," wrote Hiroshi Kaburaki, then deputy head of the Tepco's construction office at the Fukushima No. 1 plant, in the January 1969 edition of Hatsuden Suiryoku, a technical magazine on power plants.
Still, Tepco believed ground level was "high enough to sufficiently secure safety against tsunami and typhoon waves," wrote Seiji Saeki, then civil engineering section head of Tepco's construction office, in his research paper published in October 1967.

Onagawa Survives:
Before building the plant, Tohoku Electric, examining historic records of tsunami reported in the region, conducted computer simulations and concluded the local coast could face tsumani of up to 9.1 meters.
Tohoku Electric had set the construction ground level at 14.8 meters above sea level — which barely spared the Onagawa plant from major damage from 13-meter-high tsunami that hit in March.

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