CORRECTED-SPECIAL REPORT-Japan engineers knew tsunami could overwhelm Fukushima plant

iesel pumps at No. 2 ran out of fuel, allowing water levels to fall and fuel to become exposed and overheat. When the Fukushima plant suffered its second hydrogen blast in three days the following Monday, Tokyo electric executives only notified the prime minister's office an hour later. Seven workers had been injured in the explosion along with four soldiers.

An enraged Prime Minister Naoto Kan pulled up to Tokyo Electric's headquarters the next morning before dawn. "What the hell is going on?" reporters outside the closed-door discussion reported hearing Kan demand angrily of senior executives.

Errors of judgment by workers in the hot zone and errors of calculation by plant managers hampered the emergency response a full week later as some 600 soldiers and workers struggled to contain the spread of radiation.

On Thursday, two workers at Fukushima were shuttled to the hospital to be treated for potential radiation burns after wading in water in the turbine building of reactor No. 3. The workers had ignored their radiation alarms thinking they were broken.

Then Tokyo electric officials pulled workers back from an effort to pump water out of the No. 2 reactor and reported that radiation readings were 10 million times normal. They later apologized, saying that reading was wrong. The actual reading was still 100,000 times normal, Tokyo Electric said.

The government's chief spokesman was withering in his assessment. "The radiation readings are an important part of a number of important steps we're taking to protect safety," Chief Cabinet Secretary Yukio Edano told reporters. "There is no excuse for getting them wrong."  

VENTS AND GAUGES

Although U.S. nuclear plant operators were required to install "hardened" vent systems in the 1980s after the Three Mile Island incident, Japan's Nuclear Safety Commission rejected the need to require such systems in 1992, saying that should be left to the plant operators to decide.

A nuclear power plant's vent represents one of the last resorts for operators struggling to keep a reactor from pressure that could to blow the building that houses it apart and spread radiation, which is what happened at Chernobyl 25 years ago. A hardened vent in a U.S. plant is designed to behave like the barrel on a rifle, strong enough to withstand an explosive force from within.

The U.S. Nuclear Regulatory Commission concluded in the late 1980s that the General Electric designed Mark I reactors, like those used at Fukushima, required safety modifications.

The risks they flagged, and that Tokyo did not heed, would come back to haunt Japan in the Fukushima crisis.

First, U.S. researchers concluded that a loss of power at one of the nuclear plants would be one of the "dominant contributors" to the most severe accidents. Flooding of the reactor building would worsen the risks. The NRC also required U.S. plants to install "hard pipe" after concluding the sheet-metal ducts used in Japan could make things much worse.

"Venting via a sheet metal duct system could result in a reactor building hydrogen burn," researchers said in a report published in November 1988.

In the current crisis, the failure of the more vulnerable duct vents in Fukushima's No. 1 and No. 3 reactors may have contributed to the hydrogen explosions that blew the roof off the first and left the second a tangled hulk of steel beams in the first three days of the crisis.

The plant vents, which connect to the big smokestack-like towers, appear to have been damaged in the quake or the tsunami, one NISA official said.

Even without damage, opening the vulnerable vents in the presence of a build-up of hydrogen gas was a known danger. In the case of Fukushima, opening the vents to relieve pressure was like turning on an acetylene torch and then watching the flame "shoot back into the fuel tank," said one expert with knowledge of Fukushima who asked not to be identified because of his commercial ties in Japan.

Tokyo Electric began venting the No. 1 reactor on March 12 just after 10 a.m. An hour earlier the pressure in the reactor was twice its designed limit. Six hours later the reactor exploded.

The same pattern held with reactor No. 3. Venting to relieve a dangerous build-up of pressure in the reactor began on March 13. A day later, the outer building - a concrete and steel shell known as the "secondary containment" -- exploded.

Toshiaki Sakai, the Tokyo Electric researcher who worked on tsunami risk, also sat on a panel in 2008 that reviewed the damage to the Kashiwazaki-Kariwa nuclear plant. In that case, Tokyo Electric safely shut down the plant, which survived a quake 2.5 times stronger than it had been designed to handle.

Sakai and the other panelists agreed that despite the successful outcome the way the ground sank and broke underground pipes needed for firefighting equipment had to be considered "a failure to fulfill expected performance".

Japanese regulators also knew a major earthquake could damage exhaust ducts. A September 2007 review of damage at the same Tokyo Electric nuclear plant by NISA Deputy Director Akira Fukushima showed two spots where the exhaust ducts had broken.

No new standard was put in place requiring vents to be shored up against potential damage, records show.

Masashi Goto, a former nuclear engineer who has turned critical of the industry, said he believed Tokyo Electric and regulators wrongly focused on the parts of the plant that performed well in the 2007 quake, rather than the weaknesses it exposed. "I think they drew the wrong lesson," Goto said.

The March 11 quake not only damaged the vents but also the gauges in the Fukushima Daiichi complex, which meant that Tokyo Electric was without much of the instrumentation it needed to assess the situation on the ground during the crisis.

"The data we're getting is very sketchy and makes it impossible for us to do the analysis," said David Lochbaum, a nuclear expert and analyst with the Union of Concerned Scientists. "It's hard to connect the dots when there are so few dots." 

In fact, Japan's NSC had concluded in 1992 that it was important for nuclear plant operators to have access to key gauges and instruments even in the kind of crisis that had not happened then. But it left plans on how to implement that policy entirely to the plant operators.

In the Fukushima accident, most meters and gauges were taken out by the loss of power in the early days of the crisis.

That left a pair of workers in a white Prius to race into the plant to get radiation readings with a handheld device in the early days of the crisis, according to Tokyo Electric.

They could have used robots to go in.

Immediately after the tsunami, a French firm with nuclear expertise shipped robots for use in Fukushima, a European nuclear expert said. The robots are built to withstand high radiation.

But Japan, arguably the country with the most advanced robotics industry, stopped them from arriving in Fukishima, saying such help could only come through government channels, said the expert who asked not to be identified so as not to appear critical of Japan in a moment of crisis. (Scott DiSavino was reporting from New York; Additional reporting by Kentaro Sugiayama in Tokyo, Bernie Woodall in Detroit, Eileen O'Grady in New York, Roberta Rampton in Washington; Editing by Bill Tarrant)