How to Ensure a Nuclear Reactor becomes a Radiological Weapon:
Engineers knew tsunami could overwhelm Fukushima plant
Reuters March 30, 2011, 2:41 pm
The tsunami research presented by a Tokyo Electric team led by Toshiaki Sakai came on the first day of a three-day conference in July 2007 organized by the International Conference on Nuclear Engineering.
It represented the product of several years of work at Japan’s top utility, prompted by the 2004 earthquake off the coast of Sumatra that had shaken the industry’s accepted wisdom. In that disaster, the tsunami that hit Indonesia and a dozen other countries around the Indian Ocean also flooded a nuclear power plant in southern India. That raised concerns in Tokyo about the risk to Japan’s 55 nuclear plants, many exposed to the dangerous coast in order to have quick access to water for cooling.
Tokyo Electric’s Fukushima Daiichi plant, some 240 km (150 miles) northeast of Tokyo, was a particular concern.
The 40-year-old nuclear complex was built near a quake zone in the Pacific that had produced earthquakes of magnitude 8 or higher four times in the past 400 years — in 1896, 1793, 1677 and then in 1611, Tokyo Electric researchers had come to understand.
Based on that history, Sakai, a senior safety manager at Tokyo Electric, and his research team applied new science to a simple question: What was the chance that an earthquake-generated wave would hit Fukushima? More pressing, what were the odds that it would be larger than the roughly 6-metre (20 feet) wall of water the plant had been designed to handle?
The tsunami that crashed through the Fukushima plant on March 11 was 14 meters high.
Sakai’s team determined the Fukushima plant was dead certain to be hit by a tsunami of one or two meters in a 50-year period. They put the risk of a wave of 6 metres or more at around 10 percent over the same time span.
In other words, Tokyo Electric scientists realised as early as 2007 that it was quite possible a giant wave would overwhelm the sea walls and other defenses at Fukushima by surpassing engineering assumptions behind the plant’s design that date back to the 1960s.
Company Vice President Sakae Muto said the utility had built its Fukushima nuclear power plant “with a margin for error” based on its assessment of the largest waves to hit the site in the past.
That would have included the magnitude 9.5 Chile earthquake in 1960 that killed 140 in Japan and generated a wave estimated at near 6 meters, roughly in line with the plans for Fukushima Daiichi a decade later.
“It’s been pointed out by some that there could be a bigger tsunami than we had planned for, but my understanding of the situation is that there was no consensus among the experts,” Muto said in response to a question from Reuters.
Despite the projection by its own safety engineers that the older assumptions might be mistaken, Tokyo Electric was not breaking any Japanese nuclear safety regulation by its failure to use its new research to fortify Fukushima Daiichi, which was built on the rural Pacific coast to give it quick access to sea water and keep it away from population centres.
“There are no legal requirements to re-evaluate site related (safety) features periodically,” the Japanese government said in a response to questions from the United Nations nuclear watchdog, the International Atomic Energy Agency, in 2008.
In fact, in safety guidelines issued over the past 20 years, Japanese nuclear safety regulators had all but written off the risk of a severe accident that would test the vaunted safety standards of one of their 55 nuclear reactors, a key pillar of the nation’s energy and export policies.
That has left planning for a strategy to head off runaway meltdown in the worst case scenarios to Tokyo Electric in the belief that the utility was best placed to handle any such crisis, according to published regulations.
In December 2010, for example, Japan’s Nuclear Safety Commission said the risk for a severe accident was “extremely low” at reactors like those in operation at Fukushima. The question of how to prepare for those scenarios would be left to utilities, the commission said.
A 1992 policy guideline by the NSC also concluded core damage at one of Japan’s reactors severe enough to release radiation would be an event with a probability of once in 185 years. So with such a limited risk of happening, the best policy, the guidelines say, is to leave emergency response planning to Tokyo electric and other plant operators.
PREVENTION NOT CURE
Over the past 20 years, nuclear operators and regulators in Europe and the United States have taken a new approach to managing risk. Rather than simple defenses against failures, researchers have examined worst-case outcomes to test their assumptions, and then required plants to make changes.
They have looked especially at the chance that a single calamity could wipe out an operator’s main defence and its backup, just as the earthquake and tsunami did when the double disaster took out the main power and backup electricity to Fukushima Daiichi.
Japanese nuclear safety regulators have been slow to embrace those changes.
Japan’s Nuclear and Industrial Safety Agency (NISA), one of three government bodies with responsibility for safety policy and inspections, had published guidelines in 2005 and 2006 based on the advances in regulation elsewhere but did not insist on their application.
Japanese regulators and Tokyo Electric instead put more emphasis on regular maintenance and programs designed to catch flaws in the components of their ageing plants.
That was the thinking behind extending the life of the No. 1 reactor at Fukushima Daiichi, which had been scheduled to go out of commission in February after a 40-year run.
On four occasions over the past four years, safety inspectors from Japan and the International Atomic Energy Agency (IAEA) were called in to review failures with backup diesel generators at nuclear plants.
In June 2007, an inspector was dispatched to Fukushima’s No. 4 reactor, where the backup generator had caught fire after a circuit breaker was installed improperly, according to the inspector’s report.
“There is no need of providing feedback to other plants for the reason that no similar event could occur,” the June 2007 inspection concluded.
The installation had met its safety target. Nothing in that report or any other shows safety inspectors questioned the placement of the generators on low ground near the shore where they proved to be at highest risk for tsunami damage at Fukushima Daiichi.
The risks also appear to have made it hard to hire for key positions. In 2008, Toshiba admitted it had illegally used six employees under the age of 18 as part of a series of inspections of nuclear power plants at Tokyo Electric and Tohoku Electric. One of those minors, then aged 17, had participated in an inspection of the Fukushima Daiichi No. 5 reactor, Tokyo Electric said then.
The magnitude 9.0 quake struck on Friday afternoon of March 11 — the most powerful in Japan’s long history of them — pushed workers at the Fukushima plant to the breaking point as injuries mounted and panic took hold.
Hiroyuki Nishi, a subcontractor who had been moving scaffolding inside Reactor No. 3 when the quake hit, described a scene of chaos as a massive hook came crashing down next to him. “People were shouting ‘Get out, get out!'” Nishi said. “Everyone was screaming.”
In the pandemonium, workers pleaded to be let out, knowing a tsunami was soon to come. But Tokyo Electric supervisors appealed for calm, saying each worker had to be tested first for radiation exposure. Eventually, the supervisors relented, threw open the doors to the plant and the contractors scrambled for high ground just ahead of the tsunami.
After the wave receded, two employee were missing, apparently washed away while working on unit No. 4. Two contractors were treated for leg fractures and two others were treated for slight injuries. A ninth worker was being treated for a stroke.
In the chaos of the early response, workers did not notice when the diesel 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.
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.
Masashi Goto, a former nuclear engineer who has turned critical of the industry, said he believed Tokyo Electric and regulators wrongly focussed 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)