TOKYO—The operator of Japan's stricken nuclear plant let pressure in one reactor climb far beyond the level the facility was designed to withstand, a decision that may have worsened the world's most serious nuclear accident in a quarter century.
Japanese nuclear-power companies are so leery of releasing radiation into the atmosphere that their rules call for waiting much longer and obtaining many more sign-offs than U.S. counterparts before venting the potentially dangerous steam that builds up as reactors overheat, a Wall Street Journal inquiry found.
Japan's venting policy got its first real-world test in the chaotic hours after March 11's earthquake and tsunami knocked out cooling systems at the Fukushima Daiichi nuclear-power complex. By the first hours of March 12, an emergency was brewing inside the plant's No. 1 reactor.
By around 2:30 a.m., the pressure inside the vessel that forms a protective bulb around the reactor's core reached twice the level it was designed to withstand. Amid delays and technical difficulties, it was another 12 hours before workers finished releasing radioactive steam from this containment vessel, via reinforced pipes, to the air beyond the reactor building.
About an hour later, the reactor building itself exploded—a blast that Japanese and U.S. regulators have since said spread highly radioactive debris beyond the plant. The explosion, along with others amid overheating at reactors 2, 3 and 4, contributed to radiation levels that led to mandatory evacuations around the plant and the government's admission that the Fukushima Daiichi disaster ranks alongside Chernobyl at the top of the nuclear-disaster scale.
Experts in the U.S. and Japan believe the venting delay may have helped create conditions that led to the blast. In one possible scenario, pressure built so high that it damaged gaskets and other parts of the venting system, through which highly explosive hydrogen gas leaked from the core into the reactor building. It was Japan's cautious approach to venting, an outgrowth of its profound concern over nuclear contamination, that may well have made the accident worse, they say.
Tokyo Electric Power Co. chief executive Masataka Shimizu defended his company's actions when asked in Parliament last week to explain the timing of venting at the plant, which Tepco operates.
"Venting means you're affecting the surrounding area," Mr. Shimizu said. "It was imperative we made completely certain that evacuation was taken care of for everyone affected."
The controversy has reached Japan's embattled Prime Minister Naoto Kan. Shortly after 7 a.m. that same morning, Mr. Kan paid a 50-minute visit to the Fukushima Daiichi grounds and held a meeting there with plant executives. Opposition lawmakers have argued that his visit distracted plant managers, while Mr. Kan's supporters say he pushed to accelerate venting.
Manufacturers of nuclear reactors build the containment vessels that surround the core at a "design pressure" the units are capable of withstanding without breaking, under all the conditions envisioned for the plant's life. Those include all but the most severe accidents.
“ Venting is a last resort. ”
Containment vessels can withstand higher pressures, some studies have indicated. Among these are studies conducted in the 1990s by Japanese operators and equipment manufacturers, in preparation for Japan's first set of severe-accident protocols, that say such vessels can withstand twice the design pressure. Many Japanese operators have adopted this as their benchmark for releasing contaminated air.
Tepco spokesman Yoshikazu Nagai confirmed that if there is a risk of releasing radiation, the company doesn't vent until pressure hits roughly twice the design limit. "Venting is a last resort," Mr. Nagai said.
General Electric Co. GE -0.58% General Electric Co. U.S.: NYSE $25.79 -0.15 -0.58% July 25, 2014 4:00 pm Volume (Delayed 15m) : 24.45M AFTER HOURS $25.76 -0.03 -0.12% July 25, 2014 7:59 pm Volume (Delayed 15m) : 179,516 P/E Ratio 17.98 Market Cap $258.62 Billion Dividend Yield 3.41% Rev. per Employee $472,222 07/25/14 Growth in Midmarket Probably W... 07/24/14 Caterpillar Scales Back Foreca... 07/23/14 Whirlpool Hit by Falling Sales... More quote details and news » , the designer of the vessel at Fukushima Daiichi, said it is unaware of any such Japanese studies or venting protocols.
The International Atomic Energy Agency said it doesn't have specific guidelines on venting and doesn't comment on the appropriateness of actions taken in member countries.
U.S. protocols on handling accidents at similar reactors call for venting before pressure exceeds the design level. The same protocol is followed by plant operators using similar types of reactors in Korea and Taiwan, industry experts in those countries say.
The U.S. approach allows for a faster response in crisis conditions, and accepts the radiation released as part of venting as the price of possibly preventing a larger release.
One significant passage in the U.S. protocol says a reactor should be vented even if that results in "exceeding offsite radioactivity release rate limits"—in other words, shooting more radiation into the environment than is allowed—say people familiar with the protocol.
Another key difference: U.S. rules put the person at the controls of the plant in charge of venting. In Japan, it is common for operators to bump the decision to company or government superiors when there is a chance radiation will be released.
Several of those factors came into play in the early hours of March 12. Industry experts in the U.S. and other countries say that during the early hours after power and cooling systems were lost at reactor No. 1, overheating fuel rods in the reactor produced hydrogen gas and increased the pressure within the reactor and its containment vessel.
Tepco says pressure in the containment vessel likely hit 840 kilopascals (the metric equivalent of 121.8 pounds per square inch) around 2:30 a.m, roughly double the maximum pressure of 427 kilopascals the vessel was designed to handle.
Following Japan's decision chain, the control-room chief was required to first notify the utility's president, currently Mr. Shimizu, to approve venting. It is unclear when he was notified of the possible need to vent the reactor.
Much seems undisturbed in Futoba, Japan, where part of the damaged Fukushima Daiichi nuclear complex is located. But one element is largely missing: people.
But in a serious nuclear accident, Japanese nuclear-emergency laws kick in, bringing a host of other requirements to consult with the government. At 3 a.m., the minister overseeing the industry regulator convened a press conference to announce the decision to vent.
Fukushima Daiichi's reactors, like those in the U.S., have a venting system that runs to a chimney outside the building and is built to withstand high pressures that would blow out weaker pipes. When a plant loses power, as Fukushima Daiichi did, the valves must be opened manually.
Those efforts didn't start until after 10 a.m., regulators say. The valves may be harder to open manually when there is a lot of pressure on them, according to one U.S. industry official with experience working with emergency protocols. Tepco has confirmed it had trouble opening the valves.
The vents were finally opened at around 2:30 p.m., and a pressure drop confirmed.
It remains unclear how much radiation was released by the venting. And there is no way to know what would have happened if Japan had followed the U.S. procedure, or even if it could have in the aftermath of the debilitating quake and tsunami.
It is also possible, industry experts said, that explosive hydrogen gases could have come not from the containment vessel, but from spent fuel rods that were also inside the reactor building.
The subsequent explosion at reactor No. 1 didn't damage the containment vessel but destroyed much of the building itself. Amid the turmoil, Tepco didn't start injecting cooling sea water into the reactor until the evening of March 12—a delay that likely led to even greater damage to the reactor core.
The blast may also have damaged other equipment, leading to more leaks and malfunctions. Rubble from the building hindered later rescue efforts.
The full reasons behind the venting delay are unclear. Tepco has said it must investigate further before it can say whether its venting policy was appropriate.
Tepco also said malfunctioning equipment meant it didn't have much reliable data on pressure within the reactor containment vessel to begin with. Nuclear-industry executives in Japan and the U.S. cautioned that it may be months before experts can piece together what happened.
A spokesman for Japan's regulator said Tepco followed its protocols and moved speedily considering that it had to consult with regulators.
The agency spokesman said it is unclear whether venting at higher pressures was connected to the reactor-building explosion.
The protocols that tell operators what to do in nuclear accidents are supposed to prevent events from spiraling out of control. The U.S.'s protocols took shape after the 1979 partial meltdown at the Three Mile Island nuclear reactor in Pennsylvania. Plant owners and equipment makers like GE banded together to create general guidelines that are then adapted for each specific reactor.
Japanese operators are under no obligation to use the same protocols as the U.S., even when using U.S. designs such as the GE-designed reactors at Fukushima Daiichi.
Japanese industry officials said they referred to the U.S. protocols when developing their own guidelines for Japan. A Tepco spokeswoman said the company didn't base its own accident protocols on the U.S. version, but that the content is "basically the same."
—Rebecca Smith and Peter Landers contributed to this report.
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