Employees of TEPCO nuclear plant operator bowed in a gesture of apology to evacuees month past.
blackout caused a sharp drop in the level of cooling water for radioactive fuel rods, according to the operator.
little-noticed incident, did not lead to damage of the reactor or the release of radiation. However, highlighted some of the vulnerabilities of the aging facility and raises questions about safety procedures and outdated equipment that the operator Tokyo Electric Power Co., or TEPCO, had in place.
According to research by TEPCO, at 2:42 pm on June 17, 2010, an automatic off an alarm in the control room of reactor No. 2, one of six in Fukushima Daiichi. Within minutes, the control rods inserted to stop nuclear fission. But without electricity, water pumps stopped supplying the core of the reactor with fresh water. levels of temperature and pressure inside the reactor increased.
TEPCO says there is no imminent crisis, and that the normal water level were restored shortly after the power outage. Company officials say the emergency generator diesel fuel were programmed to start automatically and if the water inside the reactor had dropped 20 inches, the core emergency cooling system, chrome, have been initiated, to avoid hazardous exposure of the fuel rods.
But some critics say the consequences could have been more severe if not for quick thinking operators control room to manually turn on diesel generators and ECCS. They say that the accident revealed shortcomings in blocking situations.
"When I found out what caused the closure last June, did not know whether to laugh or mourn," said Kazuyoshi Sato, a local assemblyman in the city of Iwaki in Fukushima Prefecture, 25 miles plant. He learned of the incident the day after the local media. Mr. Sato, a longtime critic of TEPCO who wrote at length about the incident last summer in his blog, added: "If an error is almost as simple as that can cause a collapse, then what does that say about the security controls of the plant and systemic failures? "
company and Japan's Nuclear and Industrial Air Safety blamed human error, in separate reports completed in July 2010. "The subcontractor did not realize he had touched the switch while doing other work and said he was completely unaware of its critical function," TEPCO spokesman Hiroshi Aizawa, based in Fukushima City, in a recent telephone interview .
John Lee, professor of nuclear engineering at the University of Michigan, said it is not unusual for a plant to experience a brief loss of external power, since "there could be a flash of energy of some kind for few minutes. "
"However, a reduction of water in the core is quite unusual, making it a major event in my mind," added Professor Lee, an expert in the type of reactor used in Daiichi No. 2.
Experts say that the plant operators do not take lightly unplanned downtime, known as automatic shutdowns, and can lead to structural changes. "This type of emergency stop in the reactor is stressful," said Robert Albrecht, professor emeritus at the University of Washington electrical engineering who has experience in nuclear engineering. "To me it sounds like you may want to change the design."
Following the incident, NISA and TEPCO called to the diligence of the workers increased, but without fundamental rethinking of the network of the power plant. To avoid repetition, Tepco posted warning signs near the critical switches in the plant for workers aware of their presence.
Wed NISA decadelong approved an extension of the plant life of 40 years of age, in February, five weeks before the March 11 earthquake.
Heads of NISA and TEPCO was grilled about the incident in the parliamentary testimony driven by the issues raised after the earthquake. At a meeting of 01 May, company president Masataka Shimizu confirmed details omitted in the official report from TEPCO. Among them: The water levels inside the reactor No. 2 was reduced by two meters (nearly seven feet) for about 30 minutes before being restored
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No. 2 reactor was shut for one month as Tepco determined that it was caused by a technician employed by a subsidiary. When replacing a meter to record the temperature behind an electrical panel, the subcontractor inadvertently beaten an auxiliary relay or an electronic switch and control, with the elbow.
That was what NISA and TEPCO described in reports as a 'momentary' flutter which was long enough to activate the circuit breaker shut the main power reactor, but too short for activate the normal supply of backup power.
As a result, electric pumps halted temporarily stop the flow of water used to cool the reactor fuel rods red hot, according to reports NISA and TEPCO.
The change was part of a stabilization device power transmission line installed on Daiichi in 1984 and also used in other plants of TEPCO. In 2009, the company determined that these devices were no longer needed, with new transmission lines are added, and plans were made to remove them. "We already had plans to get rid of them because they are outdated," said Aizawa.
In 2010, the damage to the reactor core was avoided when the control room operators powered by diesel generators with external ventilation of the reactor vessel to relieve pressure in a suppression pool. That move, which did not release any radiation outside the plant, replenish water levels during the next hour, the company said in its report.
fuel rods coated with zirconium, full ceramic ball containing uranium oxide, could disintegrate if not continuously surrounded by water, experts say nuclear energy. The No. 2 reactor fuel rods was nearly four meters high and is usually submerged in the water twice as deep. During the incident in June 2010, the water level dropped to about six meters.
This year, after the earthquake outage prompted by Daiichi off water pumps, fuel rods are exposed in one of the three active reactors began to break within five hours later and had completely melted within 15 hours. The rods of the reactors of other fuels including No. 2 on the inside, cast in four days.
Tepco, said last month that the containment vessels damaged reactor probably been violated as a result of mergers that may have resulted in additional radiation leaks.
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