One of the striking points about the final report of the government's investigation panel on last year's nuclear disaster is the comparison of the crisis response at the Fukushima No. 1 nuclear power plant and its sister, the Fukushima No. 2 nuclear plant.
One of the striking points about the final report of the government's investigation panel on last year's nuclear disaster is the comparison of the crisis response at the Fukushima No. 1 nuclear power plant and its sister, the Fukushima No. 2 nuclear plant.
Both nuclear plants are operated by Tokyo Electric Power Co.
The panel praised the positive response and communication at the Fukushima No. 2 plant, where the Great East Japan Earthquake and tsunami also knocked out the regular cooling systems at its No. 1 and No. 2 reactors, but a nuclear accident was averted.
At the same time, the government's Investigation Committee on the Accident at the Fukushima Nuclear Power Stations disputed a theory raised by a Diet investigatory committee that tremors from the March 11 earthquake could partly account for the nuclear disaster at the Fukushima No. 1 plant.
Core meltdowns occurred at the Fukushima No. 1 plant's No. 1, No. 2 and No. 3 reactors after the power supply was lost due to a tsunami spawned by last year's quake. The Fukushima No. 2 nuclear plant was also swamped by a tsunami, but its reactor cores were not damaged.
The government's investigation panel focused on the crisis response at the Fukushima No. 1 nuclear plant's No. 2 reactor. An emergency cooling system, called the reactor core isolation cooling system (RCIC), remained operational at the No. 2 reactor for more than two days despite the loss of power, but there was no means to control the RCIC, which could have stopped at any moment.
The situation worsened while the RCIC was operating at the No. 2 reactor. Explosions occurred at the No. 1 and No. 3 reactor buildings. There was little progress in the efforts to restore the external power supply, and radiation levels rose sharply on the premises of the nuclear plant.
If the RCIC stopped, the only alternative was injection of water by fire engines into the overheating reactor cores.
The government panel's report said the RCIC would have eventually stopped operating if it remained in use for a long time. The report criticized the failure to prepare an alternative method for injecting water before the RCIC stopped.
The report pointed out temperature and pressure data should have been monitored to prepare for the alternative pumping of water into the reactors. But the data remained unmonitored until 4:30 a.m. on March 14, more than two days into the crisis.
The RCIC stopped first, creating more than seven hours of disruption in the injection of water into the reactor core. The report criticized the response of TEPCO workers on site and those in the TEPCO emergency headquarters.
"(TEPCO) had a more optimistic view of the situation at the No. 2 reactor than what it was in reality," the report said.
In the meantime, the earthquake and tsunami also knocked out the regular cooling systems at the No. 1 and No. 2 reactors of the Fukushima No. 2 plant, where emergency cooling systems took over the injection of water into the reactors. Unlike at the Fukushima No. 1 plant, operators at the Fukushima No. 2 plant continued monitoring the injection of water and created no disruption of the process. The operators conveyed pressure and water temperature readings every hour to the plant's emergency headquarters.
The government panel gave a positive evaluation to their response.
"It is thought that basic work procedures in the event of a serious accident were shared on the site of the Fukushima No. 2 nuclear plant," the panel report said.
Concerning the different responses at the two nuclear plants, the report pointed out that the available emergency response options may have hampered employees' judgments at the Fukushima No. 1 plant.
An external power supply was available at the Fukushima No. 2 plant. That made the situation there decidedly different from that at the Fukushima No. 1 plant, where the only available alternative was water injection by fire engines.
But it remains unknown if it would have been possible to prevent damage to reactor cores even if there had been no halt in the water injection at the Fukushima No. 1 plant.
Different investigation panels have produced different appraisals of the operations to cool the reactors.
"When no measures are available to deal with a severe accident, there are only very limited options to what people can do on the ground in the event of a total loss of power supply," said the report of the Diet's Fukushima Nuclear Accident Independent Investigation Commission. "One cannot simply put the blame on judgments and operations of the (nuclear reactor) operators at the time of the accident."
The civil sector's Independent Investigation Commission on the Fukushima Nuclear Accident was less forgiving.
"Knowledge and ideas may have been insufficient in the light of safety philosophy in some of the judgments made," the independent panel's report concluded.
The government panel's report dismissed the possibility that tremors from the Great East Japan Earthquake damaged the nuclear reactor pressure vessels and surrounding equipment at the Fukushima No. 1 nuclear plant's No. 1, No. 2 and No. 3 reactors and impaired their functions to contain radioactive substances.
Emergency cooling systems were set into operation at all three reactors immediately following the earthquake. Air radiation level readings on the grounds of the nuclear plant showed no conspicuous change.
Readings of pressure gauges, radiation monitors and other measurement instruments that were operating normally in the nuclear reactors were also used as evidence to refute the claim that tremors damaged the pressure vessels and other equipment.
The panel also conducted numerical analysis. If tremors from the quake had created a 3-square-centimeter rupture in the piping of the emergency cooling system for the No. 1 reactor, that would have lowered pressure in the pressure vessel below the measurements that were actually being taken, the report said.
However, the panel report did not rule out the possibility that tremors from the earthquake created a tiny rupture of 0.3 square centimeter or less, which later grew larger when the reactor temperature and pressure rose and radioactive substances leaked from there.
The Diet investigation commission's report pointed out that a rupture of 0.3 square centimeter in the piping of a pressure vessel would not have led to immediately recognizable changes in the pressure and water level readings in the nuclear reactor, but that continued leakage of reactor coolant water from the rupture could cause an accident involving fuel damage.
But the independent investigation commission, like the government investigation committee, concluded it was unlikely that tremors from the quake caused damage.
The Fukushima No. 1 nuclear plant was swamped by a tsunami less than one hour following the tremors. That means detailed inspections are necessary to identify the extent of damage that is due directly to the tremors of the earthquake. But even today, 500 days from the onset of the disaster, high radiation levels in the reactor buildings are keeping employees from working inside for long periods of time.
The government panel's final report failed to come up with a clear-cut conclusion on the longstanding debate over whether TEPCO planned to evacuate all its workers from the Fukushima No. 1 nuclear plant on the night of March 14-15.
The panel said the argument was a "major issue that concerns TEPCO's standpoint as an operator of nuclear power plants," and analyzed records of TEPCO's in-house teleconferences and interviewed concerned parties in order to draw up its conclusion.
As a result, the government panel decided that Masao Yoshida, the general manager of the Fukushima No. 1 plant at the time, and other TEPCO employees who dealt with the disaster either at the Fukushima No. 1 plant or at TEPCO headquarters, intended to leave on the ground the minimum personnel that were necessary to control the nuclear reactors, the panel report said.
Otherwise, the report said that Cabinet ministers who received phone calls from Masataka Shimizu, TEPCO president at the time, believed that Shimizu was advocating a total withdrawal. It also said senior TEPCO officials hinted at evacuating all workers during in-house teleconferences. However, Shimizu flatly denied that as a possible scenario during an interview with the government investigation panel.
The panel said that it could not conclude with certainty that Shimizu and other senior TEPCO officials had the total withdrawal of workers in mind.
Concerning the debate, the Diet investigation committee decided that the total withdrawal belief stemmed from a misunderstanding from the prime minister's office, but attributed the root cause of that to Shimizu's ambiguous communications.
The independent investigation commission refuted TEPCO's assertion that the utility never considered a total evacuation of its workers from the plant.
"It is difficult to say there is sufficient supporting evidence for TEPCO's argument, given that a large number of employees at the prime minister's office, whom we interviewed, unanimously said they understood TEPCO's proposal as referring to a total withdrawal," the independent panel's report concluded.
(This article was compiled from reports by Naoya Kon, Takashi Sugimoto and Ryuta Koike.)