The thousands of families wanting to know what specifically led to the Fukushima nuclear disaster that turned their lives upside down might have to wait decades for an answer.
The thousands of families wanting to know what specifically led to the Fukushima nuclear disaster that turned their lives upside down might have to wait decades for an answer.
The different panels that investigated the cause of the triple meltdown could not even agree on the time the tsunami struck the Fukushima No. 1 nuclear power plant on March 11, 2011.
The government and Tokyo Electric Power Co., operator of the stricken plant, maintain that a tsunami of unforeseeable size was solely to blame. TEPCO also said the accident was inevitable after it started to unfold.
However, a Diet investigation panel raised the possibility that shocks from the magnitude-9.0 Great East Japan Earthquake were also partly responsible, an argument that could have serious implications for safety measures at nuclear plants and the extent of responsibility of TEPCO and the government.
Seismic ground motions largely fell within anticipated levels, although they slightly exceeded maximum expected levels at some locations. However, the tsunami heights far outstripped levels envisaged by government regulators.
The Nuclear Regulation Authority and TEPCO are continuing with their respective investigations.
One point of contention concerns workers’ testimonies that they saw water leaking near the isolation condensers in the No. 1 reactor building at the time of the accident.
The Diet’s Fukushima Nuclear Accident Independent Investigation Commission said seismic shocks could have damaged a piping system for the isolation condensers, which lost water and were no longer able to sufficiently cool the reactor.
The NRA inspected the site in May 2013 and said the water leaks likely did not come from damaged isolation condensers. The inspectors instead said the water probably sloshed out of a spent fuel storage pool during the shaking from the quake and entered a duct. TEPCO espouses the same theory.
No consensus has been reached on the matter.
“I found (the isolation condensers) were damaged to a lesser extent than I had expected,” said Atsuhiko Kosaka, a nuclear safety liaison officer with the NRA secretariat. “But the matter should be studied further.”
The tsunami arrival time is also being contested.
The Diet investigation panel said seismic shocks could have damaged at least one of the backup diesel generators for the No. 1 reactor, which it said had stopped operating before the tsunami hit.
A wave gauge installed 1.5 kilometers off the coast of the nuclear plant recorded a second, bigger tsunami wave at 3:35 p.m. The Diet panel argued the tsunami reached the plant compound at 3:37 p.m. or later, after the backup diesel generators stopped working and all power supply was lost.
TEPCO countered that theory by presenting a new statement in October based on a reanalysis of photos of the tsunami and power supply records.
The utility said the camera clock was off by 6.5 minutes or so and argued that the tsunami arrived at 3:36 p.m., around the same time the backup diesel generators went offline. TEPCO concluded the generators stopped working because they were inundated by the tsunami.
COULD MELTDOWN HAVE BEEN AVERTED?
Another question is whether the plant’s operators and chief could have prevented the meltdowns of the No. 1, No. 2 and No. 3 reactors if they had made different decisions or taken other measures.
The government’s Investigation Committee on the Accident at the Fukushima Nuclear Power Stations pointed to operations of the high pressure coolant injection (HPCI) system, one of the emergency core cooling systems for the No. 3 reactor.
Operators halted the HPCI system at 2:42 a.m. on March 13 at their own discretion, fearing the system would fail under the abnormal circumstances at the time. They later attempted to reactivate the HPCI system, but to no avail.
The government investigation panel said water injection into the reactor was suspended for nearly seven hours because the operators stopped the HPCI system without securing an alternative means to cool the reactor. The stoppage fueled the progression of the core meltdown process, the panel said.
The government panel also noted that fire engines were late in providing an alternative way to inject water into the reactor.
TEPCO, however, rebutted this theory when it presented new analysis results in December that reversed its previous views.
The utility said the HPCI system was actually injecting less water into the reactor than it had previously believed, causing water levels to drop and the reactor core to melt sooner than earlier thought.
In other words, TEPCO argued that the core meltdown could probably not have been averted even if the workers had kept the HPCI system running at the No. 3 reactor.
The utility also said fire engines were not injecting enough water into the reactors after the cooling systems had stopped.
TEPCO connected fire engine pumps to conduits of the reactor buildings to pump water into the reactors after the cooling systems had failed.
But the utility said part of the water flowed into condensers through branch pipes along the way and never reached the reactors. It never occurred to TEPCO workers at the time that there were such escape routes for the water, the utility added.
“Failures in water injection from the fire engines present a major problem, but in the first place, there should have been an alternative arrangement, alongside the cooling systems, to allow water to be injected into the reactors as soon as the accident started,” said Fumiya Tanabe, a former chief research scientist with the Japan Atomic Energy Research Institute, one of the predecessors of today’s Japan Atomic Energy Agency.
“Such an arrangement could have cut the chain of events that led to the (hydrogen) explosions.”
(This article was written by Akira Hatano and Senior Staff Writer Hisashi Hattori.)