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Friday, March 9, 2012
QUEST FOR RECOVERY
No. 1 plant's stability doubted, as damage to melted cores, quake vulnerability remain unknown
Noda's definition of 'safe' questioned
Third in a series
Prime Minister Yoshihiko Noda assured the nation in December that the Fukushima No. 1 nuclear plant crisis had been reined in, but as the true extent of the damage inside the crippled reactors remains unknown a year on and with the complex still appearing vulnerable to another major quake, the government and Tepco's claims that the facility is secure are being questioned.
Reactor engineers and seismologists believe another nightmare meltdown scenario is unlikely, but say the government and Tokyo Electric Power Co. still face massive obstacles and a host of challenges in decommissioning the plant, including decontaminating the wrecked reactor buildings and plugging cracked containment vessels. Even then, scrapping the plant conceivably could take more than 30 years.
It wasn't until Dec. 16, around nine months after the crisis first erupted, that Noda declared his government and Tepco at long last had tamed the nuclear disaster.
"We have confirmed today that reactors (1, 2 and 3) are now in a state of cold shutdown, and the crisis at the plant thus has been contained," Noda announced.
According to the government and Tepco, the melted nuclear fuel in the wrecked reactor cores is being kept in a cool, stable state and the temperatures at the bottom of their pressure vessels remain below 100 degrees, while the leakage of radioactive materials is being closely monitored and has been drastically reduced — criteria that meet the definition of a cold shutdown.
Now that the Fukushima No. 1 plant has been stabilized, a preparatory phase for decommissioning the reactors has begun, they said.
Even so, widespread public concerns remain over just how stable the facility really is, centered on two major factors: No one has yet been able to determine the exact location of the melted fuel inside the containment vessels or to ascertain just how badly damaged the fuel rods are, while the plant also is not viewed as sturdy enough to withstand another powerful earthquake in the near term.
Scientists and nuclear experts, however, believe the government's assessment probably is accurate, more or less.
Crucially, the melted fuel in the reactor cores has stopped generating dangerously high levels of heat, they noted.
Hisashi Ninokata, a professor of reactor engineering at Tokyo Institute of Technology, explained that although the melted fuel is continuing to produce a certain amount of "decay heat" — a phenomenon caused by radioactive decay and part of the standard process for shutting down a reactor — the levels have steadily declined over the past year.
Calculations estimate that since last March the heat emanating from each reactor has fallen to the equivalent of about 0.05 percent of the predisaster power they were generating while fully operational, suggesting temperature levels have been brought under control, Ninokata said.
So even though the location and state of the damaged fuel rods remain unclear, "I can hardly imagine that the melted fuel will suffer further damage" through a surge in temperature, he said.
The chief of the Fukushima No. 1 plant, Takeshi Takahashi, also stressed that temperatures in the reactors have remained at a safe level during a Feb. 20. press tour of the complex.
And even if another massive temblor rocks the shaky reactor buildings, Tepco has prepared various contingency plans to enable its engineers to continue injecting coolant water into the reactors, Takahashi stressed.
"We have multiple backup cooling systems, so on top of the fact that the decay heat has been falling, if a cooling system is knocked out, we are prepared for such a situation and will be able to handle it," he said.
Tepco's medium- to long-term management plan for the plant claims that even if multiple accidents occur, engineers would be able to resume injecting water within three hours.
The possibility of the plant being ripped apart by a sustained nuclear chain reaction, or recriticality, also is unlikely, according to Ninokata of the Tokyo Institute of Technology.
A large quantity of water is required to cause such a phenomenon, but the amount being injected into the reactors is too small to spark recriticality and in any case is leaking out of the reactors' cracked containment vessels.
On the potential vulnerability of the wrecked reactor buildings if another megaquake rocks the area, the public is especially concerned about the risk to the reactor 4 building, which was badly damaged by a hydrogen explosion last March.
Tepco reinforced the unit's earthquake resistance by erecting support structures last May, and checked its resilience to large temblors through a computer simulation.
The simulation confirmed that the reactor 4 building can withstand a 9.0-magnitude temblor, like the one that struck on March 11, Tepco said.
Akira Wada, an expert in seismic technology and a professor emeritus at Tokyo Institute of Technology, said the accuracy of such computer simulations depends on one key factor — how conservative their premise is.
Tepco said the premise of its simulation was quite conservative. For instance, the potential resilience of some of the reactor building's damaged walls and distorted reinforced steel was excluded from the computer model.
Wada added that even though some of the reactor units were damaged by hydrogen explosions, considering how strongly they were built in the first place it remains unlikely they would collapse during another big quake.
But while experts do not anticipate a worst-case scenario unfolding at the No. 1 plant, this does not necessarily mean the wrecked reactors are fully under control.
Even after Noda declared that cold shutdown had been achieved, the facility has continued to experience a host of technical troubles. Frozen pipes caused numerous leaks of radioactive water during the winter, while a thermocouple device at reactor 2 showed abnormally high readings of around 285 degrees in mid-February, only for Tepco to subsequently find that the temperature sensor was broken.
Meanwhile, workers at the plant are still battling several crucial problems, including the thousands of tons of contaminated water that continue to flood the reactor buildings. As of Feb. 28, Tepco had processed about 254,000 tons of radioactive water since last June, but around 93,000 tons still remain on-site.
Tepco originally had planned to remove all contaminated water at the plant by mid-January, but later discovered that between 200 to 500 tons of water are flowing into the reactor buildings' underground levels every day.
The utility still has not come up with an effective solution to dispose of the water, saying only that it plans to remove it within 10 years.
In the meantime, more tanks are being brought in to store the processed water, resulting in a shortage of available space.
Projections show that the power station will reach its maximum storage capacity this fall, at which point Tepco may have no other choice than to start discharging highly radioactive water into the Pacific Ocean.
While striving to keep the reactors in a stable state and to process and store the contaminated water, the government and Tepco at the same time are moving forward with plans to decommission the plant, a process that will require the creation of technologies the world has never seen to overcome obstacles the nuclear power industry has never previously faced.
Tepco plans to start removing fuel rods from reactor 4's spent-fuel pool within two years, and those in the unit 3 spent-fuel pool within three years.
But plans to extract fuel rods from units 1 and 2 are far more vague, with the utility aiming to tackle their spent-fuel pools at some point in the 10 years.
As for the melted fuel that lies deep inside the reactor cores, Tepco says it will take up to 10 years before it can start extracting damaged rods. The utility's engineers first will have to plug cracks and holes in the three containment vessels and to then flood them, hoping the water will block radioactive emissions.
Such work, however, has never been attempted anywhere in the world.
As a reference point for the decommissioning, the government, Tepco and reactor manufacturers have been studying measures and techniques employed during the Three Mile Island accident in Pennsylvania, where one of the power plant's reactor cores suffered a meltdown in 1979. But the containment vessel did not crack in that accident, making it easier for engineers to flood it compared with Fukushima No. 1.
Even before work can start to check the condition of the melted fuel in the reactor cores, Tepco faces the huge obstacle of decontaminating the reactor buildings, experts said.
Radiation levels inside the buildings are extremely high in general but in some areas they are positively lethal, ranging from several hundred millisieverts per hour to more than 1 sievert.
Satoru Tanaka, a professor at the University of Tokyo who chairs the Atomic Energy Society of Japan, stressed the importance of accurately mapping the insides of the reactor buildings and identifying precisely where radiation is emanating from.
"There are large amounts of debris scattered inside the reactor buildings" due to the hydrogen explosions, Tanaka said. "If radioactive materials are stuck to the debris, decontamination can progress by removing the rubble.
"But the high levels of radiation may be coming from the containment vessels themselves," he warned.
The decontamination process could even turn into an insurmountable hurdle if the containment vessels are confirmed as the source of searing radioactivity, as decontaminating the inside of a containment vessel is at present considered virtually impossible.
In light of this, Noda's administration in December created a research and development team to come up with the technologies necessary to decommission the complex. The team's members include officials from the government, Tepco, reactor manufacturers and semipublic nuclear research bodies.
The team gave a public presentation Feb. 24 of the kinds of new technologies that will be required to scrap the wrecked reactors, to seek the public's help and input.
These technologies include a remote decontamination system to cleanse the reactor buildings, and a waterproof robotic machine able to function amid prohibitively high radiation levels and repair damage to the containment vessels.
To build such technologies, it is crucial that a continuous, "all-Japan" research and development system is created, Tanaka of the University of Tokyo said.