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Tuesday, April 19, 2011
Success no given in Tepco road map
By MINORU MATSUTANI and KAZUAKI NAGATA
Too many uncertainties cloud the feasibility of Tokyo Electric Power Co.'s plan to achieve a cold shutdown of the damaged reactors at the Fukushima No. 1 power plant in six to nine months, experts said Monday.
Tepco announced Sunday it aims to stabilize reactors 1, 2 and 3 to make sure radiation emissions decline in three months and then go for a cold shutdown — in which the temperature of the reactor-core coolants is brought below 100 degrees — in six to nine months.
"There are too many uncertain elements to guarantee the work will be finished in the given time frame," said Tadashi Yoshida, professor at the Nuclear Reactor Laboratory of Tokyo City University. "Tepco is probably aware of the uncertainties. But by announcing the plan, Tepco gave itself a commitment. That is a meaningful thing."
The plan includes a new tactic — filling the containment vessels of reactors 1, 2 and 3 with enough water to cover the pressure vessels, which contain the fuel rods.
Past actions, including pumping nitrogen into the containment vessels to prevent hydrogen explosions, restoring the cooling systems with circulating water, and pumping radioactive water out of the facilities, will all continue.
Tepco said it is aware of the risk of increasing the amount of radiation-tainted water if it goes ahead with the plan.
But Tepco is running another risk, Yoshida said.
By effectively filling 80 or 90 percent of the containment vessels, each of which can hold from 6,000 to 7,400 tons of water, the weight will make them more vulnerable to aftershocks, he said.
Yoshida's worry is shared by Hisashi Ninokata, professor of reactor engineering at Tokyo Institute of Technology.
"There is a question of how much weight the containment vessel and the suppression chamber can endure," Ninokata said. "If aftershocks as strong as magnitude 7 or 8 strike, it is completely unknown whether they can hold up," he said.
Also, if the containment vessels of reactors 1 and 3 have cracks or holes, filling them with water will reveal this, Yoshida said. If that happens, Tepco will have to fix them without putting workers at risk, he said.
"Who knows how close to the containment vessels workers can get? Who knows how much work robots can perform?" he asked.
Reactor 2 has a damaged suppression chamber, a doughnut-shaped water-filled vessel at the bottom of the containment vessel that is designed to relieve the pressure vessel when the pressure gets too high. The suppression chamber must be repaired before the entire containment vessel is filled with water.
The radiation level is presumably lethal near the suppression chamber because the water leaking from it is probably the source of the dangerously radioactive water (more than 1,000 millisieverts per hour) that is flooding its turbine building and a trench near to it that leads outside.
Tepco did not spell out how it plans to repair the chamber.
Yoshida and Ninokata said it will be hard to fix the leak, because the repair crews can't approach the containment vessel and have no idea how to fix the leak at this point.
To reduce the risk of?radiation exposure, Tepco said it will continue pumping water into the pressure and containment vessels and recycling the water in the turbine buildings to keep the pressure vessels cool.
Circulating water is the only way to re-establish a stable cooling capability, experts said. The beleaguered utility also said it plans to set up decontamination facilities for the water within three months to achieve cold shutdown in nine months, although it warned that there is plenty that can go wrong..
"Tepco knows of many uncertainties. But Tepco had to come up with the plan because the government and public want to know when the crisis will end," Yoshida said.
"I am also worried about workers' health in general. If Tepco loses skilled workers, it will be in big trouble," he said.
Commenting on reports that Tepco subcontractors are recruiting inexperienced workers with offers of remunderation as high as ¥10,000 per hour, he said: "They have to receive really thorough education and specific instructions or they'll do more harm than good."