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Thursday, Dec. 1, 2011

Containment vessels held: simulation test

Worst scenario thought avoided in meltdowns at Fukushima plant


Staff writer

The primary containment vessels of reactors 1, 2 and 3 at the Fukushima No. 1 power plant probably managed to contain the melted nuclear fuel, according to the findings of a Tokyo Electric Power Co. computer simulation released Wednesday.

If the findings are accurate, they suggest the crippled power plant has avoided the worst-case scenario, in which nuclear fuel would have burned through the concrete floors of the primary containment vessels and emitted massive levels of radioactive materials into the environment.

But Tepco's simulation also underlined the severity of the accident at the Fukushima No. 1 plant, as the results indicated that the nuclear fuel in reactor 1 likely penetrated through as much as 65 cm of the containment vessel's concrete floor, reaching as close as 37 cm to the vessel's outer steel shell, the last line of defense. Reactor 1 suffered the worst damage from the March 11 earthquake and tsunami.

The findings also suggest that the nuclear fuel in reactor 2 melted through 12 cm of the vessel's concrete floor, while fuel from reactor 3 burned through 20 cm.

But despite the near catastrophe, "the melted fuel remained inside the containment vessels," Tepco said in a report on the simulation's findings, which was released Wednesday during a meeting of experts assessing the damage to the plant's reactor cores.

"The extent of the damage to reactors 2 and 3 is believed to be limited compared with that in reactor No. 1, and the amount of nuclear fuel that melted through to the containment vessels is limited. We believe that much of the nuclear fuel still remains in the pressure vessels (of reactors 2 and 3)," Junichi Matsumoto, acting general manager at Tepco's nuclear power and plant siting division, said during a news conference Wednesday evening.

Tepco said the calculations were based on the pessimistic assumption that 100 percent of the fuel in reactor 1 melted through the pressure vessel and into the primary containment vessel, and that 57 percent of the nuclear fuel in reactor 2 and 63 percent of the fuel in reactor 3 penetrated through to the containment vessels.

The simulation also was based on projections Tepco made, including its calculations of the decay heat released by the melted nuclear fuel. The utility used the Modular Accident Analysis Program developed by Fauske & Associates LLC, a subsidiary of U.S.-based Westinghouse Electric and a pioneer in computer models that analyze severe accidents at nuclear power plants.

Based on analyses of gas samples taken from inside the containment vessels, Tepco said it believes the nuclear fuel has stopped melting through the concrete floors, as its temperature is being kept at a cool and stable level by the injection of coolant water.

The government and Tepco aim to achieve a cold shutdown of the three damaged reactors at the plant by the end of the year, and Wednesday's report is viewed as a key step toward that goal.

The report has been handed over to a group of the government's Nuclear and Industrial Safety Agency that is working to establish how much damage the reactor cores have suffered.

Water is currently being injected into all three reactors, the utility added, while the temperatures in the containment vessels and at the bottom of the pressure vessels have been stable and recently measured less than 100 degrees as of Wednesday. The temperatures in the containment vessels' concrete floors, however, are not known.

The March 11 earthquake and tsunami knocked out the power supply and backup generators for reactors 1, 2, and 3, preventing coolant water from being circulated into the pressure vessels for hours and causing the nuclear fuel to melt down in the three reactors.



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