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Friday, Dec. 2, 2011

Nuclear experts back Tepco meltdown simulation findings


Staff writer

While disputing the details, nuclear experts basically concurred Thursday with the findings of a computer simulation run by Tokyo Electric Power Co. that concluded nuclear fuel in reactors at the Fukushima No. 1 plant had melted into but not breached their containment vessels.

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The announcement by Tepco Wednesday was the latest indication that three overheated cores at the plant had not penetrated their concrete and steel containers and melted deep into the earth — a worst-case scenario China syndrome.

However, there is reason to dispute Tepco's claim that the nuclear fuel in reactor 1 came within 37 cm of melting through surrounding concrete and reaching the containment vessel's steel shell, said Kenji Sumita, a professor emeritus of nuclear engineering and a former deputy chief of the Nuclear Safety Commission.

"There are many computer simulation methods. I want to see simulations conducted by organizations other than Tepco," Sumita said.

Commenting on the unknown margin of error in Tepco's simulation, "I don't believe the fuel got as close as 2 or 3 cm to the steel shell, but I am not sure how credible their details are," he said.

However, Sumita claimed it was certain nuclear fuel had not escaped the containment vessels because no radioactive substances have leaked into the groundwater near the reactors.

Toshihiro Yamamoto, a specialist in reactor safety management at Kyoto University Research Reactor Institute, pointed out the difficulty of collecting data to base the computer simulation on.

"You cannot have experimental meltdowns. Thus the simulation will have to rely on many assumptions," he said.

Meanwhile, Hiromi Ogawa, a former engineer at Toshiba Corp. who managed its nuclear power generation project, effectively guaranteed the credibility of the simulation.

"The simulation referred to lots of data collected from many experiments, and thus I think the results are very precise," he said. "Still, the situation will have to be confirmed visually."

According to Tepco's simulation, the nuclear fuel in reactor 1 probably 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 in the accident triggered by 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.



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