Tokyo Electric Power Co. admitted Tuesday what many experts had long suspected: The cores of reactors 2 and 3 at the Fukushima No. 1 nuclear plant likely melted down and dropped to the bottom of their pressure vessels, just as happened at unit 1.
However, temperature readings taken in the two units, now ranging from about 100 to 110 degrees, suggest that most of the melted cores remain inside the pressure vessels and have been cooled by injected water.
“Although the simulation says that the melted cores would damage the pressure vessels if the water level was lower, we think the damage is limited considering the temperature data of the pressure vessels,” Tepco spokesman Junichi Matsumoto said at a news conference.
“Most of the melted cores appear to be at the bottom of the pressure vessels, and we don’t think there are any big holes” in the vessels, Matsumoto said.
However, the melted fuel may have damaged some parts of units 2 and 3, such as pipelines, he added.
Tepco’s latest simulation considered two scenarios. The first assumes the water level indicators have malfunctioned and that coolant water has not reached the original position of fuel rods, as was the case with reactor 1.
The second scenario assumes at least part of the rods are covered by water, as the gauges indicate.
In the first scenario, the exposed rods in reactor 3 would have melted and fallen to the bottom of the pressure vessel about 60 hours after the quake hit March 11, while the rods in 2 would have melted in about 101 hours, computer simulations showed.
If the water remained at the level where gauges now indicated it is, only a part of the fuel rods would have melted, Tepco said.
Experts weren’t surprised by Tepco’s announcement.
“We had already expected that the situation at the No. 2 and 3 reactors was similar to the No. 1 reactor,” said Kazuhiko Kudo, a professor of nuclear engineering at Kyushu University.
Tepco’s announcement “does not mean that the situation has gotten more serious. It just clearly confirmed what we had already anticipated.”
Kudo said factors including the shutdown of the cooling system at the plant and the series of explosions that occurred within a few days of the quake and tsunami showed that the fuel rods were damaged or had at least partially melted.
He said the situation won’t worsen as long as the reactors continue to be cooled with water.
“Right now, the most important thing for reactors 1 through 3 is to continue cooling them,” Kudo said. “I will not say that there is no need to worry, but I don’t think the situation will deteriorate as long as they continue to be cooled.”
Tepco conducted the computer simulation for units 2 and 3 after discovering last week that the water level indicator for the pressure vessel in reactor No. 1 was not working properly, and that coolant water had not fully covered the fuel rods.
After fixing the indicators, the utility found the water level was much lower than believed, and that the rods were fully exposed and likely melted down.
Consequently, Tepco said it began to doubt that the water level indicators for the pressure vessels in reactors 2 and 3 were working.
The results of the simulation were included in a report Tepco submitted to the Nuclear and Industrial Safety Agency.