Containment vessels also damaged

The Yomiuri Shimbun

Not only the pressure vessels, but the containment vessels of the Nos. 1 to 3 reactors at the Fukushima No. 1 nuclear power plant were probably damaged within 24 hours of the March 11 earthquake and tsunami, according to Tokyo Electric Power Co.'s analysis of the nuclear crisis.

In a report on the analysis, the utility said it carried out minute calculations on internal pressure and other measurements in the nuclear reactors after the earthquake.

The report was submitted to the Economy, Trade and Industry Ministry's Nuclear and Industrial Safety Agency on Monday night.

TEPCO said it found that an isolation condenser, a type of emergency cooling device, did not work properly at the No. 1 reactor. This caused the core meltdown to progress until it damaged the bottom of the pressure vessel about 15 hours after the earthquake.

Along with the meltdown, the temperature inside the steel containment vessel, which contains the pressure vessel, rose until it reached 300 C in 18 hours after the quake, much higher than 138 C the vessel was designed for. It is believed the internal temperature continued to rise after that.

Containment vessels are designed for a much lower temperature and pressure than pressure vessels, which can be exposed to temperatures close to 300 C and pressure reaching 70 bars when a reactor is in operation.

Rubber and metal parts used to seal pipes and other devices in the containment vessel apparently deteriorated quickly under temperatures exceeding 300 C. This may have caused steam containing radioactive materials to leak, the report said.

The pressure inside the containment vessel reached 8.4 bars 12 hours after the quake, which is nearly two times the pressure it was designed for. Damage to the containment vessel might have progressed faster than the analysis, TEPCO said.

On the basis of fluctuations in pressure measurements inside the containment vessel of the No. 2 reactor, it is assumed a 10-centimeter-diameter hole appeared in the side of the vessel, causing steam to leak 21 hours after the quake.

If there was no leakage, the rise of internal pressure would have been much sharper. This makes it difficult to explain the actual pressure measurements.

Damage in a pressure control chamber has already been acknowledged, but damage to the containment vessel itself has not been confirmed.