– ‘Melt-through’ at Fukushima? / Govt report to IAEA suggests situation worse than meltdown (The Daily Yomiuri, June 8, 2011):
Nuclear fuel in three reactors at the Fukushima No. 1 nuclear plant has possibly melted through pressure vessels and accumulated at the bottom of outer containment vessels, according to a government report obtained Tuesday by The Yomiuri Shimbun.
A “melt-through”–when melted nuclear fuel leaks from the bottom of damaged reactor pressure vessels into containment vessels–is far worse than a core meltdown and is the worst possibility in a nuclear accident.
The possibility of the situation at the plant’s Nos. 1 to 3 reactors was raised in a report that is to be submitted to the International Atomic Energy Agency.
If the report is released as is, it would be the first official recognition that a melt-through has occurred.
It was revealed earlier that sections of the bottom of the pressure vessels where control rods go through have been damaged. Highly radioactive water from inside the pressure vessels was confirmed to have leaked out of the containment vessels, even outside the buildings that house the reactors.
The report also acknowledges problems with the vertical administrative structure concerning nuclear safety regulations. As a result, the report says, who was responsible for keeping people safe in the event of a nuclear accident was not clear.
The report proposes separating the Nuclear and Industrial Safety Agency from the Economy, Trade and Industry Ministry and making it an independent organization. The report also proposes drastic reform of the nation’s nuclear administration, including the Nuclear Safety Commission.
Vessel damaged 5 hours later
The pressure vessel of the No. 1 reactor at the Fukushima No. 1 nuclear power plant is believed to have been damaged five hours after the March 11 earthquake, according to an analysis by the Nuclear and Industrial Safety Agency.
The finding differs with a provisional analysis earlier released by plant operator Tokyo Electric Power Co., which stated the the pressure vessel was believed to have been damaged 15 hours after the March 11 Great East Japan Earthquake.
On Monday, NISA, a nuclear watchdog body run by the Economy, Trade and Industry Ministry, disclosed the results of a detailed analysis regarding damage at the Nos. 1 to 3 nuclear reactors at the Fukushima facility. NISA estimates that the No. 2 reactor’s pressure vessel was damaged 80 hours after the disaster. TEPCO’s analysis contends the No. 2 reactor’s pressure vessel was damaged 109 hours after the quake.
According to NISA’s analysis, the No.1 reactor’s core began suffering damage three hours after the earthquake.
The No. 1 reactor’s pressure vessel was damaged at 8 p.m. on March 11, five hours after the earthquake. The No. 2 reactor’s pressure vessel suffered damage at 10:50 p.m. on March 14, while the No. 3 reactor’s pressure vessel suffered damage at 10:10 p.m. on March 14. NISA data showed the pressure vessels at the Nos. 1 and 2 reactors were damaged earlier than TEPCO’s analysis showed.
On the other hand, the No. 3 reactor’s pressure vessel was found to have been damaged 13 hours later than TEPCO’s data showed.
NISA presumed the vessels failed when there was almost no water in the reactor cores of the Nos. 2 and 3 reactors.
(Jun. 8, 2011)