Core of reactor 1 melted 16 hours after quake | The Japan Times Online
Monday, May 16, 2011
Core of reactor 1 melted 16 hours after quake
New analysis shows damage to fuel rods was surprisingly quick
The meltdown at reactor No. 1 in Fukushima happened more quickly than feared, Tokyo Electric Power Co. said Sunday in a new analysis.
The core of the heavily damaged reactor at the Fukushima No. 1 power plant is believed to have melted 16 hours after the March 11 mega-quake and tsunami rocked the complex in northeastern Japan.
Preliminary analysis shows that No. 1 had already entered a critical state by 6:50 a.m. on March 12, with most of its fuel having melted and fallen to the bottom of the pressure vessel, the plant operator said. Tepco released data Thursday showing some of the fuel rods had melted.
The reactor automatically halted operations immediately after the 2:46 p.m. quake, but the water level in the reactor dropped and the temperature began rising at around 6 p.m. The damage to the fuel rods had begun by 7:30 p.m., with most of them having melted by 6:50 a.m. the following day, the utility said.
While the utility had planned to bring the nation’s worst nuclear accident under control in around six to nine months from mid-April, it now has no choice but to abandon a plan to flood the containment vessel of reactor 1 because holes have been created by the melted fuel, an adviser to Prime Minister Naoto Kan said earlier Sunday.
Nevertheless, Goshi Hosono, the top official tasked with handling the nuclear crisis, told TV programs the government had yet to revise the timetable for bringing the crisis to an end.
Asked about initial plans to completely submerge the 4-meter-tall fuel rods by entombing the vessel in water, Hosono said, “We should not cause the (radioactive) water to flow into the sea by taking such a measure.”
Hosono said the government will instead consider ways to decontaminate the water being used to cool the fuel so that it can be recirculated instead of letting it flood the facility.
Hosono made the remarks after Tepco discovered a pool of water more than 4 meters deep and exceeding 3,000 tons in the basement of reactor No. 1. This suggests that the water, which is likely highly radioactive, is seeping through the holes after being injected into the reactor core.
From there, it is probably leaking from either the containment vessel or the suppression pool, which enclose the pressure vessel, and into the piping.
In a related revelation concerning a major mixup after the six-reactor complex lost power, Tepco and other sources said the same day that the utility had assembled 69 power supply vehicles at the plant by March 12 but that these proved virtually useless.
The inability to use the vehicles delayed the damage control work at the plant, significantly worsening the emergency.
Tepco earlier said it had tried to connect the vehicles to power-receiving equipment needed to operate the water pumps intended to cool down the reactors. But this failed because the equipment was submerged in seawater from the tsunami, which posed the risk that the equipment would short out.
Tepco’s account conflicts with the one detailed by the Nuclear and Industrial Safety Agency, which mentioned the first arrival of such a vehicle on the evening of March 11 but stopped mentioning it the following day, as the focus of attention had shifted to the need to release radioactive steam to relieve pressure that had built up inside the containment vessel of reactor 1.
The different versions of the story given by Tepco and the agency might come to a head as investigations progress to determine why efforts to immediately contain the crisis failed.