Updated 8/9/14: Fukushima reactor 3 meltdown was worse than estimated: Tepco — The Japan Times; Reactor No. 3 meltdown occurred 4 hours earlier — Yomiuri Shimbun; NHK World

Updated 8/9/14, The Japan Times: ” Tokyo Electric Power Co. said Wednesday that its new estimate shows that all the fuel rods in reactor 3 at the Fukushima No. 1 nuclear power plant apparently melted down and fell onto the bottom of the containment vessel.

In November 2011, the company had said it believed only about 63 percent of reactor 3′s fuel core had melted.

The utility updated its estimate as part of an effort to probe unclear points about the triple meltdown at the Fukushima No. 1 plant caused by a megaquake and monstrous tsunami in March 2011.

The revised estimate is based on the finding that an emergency cooling system, known as HPCI, of reactor 3 stopped working six hour earlier than previously thought, and that the meltdown had also started more than five hours earlier.

Tepco had previously said that the HPCI had shut down at 2:42 a.m. on March 13, 2011. But further investigation over the past year determined that the HPCI appeared to have lost its cooling function about at 8:00 p.m. on March 12.

According to the new estimate, all the melted fuel penetrated the pressure vessel, fell onto the bottom of the containment vessel and melted about 68 cm into the concrete.

The pressure vessel is located inside the massive containment vessel.

The analysis shows that the fuel did not penetrate the containment vessel, according Tepco.

While the new analysis announced on Wednesday, based on temperature, pressure and other data, shows that all the fuel had melted down to the containment vessel, Tepco has a more optimistic view.

“We think some fuel still remains at the core part based on the actual plant data,” said Shinichi Kawamura, a Tepco spokesman, during a news conference.

According to Kawamura, this is because the temperature of the pressure vessel decreased when the water was injected, meaning some warm fuel was still there. ”

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Originally posted 8/7/14, Yomiuri Shimbun: ” A 2011 meltdown inside one of the reactors at the Fukushima No. 1 nuclear plant may have started about four hours earlier than was previously believed, plant operator Tokyo Electric Power Co. said Wednesday.

In its latest findings, TEPCO also said that most of the nuclear fuel in the No. 3 reactor at the plant in Fukushima Prefecture melted through the pressure vessel and continued down to the bottom of the outer containment vessel. The finding may make it even more difficult to decommission the plant.

According to previous findings, including the final report made in July 2012 by the government’s Nuclear Incident Investigation and Verification Committee, TEPCO manually stopped an emergency high-pressure coolant injection apparatus at the No. 3 reactor and tried to switch to another water injection system as operation of the apparatus became unstable before 3 a.m. on March 13, 2011. The attempted switch failed, as TEPCO was unable to secure power source.

Initially, TEPCO had assumed that from that time on, a “hiatus of coolant injection” had started, leading to a fall in the water level inside the pressure vessel and triggering a core meltdown inside the vessel, sometime after 9 a.m. on the same day.

However, a record made by a worker at the plant that details water levels inside the reactor was found in the autumn of 2012. The record suggests it is possible that the coolant apparatus had already ceased functioning nearly seven hours before TEPCO stopped the coolant-injection device.

A new analysis of conditions inside the reactor, made on the basis of the uncovered record, led to the latest finding that the temperature in the reactor core reached the fuel’s melting point of 2,200 C at around 5:30 a.m. on that day.

TEPCO has come to assume that the core meltdown was highly likely to have started in the early morning of March 13.

As the core meltdown is now believed to have started earlier than was previously thought, the amount of melted nuclear fuel that passed into the containment vessel through the pressure vessel is considered to have been greater, making it technically more difficult to extract the melted fuel and dispose of it.

At 9:25 a.m on March 13, 2011, two days after major earthquakes and tsunami hit the area that includes Fukushima Prefecture, TEPCO began injecting water into the No. 3 reactor, with the use of a firefighting vehicle. Shortly after 11 a.m. on March 14, however, a hydrogen explosion occurred in the reactor-housing building, blowing away the upper part of the building. ”

source

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Read a related NHK World article, “Meltdown at Fukushima reactor 3 worse than thought,” HERE.

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