Revelations from Fukushima

Not exactly plastered over the world’s headlines - not so mcuh as a word from the NZ media and I couldn’t find anything on Reuters.

Reactor Core Melted Fully, Japan Says

Fuel Breached Vessel Floor, Operator Says, In Its Gravest Fukushima Status Report

Wall Street Journal,

1 December, 2011

TOKYO—Japan's tsunami-stricken nuclear-power complex came closer to a catastrophic meltdown than previously indicated by its operator—who on Wednesday described how one reactor's molten nuclear core likely burned through its primary containment chamber and then ate as far as three-quarters of the way through the concrete in a secondary vessel.

The assessment—offered by Japan's government and Tokyo Electric Power Co., the operator of the Fukushima Daiichi nuclear complex—marked Japan's most sobering reckoning to date of the nuclear disaster sparked by the country's March 11 earthquake and tsunami.

But it came nearly six months after U.S. and international nuclear experts and regulators had reached similar conclusions. That lag echoes international allegations, in the tense weeks following the disaster, that Japan was underplaying the severity of the contamination and was slow to provide information to outside nuclear regulators.

For the first time, Tokyo Electric, known as Tepco, said that nuclear-fuel rods in the complex's No. 1 reactor had likely melted completely, burning through their so-called pressure vessel and then boring through concrete at the bottom of a second containment vessel. Tepco estimates the fuel then eroded about 65 centimeters (about two feet) deep into the 2.6-meter (8.5-foot) concrete bottom. The government model estimated the erosion at up to 2 meters.

The molten core stopped short of reaching the vessel's steel casing, under which lies an additional 7.6 meters of concrete foundation, Tepco said.

That brought the fuel closer than previously believed to breaching the containment vessel and foundation and continuing to burn through the ground below—a scenario sometimes described as the "China Syndrome," from the fanciful notion, popularized in a U.S. film by the same name, that in a catastrophic meltdown, molten reactor fuel could sink through the earth until it reached China.

The findings are the latest reminder of how much remains unknown about the extent of the mid-March Fukushima Daiichi accident: Workers still can't get close enough to the stricken reactors to make first-hand reckonings. Wednesday's assessment was based on separate analyses by Tepco and the government of the latest radiation and temperature data from around Fukushima Daiichi's reactors.

Tepco said there is no danger of further damage now.

In its last major update on reactor No. 1, in May, Tepco said the reactor's fuel had more than half melted, and some had fallen into the containment vessel.

Around the same time, models run by the U.S. Nuclear Regulatory Commission had already pointed to a complete melting and containment breach. "This was not at all unexpected," said Eliot Brenner, a spokesman with the U.S. Nuclear Regulatory Commission. "It really does nothing to change our assumptions—because we based our decisions on very pessimistic scenarios."

Officials at the International Atomic Energy Agency have been frustrated by their slow access to information concerning events at Fukushima Daiichi, according to a Vienna-based diplomat familiar with the United Nations nuclear watchdog. An IAEA spokesman declined to comment on Japan's report.

Fuel rods in Fukushima Daiichi's No. 1 reactor, in March aerial photo, melted fully out of their pressure vessel, Tepco said Wednesday.

The precise timeline of melting remains unclear, but it likely stopped as Tepco began dousing the complex's overheating reactors with seawater about a day after the quake and tsunami cut power to its cooling pumps. Edwin Lyman, a senior scientist with the Union of Concerned Scientists, says it remains unclear why the fuel rods didn't also breach the containment wall. 

"Why this didn't happen is still unknown," Mr. Lyman said.

Steven Kraft, a nuclear-industry engineer who participated in a briefing with Tepco officials on Wednesday said that even if the molten fuel, known as corium, did reach and breach the containment vessel's steel lining, it had several meters of steel-reinforced cement to melt through before reaching soil.

As for a so-called China Syndrome, "They were a great distance and a long time away from that scenario," said Mr. Kraft, the senior director of Fukushima response for the Nuclear Energy Institute, the nuclear-power industry's policy arm.

Had the corium eroded enough concrete, ground-water contamination could become an issue, according to an analysis by Argonne National Laboratory, a federally funded research lab outside of Chicago. Argonne said such a failure, while serious, would pose less of a public health risk than airborne releases of radioactive iodine, which can be spread widely by wind.

Tepco said Wednesday the damage in reactors No. 2 and No. 3—the others among Fukushima Daiichi's six reactors that overheated dangerously—was less severe than in No. 1. It said their cores had partially melted, and some fuel had burned through the reactors' surrounding vessels to the concrete base of their containment vessels.

In all three units, the fuel has now cooled to below the critical temperature of 100 degrees Celsius, and thus poses no further threat, officials said. "The fuel is now being kept safely cooled at all three reactors," a government spokesman said at a briefing following the Tepco report.

Officials have said the fuel in all reactors is approaching a state of cold shutdown, by year's end, at which point there would be no nuclear reaction or radiation release. It is then expected to take decades to dismantle and clean the site.

In April, Japan's Nuclear and Industrial Safety Agency rated the Fukushima Daiichi episode a "major accident," or a Level 7 emergency, the highest level on the International Nuclear and Radiological Event Scale. It said in June the total radiation release was roughly 1/10th that from the accident in Chernobyl in the former Soviet Union, the only accident that exceeds Fukushima Daiichi in severity.

In a recent safety assessment, Tepco said the biggest risk to the plant remains another large tsunami, which could destroy water-supply lines and prevent further cooling of the reactors. The company stressed, however, that the availability of multiple water-supply sources, including on-site fire trucks, reduces the risks.

Corrections & Amplifications 

A photo of smoke rising from a reactor at Fukushima Daiichi nuclear-power plant showed the No. 3 reactor. A caption that accompanied the photo in an earlier version of this article incorrectly identified it as reactor No.1.

Steven Kraft, a nuclear-industry engineer, participated in a face-to-face briefing with Tokyo Electric Power Co. officials on Wednesday. An earlier version of this article about the accident at the company's Fukushima Daiichi nuclear plant incorrectly described it as a phone briefing.

and from the Australian

Fukushima nuclear catastrophe closer than thought

the Australian,

 2 December, 2011

MOLTEN nuclear fuel in one reactor at Japan's stricken Fukushima Daiichi plant burned through the steel pressure vessel and three-quarters of the surrounding concrete containment vessel that formed the reactor's last substantial internal barrier.

The revelation of the near "China Syndrome" meltdown is yet another revision of the severity of the disaster following the March 11 earthquake and tsunami.

Had the fuel, which would have reached more than 2000C, burnt through the containment vessel it would have escaped into the basement of the reactor, where it would have been far more difficult to contain. The plant's operator, TEPCO, and the Japanese government yesterday released the results of simulations that showed for the first time how close the No 1 reactor's defences came to being breached.

TEPCO said the damage in reactors No 2 and No 3 was less severe than in the No 1 reactor, although it believed a small amount of fuel did burn through the stainless steel and on to the concrete base of the secondary containment vessels.

Officials say the fuel in all three units has now cooled to below the critical temperature of 100C and poses no further threat.

"The fuel is now being kept safely cooled at all three reactors," a government spokesman said at a briefing where the results were announced.

In the case of the No 1 reactor, any fuel that escaped the secondary concrete vessel would have landed on a protective steel plate. Had it burned through that, it would then have confronted a 10m thick steel foundation beyond which lays ordinary soil.

The TEPCO and government analyses - which may not be the final word on the extent of the meltdown - showed that the fuel burned through a total of about 2m of the 2.6m thick secondary containment vessel.

The operator and the government agencies in charge of regulating the nuclear industry have consistently underestimated the severity of events at the plant.

The meltdowns at Fukushima Daiichi occurred in the days after the deadly tsunami cut power to the plant.

Subsequent explosions amid the venting of gas radiation spewed out of the plant and into the surrounding air, sea and soil.

More than 80,000 people have been forced out their homes as a result of the disaster, which ranks second only to the 1986 Chernobyl incident in terms of severity. Japan has also been forced to endure a run of food scares thanks to radioactive cesium deposited across wide areas around the plant.

The Japanese government has conceded that it may take 30 years to fully decommission the plant and that some areas around it may be uninhabitable for decades.

Keiji Miyazaki, a professor emeritus of nuclear engineering at Osaka University, told The Wall Street Journal that questions now had to be posed about why it took so long to come up with a way to cool this reactor.

"There has to be a long period of time without any (cooling) water being injected into the reactor for the fuel to melt through the concrete bottom," he said.

Reactor No 4 at the plant had its fuel removed for periodic maintenance while the No 5 and No 6 reactors were able to be shut down without major problems.

Fukushima guilty of world's worst sea contamination

al-Jazeera