Reactor Core Melted Fully, Japan Says

Reactors Monitor - (Click on image)

by Mitsuru Obe and Tom Fowler - The Wall Street Journal November 30, 2011

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

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.

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 phone 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.

—Phred Dvorak in Tokyo, David Crawford in Berlin and Deborah Solomon in Washington contributed to this article.

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