Japan now assumes 'possibility of a meltdown' at troubled reactors

Japanese workers raced against the clock to pump seawater into two damaged nuclear reactors. It’s a last-ditch effort to cool them enough to avert the kind of core meltdowns that happened at Chernobyl and Three Mile Island.

Japanese workers and nuclear experts raced against the clock to pump seawater into two damaged nuclear reactors. It’s a last-ditch effort to cool them enough to avert Chernobyl- or Three Mile Island-like core meltdowns even as government officials admitted partial meltdowns are already assumed to have occurred in both units.

Japanese authorities now list six reactors at two different nuclear power plants – Fukushima I and nearby Fukushima II – in a state of emergency following the massive earthquake and tsunami waves that hit Japan Friday. A total of 11 of the nation’s 54 reactors shut down following the quake, knocking out about 30 percent of Japan’s power.

At a Sunday morning press briefing, Chief Cabinet Secretary Yukio Edano said experts were "assuming the possibility of a meltdown" at the No. 3 reactor at the Fukushima I plant, about 150 miles north of Tokyo, as well as at its No. 1 reactor.

In the case of the No. 3 unit, though, Mr. Edano said the decision had been made far sooner to inject seawater directly into the No. 3 reactor than it had on the No. 1 reactor – a previously unprecedented step that some US experts described as a desperate measure. Despite that, coolant levels at the No. 3 reactor dropped so that part of the fuel rods became exposed, Edano said.

"Unlike the No. 1 reactor, we ventilated and injected water at an early stage," Edano said, according to wire reports of the press conference. Asked if fuel rods were partially melting in the No. 1 reactor, he acknowledged "there is that possibility.”

“We cannot confirm this because it is in the reactor,” he continued. “But we are dealing with it under that assumption. We are also dealing with the No. 3 reactor based on the assumption that it is a possibility."

A nuclear reactor core meltdown occurs when fuel rods in the reactor’s core overheat and begin to melt. The rods are filled with uranium oxide ceramic pellets wrapped in zirconium cladding. It is possible for that molten material to get so hot that it could melt through the primary steel confinement shell – and then, even through the floor of the reactor building, US experts say.

More people are being evacuated

Underscoring the seriousness of the situation, the Fukushima prefecture government Sunday widened its mandatory evacuation zone to 20 kilometers (12.4 miles) around both nuclear plants – ordering an additional 80,000 people to leave, in addition to the 62,000 residents who had already left, Japanese press reports said. Up to 450,000 residents could be evacuated overall, Kyodo News reported.

Although all three of the Fukushima I’s then-operating reactors shut down successfully – with control rods inserted into the fuel core after the quake – it takes at least 24 to 48 hours for them to cool under the best of conditions, with proper circulation of cooling water. But diesel emergency generators were knocked out within an hour after the quake, apparently by the tsunami that followed. Another battery-powered cooling system backup soon lost power.

With few alternatives, Tokyo Electric Power Company, which owns the plant, began pumping seawater into the No. 1 reactor on Saturday. Even so, an enormous explosion that morning destroyed most of a secondary containment building housing the No. 1 reactor, but apparently left intact a critical steel shell – the reactor’s primary containment. Efforts to cool that unit with seawater continued.

At the Sunday press conference, however, Edano warned that a further, similar hydrogen-based explosion could also occur at the No. 3 reactor at Fukushima I, since the fuel rods had been briefly exposed, raising the risk of meltdown and explosion. Even so, he said, if there was a hydrogen-based explosion like the one Saturday, he did not expect it to carry with it significant radioactive debris from the fuel core.

"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said, according to the Associated Press. "If there is an explosion, however, there would be no significant impact on human health."

Meanwhile, radiation levels well in excess of government standards – including cesium 137 – were reported around the Fukushima I plant with its three troubled reactors, corroborating the view that partial meltdowns had occurred, government officials said.

Tokyo Electric Power Corporation reported radiation levels near the front gate of the plant gradually went down after peaking on Saturday afternoon. But those levels rose again nearly as high Sunday morning, Japan Broadcasting Corporation’s NHK World website reported – about double the levels required to be reported to the government.

Radiation screening centers

Radiation screening centers have been set up outside the evacuation zones. So far, 22 people are confirmed to have been exposed to radiation after the hydrogen explosion at the No. 1 reactor building, NHK World website reported. As many as 160 may have been exposed overall, Japanese officials said.

US experts said Japanese efforts to cool the reactors had clearly had at least some effect – although it’s not yet clear if they will succeed in averting a complete meltdown.

“If there had been no attempt to cool these reactors, the cores would have melted by now,” said Edwin Lyman, a nuclear physicist with the Union of Concerned Scientists. “The fact that they have immersed the whole thing in saltwater means that, if it ruptures, it might limit somewhat the amount of radioactive material going into the atmosphere.”

Water leak could release radioactivity

But Lyman and others also worry about the spent-fuel pools near these reactors that contain tons of expended yet very hot fuel. If the water leaks from the pools, that could also lead to a major release of radioactivity, he says.

“The spent-fuel pools are not as big a threat as the core,” he says. “But they are still a significant and dangerous part of the equation.”

As Japan struggled to deal with the nuclear crisis, US experts said that the type of reactor involved was less likely to release as much radioactive materials as the very different Chernobyl-type of plant. Yet, any major releases of radioactivity could be especially dangerous because the reactors are so close to masses of people.

Other lessons will be learned from this crisis – including why all the backup systems failed, says Ken Bergeron, a physicist who worked for Sandia National Laboratories. He described what happened at Fukushima I as a “station blackout” – the loss of both off-site electricity and on-site backup power from diesel generators and battery backups.

“We're in uncharted territory,” he said in a Saturday press teleconference organized by US anti-nuclear activists. “We're in the land where probability says we shouldn't be. And we're hoping that all of the barriers to release of radioactivity will not fail.”