Meltdown in Japan? The Experts Differ On Risk Of Containment Breech

By Steve Hynd

AP reports nothing but bad news for Japan's nuclear industry:

Water levels dropped precipitously Monday inside a stricken Japanese nuclear reactor, twice leaving the uranium fuel rods completely exposed and raising the threat of a meltdown, hours after a hydrogen explosion tore through the building housing a different reactor.

Water levels were restored after the first decrease but the rods remained exposed late Monday night after the second episode, increasing the risk of the spread of radiation and the potential for an eventual meltdown.

So, how possible is a meltdown? In the Wall Street Journal, William Tucker, a pro-nuclear advocate, says it isn't possible at all.

If the coolant continues to evaporate, the water level can fall below the level of the fuel rods, exposing them. This will cause a meltdown, meaning the fuel rods melt to the bottom of the steel pressure vessel.

Early speculation was that in a case like this the fuel might continue melting right through the steel and perhaps even through the concrete containment structure�the so-called China syndrome, where the fuel would melt all the way to China. But Three Mile Island proved this doesn't happen. The melted fuel rods simply aren't hot enough to melt steel or concrete.

The decay heat must still be absorbed, however, and as a last-ditch effort the emergency core cooling system can be activated to flood the entire containment structure with water. This will do considerable damage to the reactor but will prevent any further steam releases. The Japanese have now reportedly done this using seawater in at least two of the troubled reactors. These reactors will never be restarted.

None of this amounts to "another Chernobyl."

But over at Scientific American, they're pointing out the differences between Three Mile Island and the Japanese plants:

But one of the disadvantages is that the containment structure is a lightbulb-shaped steel shell that's only about 30 or 40 feet across�thick steel, but relatively small compared to large, dry containments like TMI. And it doesn't provide as much of an extra layer of defense from reactor accidents as containments like TMI. So there is a great deal of concern that, if the core does melt, the containment will not be able to survive. And if the containment doesn't survive, we have a worst-case situation."

And just what is that worst-case scenario? "They're venting in order to keep the containment vessel from failing. But if a core melts, it will slump to the bottom of the reactor vessel, probably melt through the reactor vessel onto the containment floor. It's likely to spread as a molten pool�like lava�to the edge of the steel shell, and melt through. That would result in a containment failure in a matter of less than a day. It's good that it's got a better containment system than Chernobyl, but it's not as strong as most of the reactors in this country."

Physicist Ken Bergeron, who did research on nuclear accident simulation for Sandia National Laboratory, adds:

"The containment, I believe, is still intact. But if the core does melt, that insult will probably not be sustained, and the containment vessel will fail. All this, if it were to occur, would take a matter of days. What's crucial is restoring AC power. They've got to get AC power back to the plant to be able to control it. And I'm sure they're working on it."

And finally, nuclear expert Joe Cirincione sets out what the worst-case scenario means:

"One reactor has had half the core exposed already," he explained. "This is the one they're flooding with sea water in a desperate effort to prevent it from a complete meltdown. They lost control of a second reactor next to it, a partial meltdown, and there is actually a third reactor at a related site 20-kilometers away they have also lost control over. We have never had a situation like this before."

"The worst case scenario is that the fuel rods fuse together, the temperatures get so hot that they melt together in a radioactive molten mass that bursts through the containment mechmisms and is exposed to the outside. So they spew radioactivity in the ground, into the air, into the water. Some of the radioactivity could carry in the atmosphere to the West Coast of the United States."

"Really?" a surprised Wallace asked. "I mean, thousands of miles across the Pacific?"

"Oh, abosolutely. Chernobyl, which happened about 25 years ago, the radioactivity spread around the entire northern hemisphere. It depends how many of these cores melt down and how successful they are on containing it once this disaster happens," Cirincione replied.

Ouch. Let's all hope it doesn't come to that.