Twenty-four hours ago I asked, “How sure are we that the containment vessels at Fukushima can contain a full meltdown?” Twenty-four hours later, the answer appears to be: Not very.

In 1972, Stephen H. Hanauer, then a safety official with the Atomic Energy Commission, recommended in a memo that the sort of “pressure-suppression” system used in G.E.’s Mark 1 plants presented unacceptable safety risks and that it should be discontinued. Among his concerns were that the smaller containment design was more susceptible to explosion and rupture from a buildup in hydrogen — a situation that may have unfolded at the Fukushima Daiichi plant…

A written response came later that same year from Joseph Hendrie, who would later become chairman of the N.R.C. He called the idea of a ban on such systems “attractive” because alternative containment systems have the “notable advantage of brute simplicity in dealing with a primary blowdown.”

But he added that the technology had been so widely accepted by the industry and regulatory officials that “reversal of this hallowed policy, particularly at this time, could well be the end of nuclear power.”

The Mark 1’s containment vessel was smaller than other types of vessels, which made it cheaper and easier to build — but also, in theory, more prone to rupture in the event of a full meltdown. More from WaPo:

In 1986, a top official at the Nuclear Regulatory Commission raised concerns about the GE containment system’s design.

“I don’t have the same warm feeling about GE containment that I do about the larger dry containments,’’ said Harold Denton, director of NRC’s Office of Nuclear Reactor Regulation during an industry conference, according to a report at the time by the publication, Inside N.R.C. “There has been a lot of work done on those containments, but Mark I containments . . . you’ll find something like a 90 percent probability of that containment failing.’’

Not only are these Mark 1 vessels in service at Fukushima, but they’re 40 years old — so old, in fact, that the reactors were going to be retired anyway later this year.

It wasn’t just industry officials who questioned the Mark 1, either. It was some of G.E.’s own technicians:

Thirty-five years ago, Dale G. Bridenbaugh and two of his colleagues at General Electric resigned from their jobs after becoming increasingly convinced that the nuclear reactor design they were reviewing — the Mark 1 — was so flawed it could lead to a devastating accident…

“The problems we identified in 1975 were that, in doing the design of the containment, they did not take into account the dynamic loads that could be experienced with a loss of coolant,” Bridenbaugh told ABC News in an interview. “The impact loads the containment would receive by this very rapid release of energy could tear the containment apart and create an uncontrolled release.”

But wait. There’s been no confirmed containment rupture (yet) at Fukushima, notwithstanding last night’s massive but temporary spike in radiation near the plant. All six containment vessels at the plant withstood a 9.0 earthquake and dozens of significant tremors; three of them have also withstood hydrogen explosions in the buildings they’re housed in, albeit with some damage to the suppression pool in reactor number two. And according to G.E., there’s never been a breach — ever — of any Mark 1 containment vessel since they were first introduced. So what gives? More from ABC:

Bridenbaugh told ABC News that he believes the design flaws that prompted his resignation from G.E. were eventually addressed at the Fukushima Daiichi plant. Bridenbaugh said GE agreed to a series of retrofits at Mark 1 reactors around the globe. He compared the retooling to the bolstering of highway bridges in California to better withstand earthquakes.

“Like with seismic refitting, they went back and re-analyzed the loads the structures might receive and beefed up the ability of the containment to handle greater loads,” he said.

He thinks the Mark 1 may be “still a little more susceptible to an accident that would result in a loss of containment,” but those retrofits might have been the difference between a frightening-yet-smallish radiation leak and a sustained radioactive belch. Or maybe I’m wrong; maybe the heroic nuclear techs still on the scene at Fukushima, against all odds, have actually managed to prevent a significant meltdown in each of the reactors thus far, in which case the containment vessels haven’t — yet — been put to the ultimate test. The longer this goes on, though, and the more sea water is flushed to the core, the cooler it gets and the less steam is produced, which means the risk of a rupture due to pressure build-up should be declining by the hour. That’s assuming, of course, that they’re still able to get sea water in there. Are they? Quote:

A senior nuclear industry executive who insisted on anonymity said that a compromised suppression pool [in reactor number two] made it much harder to bleed high-pressure steam from an overheating nuclear reactor so as to pump more seawater into it. “How are you going to bleed into something that has got a big hole in it?” he said.

Mr. Friedlander was more optimistic, saying that the rupture in the primary containment building was much more likely to have occurred above the water line of the suppression pool than below it. “The likelihood is that it is still holding water,” and so can be used for some venting of vapor from the reactor, he said.

Believe it or not, reactor two isn’t even the biggest crisis they’re facing right now. Potentially, that’s the spent fuel rods that have started to boil in a rooftop storage pool that’s lost water. If those start to melt, you’ll get the same sort of radioactive release as you would from a containment vessel breach. A Japanese source quoted in that last Times link says they have just a single day to do something about it, or possibly two, and they’re so desperate that they may send helicopters to try to douse the rods from above. I wonder, though, if the rods have already started to melt: This ominous report from Reuters timestamped at 2:43 p.m. says Japanese officials admit that radiation is now being released directly into the air without saying precisely why. Presumably it’s due to the spent fuel rods and not a containment vessel rupture; either way, the leak is significant enough to drive radiation levels around the plant up to 400 millisievert per hour. For comparison purposes, cancer has been linked to levels of 100 millisievert per year. What that means for Tokyo, if those radiation levels don’t start to drop seen, I can’t begin to imagine.

Exit question: If there’s a meltdown and the Mark 1 breaches, what happens if the core ends up in the area’s water table?

Update: A bulletin from Reuters:

FLASH: Japan nuclear safety agency says Fukushima No. 4 reactor roof is cracked

By “reactor roof,” do they mean the containment vessel itself or merely the building in which number four is housed? Losing the building is no big deal. Losing the vessel would be … oh boy. Stand by for updates.

Update: What will happen to Tokyo?

Toxicologist Lee Tin-lap at the Chinese University of Hong Kong said such a radiation level was not an immediate threat to people but the long-term consequences were unknown.

“You are still breathing this into your lungs, and there is passive absorption in the skin, eyes and mouth and we really do not know what long-term impact that would have,” Lee told Reuters by telephone…

“The possibility of further radioactive leakage is heightening,” a grim-faced Kan said in an address to the nation earlier in the day.

Update: Reuters clarifies: It’s the reactor building whose roof is cracked, not the containment vessel. Whew.