The warnings were stark and issued repeatedly as far back as 1972: If the cooling systems ever failed at a Mark 1 nuclear reactor, the primary containment vessel surrounding the reactor would probably burst as the fuel rods inside overheated. Dangerous radiation would spew into the environment.
Now, with one Mark 1 containment vessel damaged at the embattled Fukushima Daiichi nuclear plant and other vessels there under severe strain, the weaknesses of the design — developed in the 1960s by General Electric — could be contributing to the unfolding catastrophe.
When the ability to cool a reactor is compromised, the containment vessel is the last line of defense. Typically made of steel and concrete, it is designed to prevent — for a time — melting fuel rods from spewing radiation into the environment if cooling efforts completely fail.
In some reactors, known as pressurized water reactors, the system is sealed inside a thick, steel-and-cement tomb. Most nuclear reactors around the world are of this type.
But the type of containment vessel and pressure suppression system used in the failing reactors at Japan’s Fukushima Daiichi plant — and in 23 American reactors at 16 plants — is physically less robust, and it has long been thought to be more susceptible to failure in an emergency than competing designs.
G.E. began making the Mark 1 boiling water reactors in the 1960s, marketing them as cheaper and easier to build — in part because they used a comparatively smaller and less expensive containment structure.
American regulators began identifying weaknesses very early on.
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.
“What are the safety advantages of pressure suppression, apart from the cost saving?” Mr. Hanauer asked in the 1972 memo. (The regulatory functions of the Atomic Energy Commission were later transferred to the Nuclear Regulatory Commission.)
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.”
In an e-mail on Tuesday, David Lochbaum, director of the Nuclear Safety Program at the Union for Concerned Scientists, said those words seemed ironic now, given the potential global ripples on the nuclear industry from the Japanese accident.
“Not banning them might be the end of nuclear power,” said Mr. Lochbaum, a nuclear engineer who spent 17 years working in nuclear facilities, including three that used the G.E. design.
Questions about the G.E. reactor design escalated in the mid-1980s, when Harold Denton, an official with the N.R.C., asserted that Mark 1 reactors had a 90 percent probability of bursting should the fuel rods overheat and melt in an accident. A follow-up report from a study group convened by the commission concluded that “Mark 1 failure within the first few hours following core melt would appear rather likely.”
In an extreme accident, that analysis held, the containment could fail in as little as 40 minutes.
Industry officials disputed that assessment, saying the chance of failure was only about 10 percent.
Michael Tetuan, a spokesman for G.E.’s water and power division, staunchly defended the technology this week, calling it “the industry’s workhorse with a proven track record of safety and reliability for more than 40 years.”
Mr. Tetuan said there are currently 32 Mark 1 boiling water reactors operating safely around the globe. “There has never been a breach of a Mark 1 containment system,” he said.
