Five of the six reactors at Fukushima No. 1 nuclear plant are so-called Mark 1 boiling water reactor (BWR) models, developed by General Electric in the 1960s and installed in Japan in the 1970s.

In the 1970s, criticism amplified that the Mark 1's concrete containment shield, which surrounds the reactor vessel, was vulnerable to explosion caused by a buildup of hydrogen gas if the reactor overheated.

The original design "did not take into account the dynamic loads that could be experienced with a a loss of coolant," Dale Bridenbaugh, who quit as a GE engineer in 1975 over the alleged problem, told ABC News on Wednesday.

Blasts attributed to hydrogen have occurred at four of the Fukushima units, and the containment vessels at the No. 2 and No. 3 reactors have reportedly been damaged but not apparently ruptured.

A partial meltdown of the fuel rods has occurred in the Nos. 1, 2 and 3 reactors but the information from Fukushima -- while sketchy -- indicates the steel shells surrounding the reactors themselves have not been breached, say French safety agencies.

Michael Tetuan, spokesman for GE Hitachi Nuclear Energy, the current GE unit developing and selling nuclear plant technology, said there were 32 Mark 1 installations in the world, in addition to 23 in the United States.

The US Nuclear Regulatory Commission (NRC) ordered US operators to retrofit Mark 1 plants in the 1980s to strengthen the containment vessel, he said.

"We shared that with our customers overseas ... but I can't tell you if they did indeed retrofit," he said.

"We understand that all of the BWR Mark 1 containment units at Fukushima Daiichi also addressed these issues and implemented modifications in accordance with Japanese regulatory requirements."

"Two potential safety concerns were identified for the GE Mark 1 design," Viktoria Mitlyng, a spokeswoman for the NRC, explained.

First was the ability of the torus -- a donut-shaped unit at the reactor base -- to withstand a high volume of steam that would be diverted to it during an accident.

The second issue was the unit's ability to vent the containment unit to prevent a build-up of hydrogen.

The NRC required the torus units to be reinforced, and hardened vents -- to safely vent any gas buildup -- were installed as well.

"These changes were implemented at all GE Mark 1 plants and inspected by the NRC," she said, referring to US plants.

Questions are also being asked about another aspect of the Mark 1 design, namely the location of cooling tanks which hold highly radioactive spent fuel rods.

They are placed outside the protection of the containment vessel.

These pools are now the source of intense anxiety in Fukushima, because pumps designed to circulate and top up the water that cools the immersed rods failed in the tsunami generated in the quake.

"At least two spent fuel pools at the Fukushima plant have caught fire and are releasing radiation into the atmosphere," said Edwin Lyman, a physicist and expert in nuclear plant design at the Union of Concerned Scientists (UCS), an NGO on nuclear safety.

"(...) The United States has 31 boiling-water reactors with similarly situated spent fuel pools that are far more densely packed than those at Fukushima and hence could pose far higher risks if damaged," Lyman said on Wednesday to the Senate Environment and Public Works Committee.

He called on US operators to withdraw some of the rods and place them in dry storage casks in order to reduce the heat load.

In Paris, Olivier Gupta, deputy director general of France's Institute for Radiological Protection and Nuclear Safety (IRSN), said the location of the fuel-rod pools outside the containment vessel was common to "many nuclear reactors, including in France."

"It's something that has been taken into account for new-generation reactors and will be modified," said Marie-Pierre Comets of another French watchdog agency, the Nuclear Safety Authority (ASN).

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