Since General Electric supplied the design four decades ago for all six nuclear reactors at the Fukushima Daiichi power plant in northeastern Japan, some regulators and critics have questioned whether the system — which was supposed to be smaller and less expensive than others — can withstand a nightmare scenario.

Their concerns focused on the reactor’s containment system that is the final line of defense against a wide release of radiation. Now GE’s technology is facing the ultimate test: Can the structure enclosing the reactor keep the hot, radioactive stew bottled up inside? And can the spent fuel pools withstand a combination of explosions and equipment failure?

There is no sign so far that GE’s design is to blame for any of the plant’s problems, which mainly have been the result of power failures after the massive earthquake and tsunami that slammed the area Friday. The containment structure is holding up.

Some experts said that if the situation deteriorates at the nuclear plant, GE’s design — known as the Boiling Water Reactor Mark 1 — may not withstand the massive amount of hydrogen gas that could be released.

“We’re not at that point yet,” said Paul Gunter, director of the Reactor Oversight Project at Beyond Nuclear. “But these vessels are brittle. They were going to retire Fukushima Daiichi in just a few more months, and so this particular Mark 1 with its substandard design was reaching its endlife, and so it raises a lot of concerns.”

GE defended its technology on Monday while it offered engineers to help Japanese officials contain the crisis.

“The BWR Mark 1 reactor is the industry’s workhorse with a proven track record of safety and reliability for more than 40 years,” GE said in a statement. “Today, there are 32 BWR Mark 1 reactors operating as designed worldwide. There has never been a breach of a Mark 1 containment system.”

GE’s design is unique. The company specializes in boiling water reactor systems — in contrast to the pressurized water technology produced by rivals such as Westinghouse and Ariva. Two-thirds of the nuclear plants in the United States rely on pressurized water technology, according to Edward Blandford, a researcher who focuses on nuclear reactor design at Stanford’s Center for International Security and Cooperation.

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.’’

“There is a wide spectrum of ability to cope with severe accidents at GE plants,’’ Denton said. “And I urge you to think seriously about the ability to cope with such an event if it occurred at your plant.’’

GE’s Mark 1 containment system was designed to withstand discrete problems known as design basis accidents, such as a broken pipe releasing hot steam, said Ken Bergeron, a physicist and former scientist with the Sandia National Laboratories, where he worked on nuclear reactor accident simulation.

“Unfortunately, some of the shortcuts that were taken to accommodate the design basis accident at a fairly low cost results in containment that does not do well for severe accidents,” Bergeron said.

There are also fresh concerns about the fuel rods, which are above the reactors. Since the Sept. 11 terrorist attacks, some nuclear experts have said that the rods should be closer to the ground to avoid exposure to terrorist attacks.

Officials are concerned that structural damage to the plant may have exposed some of the rods to the air, which would spread radiation.