The United States is headed toward a major nuclear disaster -- one that could mirror what happened last year in Japan -- unless the government more closely monitors aging power plants, safety advocates and activists said.

"This is like the Titanic that is headed toward an iceberg," said Paul Gunter, the co-founder of the Clamshell Alliance anti-nuclear group and a longtime activist.

Gunter's concern centers on the 23 "Mark I" nuclear reactors in the United States, which are identical to the containment vessels used at Fukushima's Daiichi nuclear power plant, where three reactors failed and went into meltdown in 2011.

With more frequent extreme weather events like Hurricane Sandy predicted, whether the facilities, which dot the landscape from New Jersey to Nebraska, could withstand a disaster as forceful as the tsunami in Japan remains unclear.

"Our facilities are responding extremely well," said Steve Kerekes, spokesman of the Nuclear Energy Institute, a policy organization of the nuclear energy said after Hurricane Sandy hit the U.S. East Coast.

He expressed no concerns over the weather challenges faced by aging reactors., saying records showed the facilities in the United States have operated safely in extreme weather.

But some experts said they fear that a nuclear disaster is unfolding in the country.

The small containment leaves Mark I reactor -- the earliest model designed by General Electric Co. -- not as robust as later designs, said Margaret Harding, an independent nuclear consultant who worked for General Electric for 27 years.

Gunter said he worries that the parallels to Japan are too similar to ignore, noting the strain the reactors could be under when there are severe weather conditions.

Tuesday's historic hurricane brought the issue into sharp focus as Oyster Creek nuclear station -- one of the oldest nuclear plants adjacent to the Oyster Creek in New Jersey using Mark I boiling water reactor -- declared an alert because of high water levels. The plant experienced power disruption but backup diesel fuel was able to provide power for cooling.

But experts said if future events become more severe, under-designed protections might fail.

If operators aren't able to connect temporary equipment in flooding, "there's another nuclear disaster," said Dave Lochbaum, director of the nuclear safety program of Union of Concerned Scientists, a nuclear oversight group.

Earlier this month, an un-redacted version of a recently released Nuclear Regulatory Commission report made its way to Greenpeace, the environmental group that couples the Mark 1 safety issues with a concern about geographic adjacencies. The report highlights the threat the NRC sees to power plants close to waterways, especially large dams.

All 23 Mark I reactors are adjacent to waters, some with major cities that have populations in the millions in the potential radioactive contamination zone. Gunter said waves generated by severe weather or a succession of dam breaks could be higher than the water wall caused by the tsunami and lead to a similar station blackout in Japan.

An earthquake April 11, 2011, created a tsunami that struck the Fukushima nuclear power plant and caused a loss of all power at the facility, imperiling the plant's capability to cool down the overheated core.

"Fukushima demonstrated that you can't be without power for a long time," said Gunter, who is also director of Beyond Nuclear, a non-profit in Takoma Park, Md. "But when these [backup] diesel generators are damaged, or the fuels are contaminated, there is very short time before the core damage occurs. It relies on a robust containment, which Mark I doesn't have."

Nuclear Regulatory Commission Chairwoman Allison Macfarlane said last week at a nuclear safety presentation that the United States is working to better prepare for station blackout and other events triggered by extreme weather.

John Lee, nuclear engineering professor at University of Michigan, said that despite the vulnerable containment, the safety measures in place would make sure Mark I reactors in the United States can survive natural disasters similar to the Fukushima tsunami.

However, a report conducted by Union of Concerned Scientists said the Nuclear Regulatory Commission ignores weaknesses in protection regulations. It allows 27 reactors to operate facing earthquakes larger than they are designed to withstand, 47 reactors violating fire protection regulations, including one Mark I plant.

"As long as luck prevents those vulnerabilities from being challenged, it's fine," Lochbaum said. "But if luck runs out, those pre-existing conditions can mean disaster."