* Spent fuel pools like Japan's are contributing factor
* Nuclear plant support systems need power in disasters
* External risks should be considered, nuclear experts say
WASHINGTON, March 21 (Reuters) - U.S. nuclear plants that share design features with Japan's stricken Fukushima plant would be vulnerable if a comparably sweeping disaster hit the United States, nuclear experts said on Monday.
The design of the reactors, with spent fuel pools at the top of the building that contained them, is only one factor adding to the risk, said David Lochbaum, a nuclear engineer at the Union of Concerned Scientists.
"The arrangement with the spent fuel pool at upper elevations of the reactor building was a contributing factor, but the larger factors were the spent fuel cooling system was not designed to withstand earthquakes," Lochbaum said at a telephone briefing.
U.S. nuclear regulators said on Monday they were launching additional inspections and considering a 90-day review of the country's 104 nuclear reactors in the wake of Japan's nuclear crisis that followed a powerful March 11 earthquake and tsunami. [ID:nN21116372] Atomic energy supplies about 20 percent of U.S. electricity.
Even after nuclear fuel can no longer reliably produce electrical power, it is still hot and must be cooled in water tanks called spent fuel pools.
While the reactors themselves are behind concrete walls 4-5 feet (1.2-1.5 metres) thick, the cooling pools at the top of the stack in this design are shielded by sheet metal siding, said Lochbaum, who worked at U.S. nuclear plants for 17 years, including three that are similar to the plants in Japan.
In addition, many support systems at those plants are not designed to draw power from any source besides the electrical grid, he said.
"So when these earthquakes and tsunami (in Japan) took out the normal power and the backup power and caused a lot of damage that was not seismically supported at the plant, the pools were left with nothing that could cool the water," Lochbaum said.
Other systems left vulnerable could include the air system that powers inflatable seals around the gates of the pools. If those deflate, water around the spent fuel rods could leak out, he said.
There are 33 U.S. nuclear reactors with elevated spent fuel pools and most are in the East and Midwest, where the risk of seismic activity is estimated to be lower than in Japan.
But Lochbaum said, "U.S. plants are equally vulnerable to that type of scenario where you lose power to the cooling system."
This particular problem was identified as long ago as 1992 but has not been remedied in U.S. plants, he said. If a disaster similar to what is occurring in Japan hit parts of the United States with this kind of nuclear plant, Lochbaum said, "We're likely to reach the same destination."
New designs for future U.S. nuclear power plants would offer little safety advantage over the current ones, said Edwin Lyman, a physicist at the Union of Concerned Scientists and an expert on nuclear plant design and the effects of radiation.
Lyman said the Nuclear Regulatory Commission did not require any plant to calculate seismic risk in its overall risk assessments. "So when you hear a vendor talk about how the risk of core damage at their plant is one in 10 million, that does not include seismic risk or any other external risk."
He also took aim at NRC officials for reassuring Congress and the public that nuclear plants are prepared for a Japan-scale event because of safety measures put in place after the Sept. 11, 2001, attacks.
Bill Borchardt, executive director of operations at the NRC, said on Monday, "We identified important pieces of equipment that ... regardless of the cause of a significant fire or explosion at a plant we would have pre-staged equipment, procedures and policies to help deal with that situation.
"All of these things are directly applicable to the kinds of very significant events that are taking place in Japan," Borchardt said.
Lyman said those plans were considered "security-related information," and were unavailable to the public.
"The public isn't able to evaluate whether these plans make sense and whether in fact they differ significantly from the kind of response that we've seen in Japan," he said. (Additional reporting by Ayesha Rascoe; Editing by Peter Cooney)