

By Scott DiSavino – Fri Mar 11, 1:10 pm ET

NEW YORK (Reuters) – The massive earthquake that forced the closure of four nuclear power plants in Japan has highlighted the grave risk of inadequate back-up generators at U.S. facilities, a leading U.S. scientist group said on Friday.

While the U.S. regulator made clear that the national nuclear fleet is built to withstand the biggest earthquakes in history, scientists said they needed to do more to ensure that future quakes don't risk the kind of reactor impact that Japan is now grappling with.

"We do not believe the safety standards for U.S. nuclear reactors are enough to protect the public today," Edwin Lyman, senior scientist, global security programs, at the Union of Concerned Scientists, told Reuters. The group supports nuclear power as a means to combat global warming, but wants tougher safety measures.

The magnitude 8.9 earthquake that rocked Japan on Friday knocked out power to the backup cooling systems of a reactor in Fukushima prefecture, north of Tokyo, forcing the evacuation of thousands of residents.

Operator Tokyo Electric Power Co said rising pressure inside the No.1 reactor had elevated the risk of a radiation leak, Jiji News reported on Saturday, although officials said earlier there had not been a leak.

Lyman said U.S. reactors also do not have enough backup power to ensure a safe shutdown during an emergency. If reactors could lose both off-site power and backup generators it could lead to a core meltdown in a short period of time.

Nuclear plants need power to keep water flowing over the fuel rods to prevent overheating.

BUILT TO STAND

But news of widespread shutdowns across the nuclear sector in Japan raised questions about how the United States' 104 reactors would respond in the event of a similar quake, one of the five biggest of the past century.

"There have been tremblers felt at U.S. plants over the past several years, but nothing approaching the need for emergency action," Scott Burnell, a spokesman at the Nuclear Regulatory Commission told Reuters.

Before any reactor is built in the United States, owners are required to conduct geologic seismic studies to determine the biggest earthquake to have occurred in that area going back thousands of years. As in Japan, U.S. reactors are designed to safely shut in the event of an earthquake.

If a bigger earthquake were to occur, Burnell said the plant safety systems would continue to provide the level of safety needed to shut the plant but there would likely be some degradation, though not more than the plant was designed for.

Two U.S. nuclear plants along the California coast made preparations for a potential Pacific Ocean tsunami on Friday, but continued to operate normally.

The reactors -- built by companies including PG&E Corp and Edison International -- are designed to safely shut in the event of an earthquake that big, Burnell said.

There are multiple and redundant safety systems at a nuclear plant used to shut the reactor and prevent the release of radiation during an accident.

These systems include an air tight steel or reinforced concrete containment building with walls between 4 to 8 feet thick that is strong enough to withstand the impact of a fully loaded passenger airliner without rupture, and a reactor vessel containing the uranium fuel rods that is made of high tensile steel four to eight inches thick.

The two biggest nuclear operators in the United States are Exelon Corp and Entergy Corp.

(Reporting by Scott DiSavino; Editing by David Gregorio)

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