The study comes almost a year after the disaster in Japan in March when an earthquake and tsunami damaged the Fukushima Daiichi nuclear power plant, causing reactor fuel meltdowns and radiation releases.

"Successful implementation of existing mitigation measures can prevent reactor core damage or delay or reduce offsite releases of radioactive material," the U.S. Nuclear Regulatory Commission said in the study.

"As a result, the calculated risks of public health consequences from severe accidents modeled in the severe accident study) are very small," the NRC said.

The study found there was "essentially zero risk" to the public of early fatalities due to radiation exposure following a severe accident. The long-term risk of dying from cancer due to radiation exposure after an accident was less than one in a billion and less than the U.S. average risk of dying from other causes of cancer, which is about two in one thousand.

The study, called the State-of-the-Art Reactor Consequence Analyses (SOARCA), looked at the radiological health consequences for potential severe accidents at Exelon Corp's Peach Bottom nuclear plant in Pennsylvania and Dominion's Surry nuclear plant in Virginia.

The NRC said it picked those two sites because Peach Bottom is representative of the nation's General Electric boiling water reactor designs and Surry is representative of the Westinghouse pressurized water reactor designs.

About a third of the 104 operating nuclear units in the United States are boiling water reactors and about two-thirds are pressurized water reactors. Nuclear power provides about 20 percent of the nation's electricity.

As part of its study, the NRC looked at the possibility of a station blackout, or loss of power, caused by an earthquake or other seismic activity. The loss of power at Fukushima prevented operators from pumping water around the reactor fuel, which allowed the uranium rods to overheat, meltdown and release radiation.

On Tuesday, the NRC told nuclear plant operators in the central and eastern United States to start using a new seismic model to reassess the potential for earthquakes in their area.

In August, a magnitude 5.8 earthquake shook two reactors at the North Anna nuclear power plant in Virginia, shutting the facility for about three months. That earthquake caused only minor damage to the plant and did not lead to the release of radiation.

WORST CASE SCENARIO

In the worst case scenario with an earthquake causing a station blackout, the study found that core damage could begin in 1 to 3 hours and reactor vessel failure could begin in about 8 hours. The NRC said that was possibly enough time to allow a plant operator to restore core cooling and prevent vessel failure.

If core cooling is not restored, the NRC said containment failure and radiological release could begin at about 8 hours for Peach Bottom and at 25 hours for Surry.

The NRC noted that the availability of portable diesel driven pumps to move water through the reactor core to cool the fuel would be "especially helpful" in mitigating the consequences of a severe accident.

The NRC said this study, which has been in the works since at least 2007, updates decades of research by the NRC, the nuclear power industry and the international nuclear energy research community on severe reactor accident phenomena, including the industry upgrades since the terrorist attacks of September 11, 2001.

The NRC noted all of the accident scenarios in the latest SOARCA study progress more slowly and release much less radioactive material than a previous severe accident study in 1982.

The NRC said the SOARCA study was nearing completion when the Fukushima accident occurred. Fukushima had many similarities with some of the Peach Bottom severe accident scenarios, the NRC said.

Some of the nation's biggest nuclear power operators include units of Exelon, Entergy Corp, Duke Energy, Progress Energy and Dominion.

(Reporting By Scott DiSavino; Editing by Bob Burgdorfer)

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