Both Thrall and Hall said that any radiation release that would bring that up to about 6 mSv would be cause for alarm and would trigger actions to mitigate the health effects, including the use of potassium iodide tablets to address iodine-131 exposure and ferric hexacyanoferrate(II) (Radiogardase) capsules to mitigate cesium-137 exposure.
But both called that scenario highly unlikely.
Kirby Kemper, a physicist at Florida State University, told MedPage Today and ABC News in an e-mail that the health risk attributable to low levels of radiation remains controversial.
But, he noted by way of comparison, "people in Denver ... have about twice the background radiation level compared to people in Florida due to cosmic rays, but actually have a longer life span than people from Florida."
Dr. Jerrold Bushberg, a radiation oncologist at the University of California Davis, said that even if there is a risk from radiation drifting over from Japan, it will be trivial relative to the overall risk of cancer.
In an e-mail to MedPage Today and ABC News, Bushberg said that an extra dose of radiation of 40 times the normal background radiation from cosmic rays and geologic sources still would be barely detectable.
"The increased cancer risk from that exposure would be less than 1 percent," he said.
According to the NRC -- the U.S. government's atomic energy watchdog -- nuclear plants must be designed to withstand "the most severe natural phenomena historically reported for the site and surrounding area. The NRC then adds a margin for error to account for the historical data's limited accuracy."
However, earthquake resistance specifications for two plants on California's coast are lower than some historic quakes in the state, and much less than the magnitude-9.0 temblor that struck Japan last week.
The Diablo Canyon station north of Santa Barbara is built to withstand a magnitude-7.5 quake, and a plant at San Onofre, north of San Diego, can tolerate a 7.0 magnitude shock.
Three earthquakes of magnitude 7.5 or more have been recorded near these facilities in the past 200 years, according to the Southern California Earthquake Data Center at Caltech.
Nevertheless, Marvin Fertel, president and CEO of the Nuclear Energy Institute declared on the television program Meet the Press on Sunday that these and all other U.S. plants are "designed to withstand the maximum credible earthquake."
He added, "We've done things post-9/11 to make sure ... if you lost all power you could get water to the core and continue to cool it."
What appears to have been the critical factor leading to the Japanese reactors' problems was the tsunami.
The onrush of seawater knocked out the onsite generators that provided the power to water pumps for cooling the immensely hot reactors. Engineers have since been struggling to bring in and install replacement generators to resupply the reactors with cooling water.
Although both Diablo Canyon and San Onofre are on the Pacific coast, where they might be vulnerable to tsunamis, they are sited on headlands about 120 and 80 feet above sea level, respectively.
The tsunami that washed ashore in Japan last week was estimated at 20 to 30 feet.
On the other hand, some East Coast nuclear plants are virtually at sea level, such as Indian Point on the Hudson River north of New York City and Seabrook Station in New Hampshire.