Washington (CNN) -- The chairman of the Nuclear Regulatory Commission said Tuesday that the situation in the wake of the Japanese nuclear reactor crisis is static but not yet stable.
On the day that Japan bumped up the seriousness of its nuclear accident from a Level 5 to a Level 7 priority, on par with the Chernobyl disaster, a Senate committee heard from U.S. environmental officials, scientists and NRC Chairman Gregory Jaczko.
"From the information we have, we believe the situation currently is static, namely we don't see significant changes on a day-to-day basis with the reactors," Jaczko told the Senate Committee on Environment and Public Works. "It is not yet, however, what we believe to be stable: namely that given additional events or other circumstances ... there would not be the potential for significant additional problems at the reactors."
Thus, Jaczko said, the focus is "on these efforts to transition from static to stable to ensure (the) long-term ... ability to cool the reactors and to provide cooling for the spent fuel pools."
California Sen. Barbara Boxer's state has a number of nuclear plants in areas with high seismic activity near millions of people. She pressed Jaczko for reassurances that enough is being done to protect people in the United States, especially California residents.
"We've got to move beyond talk and get to the serious question (of) what do we do, to do everything in our power to make it safe," Boxer said.
Boxer pointed out that officials once said it was "very unlikely" Japan would ever face the kind of crisis it finds itself in today.
Questions were raised at the hearing about how far to extend evacuation zones in the United States in the event of a nuclear incident. The NRC has recommended that Americans in Japan stay at least 50 miles away from the damaged reactors. The current standard in the United States is a 10-mile evacuation zone.
"As we've seen in Japan, nuclear events tend to develop over a long period of time," said Jaczko. "This is three weeks into this event, and we've had the time and the ability to make protective action recommendations, and to update and modify them as conditions of the plant changed. So that 10 miles is really based around the idea of what do you need to have prepared right away so that if you have an event that develops quickly, you can address that and have prestaged and prepared what to do," he said.
Boxer pointed out that two of the nuclear plants in California were built to withstand a certain level of earthquake, pointing out in Japan it was 7.5 but the devastating quake was much more powerful. She said nuclear regulatory officials are being too conservative.
"It's eerie to me, because I don't sense enough humility from all of us here. You know, as some great scientists once said, we think we have all the answers, but Mother Nature may not agree with us. So a lot of what you're saying is the same thing that they said," Boxer pointed out. "You can't know for sure what's going to happen," she said.
Lisa Jackson, administrator for the Environmental Protection Agency, reassured senators that air and water sampling for radiation contamination continue in the United States and that so far there is nothing to worry about.
"Let me be clear, EPA has not seen and does not expect to see radiation in our air or water reaching harmful levels in the United States," said Jackson. "All of the data that we have seen, which we continue to make public and available on our website, indicates that while radiation levels are slightly elevated in some places, they are significantly below problematic levels."