Early fears that a pool housing spent nuclear fuel at Unit 4 of Japan's crippled Fukushima Daiichi plant went dry and triggered a large radioactive release were unfounded, the Nuclear Regulatory Commission said today.

Video footage of the Japanese plant that was rocked by an earthquake and tsunami on March 11, as well as water samples from the pool, shows it is unlikely that the pool ever went dry, NRC Director of Operations Bill Borchardt said during a commission meeting today.

"Staff welcomes this as very good news and just one indication the event may not have been as serious as previously believed for Unit 4," he said.

Japanese officials believe that fuel at all three reactors at the site has breached its reactor vessels -- one of the first lines of defense -- but no concrete information is available on the extent of that damage, Borchardt said.

The Japanese crisis prompted NRC Chairman Gregory Jaczko to recommend that Americans surrounding the Fukushima plant be evacuated within a radius of up to 50 miles, and NRC staff said there is no reason to think that recommendation was inappropriate with the change in information.

The accident also prompted NRC to launch a short- and long-term safety review of all 104 U.S. plants. The agency task force undertaking the reviews said today it is reconsidering major safety assumptions that underlie the commission's current regulatory requirements.

Most pointedly, the task force -- which is slated to release its first set of findings next month -- may require operators to have more extensive backup power if on-site and off-site power is lost. It is also reconsidering the standards that older reactors have been required to meet and whether or not their procedures reflect new safety information.