The Nuclear Regulatory Commission moved closer yesterday to imposing new safety standards on U.S. reactors in the wake of Japan's nuclear crisis.

Four NRC commissioners voted on top-tier safety orders to protect reactors from earthquakes and other natural disasters, ensure spent-fuel pools have appropriate equipment, and ensure the reliability of reactor vents.

NRC Chairman Gregory Jaczko and Democratic Commissioner William Magwood voted to approve the orders, while Republican William Ostendorff and Democrat George Apostolakis favored but also asked for revisions. Republican Kristine Svinicki has not posted her decision on the NRC website.

The orders stem from the March 11, 2011, earthquake and tsunami in Japan that crippled the Fukushima Daiichi nuclear complex, triggering core meltdowns at three reactors, hydrogen explosions and radioactive leaks.

If NRC approves the orders, nuclear companies will have until the end of 2016 to comply.

The commissioners must work to reach an agreement in coming weeks on how to implement the safety proposals. A final vote will be held once their views align. NRC staff have said they hope to release technical plans for the rules by August.

In his vote, Jaczko again pushed for all safety upgrades to be made within five years and urged the agency to hold licensees accountable for making changes and explain how the safety upgrades will be made.

"While the Fukushima accident occurred in another country and under a different regulatory structure, I hope we recognize the importance of applying lessons we learn from this nuclear accident and make necessary safety improvements in the United States," he wrote.

Jaczko also said he was concerned about time frames proposed by NRC staff that would give plant operators until 2015 to reanalyze their sites' vulnerability to earthquakes. Operators would then have until 2019 to identify any additional risks to their plants, Jaczko said.

The chairman questioned that timeline because the latest seismic data indicate that plants in the central and Eastern United States could be exposed to seismic hazards that exceed the facilities' limits.

"I understand the complexity of these seismic analyses and the limited seismic expertise available to perform them, but I simply cannot accept a timeline that puts this issue well into the later part of the decade -- without even addressing implementation of any required action," Jaczko wrote. "A fundamental lesson from Fukushima was that there was evidence the design-basis protection may not bound the most severe events predicted for the site, and action wasn't taken to address that gap."

But Magwood said the agency should consider a broad range of hazards that could cripple a plant and not "implement a laserlike focus on seismic." He also pointed out that the tsunami -- not the earthquake -- caused the bulk of damage at the Fukushima plant.

Magwood said the agency should provide operators flexibility in reassessing seismic risk and said nuclear plants in the southeastern United States should "perhaps be more concerned with extremely high temperature events than either earthquakes or floods."