Gaps in safety regulations have left U.S. nuclear power plants vulnerable to accidents when they lose power to their cooling systems, a prominent nuclear watchdog group told a congressional hearing Friday.

If U.S. nuclear power plants should lose primary and backup power — as happened in March at the Fukushima Daiichi facility in Japan — batteries are designed to kick in and prevent nuclear fuel from melting.

But some plants keep just four hours of battery power, which would force workers in a disaster to play “a very high-stakes version of ‘beat the clock,’ ” David Lochbaum of the Union of Concerned Scientists told members of the House Committee on Science, Space and Technology. “If they restore normal or backup power within a few hours, they win. If not, many may lose.”

Lochbaum recommended that plants extend battery capacity to 16 hours, giving workers more time to restore cooling power.

At Fukushima, a powerful earthquake knocked out primary power, and a subsequent tsunami wiped out backup diesel generators. On-site batteries depleted within eight hours, leaving workers with no power to cool the cores of three nuclear reactors. All three reactors suffered serious damage, and the facility continues to release radioactive debris into the air and water.

On Thursday, Fukushima’s operator, Tokyo Electric Power Co., announced that the Unit 1 reactor suffered worse damage than the company had previously acknowledged. New data show that the reactor’s uranium fuel melted, slumped to the bottom of the primary containment vessel and burned a hole that released radioactive water, a spokesperson said at a news conference in Tokyo.

At the congressional hearing, Brian Sheron of the Nuclear Regulatory Commission said that regulations require U.S. nuclear plants to maintain two backup diesel generators for each reactor. Last month, such generators worked as designed when severe storms knocked out primary power at the Brown’s Ferry nuclear plant in Alabama.

A contentious, decades-long debate over how and where to store the nation’s used nuclear fuel also drew attention at the hearing, as Lochbaum pointed out that U.S. facilities are continually adding spent uranium fuel rods to on-site water pools.

Such “re-racking” pushes the rods closer together, increasing the risk of an accident, Lochbaum said. “This practice . . . exposes millions of Americans to elevated and undue risk.”

At Fukushima, water drained from at least one such pool and the fuel heated up, releasing intense radiation that hampered work at the site. The exposed fuel also might have triggered the explosion that seriously damaged the building housing the Unit 4 reactor.

At the hearing, Lochbaum recommended that U.S. plant operators transfer fuel from water-cooled pools to safer, passively air-cooled storage units called dry casks. A dry cask storage unit at Fukushima, which contains more used fuel than the water pools, has remained safe and has released no radiation despite the disaster, Lochbaum added.

Sheron, of the NRC, responded by saying that at U.S. facilities, “it would take a very long time to drain the pool and uncover the fuel. That gives [workers] ample time to bring in equipment or restore cooling.”

Debate over handling of used nuclear fuel flared in 2009, when President Obama halted construction of the long-planned nuclear waste depository at Nevada’s Yucca Mountain. On Tuesday, a Government Accountability Office report concluded that the decision was made “for policy reasons, not technical or safety reasons.” Senate Majority Leader Harry Reid (D-Nev.) has long opposed the facility.

The decision to shutter Yucca Mountain leaves the country with no plan for permanently storing its growing cache of used radioactive fuel.

A “blue ribbon” Energy Department commission established by Obama in January 2010 is studying how the country should manage its nuclear waste. In draft recommendations unveiled Friday, commission members said that the country should build an above-ground dry cask facility for “interim” nuclear waste storage.

Such a facility would consolidate nuclear waste and relieve pressure to pack more fuel into the pools at each of the nation’s 104 nuclear plants. Ultimately, though, the country still needs a deep mountain storage facility for permanent disposal, commission members said.