By Tom Doggett
WASHINGTON (Reuters) - The top U.S. nuclear regulator warned on Wednesday that one pool holding spent fuel at Japan's stricken nuclear plant may have run dry and a second could be leaking, something experts say could accelerate the release of radiation.
"We believe at this point that Unit Four may have lost a significant inventory, if not lost all, of its water," Gregory Jaczko, head of the U.S. Nuclear Regulatory Commission, told lawmakers at a House energy and commerce subcommittee hearing.
Officials in Japan have not said how much water remains in the pool.
Jaczko said there was the possibility of a crack in the spent fuel pool in reactor No.3, "which could lead to a loss of water in that pool".
The NRC chairman also said the spent fuel pool level in reactor No.2 "is decreasing."
While the NRC has 11 experts in Tokyo monitoring the situation, he added that the NRC's information "is limited".
"We've been very careful only to provide information that we believe is very reliable," he said.
Japan is struggling to contain a deepening crisis at the Fukushima plant, about 240 km (150 miles) from Tokyo, with the focus shifting this week to the potential for radiation to be released from the storage pools holding old fuel rods.
Officials have raced to pour water back into the pool at the No.4 reactor via helicopter or hose after a fire and blast on Tuesday.
Because the rods continue to release heat for months after removal from the reactors, they must be continually cooled. But the plant's cooling system and generators were incapacitated by the quake and tsunami.
The pools are dangerous for two reasons: being outside a containment wall protecting the nuclear core, they are more easily exposed to the atmosphere; and the building housing the No. 4 reactor's pool has suffered hydrogen gas explosions.
They also hold radioactive elements that could quickly heat up again if water burns off. Experts worry that this could expose the used nuclear fuel and start a fire that would release more radioactivity.
"There is no water in the spent fuel pool and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures," said Jaczko, making his first appearance before Congress since the crisis began.
Jaczko said the United States would not be hit by harmful radiation from the reactors and that the evacuation area around them was smaller than what the NRC would recommend.
He said radiation levels around the plant could give emergency workers "lethal doses" of radiation, forcing them to stay away.
"We believe that around the reactor site there are high levels of radiation," Jaczko said.
"It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time."
(Editing by Alden Bentley and Dale Hudson)