The top U.S. nuclear official in Japan said Thursday that his team warned higher-ups that a spent fuel pool at a malfunctioning nuclear plant could be at risk of running dry, an issue that created a political controversy between U.S. officials and their Japanese counterparts.

U.S. Nuclear Regulatory Commission official Charles Casto was assigned to direct site operations for the U.S. government's response to the nuclear disaster at the Fukushima Dai-ichi nuclear plant shortly after the accident last March. The plant was struck by a massive earthquake and a tsunami on March 11 that disabled its emergency cooling systems, leading to meltdowns, explosions and radioactive releases.

Less than a week later, NRC Chairman Gregory Jaczko told U.S. lawmakers that all the water from a spent fuel pool was gone — a development that if true raised the risk that the used fuel could ignite and spread more radiation into the environment.

Japanese officials denied Jaczko's statement at the time. NRC officials have since acknowledged that recent evidence shows that the pool probably did not go dry.

Casto said his team was operating in what he likened to the fog of war. They had the greatest concerns about the spent fuel pool on the Unit 4 reactor, which had the hottest fuel. Without reliable information from plant sensors, his team believed a combination of circumstantial evidence showed that Unit 4 pool could be empty. That information included the massive damage observed at the plant, a spike in radiation readings near the building and water vapor plumes.

Casto said there were also concerns that one of the many aftershocks that followed the initial quake could trigger another tsunami, making conditions at the plant even worse.

"It was a possibility that they may be empty," Casto said, speaking about the pools during an interview in Atlanta with The Associated Press. "It wasn't unreasonable to think there was damage in that liner."

The condition of the plant's spent fuel pools was a concern throughout the crisis. Utility companies must remove nuclear fuel rods from a reactor when they can no longer sustain the nuclear reactions that produce heat and, ultimately, electricity. Those used fuel rods remain extremely hot and radioactive. They must be submerged in pools for a minimum of roughly five years.

While reactors are encased in protective steel and concrete, spent fuel pools are not. If the water drains from a pool, the exposed fuel rods can emit lethal doses of radiation to anyone nearby and even ignite, causing a fire that spreads radioactive toxins.

Working from the U.S. embassy in Tokyo, Casto said NRC officials had very limited information about plant conditions. Without electricity, many plant sensors no longer worked. Information from the remaining equipment was suspect since so much of it was badly damaged. Casto said his team focused instead on radiation readings collected by the U.S. military.

He said he worried that a pool liner could have been punctured by debris flung by the natural disasters or the explosions rattling the plant.

"You see all that damage on the top of that building and you're thinking, 'There's probably some damage on the spent fuel pools,'" Casto said.

U.S. officials were aware that no one had poured water onto the pool for three to four days after the tsunami, Casto said. Images from flying drones and even TV cameras showed white smoke — likely water vapor — coming from the area of the Unit 4 pool. Casto's team interpreted that as a sign water from the spent fuel pool was boiling.

"And then suddenly it stopped," he said.

Plant workers also reported high radiation levels from debris in-between the Unit 3 and Unit 4 reactor buildings. Casto said his team thought those radiation readings could indicate that damaged nuclear fuel had spread on the site.

"You put that together and you say, 'We're worried that there may not be water in that spent fuel pool,'" he said.

Some information was open to debate. Japanese officials once called Casto to an emergency center where he watched video taken from a helicopter that flew over the Unit 4 building. Japanese officials told Casto that they saw a reflection among the rubble, indicating there was water in its pool.

"I couldn't see it," he said.


Ray Henry can be reached at http://www.twitter.com/rhenryAP.