Top officials at the Nuclear Regulatory Commission heavily debated a key assumption that prompted the agency's chairman to call for a 50-mile evacuation around a crippled Japanese nuclear plant last year, according to newly released transcripts.

NRC Chairman Gregory Jaczko told a House Energy and Commerce subcommittee in the immediate aftermath of the March 11, 2011, earthquake and tsunami that crippled the Fukushima Daiichi nuclear power plant, triggering explosions, radioactive releases and core meltdowns in three reactors, that a pool holding spent fuel at the Japanese plant was dry, which would have triggered a larger radioactive release and warranted expanded evacuation.

Months later, NRC officials publicly acknowledged that statement was incorrect.

But at the time, NRC officials questioned Jaczko's public statement about the spent fuel pool, according to more than 3,000 pages of redacted phone conversations the agency released yesterday.

The status of the pool was crucial because it factored into the chairman's recommendation for the U.S. Embassy in Tokyo to advise Americans living within about 50 miles of the plant on Japan's northeast coast to evacuate. Japan, at the time, was only calling for an evacuation within about 12 miles of the plant.

Two top NRC officials told Jaczko during a March 16 phone call that Fukushima owner Tokyo Electric Power Co. (TEPCO) told commission members the pool was dry, according to the transcripts.

Even so, Chuck Casto, who led NRC's delegation in Japan, said he was struggling to determine whether there was water in the pool from a split-second of a video taken from a helicopter. Casto repeatedly said he was given confusing information about the location of the pool, and that "we don't even know that that's the damned spent-fuel pool that we was looking at."

Jaczko then asked repeatedly if his public statement was incorrect.

"Well, so it's inaccurate for me to say it's dry? Is that what you're saying?," Jaczko asked Casto on March 16. "It's OK if that's the case; just tell me."

"I would say it's probably inaccurate to say it's dry," Casto said.

In the months following the Japanese crisis, NRC staff were unable to tell the Advisory Committee on Reactor Safeguards -- an oversight panel within the agency -- who vetted the chairman's recommendation.

Instead, staff said the "conservative" decision to call for a 50-mile evacuation zone was based on assumptions that the spent fuel pool was full of fuel, as U.S. spent fuel pools are (E&ENews PM, April 7, 2011).

More than 35 environmental and anti-nuclear groups have seized on Jaczko's recommendation to call for larger evacuation zones surrounding the United States' 104 operating nuclear plants (Greenwire, Feb. 15).

Jaczko told reporters during a conference call yesterday that the early days of the accident were "very hectic" and there was little information, likening the experience to the "fog of war."

"Much of what we knew came from a variety of sources, some from the Japanese, some from the International Atomic Energy Agency and a great deal from the news media," Jaczko said.

Ultimately, Martin Virgilio, NRC executive director for reactor and preparedness programs, assured the chairman he was on solid footing in calling for a 50-mile evacuation.

NRC Operations Director Bill Borchardt also supported the chairman's recommendation, according to the transcripts. "If this happened in the U.S., we would go out to 50 miles," Borchardt said. "That would be our evacuation recommendation."

Despite the confusion, the transcripts also show Jaczko and NRC officials correctly gauged the severity of the event.

"At this point, I would see a worst scenario probably being three reactors eventually having, for lack of a better term, a meltdown," Jaczko told White House officials during a March 16 call. "So, the reactors would likely eventually, you would eventually breach primary containment and have some type of release."