Transcripts of phone conversations in the days after last year’s tsunami in Japan, released Tuesday by the Nuclear Regulatory Commission, show an early sense of urgency, as well as confusion, about the emergency at the Fukushima Daiichi nuclear reactor complex.

The transcripts also include lengthy discussions justifying NRC Chairman Gregory Jaczko’s controversial decision to urge Americans within a 50-mile radius of the Japanese nuclear plant to evacuate. They show that the decision was based in part on an assessment, now thought to be false, that one of the Fukushima Daiichi spent fuel pools was dry and that its walls had, in the words of one official, “crumbled,” releasing radioactive elements.

NRC officials said Tuesday that the agency’s assessments about radioactivity release turned out to be substantially correct in the end because of releases from three of the reactors.

In conversations recorded March 16, NRC staff members debated the evacuation issue, including how it would be carried out.

The NRC’s executive director for operations, Bill Borchardt, said that “if this happened in the U.S. we would go out to 50 miles.” Adm. Kirkland H. Donald of the National Nuclear Security Administration’s office of naval reactors agreed. Jaczko also said that the situation could get worse.

At the time, Japanese officials had urged evacuation within a radius of 12 miles from the plant. Also, the NRC-recommended 50-mile evacuation radius far exceeds the radius that U.S. nuclear plant operators are expected to plan for in case of an emergency.

The transcripts were released in response to a Freedom of Information Act request and are available on the agency’s Web site. The conversations in the transcripts start the morning of the earthquake in Japan, on March 11, and run through March 20. They are redacted in many places, including a 16 1 / 2-page section from March 12.

At first, Jaczko and his staff members focused on the limited impact the earthquake-triggered tsunami could have on U.S. plants, especially the one at Diablo Canyon in California. But their focus changed mid-morning when the agency received a cable from the International Atomic Energy Agency that an NRC operations room official said had “some somewhat alarming language” about the failure of the Fukushima Daiichi backup power and cooling system.

By late afternoon, an NRC official on a conference call said that given the likelihood that the Fukushima Daiichi complex was suffering a blackout, “we’re about at the time where they could start to see core damage.”

But Bill Ruland, the official, added that information remained “meager.” Speaking later in a conference call with Jaczko, he said, “We’re dying in a sea of silence here actually.”

The agency remained heavily dependent on news releases from Tokyo Electric Power Co. and media reports, especially in the first days of the crisis. Jaczko said Tuesday that those early days “reflect the fog of war, so to speak.”

The phone transcripts convey an impression similar to that conveyed by NRC e-mails that have been on the agency’s Web site for some time.

The NRC response to the Japanese nuclear emergency has been the focus of attention because U.S. nuclear power experts are debating what lessons the United States should draw for its power plants, as well as how the NRC might be able to better help other nations in the future.

Dan Dorman, deputy director for engineering at the NRC’s office of nuclear reactor regulation, said that the confusion about what was happening in Japan was “exacerbated by the event being halfway around the world in another country.” He said that in a U.S. incident, the NRC would have direct lines to the reactor’s operator and a clearer idea of what was happening.

The NRC has laid down requirements for U.S. nuclear plants to bolster their defenses against multiple power failures of the sort that hit Fukushima Daiichi. The earthquake knocked out the plant’s normal power supplies from the electricity grid, and the tsunami flooded and disabled backup diesel generators.

In addition, the agency is drawing up guidelines to improve the safety of spent fuel pools.

Last week, the NRC’s five commissioners approved a construction and operating license for a new nuclear power plant for the first time since 1978. Jaczko was the sole dissenting vote, saying that he wanted clearer assurances that the two new reactors in Georgia would make changes in plans to meet any further post-Fukushima recommendations that the commission may issue. Those additional recommendations might be issued as early as this or next week.