Nuclear Regulatory Commission Chairman Gregory Jaczko defended his recommendation to evacuate Americans 50 miles from a crippled Japanese nuclear reactor in a letter to Sen. Jim Webb (D-Va.) last week.

House Republicans have criticized the chairman's recommendation for all Americans within 50 miles of the Fukushima Daiichi nuclear plant to be evacuated after the complex was hit by a powerful earthquake and tsunami on March 11.

His recommendation raised questions because NRC only evacuates Americans within 10 miles of damaged U.S. nuclear plants, and Japanese officials were evacuating individuals within 12 miles of the Fukushima plant.

Jaczko said in his June 17 letter that the recommendation came amid confusion and sparse information from Japan and that it was made based on the assessment of conditions at the site as they were understood at the time.

"Since communications with knowledgeable Japanese officials were limited and there was a large degree of uncertainty about plant conditions at the time, it was difficult to accurately assess the potential radiological hazard," Jaczko said. "The U.S. emergency preparedness framework provides for the expansion of emergency planning zones as conditions require."

NRC considered two scenarios based on computer models that assessed possible offsite consequences, Jaczko said in the letter.

The first scenario assumed an "ex-vessel, unfiltered release" from a totally failed containment from one unit for about 10 hours. The second calculation assumed multiple units failed and 100 percent damage to the spent fuel pool at Unit 4, with a duration of about 15 hours, according to the letter. The scenarios also incorporated different wind speeds and weather patterns.

Moreover, those calculations demonstrated that U.S. EPA's protection action guidelines could be exceeded at a distance of up to 50 miles from the Fukushima site, if a large-scale release occurred from the reactors or the spent fuel pools, the chairman said.

"Even though these recommendations were made during a time of uncertainty and rapidly changing conditions during the first few days of the accident" they were appropriate, and the Japanese government significantly revised its estimate upward for the amount of radiation released from the plant in the first week of the disaster, Jaczko said.

Webb initially asked the chairman on April 15 for data that fed into his recommendation. In early April, NRC staffers were unable to tell the commission's Advisory Committee on Reactor Safeguards who vetted Jaczko's recommended evacuation (E&ENews PM, April 7).

The senator was not immediately available to comment on the chairman's letter.

The chairman had repeatedly cited severe damage to the spent fuel pool at Unit 4, but NRC staff said this month that the pool never went dry (Greenwire, June 15).

Jaczko said that the new information did not invalidate his recommendation. "The more reassuring recent assessment of the situation in the Unit 4 spent fuel pool is countered by the confirmation of significant core damage to Units 1, 2 and 3 and does not invalidate our earlier decision," he said.

Click here to read Jaczko's letter to Webb.