"Today, spent fuel is better protected than ever. The results of security assessments completed to date clearly show that storage of spent fuel in both spent fuel pools and in dry storage casks provides reasonable assurance that public health and safety, the environment and the common defense and security will be adequately protected," adds the NRC.

Some of the agency's responses show where the NRC and the NAS disagree.

The NAS supported analyzing "maximum-credible scenarios" as a particularly useful tool to understand the damage that could be inflicted by a terrorist attack on spent fuel storage facilities, noted the NRC.

"In characterizing a maximum-credible scenario, the NAS report indicates that terrorists must be able to carry out the attack as designed — for example, to hit a spent fuel storage facility with an aircraft at its most vulnerable point."

The NAS criticized the NRC for not adapting that sort of analysis. In its reply, the commission defended that position.

"The NRC considers that the maximum-credible scenario method is not an effective and efficient use of NRC and licensee resources," said the NRC document. "This method would direct analysis at an overly large scope of scenarios, including some unrealistic scenarios.

"Rather, the NRC focuses on realistic or credible scenarios."

The NRC report said the agency's analysis employs what it calls reference scenarios. These are "based upon either the characteristics of previous terrorist attacks or upon qualitative judgements of the technical means and methods terrorists might employ to attack a spent fuel facility."

The commission believes that analysis of unrealistic scenarios could lead to a misinterpretation of the actual risk, "and this can cause confusion among the public and other stakeholders," wrote the NRC.

Lochbaum commented, in an e-mail to the Deseret Morning News, "The NAS did not recommend 'unrealistic' scenarios be analyzed. They recommended that maximum credible, repeat credible, scenarios be considered."

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E-mail: bau@desnews.com