The panel in a largely classified 130-page report concluded that if terrorists succeeded in partially draining water from a reactor spent-fuel pool an intense fire likely would release large amounts of radiation into the environment.
The panel said that neither federal regulators nor the industry have fully determined the vulnerabilities and consequences of such an attack and that specific risks "can only be understood by examining ... spent fuel storage at each plant."
The report, a declassified version of which was released Wednesday, has been the subject of intense internal debate between panel members and the Nuclear Regulatory Commission which had opposed its release and has called some of its recommendations unnecessary.
The agency said in a statement that it considers the NAS study important and is giving its recommendations "serious consideration."
But it also said it considers reactor spent fuel pools "well protected by physical barriers, armed guards, intrusion detection systems, area surveillance systems" and limits on access by workers at power plants.
After the classified document was provided to members of Congress last month, NRC Chairman Nils Diaz in a letter to lawmakers called some of the panel's assessments "unreasonable" and said that some of its conclusions "lacked sound technical basis."
"Today spent fuel is better protected than ever," Diaz wrote.
But the NAS panel in its report said that spent-fuel pools, 100,000 gallons of circulating water designed to store used fuel rods after they are removed from the reactor, remain tempting potential targets of terrorists.
Protecting them is "a critical national security issue," said Bruce Alberts, president of the National Academy of Sciences.
The panel of scientists found that an attack using an aircraft or high explosive could under some conditions lead to a draining of the spent-fuel pool, unleashing a high-temperature fire and release of large amounts of radiation.
It urged the NRC to require industry to take short-term measures that it said would mitigate some of the danger, including reconfiguring the position of fuel assemblies to more evenly distribute decay-heat loads and installing a water spray system to cool the fuel should the facility be damaged in an attack.
"Such measures should be implemented promptly," the report said.
The report said that the NRC should consider getting industry to move more of its spent fuel rods from pool storage to dry casks that are considered less vulnerable to a terrorist attack. But it stopped short of recommending such action.
The panel acknowledged that cask storage is more expensive and that, in any case, significant amounts of spent fuel always will have to be kept in pools for up to five years to allow the fuel rods to adequately cool. About a fourth of the country's commercial power plants, with 103 reactors, have begun storing some of their spent fuel in dry casks as pool storage space has filled to capacity.
The nuclear industry maintains the pool storage is safe and protective.
The NAS panel said the likelihood of terrorists stealing spent fuel rods to make a radioactive bomb "is small" but that the NRC, nevertheless, should review and upgrade its security requirements for protecting spent-fuel pools.
While acknowledging that the NRC has made improvements in nuclear power plant security since the Sept. 11, 2001, attacks, the report said "an assessment of current measures should be performed by an independent organization" outside of the NRC.