But a new study by the private group said it would be hard to stage such an attack, and terrorists had little chance of stealing enough spent nuclear fuel from a plant to produce a radiological -- or "dirty" -- bomb, a concern expressed by Congress, which ordered the report.
"Terrorists view nuclear power plant facilities as desirable targets because of the large inventories of radioactivity they contain," the study said.
"While it would be difficult to attack such facilities, the committee judges that attacks by knowledgeable terrorists with access to appropriate technical means are possible," it said.
The US Nuclear Regulatory Commission, commenting last month on a classified version of the report, said some of the academy's scenarios were "unreasonable" and that some recommendations "lacked a sound technical basis." It said spent fuel was better protected today than ever before.
The study's release coincided with a major US exercise this week to test the readiness of officials, companies and citizens in case of a terrorist attack. Mock drills include a chemical attack in Connecticut and the release of a biological agent in New Jersey involving some 10,000 participants.
Past readiness exercises revealed problems including confusion about actions agencies needed to take and overwhelmed communications capabilities.
US officials fear militant groups such as al Qaeda are bent on eclipsing the Sept. 11, 2001, strikes with a nuclear, chemical or biological attack, but acknowledge that more low-tech strikes are more likely.
The National Academy of Sciences report did not look at the overall threat of a terrorist attack on nuclear facilities, but focused exclusively on nuclear waste stored at the sites.
It said nuclear plants could reduce the risk of a spent fuel blaze significantly by redistributing the waste kept in storage pools and by installing heavy-duty water sprinklers to cool the pools even if they are damaged.
The study said nuclear power plants had made many security improvements since the Sept. 11 attacks, but said the Nuclear Regulatory Commission failed to give the National Academy of Sciences team enough information to assess their effectiveness.
More than 90 percent of the United States' nuclear reactors are privately owned and operated.
The study also urged the Nuclear Regulatory Commission to share more information with the industry, saying current secrecy requirements were hurting plants' ability to reduce their vulnerabilities to terrorist attacks.
The commission said on Wednesday it was giving the study serious consideration and had a plan to address its recommendations.
The study said that while it was safer to store the waste in heavily fortified, dry casks, they could only be used after the spent fuel has already cooled off in pools.
It called on the Nuclear Regulatory Commission and plants to conduct case-by-case vulnerability checks, and urged the commission to ensure operators take prompt and effective measures to mitigate the possible consequences of an attack.
The classified version of the study was provided to the government in July 2004. The National Academy of Sciences said the public report contained all of the findings and recommendations of the original, but some had been slightly reworded.
(Additional reporting by Larry Fine in New York)