Highly radioactive material could fall into the hands of terrorists because the nation's nuclear plants are not keeping close enough track of spent fuel, the Government Accountability Office (search) said Monday.

The attacks of Sept. 11, 2001, raised concerns that radioactive materials "could be diverted or stolen and used maliciously," said the report, which also questioned the level of plant oversight by the federal Nuclear Regulatory Commission (search).

NRC spokesman David McIntyre called the risk of the spent fuel ending up in terrorists' hands "extremely low."

The report was requested by Vermont's two U.S. senators and others following news a year ago that two pieces of spent nuclear fuel had been reported missing at the Vermont Yankee nuclear plant. The pieces were later found in the plant's spent fuel storage pool, but not where records had indicated they were.

Spent nuclear fuel also was reported missing from the Millstone Nuclear Power Station in Connecticut in 2000 and from the Humboldt Bay Power Plant in California last year. None of that fuel has been found.

"NRC inspectors often could not confirm that containers that were designated as containing loose fuel rods in fact contained the fuel rods. ... Thus, spent fuel may be missing or unaccounted for at still other plants," said the GAO, which is the investigative arm of Congress.

The report recommended that the NRC establish new control and accounting rules for nuclear plants' handling of loose spent fuel pieces and an inspection program to make sure the rules are being followed.

The NRC's McIntyre said the commission beefed up such controls and inspections beginning in November 2003. He said those efforts led to the discovery of pieces unaccounted for at Vermont Yankee and Humboldt Bay.

He said the agency is following up at other plants and waiting to learn the scope of any other problems before deciding on new rules or inspection regimens.

The risk of the spent fuel ending up in terrorists' hands is negligible, he said, because it would be very difficult to get the material past the radiation alarms at nuclear plants.