WASHINGTON -- Nearly a third of the nation's nuclear power plants may not be setting aside enough money to clean up their sites after they are shut down -- including the Columbia Generating Station north of Richland -- according to a new federal report.

That could leave taxpayers on the hook for billions in decontamination costs.

However, a spokesman for the Nuclear Regulatory Commission, which is charged with making sure plants set aside enough money to cover the cost of decontamination, rejected the General Accounting Office report's findings and insisted its methodology was flawed.

Officials at Energy Northwest believe it will cost about $600 million to decommission their 1,150-megawatt plant in southern Hanford, one of the nation's newest reactors.

So far, about $80 million has been set aside.

The plant is licensed to run until December 2023. Energy Northwest and the Bonneville Power Administration, which buys all the power from the reactor, pay into the decommissioning fund annually. Payments increase by 6 percent plus inflation each year.

"If you look at the graph, it's going almost straight up," said Energy Northwest spokesman Don McManman.

The payments were structured to be highest toward the end of the plant's operating life to correspond with the retirement of construction bonds issued to build it. The last of the annual debt payments is scheduled to be made in 2018.

U.S. Rep. Ed Markey, D-Mass, asked the GAO to study the accumulated funds out of fear that taxpayers could be stuck with the bill if a power company goes bankrupt and leaves a funding gap.

The question was worth asking, he said, because deregulation -- which gave plant owners freedom to operate separate from utilities -- also raised the risk of bankruptcies with no assets left behind.

The GAO report measured funds collected by the end of 2000 by each owner or part-owner of a plant, as well as the rate of more recent contributions. Of the 122 plants that have been built in the United States, the report said the trust funds for 42 plants were smaller than necessary, and the rate of contribution for 31 plants was too small.

Markey echoed the report's call for the NRC to get tougher on owners by not relying on their statements of future funding plans and to evaluate the funds for plants with multiple owners on a per-owner basis, because owners with excess funds are generally not required to transfer money to partners.

"When a nuclear power plant is shut down, it leaves behind a huge radioactive public health hazard," Markey said in a statement. "The NRC is charged with protecting public health and safety. If it can't effectively monitor the trust funds for decommissioning, it can't guarantee that the companies that profited from selling nuclear energy will do their duty and clean up their mess."

But NRC spokesman Scott Burnell said the GAO report failed to account for the fact that different plant owners contribute at different rates on different years. Although he conceded the NRC never has fined an owner for failing to put enough money into its trust fund and has not established a system of punishment, he said the agency has enough power to ensure the job gets done.

"The bottom line is that the NRC will take whatever actions are appropriate to ensure that the funding mechanisms the licensee has in place will support decommissions," Burnell said. "All those sites which have already shut down do have sufficient funds to carry out decommissions."