Environmentalists charged yesterday that the Nuclear Regulatory Commission has underestimated the costs and risks of shutting down nuclear power plants.

"The NRC is taking a cavalier attitude toward decommissioning, falsely suggesting that it will be cheap, safe and easy," said Ruth Caplan, energy policy analyst for Environmental Action.

Rules proposed by the NRC for dismantling the plants and removing their radioactive contents would "help the financially depressed nuclear industry, while public health and safety are being shunted aside," said Joan Claybrook, president of the Public Citizen environmental group.

Caplan told a news conference that the proposed regulations would allow utilities to spend money intended for decommissioning on other projects, including construction of new plants.

The environmentalists' report will be submitted to the NRC in response to the proposed regulations, and NRC spokesman Frank Ingram said the agency would review the comments "just as we would any reaction to any proposed rule." The agency will have no further comment on the report, he said.

Under the NRC's proposed regulations, utilities would be required to collect $100 million to finance the decommissioning of each of their reactors.

But the report said other experts predict that the cost could be as high as $3 billion and accused the NRC of failing to set realistic cost estimates.