A citizens' group says it plans to challenge the Nuclear Regulatory Commission's decision permitting the operator of the Three Mile Island nuclear plant to boil away 2.3 million gallons of radioactive water.

Some of the water is left over from the nation's worst commercial nuclear accident, which occurred at the Middletown, Pa., plant in 1979.In a 5-0 vote, the commission on Thursday upheld a February decision of its Atomic Safety and Licensing Board, which concluded that the evaporation plan "will have extremely small radiation exposure consequences, both to workers and the general public."

However, a citizens' group based near the plant said it would continue to fight the plan.

Frances Skolnick of the Susquehanna Valley Alliance said the decision wasn't unexpected.

"It enables us to go outside of the Nuclear Regulatory Commission and to the courts," she said. "We will definitely seek relief."

She said it had not decided where the challenge would be filed.

The alliance joined a second group, Three Mile Island Alert, as an intervenor in the NRC's review of the matter last year. The groups had asked General Public Utilities Nuclear, the plant operator, to consider other alternatives, including storing the water in tanks for perhaps 30 years to allow radiation to decay.