The city of Lancaster, Pa., asked a federal judge yesterday to bar the Nuclear Regulatory Commission from approving the discharge of "any radioactive wastewater" from the Three Mile Island nuclear power until an environmental impact statement is filed.

In a suit filed here in U.S. District Court, the city - located downstream from the Three Mile Island plant on the Susquehanna River - charged that the commission "intends to approve the decontamination and dumping of radioactive wastewater without prior warning" to the city or any other persons located downstream from the plant.

According to the suit, about 950,000 gallons of radioactive wastewater are still being held in the No. 2 reactor of the Three Mile Island plant as a result of a nuclear accident. Lancaster gets its drinking water from the Susquehanna.

Frank Ingram, a spokesman for the commission, said that commission officials met May 14 with Pennsylvania Gov. Richard Thornburgh and told him that nuclear wastewater generated during the accident "has not been processed and will not be processed in the near future."

Ingram said Thornburgh was told that before Metropolitan Edison Co., the plant's operator, takes any action, the commission will "review the options" available for treatment and disposal of the wastewater and will make the result of the review public and will discuss them with the state. Ingram said the commission will approve a final processing plan for the wastewater before it is put into effect.