More than 400,000 gallons of contaminated water from the disabled Three Mile Island muclear power plant is being discharged into the Susquehanna River, federal and state officials said late today.

The discharge can be made without harmful radioactive pollution of the river, said Clifford Jones, head of the state Department of Environmental Resources.

"There is an urgent need to begin discharging," Jones said.

The water is contaminated with a very slight level of exnon, a low-level contaminant, according to DER officials.

Officials from the Nuclear Regulatory Commission and the U.S. department of Energy said such diluted waste water is discharged rountinely from the plant under normal operating conditions.

Jones said plant officials fear that unless the diluted water is discharged, holding tanks would fill up and undiluted waste water would run directly into the river through storm drains.

"Most of it [exnon] will be dissipated like bubbles in soda pop," said DER spokesman Dave Milne. He said the gas decays after a few hours.

"The problem is that 400,000 gallons of the waste water have accumulated in the sump of the plant's turbine building. The water apparently became contaminated because of radioactive gases released into the plant," said Jones.

The NRC said it expects no detectable levels of xenon in the river a few miles downstream of the discharge.

The plant began discharging the waste water at about 2:30 p.m., but the discharge was suspended four hours later for review by federal and state officials. It resumed after tests showed exnon was the only element in the discharge.

"DER is notifying downstream municipal water systems of the discharge, but there is no cause for concern," said Jones. "The state of Maryland is also being notified." The Maryland border is about 35 miles south of the plant.