GOLDEN, Colo. (AP) _ A steel-lined boxcar loaded with low-level nuclear waste from Rocky Flats nuclear weapons plant returned to the plant today after being ordered out of Idaho over the weekend.

But Colorado Gov. Roy Romer has urged that the boxcar remain on railroad tracks and its cargo of rags, clothing and equipment contaminated by plutonium residue be kept aboard until another storage site can be found elsewhere.

Ed Heintz, director of communications for Rockwell International which operates Rocky Flats for the Department of Energy, said the boxcar will remain on a siding until officials can decide where to send the waste.

Idaho Gov. Cecil D. Andrus on Saturday turned the unwanted boxcar back from a temporary storage site at Idaho Falls, and banned further shipments to his state. Before then, it had spent several days in limbo at a railyard in Blackfoot, Idaho.

Romer on Sunday then rejected a Department of Energy request to expand low- level radioactive waste storage at Rocky Flats.

''I'm not going to authorize an enlargement of that storage, the reason being I do not want Colorado to be the escape valve for everybody's failure to provide a permanent site,'' Romer said.

DOE had planned to transfer the waste from the Idaho site to New Mexico when the Waste Isolation Pilot Plant permanent disposal site there opens. The WIPP facility was scheduled to be open this month, but the date has been pushed back at least until early next year.

If the WIPP site opens in January as the DOE now plans, Rocky Flats wastes will account for more than 70 percent of the debris to be buried in salt deposits 2,500 feet underground.

''I met with DOE and they said, 'Would you expand the authorization to store (at Rocky Flats)?' And my answer was 'No 3/8,' '' Romer told The Associated Press in a telephone interview.

''I said, 'You can keep the authorization for storage you now have. But I will not enlarge the storage.' There is an interim arrangement which provides for about 1,600 cubic yards of material ... I said, 'You can bring the railcar back. But the capacity that the railcar takes has to be a part of the original interim agreement,''' Romer said.

''I also asked them not to unload that car but to leave it loaded and to leave it on the track so we can work to find another destination for it,'' Romer said.

Romer said he hoped to confer with other governors in the region on finding a way to get the New Mexico site open as soon as possible.

Romer said about 650 yards of low-level waste has been stored at Rocky Flats, leaving about 950 cubic yards to go until its storage limit is reached.