Mirant Corp. officials announced a limited reopening of their power plant in Alexandria yesterday but said they had not yet made changes to the plant to reduce pollution levels that alarmed state environmental officials and prompted closure of the plant last month.

Instead, the company reworked a computer study using more "realistic" scenarios that ended up with numbers its officials feel will bring it within air quality standards, said Steven Arabia, a Mirant spokesman.

The company notified the Virginia Department of Environmental Quality late yesterday of its intention to reopen the troubled plant, which supplies power to thousands of homes in the District. The Department of Environmental Quality has no authority to stop the plant's reopening because its shutdown was voluntary, department spokesman Bill Hayden said. However, the department could impose hefty fines later or revoke the plant's permit to operate. The plant was shuttered after a study showed it could exceed national air quality standards under certain circumstances.

Plans call for running one of five generators at the plant up to 16 hours a day, with an eight-hour break. Using one unit will produce less than a quarter of the power the plant can generate at full capacity, Arabia said.

"We were going to move as quickly as we could to get the plant back into service . . . and this is the first step in that process," Arabia said.

Critics of the plant were furious at the move. "They simply unilaterally announced they were reopening the plant without making any improvements or installing any modern pollution controls," said U.S. Rep. James P. Moran Jr. (D-Va.). "It's par for the course: Do it on the cheap, don't invest any money and finagle a way to get around the regulations."

Arabia countered that Mirant will have to make changes to the plant in order to fully reopen. "Those kinds of things will be necessary in order to get additional units online," Arabia said. "This is a significant step but one of many that are to come."

State and local officials were trying to analyze the ramifications of the decision late yesterday. The Department of Environmental Quality responded with a letter saying that it had "serious reservations" with Mirant's new analysis, according to Hayden.

Richard J. Baier, Alexandria's director of transportation and environmental services, said the city -- which has been trying to get the plant closed for more than a year -- was combing through Mirant's new model late yesterday.

"We're concerned there may be a numbers game here," Baier said.

When Mirant shut down Aug. 24, other public safety officials across the region protested the decision, calling it a threat to the region's power grid and national security.

"The concern is that the plant supplies a significant source of power for the District of Columbia," said Richard E. Morgan, D.C. Public Service commissioner. "Without that plant, the lights will stay on as long as we don't have a major heat wave along with equipment failures, which can and do happen. We're concerned that it could be only a matter of time before there is a power outage."

Activists in Alexandria, who have long lamented the aging plant's existence and have seen it as a major polluter, decried Mirant's tactics.

Neighborhood activist Poul Hertel said that Mirant was "obfuscating" by inserting its own numbers into accepted testing methods, getting results that were "not scientifically valid."