The shutdown of the Mirant Corp. power plant on the Potomac River in Alexandria might have been a big victory for activists fighting for a cleaner environment, but industry analysts say it probably won't be long before the plant is back in business.

Power generated by the 56-year-old coal-burning plant -- which supplies 400,000 homes in the District and Maryland -- is too important to the long-term stability of the power grid and too profitable for its owners to close it for good, industry experts said.

"If you're a utility executive, you'll do anything you can to keep them running," said Dick Munson, executive director of the Northeast-Midwest Institute, a nonprofit research group representing the interests of states in those regions. "Unless they get additional costs imposed on them," such as for major upgrades, "there's little incentive for a utility executive to close them, even though they're inefficient."

Atlanta-based Mirant, which filed for bankruptcy protection in 2003, was ordered Aug. 19 by Virginia officials to take immediate steps to reduce pollution, shutting down the plant if necessary. The directive by the Department of Environmental Quality followed an analysis that found that sulfur dioxide, nitrogen dioxide and particles in the vicinity at times considerably exceeded national standards.

Mirant officials have declined to say how long it will take to deliver a plan for compliance to the state. But they say they are committed to finding a fix, which could include burning cleaner coal, installing new technology or raising the height of the 160-foot smokestacks to disperse the pollutants. Mirant's troubles have been intensified by its proximity to a densely populated area, where polluted downwash from the smokestacks has been shown to settle.

"The company is anxious to get out of bankruptcy and to bring itself into compliance with the environment," said Daniele Seitz, an analyst with Maxcor Financial Group who follows Mirant. "This should happen in a matter of months."

Although Wednesday's action was characterized by state officials as rare -- perhaps the first closure of a coal-fired plant in Virginia -- experts said the problems that led to the shutdown are typical of aging plants.

According to the Edison Electric Institute, there are 626 coal-fired power plants in the United States. Nearly three-quarters of the plants' boilers are more than 30 years old, and most operate without pollution-control technology, according to testimony in Congress this year. In a decade, almost 90 percent of the nation's boilers will be more than 30 years old.

Cheap to run and expensive to upgrade with the latest pollution controls, coal-fired plants generate about 51 percent of the nation's electricity. They have earned a reputation as big polluters -- sources such as gas are more efficient and cleaner -- but their low-cost energy makes them desirable.

Alexandria officials have said they will strenuously oppose any attempt to reopen the plant, but neighborhood activists are remaining cautious. "It's a victory in that the science has proven the truth about the health effects occurring in this neighborhood," said Elizabeth Chimento, who lives a block from the plant and has spearheaded the effort against it since 2001. "But this is not over by any means. The most important thing to this plant is profit, and when it's closed, they're not making profit, and that's the motivating factor."

Neighbors of the plant were the first to suspect that the odd dust that coated nearly everything in the northern part of Old Town was from the power plant. Their prodding and research got city and state officials to investigate.

But the D.C. Public Service Commission has worked against activists' efforts, warning of a "potentially dangerous and security-threatening" interruption of electrical service unless the plant reopens. The declaration contradicts statements by Pepco and officials overseeing the power grid, who say contingency plans are in place to deal with the shutdown in the short term.

The District filed a petition with the U.S. Department of Energy and the Federal Energy Regulatory Commission, which will take comments until 5 p.m. tomorrow and later decide whether to intervene.

Industry analysts say one thing seems certain: Whether Mirant goes forward with the costly upgrades or stays shuttered, the price of electricity for consumers is likely to go up.

"You have a company that cannot sell its power and customers who are going to have higher prices to pay," said Seitz, the analyst. "That's the price of shutting down the plant. I hope people are happy they can breathe better, but nothing comes cheaply in energy."