WASHINGTON — The operators of 20 of the nation’s aging nuclear reactors, including some whose licenses expire soon, have not saved nearly enough money for prompt and proper dismantling. If it turns out that they must close, the owners intend to let them sit like industrial relics for 20 to 60 years or even longer while interest accrues in the reactors’ retirement accounts.

Decommissioning a reactor is a painstaking and expensive process that involves taking down huge structures and transporting the radioactive materials to the few sites around the country that can bury them. The cost is projected at $400 million to $1 billion per reactor, which in some cases is more than what it cost to build the plants in the 1960s and ’70s.

Mothballing the plants makes hundreds of acres of prime industrial land unavailable for decades and leaves open the possibility that radioactive contamination in the structures could spread. While the radioactivity levels decline over time, many communities worry about safe oversight.

Bills that once seemed far into the future may be coming due. The license for Vermont Yankee in Vernon, Vt., at 40 the nation’s oldest reactor, expires on Wednesday, for example. And while the Nuclear Regulatory Commission has granted its owner, Entergy, a new 20-year permit, the State of Vermont is trying to close the plant.