ST. PETERSBURG, Russia — A 20-year program to convert highly enriched uranium from dismantled Russian nuclear weapons into fuel for U.S. power plants ended Thursday, with the final shipment loaded onto a vessel in St. Petersburg’s port.

The U.S. Energy Department described the program, commonly known as Megatons to Megawatts, as one of the most successful nuclear-nonproliferation partnerships ever undertaken.

The agreement, signed in 1993 shortly after the collapse of the Soviet Union and always scheduled to end in 2013, gave Russia the financial incentive to dismantle thousands of nuclear weapons. The initial aim was to help keep the vast stockpiles of weapons-grade uranium out of the hands of terrorists and to make sure Russia’s nuclear workers got paid at a time the country was nearly bankrupt.

Under the program, 500 metric tons of highly enriched uranium, the equivalent of about 20,000 nuclear warheads, was converted into fuel for U.S. nuclear reactors. During the past 15 years, the fuel has generated 10 percent of U.S. electricity, or nearly half of all commercial nuclear energy.

Unlimited Digital Access. $1 for 4 weeks.

“For two decades, one in 10 light bulbs in America has been powered by nuclear material from Russian nuclear warheads,” U.S. Energy Secretary Ernest Moniz said in a statement.

While monitored by the U.S. and Russian governments, the contract has been carried out by two commercial companies: the United States Enrichment Corp. and Techsnabexport.

The commercial value of the agreement was $17 billion. The money came from U.S. utilities, and it provided a significant income flow to a struggling Russia.

Rick Shannon, president of Atlantic Ro-Ro Carriers, the company in charge of shipping for this program, said he expects the U.S. will continue to purchase low-enriched uranium derived from Russian nuclear stockpiles.

“Shipments of this type are critical to continue,” he said. “As to how they continue and what the commercial version will be, it will a little bit different than right now.”

Russia is estimated to have hundreds of tons of highly enriched uranium, far more than is needed to maintain its nuclear arsenal. The New START arms-reduction treaty limits the number of deployed strategic warheads to 1,550 for each country.

U.S. and Russian officials watched Thursday as 40 cylinders containing low-enriched uranium were loaded onto a ship bound for Baltimore.

The ship is under the command of Roman Elokhin, a Russian sea dog with a full head of white hair.

And in addition to the 60 tons of uranium canisters, it’s carrying the usual load of aluminum, steel and containers.

In early December, it will call at Ruckert Terminals in Baltimore, across the water from Fort McHenry.

The uranium, last loaded, will be first to go ashore. Then it will be taken to one of three plants to be fabricated into usable fuel.

“The North Atlantic in winter, it gets a little bit hairy out there,” said Shannon, of Ro-Co. “But the old captains in the Russian fleet are some of the best in the world.”

Material from The Washington Post is included in this report..