ST. PETERSBURG, Russia — The final shipment of civilian nuclear reactor fuel made from Soviet atomic bombs left port for the United States on Thursday, ending a post-Cold War program that has been a long-running boon for the American nuclear power industry.

Over its 20-year course, the program, known as Megatons to Megawatts, supplied the energy for about 10 percent of all the electricity generated in the United States, far surpassing the electricity derived from solar, hydro and biofuels as well as other alternative sources. Though not well known, the program, which began in 1993, has shaped the American uranium fuel market for two decades.

In that time, the Russians dismantled about 20,000 nuclear warheads, processed their high-enriched uranium cores into low-enriched fuel, and sent it to the United States. Russian nuclear fuel is now expected to drop from about 50 percent of the American market to 20 percent. Prices could rise for utilities.

In a cold rain on a pier at the St. Petersburg port on Thursday, dignitaries from the United States and Russia signed placards attached to the 10 final pallets. Each pallet held four cylinders of low-enriched uranium. It took about two nuclear bombs to make each of the chunky cylinders, which look like oversize water heaters.