BANGKOK — In Thailand, a steel canister with dangerously radioactive contents has vanished from a coal power plant and may have been missing for three weeks — or possibly longer. The search for it, which began a week ago, has expanded repeatedly but so far yielded nothing.

Globally, radioactive objects are lost fairly often, sometimes improperly discarded and posing a lethal hazard to people who find them. Their potential value to terrorists is another worry.

In this case, the power company operating the plant where the canister vanished has expressed concern that a scrap metal scavenger might have taken it to sell. Despite a cash reward for information leading to its recovery, there have so far been no signs of the 55-pound canister that is eight inches long and five inches in diameter.

It contained cesium-137, a dangerous isotope, said Narong Nakornjinda, the governor of Prachin Buri Province. Exposure to the substance, which is used in small amounts in various industrial devices in the form of a white crystalline powder, can cause cancer, burns, radiation sickness and death, according to the U.S. Centers for Disease Control and Prevention.

The isotope can last in detectable quantities for up to 300 years.

The missing canister, one of 14, was being used to measure steam pressure at the plant and had been there since 1995. The National Power Supply Public Company, which owns the plant, said it did not know when the canister vanished. But on Feb. 23, some workers heard a noise that sounded like the canister falling to the floor, said Kittiphan Chitpentham, the company’s spokesman.

Plant workers first noticed the canister was missing from its mount during a monthly inspection on March 10. They reported the loss to the Office of Atoms for Peace, the country’s main authority for nuclear research, the same day.

The nuclear agency deployed drones, robots and search teams with radioactive detection equipment in the days that followed. Along with the local police, they searched more than a dozen scrap metal shops in the area on Tuesday. They examined security camera footage from the power plant, and they checked local hospitals for patients showing signs of radiation sickness. But they failed to find the canister.

On Thursday, the search was expanded to neighboring Chachoengsao Province, doubling the area being scoured to nearly 4,000 square miles. As of Friday, the search continued.

“We are now confused,” Mr. Kittiphan said, “wondering how it went missing, when and where.”

Several radioactive objects have gone missing recently, including in the United States. A camera containing radioactive material has been missing in the Houston area since last week, Texas officials said on Monday. In another recent case, more than 2.5 tons of natural uranium were discovered to be missing in Libya on Tuesday.

The U.S. government recently began a nationwide effort to remove certain highly radioactive materials from hospitals and other civilian sites to counter the threat of terrorists acquiring them.

This is not the first civilian radiation accident in Thailand. In 2000, 10 people developed symptoms of radiation poisoning after opening canisters containing cobalt-60 that had been illegally discarded at a junkyard in a suburb of Bangkok, according to a report by the International Atomic Energy Agency. Three of them eventually died from that exposure.

The area where the current canister went missing, Prachin Buri Province, is home to nearly half a million people, as well as several national parks that are popular travel destinations near Bangkok, three hours away by car.

Those circumstances make it a contrast with a similarly frantic search in Australia in January. There, a much smaller container of cesium-137 was lost in a remote outback but found in only six days.

Muktita Suhartono reported from Bangkok, and John Yoon from Seoul.

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