As the vast operation to tame the overheating Fukushima Daiichi nuclear plant enters its third week, hundreds of emergency workers and residents from the region have been sent to a specialized medical center to be tested for possible radiation exposure, officials said Monday.

Since March 15, doctors at the National Institute of Radiological Sciences have screened 862 workers and 230 residents who live within three kilometers (nearly two miles) of the Daiichi complex, which was badly damaged during the massive March 11 earthquake and tsunami. So far, none has been found to have dangerously high levels of radiation, said Bummei Nakayama, a physician who helped with the screening.

On Monday, the center released three workers who suffered leg burns after stepping in irradiated water last week while laying electrical cables at the Daiichi plant. They had no internal injuries or skin abnormalities and are expected to make a full recovery, Nakayama said.

The three workers will return to the hospital for a checkup in two to three weeks, he said, adding that a rash could develop on the areas of their ankles that were exposed to the water.

“We saw nothing in four days,” Nakayama said. “As of today, there are no symptoms. We determined this is not very serious.”

The doctor said the workers, who returned home, had no pain and seemed in good spirits during their four nights at the medical center.

“They complained about nothing,” Nakayama said.

The mass radiation screenings come amid concerns from outside experts that the Japanese government and the Tokyo Electric Power Co., which operates the Daiichi facility, are not doing enough to protect workers from the potentially dangerous exposure.

The Health Ministry raised the limit of allowable exposure for plant workers from 100 millisieverts to 250. After the three workers were injured, government authorities ordered Tepco to remove pools of standing water, which was found to have radiation levels 10,000 times higher than normal.

The government has designated the NIRS and Hiroshima University as the nation’s emergency medical facilities for radiation victims. But the NIRS, which focuses largely on radiation research and cancer treatment, has little experience treating patients who have been exposed to dangerous radiation because it happens so infrequently.

The last time radiation patients checked into the NIRS was in 1999 after two employees working for a fuel-processing company in Tokaimura poured too much uranium into a processing tank and caused a nuclear fission chain reaction.

Despite treatment at the NIRS, both died within several months; a third employee who spent time at the medical center survived with no lasting health problems, doctors said. More than 660 others were exposed to smaller amounts of radiation during the incident.

Yoshinobu Harada, a spokesman for the medical center, said workers who have been screened include support staff, such as truck drivers who work outside the Daiichi plant, along with the technicians who have toiled inside it. The screenings, which are conducted with either a full-body radiation scanner or a hand-held version, take about 15 minutes, Harada said.

With nearly 100 screenings per day, the NIRS is at risk of being overwhelmed. When patients first began to arrive, Harada said, the medical center turned them away.

“Now we are forced to say yes,” he said. “We made a special exception, but we can’t just accept everyone. We can only take the people in really bad need. In principle, those within 20 to 30 kilometers of the plant are the ones who really qualify.”

Special correspondent Tetsuya Kato contributed to this report.