A small amount of radioactive materials — plutonium and americium — was found in the urine of five workers exposed to radiation in an accident earlier this month at a nuclear research facility in Ibaraki Prefecture.

The result shows that the five workers have suffered internal radiation exposure following the June 6 accident at the Oarai Research & Development Center, operated by the Japan Atomic Energy Agency (JAEA).

The workers were administered medication to facilitate the discharge of radioactive materials from their bodies for five days after the accident.

All five, although showing no signs of deterioration or notable change in their health, were hospitalized again Sunday for another round of medication from Monday, according to the National Institutes for Quantum and Radiological Science and Technology, which operates the hospital treating them.

In the accident, radioactive materials were released into the air in the room where the five were working when one opened a metal container holding plutonium and uranium powder samples and a plastic bag containing the samples inside suddenly ruptured.

Initially, the agency said up to 22,000 becquerels of plutonium-239 were found in the lungs of one of the workers, while 5,600 to 14,000 becquerels of the radioactive substance were found in the lungs of three other workers. It said at the time that the four had suffered internal radiation exposure.

But the facility operator has since said a subsequent check by the National Institute of Radiological Sciences has found no plutonium in the lungs of any of the five workers. It has not ruled out the possibility that what was actually detected was radioactive substance left on the workers’ bodies after decontamination.

The national institute, meanwhile, has said that americium was found in the lungs of at least one of the five workers.

Also on Monday, JAEA President Toshio Kodama again apologized over the accident, saying at a news conference that “the agency as a whole had problems in the prediction of risks.”

He said he has no intention of resigning for now but will take “appropriate” responsibility depending on the cause of the accident.

The agency submitted a report compiling the details of the accident and measures to be taken to prevent a recurrence to the Nuclear Regulation Authority, the state’s nuclear safety watchdog.

At the prefectural office building in Ibaraki Prefecture, Gov. Masaru Hashimoto urged Kodama in a face-to-face meeting to launch an early investigation into the cause of the accident.

“There has been reputational damage” due to the accident just as the summer vacation season is starting in Oarai, which is popular for its beaches, Hashimoto told Kodama.