The Howard University campus is in Northwest Washington. (Kate Patterson/for The Washington Post)

Howard University violated a federal safety rule by allowing a hazardous radioactive substance to be stored in one of its laboratories without permission, according to a letter from the Nuclear Regulatory Commission.

The university notified the federal agency last year that it had discovered radioactive powder in a small jar in an unused physics lab. The lidded jar had been kept inside a lead-shielded container that was clearly marked as a storage unit for radioactive materials.

NRC investigators determined that the powder was actinium-227, a radioactive isotope, according to a Jan. 31 letter summarizing the incident. The jar contained about 79.5 microcuries of the substance, the letter said. The NRC said the university was never authorized to possess or use the substance.

A microcurie is a millionth of a curie, which is a unit of measurement for radioactivity.

The letter gave no indication that anyone was exposed to radiation related to the jar. Howard said Sunday that “no one was exposed or harmed.”

The powder had been stored in the locked lab since at least 2014, the letter said, but a label on the jar indicated that the material had been received in 1942.

“The scientist who had used the laboratory had retired in 2014, and it is not known if he had used the material or how it otherwise came to be at Howard,” the NRC letter said.

The NRC found no evidence that the lab had been contaminated by the actinium. But it faulted the university for failing to store the material in a ventilated hood or other containment device to prevent an airborne release. If the material had been mishandled, the letter said, “it could have resulted in unintended internal radiological dose.”

The NRC categorized Howard’s violation as a “Severity Level III” incident. A spokesman for the agency said violations are rated on a four-level scale, with Level I the most severe.

The NRC could have fined Howard $7,000 for the violation. But it waived that penalty because it said the university had taken “prompt and comprehensive” steps to address the problem.

“While there was no indication that the substance presented a safety risk, as it was stored in a lead vessel, the University adhered to all safety precautions and protocols in collaboration with the NRC following the discovery,” Howard Provost Anthony Wutoh said in a statement.

“Our priority remains the safety of our entire campus community. Once a potential risk was identified, the University adhered to all policies and best practices to assure the safety of our students, faculty and staff,” the statement said.

The NRC letter, addressed to Howard President Wayne A.I. Frederick, noted that the university had recently ordered all academic department chairs to take an inventory of the radioactive material in their labs and “confirm that they do not possess any unauthorized material.”

In 2015, the NRC found that Howard had violated safety rules in a 2008 incident involving its handling of the radioactive isotope cesium-137.

Last year, the NRC fined Idaho State University $8,500 for failing to maintain control and surveillance of a gram of plutonium.

Correction: A previous version of this story incorrectly described the curie as a unit of measurement for radiation. It actually measures radioactivity. This version has been updated.