BOISE, Idaho (AP) — A small amount of radioactive, weapons-grade plutonium about the size of a U.S. quarter is missing from an Idaho university that was using it for research, leading federal officials on Friday to propose an $8,500 fine.

The U.S. Nuclear Regulatory Commission said Idaho State University can't account for about a 30th of an ounce (1 gram) of the material that's used in nuclear reactors and to make nuclear bombs.

The amount is too small to make a nuclear bomb, agency spokesman Victor Dricks said, but could be used to make a dirty bomb to spread radioactive contamination.

"The NRC has very rigorous controls for the use and storage of radioactive materials as evidenced by this enforcement action," he said of the proposed fine for failing to keep track of the material.

Dr. Cornelis Van der Schyf, vice president for research at the university, blamed partially completed paperwork from 15 years ago as the school tried to dispose of the plutonium.

"Unfortunately, because there was a lack of sufficient historical records to demonstrate the disposal pathway employed in 2003, the source in question had to be listed as missing," he said in a statement to The Associated Press. "The radioactive source in question poses no direct health issue or risk to public safety."

Idaho State University has a nuclear engineering program and works with the U.S. Department of Energy's Idaho National Laboratory, considered the nation's primary nuclear research lab and located about 65 miles (105 kilometers) northwest of the school.

The plutonium was being used to develop ways to ensure nuclear waste containers weren't leaking and to find ways to detect radioactive material being illegally brought into the U.S. following the Sept. 11, 2001, terrorist attacks, the school said in an email to the AP.

The university, which has 30 days to dispute the proposed fine, reported the plutonium missing on Oct. 13, according to documents released by the Nuclear Regulatory Commission. The agency said a school employee doing a routine inventory discovered the university could only account for 13 of its 14 plutonium sources, each weighing about the same small amount.

The school searched documents and found records from 2003 and 2004 saying the material was on campus and awaiting disposal. However, there were no documents saying the plutonium had been properly disposed.

The last document mentioning the plutonium is dated Nov. 23, 2003. It said the Idaho National Laboratory didn't want the plutonium and the school's technical safety office had it "pending disposal of the next waste shipment."

The school also reviewed documents on waste barrels there and others transferred off campus since 2003, and opened and examined some of them. Finally, officials searched the campus but didn't find the plutonium.

The nuclear commission said senior university officials planned to return the school's remaining plutonium to the Energy Department. It's not clear if that has happened.

Energy Department officials didn't return calls seeking comment Friday.

Dricks, the commission spokesman, said returning the plutonium was part of the school's plan to reduce its inventory of radioactive material.

He said overall it has "a good record with the NRC."

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This story has been corrected to show that the missing plutonium is about a 30th of an ounce, not a third of an ounce.