Papers released under freedom of information laws describe reports by a former security officer at the power plant describing the strange incident.
The unnamed officer worked at the Cooper Nuclear Station near Brownville, Nebraska, according to the files released by the Nuclear Regulatory Commission (NRC).
The incident was described as an “unidentified flying object violating the protected area at the station”.
According to the documents, the report was only received by the former employee in June 2010.
The report said: “An unidentified flying (UFO) object violated the protected area at Cooper Nuclear Station between 1986 and 1989, but the event was not reported to the NRC as required.
“The CI [confidential informant] described an event that occurred during his employment as a security officer.
“He was employed there from 1986 through 1989 and did not remember specifically when during that time the event occurred.
“While posted at the intake structure one night, he observed an ‘unidentified flying object’ fly down the Missouri River about 150 feet in the air and hover in front of the intake.
“He observed it for a few moments and then contacted a fellow security officer who also observed it.”
It said that after they watched it together, the object went back up the river.
Colleagues did not believe the pair, it was reported.
It said the following evening, the officer saw the UFO return.
He did not tell anyone until it came into the protected area and hovered just north of the reactor building.
The man described it as triangular in shape with a rotating circle of lights on the bottom.
This matches the description of the alleged TR-3B triangular UFO, which some conspiracy theorists claim is a secret US spy craft developed using reverse engineered alien technology taken from crashed flying saucers.
It was also reportedly silent and a third of the size of the reactor building.
He the called the security room and most officers on shift reportedly saw it.
The report addedL “These individuals included (names reacted), all of whom still work at the plant today.
“After hovering there for a few minutes, the UFO exited the protected area and returned back up the river to the north as it had the previous night.
“The CI said that he never saw the UFO at the plant again after that evening.”
The whistleblower said the incident should have been reported as a violation of the protected area space but was not reported.
The documents show that Nick Taylor, the NRC senior resident inspector, searched for corroborating documents from the time, but found nothing.
However, he said: “I’d be careful about concluding that if an event wasn’t recorded in CAP [Corrective Action Program] that it did not occur.”
The files were published on UFO website TheBlackVault.com, whose founder John Greenewald requested they be released.
The website includes millions of declassified documents, many obtained by Mr Greenewald.