SEOUL — At the same time the Carter administration was mulling options to pull U.S. forces from South Korea in the late 1970s, U.S. officials were studying the use of nuclear weapons to repel a possible North Korean invasion, recently unclassified documents show.

According to a lengthy study funded by the U.S. Nuclear Defense Agency — obtained last week under a Freedom of Information Act request by the Nautilus Institute for Security and Sustainability — the options amounted to using 30 airburst nuclear weapons on invading armor units.

“Vulnerability of North Korean Forces, Vol. I: Evaluation of Vulnerability of North Korean Divisions to Tactical Nuclear Weapons” found that use of tactical nuclear weapons on the battlefield would be effective against North Korean armor units in a scenario where they penetrated 15 miles south of the Demilitarized Zone, just nine miles north of Seoul.

“While this analysis is focused upon attacks against acquired targets, some consideration is also given to the use of deduced targeting against suspected enemy positions or terrain targets,” the 1978 report read.

“The evaluation of NK division vulnerability centers on individual enemy targets or units deployed on the terrain, the capability of allied forces to acquire these targets, the damage achieved against these targets from an appropriate combination of weapons effects, and the significance of this damage on the performance of combat missions.”

The U.S. military acknowledged keeping nuclear weapons on the Korean peninsula until an October 1991 decision to withdraw them as part of a “worldwide drawdown” of tactical nuclear weapons, according to the National Security Archives.

A previous study released by Nautilus detailed delivery systems of the former U.S. nuclear force in South Korea; an upcoming release will focus on old military exercises involving nuclear-tipped artillery, the group said.

“These documents suggest that North Koreans may have deeply entrenched nuclear threat perceptions that drive their own strategy to acquire nuclear weapons today,” Nautilus researchers said.