Traces of Xenon, a rare isotope linked to nuclear activities, was detected by the Korea Institute for Nuclear Safety three times in South Korea's atmosphere in June, Yohnap news agency reported on Tuesday.
There were no signs of a North Korean nuclear bomb test, and no seismic activity had been observed, though it remains unclear where the Xenon originated.
"Relevant agencies are conducting an analysis, and that's what we know," a South Korean Defense Ministry spokesman said.
Fear still lingers after North Korea detonated its third underground nuclear bomb in February — when various nations noticed suspicious seismic action — especially with the possible restart of its nuclear enrichment plant in Yongbyon.
Reasonable speculation suggests the Xenon may have come from Japan's wrecked Fukushima Daiichi nuclear power plant, according to Lee Ho-ryung, a nuclear specialist at the Korea Institute of Defense Analyses, who spoke with the Wall Street Journal.
The mystery of the isotope comes as diplomatic relations between North and South Korea appear to be improving, as least compared to the high tensions that followed the North's nuclear bomb test, when militaries mobilized and heated rhetoric passed almost daily between the two nations.
Indeed, the Kaesong Industrial Complex, a rare symbol of inter-Korean cooperation that closed in April, is on track to reopen. On Wednesday, South and North Korea reached a five-point agreement to reopen the shuttered complex. The Koreas agreed on safeguards to prevent another work stoppage at the joint factory park and will move to set up a joint committee to oversee future operations — though as South Korea's chief delegate Kim Ki-woong noted on Tuesday, much remains to be done.
"The fact that the two sides have sat down for the seventh time shows the difficulties involved in the negotiations, but if both sides concur on the need for 'progressive development' of the Kaesong complex and work together, all problems and difficulties can be overcome," he said.