South Korea said nuclear material detected but unsure of origin SEOUL, Aug. 13 (UPI) -- The South Korean government said Tuesday it detected traces of nuclear activity in its atmosphere though it was unclear if it came from North Korea. North Korea in February conducted an underground nuclear test, its third since 2006. North Korean analysts at Johns Hopkins University have detected some level of work at various missiles sites so far this year. Missile tests in the past have preceded all of North Korea's nuclear tests. The Korea Institute of Nuclear Safety in June detected trace amounts of Xenon in the South Korean atmosphere, the Yonhap news agency reported. Xenon is present in trace amounts following nuclear detonations and related activity. A government source said no nuclear activity was observed from North Korea in the weeks leading up to the June detection. So far, investigators have been unable to track down the source of the Xenon. Sen. Robert Menendez, D-N.J., is scheduled to visit South Korea for two days starting Saturday. He's scheduled to discuss bilateral concerns with South Korean leaders and visit U.S. service personnel stationed in Seoul. It's his first visit to the region as chairman of the Senate Foreign Relations Committee.