seoul south korean nuclear experts check contamination north korea sixth large nuclear test say friday find minute trace radioactive xenon gas early specify source nuclear safety security commission nssc say conduct test land air water sample since shortly north korea nuclear test sunday statement say commission analyze xenon enter south korean territory make decision late time whether material link north korea nuclear test xenon naturally occur colorless gas use manufacture sort light nssc say detect xenon-133 radioactive isotope occur naturally past link north korea nuclear test chance xenon impact south korea territory population statement say south korea find trace radioactive gas 'ca n't yet link nuclear test