A nuclear monitoring station in Japan has recorded what could be the first detection of radioactive noble gases from February’s nuclear test in the DPRK.

The station is in Takasaki, about 100 kilometers northwest of Tokyo and about 1,000 kilometers from North Korea’s Punggye-ri nuclear test site, and is operated by the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO), which made the announcement on Tuesday.

Detected were two radioactive isotopes of the noble gas xenon: xenon-131m and xenon-133 — something the CTBTO called “rather unusual.”

Noble gases are one of four things the CTBTO looks out for in its nuclear monitoring process. That’s because the gases can be released by either More >