A new reactor at North Korea’s primary nuclear weapons development site has been in regular operation since last year and can likely produce enough plutonium to craft around a dozen nuclear weapons annually, a think tank assessed in a report this week. 

The Institute for Science and International Security (ISIS) found that the experimental light water reactor at the Yongbyon nuclear complex has discharged “significant amounts of water” into a nearby river as part of a cooling process since October, with hot water melting ice near the discharge point in the winter season indicating the reactor’s operation.

Snow melt visible