When nuclear plants were designed, says Wald, regulators anticipated that spent fuel rods would sit in cooling water pools for five years and then be chemically processed and turned into plutonium.

"Reprocessing of spent fuel rods is at its final phase and extracted plutonium is being weaponised, " it said.

The Yongbyon facilities include a 5mw graphite-moderated reactor and a plant used to reprocess spent nuclear fuel from this reactor, including the extraction from it of weapons-grade plutonium.