A U.S. group tracking data on weapons of mass destruction has ranked North Korea last in the world in terms of safeguarding its nuclear stockpile, according to KBS World Radio.Washington-based Nuclear Threat Initiative (NTI) gave North Korea a score of 30 out of 100 in its latest Nuclear Materials Security Index of 25 countries.The index surveyed countries with more than one kilogram of weapons-usable nuclear materials, including highly-enriched uranium and plutonium.The NTI categorized nine countries, including North Korea, the United States, China, France and India, as nuclear-armed states. It said these countries possess 95 percent of the worlds nuclear materials, including military and civilian-purpose materials.Australia ranked first with a score of 92 followed by Canada and Switzerland.