North Korea may be one test shy of developing a technology to miniaturize a nuclear warhead small enough to fit on its long-range ballistic missile, a nuclear policy researcher said Wednesday. \"In the last (third) nuclear test, they could not finish the task of miniaturization ... but if they have a chance for more nuclear tests, maybe one more, they would be able to have small and more reliable device for their missile,\" Li Bin, a nuclear policy expert at the Carnegie Endowment for International Peace and a professor at Tsinghua University, said during an international forum on North Korea, hosted by the Asan Institute for Policy Studies. In its first nuclear test in 2006, \"(the North) began with a small device with a small amount of explosives, and it was not so successful,\" the Chinese expert said. \"Then they had to add more chemical explosives because the yield was not good enough. Eventually they got full yield (in the third test), but the device is not small enough,\" according to Li, who also joined the Chinese delegation on the Comprehensive Test Ban Treaty negotiations. The analysis came as China is moving quickly to revive the long-stalled six-party talks aimed at persuading the communist country to stop its nuclear weapons program. After the disarmament dialogue held their last meeting in late 2008, the North conducted two more nuclear tests including one in February and several rocket launches, which the international community believes were to test the country\'s long-range missile technology. Pyongyang is believed to be developing intercontinental ballistic missiles (ICBMs), and the country has repeatedly threatened nuclear attacks on the continental United States and South Korea. The Chinese expert also noted that the North may have tested a plutonium-based nuclear weapon, instead of uranium-based one, because it is more difficult to miniaturize bombs using uranium. Joshua Pollack, a nuclear expert at Science Applications International Corp., said the North is presumed to be internally producing crucial components for gas centrifuge, used for uranium enrichment, given the progress the North has made in its nuclear facilities in Yongbyon despite little indication that the country imported the crucial parts since 2003. \"If North Korea is already making crucial components for centrifuges inside the country, then the current policy based on export controls, sanctions and interdictions has probably reached its limit of effectiveness,\" Pollack said. \"If that\'s the case we cannot easily stop the expansion of the enrichment program,\" he said, adding that raises a serious question about whether there can be viable strategies to denuclearize the North