How will we know? A successful nuclear test will be easily detected because its seismic signals will be monitored around the world by the International Monitoring System established under the Comprehensive Test Ban Treaty to monitor potential clandestine nuclear tests anywhere in the world. Both the 2006 and 2009 tests gave indisputable seismic evidence of nuclear tests. This one may be even easier to detect because Pyongyang has vowed to test at a higher level.
But what exactly did Pyongyang mean by a "higher level?" Was it just a higher explosion yield? That is possible, because much of the international community dismissed the 2006 test as a failure and the 2009 test as not very successful. The yield of the 2006 test is estimated at somewhat less than 1 kiloton (1,000 tons of TNT equivalent). Experts are still divided on the yield of the 2009 test; our best estimate is between 2 and 7 kilotons. In any case, if the North Koreans can explode a device with a yield in that range, then they most likely can produce a Nagasaki-like bomb with a yield of 20 kilotons. Perhaps that is what Pyongyang means by a higher level.
More likely, however, and consistent with Pyongyang's pronouncement that it will also increase its nuclear deterrent qualitatively, is an attempt to test a more sophisticated, miniaturized design. How will we know? Pyongyang will almost certainly claim that the test was successful and will tout its sophistication. It will be difficult to distinguish truth from propaganda, but experience shows there is often a nugget of truth in North Korea's claims. It will also be difficult to discern from seismic signals if one or two devices were tested if they are simultaneous and closely spaced.
Aside from seismic signals, which tell us only the size of the explosion and do not allow us to differentiate between plutonium and HEU, nor tell us anything about the sophistication of the device, there are only a few other signals that can be monitored. If the nuclear blast carried out in the tunnel deep underground causes sufficient fissures in the overburden rock, then gaseous fission products can escape and may be detected by airborne instruments or radiological monitoring stations around the world. The U.S. government reported that it picked up such signals after the 2006 test with offshore airborne monitors. It announced that these signals gave definitive proof that North Korea had detonated a nuclear device, but did not specify whether it was plutonium or HEU. There are different telltale signatures for HEU and plutonium devices, but they must be detected and analyzed very rapidly to allow conclusive identification. There were no reports that anyone detected radiological signals after the 2009 test. This could likely be a result of better containment or just bad luck of not having the detectors in the right place at the right time.
If a next test is well contained, then we may learn nothing about the device detonated. However, one of the risks Pyongyang takes in trying to demonstrate a test at a higher level is that they may produce fissures that allow radioactive seepage or possibly cause a major blowout from the tunnel. The U.S. testing program experienced such problems even after having conducted hundreds of tests. Unrecognized complex geological conditions apparently led to a blowout during the 1970 underground Baneberry nuclear test at the Nevada Test Site. The blowout released a radioactive cloud nearly 10,000 feet high. Were something similar to happen in North Korea's next test, we would be more likely to learn technical details about the type of device detonated due to radiological contamination. However, spewing a radioactive cloud over the skies of Northeast Asia would create an enormous political storm from the nearby countries.
When will they test? Overhead imagery of the Punggye-ri nuclear test site demonstrates conclusively that North Korea is prepared to test. A third test tunnel, identified by the south portal, has been ready for nearly a year. It has been kept prepared through summer floods and winter snow. There has been a flurry of recent activity there and at the west portal, site of the 2009 test, and a nearby support area. Security appears particularly strict around the west portal, potentially indicating that the test device is or will be housed there until emplacement into the south tunnel. Everything we can see indicates North Korea is technically ready to test with little notice. When to test is now largely a political decision.
What difference will a test make? A successful test will make Pyongyang's nuclear weapons appear more threatening and make its deterrent more credible because it may then possess a missile-deliverable nuclear weapon. It may also set North Korea on a path of substantially expanding its nuclear arsenal through stepped-up HEU production. It may make Pyongyang more aggressive and provocative in dealing with South Korea and Japan. However, one more test does not fundamentally change the security threat North Korea poses. Pyongyang can threaten South Korea, Japan, or U.S. regional assets, but it can only use its nuclear weapons if it is prepared to accept the destruction of the regime.
A successful test will, however, destabilize the region -- precisely the scenario China has tried to avoid by supporting Pyongyang over the years, and the reason it is in China's interest to use all its influence to stop the test. The combined military forces of South Korea, Japan, and the United States will be forced into higher alert status. A test will likely drive them to increase their ballistic missile defense protection against North Korea, which will clearly complicate relations with China.
One of the most damaging results of another test will come from potential cooperation with Iran. Sharing Pyongyang's nuclear test experience with Tehran similarly to how it has shared missile technologies will greatly increase the Iranian nuclear threat. Iran now has the capacity to enrich uranium to weapons grade, although it has claimed to have enriched it only to lower levels for peaceful purposes. It would be very difficult for Iran to continue its peaceful nuclear façade if it tested to further its nuclear weapons capabilities. However, if Pyongyang were to involve Iran or share its testing experience, that would change the picture dramatically. Should Iran make the decision to build nuclear weapons, it is more likely to do so without necessarily testing its own device.
But perhaps the greatest impact of another North Korean nuclear test is that it will signal that the new regime, like its predecessors, has chosen bombs over electricity. Another nuclear test will make it impossible for the new South Korean government or the second Obama administration to look for resolution of long-standing enmities by focusing on issues beyond the nuclear dispute. Normalization of relations, a peace treaty, access to energy and economic opportunities -- those things that come from choosing electricity over bombs in the nuclear arena and have the potential of lifting the North Korean people out of poverty and hardship -- will be made much more difficult, if not impossible, for the next five years, if not longer.