This report presents what is known publicly about the tests, discusses detection and containment of nuclear tests, explores the possible significance of containment for North Korea, and raises, as issues for Congress, implications for the Comprehensive Nuclear-Test-Ban Treaty (CTBT) and possible means of improving U.S. and international ability to monitor nuclear testing. It establishes an International Monitoring System (IMS) to monitor signs of an explosion. As of November 2010, 182 nations had signed the treaty and 153 of them had ratified. In April 2009, President Obama pledged to pursue U.S. ratification of the CTBT "immediately and aggressively." It was clearly nuclear because it released radioactive materials. The 2009 Test

North Korea announced on May 25, 2009, that it had conducted a second nuclear test. The lack of certainty as to whether the test was nuclear arises because seismic signals, including those detected by 61 stations of the IMS, were consistent with a nuclear test, and seismic signals from the 2006 and 2009 events were very similar, but open sources did not report the detection of physical evidence that would provide conclusive proof of a nuclear test, such as certain radioactive isotopes of noble gases or radioactive particulates (i.e., fallout). The United States conducted large aboveground and underground tests using chemical explosives to simulate some effects of nuclear explosions. How might North Korea have contained its second test? Of course, OSIs pursuant to the treaty could only occur after the treaty had entered into force, though it is possible that OSIs could be done outside the treaty regime, such as if requested by one country before the treaty enters into force to prove that it had not conducted a nuclear test, or pursuant to a bilateral agreement permitting one state to monitor another state's nuclear test site. The United States has its own technical means of detecting nuclear explosions, which the Air Force Technical Applications Center (AFTAC) operates. Either may prove that a nuclear test occurred. Some believe that argon-37 could be detected at long range. How Can Radioactive Material Be Contained? Better techniques can greatly improve containment. Some U.S. nuclear tests that were not reported as releasing radioactive material might have done so, but the amount released may have been below the detection threshold for instruments available at the time. While there is no publicly-available information on whether North Korea attempted to contain its second test, and if so what methods it used, containment could have resulted from one of the following factors, or a combination of several:
Lessons learned from the first test: As noted, a nuclear test provides data for many disciplines involved with the test. Use of a higher-yield nuclear device: It can be harder to contain lower-yield underground nuclear explosions than higher-yield ones. Nonnuclear explosion: The May 25 event may not have been a nuclear test, which would explain the lack of radioactive effluents. Fallout on Japan or South Korea would likely antagonize them. Fallout on North Korea could contaminate land. Of course, the surer way for North Korea to avert OSIs would be for that nation not to ratify the CTBT, keeping it from entering into force. For example, detecting uranium in debris from a nuclear test conducted inside North Korea would not be proof that the weapon was North Korean, even if it could be determined that the uranium was of North Korean manufacture: "David Asher, who helped direct efforts to counter North Korea's proliferation activities in the George W. Bush administration … [said] 'My fear is that just as Iran's demands for enriched uranium for a bomb are expanding, North Korea may be in the position to begin supplying.'" Opponents might argue that without detection of radionuclides there is no proof that North Korea conducted a nuclear test. Suspicious seismic signals and an absence of radionuclides, it is argued, would surely lead to calls for an OSI. Entry into force requires ratification by North Korea, among others, yet that nation's ratification may be difficult to obtain. In examining budgets and programs, Congress may wish to consider various means of improving U.S. and international ability to monitor nuclear testing by North Korea and other nations.
On May 25, 2009, North Korea announced that it had conducted its second underground nuclear test. Unlike its first test, in 2006, there is no public record that the second one released radioactive materials indicative of a nuclear explosion. How could North Korea have contained these materials from the May 2009 event and what are the implications?
As background, the Comprehensive Nuclear-Test-Ban Treaty (CTBT) would ban all nuclear explosions. It was opened for signature in 1996. Entry into force requires ratification by 44 states specified in the treaty, including the United States and North Korea. As of November 2010, 153 states, including 35 of the 44, had ratified. North Korea has not signed the CTBT. President Clinton signed it in 1996; in 1999, the Senate voted not to consent to its ratification. In 2009, President Obama pledged to press for its ratification.
The treaty establishes a verification mechanism, including an International Monitoring System (IMS) to detect nuclear tests. Three IMS technologies detect waves that pass through the oceans (hydroacoustic), Earth (seismic), or atmosphere (infrasound); a fourth detects radioactive material from a nuclear test. Scientists concur that only the latter proves that an explosion was nuclear. Some believe that deep burial and other means can contain radioactive effluents. Another view is that containment is an art as much as a science. The United States learned to improve containment over several decades. Yet by one estimate, North Korea contained over 99.9% of the radioactive effluents from its 2009 test. It might have done so by application of lessons learned from its 2006 test or the U.S. nuclear test experience, use of a higher-yield device, release of material below the detection threshold, good luck, or some combination. Alternatively, the 2009 event may have been a nonnuclear explosion designed to simulate a nuclear test.
Containment could be of value to North Korea. It could keep radioactive fallout from China, Japan, Russia, or South Korea, averting an irritant in relations with them. It could prevent intelligence services from gathering material that could reveal information about the weapon that was tested. It could permit North Korea to host nuclear tests by other nations, such as Iran; while such tests would be detected by seismic means, they could not be attributed to another nation using technical forensic means if effluents, especially particles, were contained.
An issue for Congress is how containment could affect CTBT prospects. Supporters might argue that explosion-like seismic signals without detected radioactive material would lead to calls for an onsite inspection. Opponents might claim that only detection of radioactive material proves that a nuclear explosion occurred. Both would note inspections could not be required unless the treaty entered into force, supporters to point to a benefit of the treaty and opponents to note that North Korea could block inspections by not ratifying the treaty. Congress may wish to consider ways to improve monitoring capability, such as supporting further research on test signatures, improving monitoring system capability, and deploying more monitoring equipment. This update reflects developments in the North Korean uranium program and prospects for another nuclear test.
Related CRS reports include CRS Report RL34256, North Korea's Nuclear Weapons: Technical Issues, which summarizes open-source information on that nation's nuclear weapons program, including fissile material and warhead estimates, and assesses developments toward denuclearization; and CRS Report R40684, North Korea's Second Nuclear Test: Implications of U.N. Security Council Resolution 1874, which analyzes possible economic effects on North Korea of sanctions and vessel inspections that Resolution 1874 puts in place.