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 April 2010, 151 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 that 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 also wish to consider options to improve monitoring capability, such as supporting further research on test signatures, improving the capability of monitoring systems, and deploying more monitoring equipment. This report may be updated, especially if North Korea conducts another 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.