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People in Seoul, South Korea, watched a news report on North Korea's announcement of its first hydrogen bomb test on Wednesday. Credit Jung Yeon-Je/Agence France-Presse — Getty ImagesNorth Korea’s claim that it set off a hydrogen bomb on Wednesday — in what would be the fourth time it has tested a nuclear weapon since 2006 — has stirred concerns among governments around the world. Below is a brief primer on some of the central issues at stake.
Q. How is a hydrogen bomb different from an atomic bomb?
A. A hydrogen bomb, also known as a thermonuclear bomb, combines hydrogen isotopes under extremely high temperatures to form helium, in a process known as nuclear fusion. It is more powerful than a conventional atomic weapon: It uses the energy released from the combination of two light atomic nuclei, while an atomic bomb uses the energy released when a heavy atomic nucleus splits, a process known as nuclear fission. American scientists developed the hydrogen bomb, which was first tested in 1952.
Britain, China, France, Russia and the United States are known to possess thermonuclear weapons. Whether the four other countries with atomic bombs — India, Israel, North Korea and Pakistan — also have hydrogen bombs is not certain.
The North Korean leader Kim Jong-Un guiding the test fire of a rocket in Pyongyang in an undated photo released by the country's Korean Central News Agency in 2014. Credit KCNA, via Reuters
Q. What, precisely, did North Korea announce?
A. The North’s government said that it had detonated a hydrogen bomb — its first — at 10 a.m. on Wednesday.
“This test is a measure for self-defense the D.P.R.K. has taken to firmly protect the sovereignty of the country and the vital right of the nation from the ever-growing nuclear threat and blackmail by the U.S.-led hostile forces and to reliably safeguard the peace on the Korean Peninsula and regional security,” it said, referring to the North’s formal name, the Democratic People’s Republic of Korea.
Q. How can North Korea’s claim be verified?
A. Governments and scientists are working to see if the claim is true.
The United States Geological Survey reported that it detected a magnitude 5.1 seismic event in the northeastern part of North Korea, where the test is said to have occurred — roughly similar to what happened in 2013, when North Korea tested an atomic bomb. But a South Korean lawmaker, Lee Cheol-woo, said that his country’s intelligence service estimated the event triggered an explosive yield of six kilotons and a magnitude 4.8 event — smaller than the 7.9 kilotons and magnitude 4.9 reported after the 2013 test.
A successful hydrogen bomb test would typically have an explosive yield of hundreds of kilotons — or tens of kilotons, for a failed test — Mr. Lee said.
Q. What if it was not a hydrogen bomb?
Yohei Hasegawa, an officer at Japan's meteorological agency, displayed a chart showing seismic activity in Tokyo on Wednesday. Credit Yoshikazu Tsuno/Agence France-Presse — Getty Images
A. When Kim Jong-un, the North’s leader, announced in December that his country had finally developed the technology to build a thermonuclear weapon, experts were skeptical. Some said that North Korea might be preparing to test a “boosted-fission weapon,” more powerful than a traditional atomic bomb. Designers can easily increase the destructive power of an atomic bomb by putting at its core a small amount of tritium, a radioactive form of hydrogen. The Yonhap News Agency of South Korea reported that the government in Seoul was leaning toward the theory of a boosted-fission weapon, “one level away from a hydrogen bomb.”
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Q. How many times has North Korea detonated a nuclear weapon?
A. This appears to be the fourth time. North Korea conducted underground nuclear tests on Oct. 9, 2006; May 25, 2009; and Feb. 12, 2013.
Q. What might North Korea be trying to accomplish with its threats?
A. In the past, United States administrations and South Korean governments managed to tamp down periodic heightened tensions with North Korea by offering concessions, including much-needed aid, in return for the North’s promising to end its nuclear weapons programs. Many analysts believe that North Korea is again seeking aid and other concessions, while some suggest that it merely wants to be recognized as a nuclear state, like Pakistan.
Still others suggest that the North genuinely fears an attack by the United States or South Korea and views the warnings as deterrence. Highlighting a perceived threat from abroad is also a favorite tool the North Korean government uses to ensure internal cohesion in an impoverished country that has experienced enormous privation, including devastating famine and continuing pervasive hunger.
Q. Could North Korea attack the United States?
A. Maybe. In 2012, North Korea launched a rocket that put its first satellite into orbit — raising the possibility of intercontinental ballistic missiles that could reach North America. The United Nations Security Council condemned the launching as a violation of several Security Council resolutions and tightened sanctions against it.
President Obama looked toward North Korea during a visit to the demilitarized zone between North and South Korea in 2012. Credit Saul Loeb/Agence France-Presse — Getty Images
Q. How might the United States, China, Japan and South Korea respond to a missile test or an attack?
A. If a missile attack went into the water, even if it passed over Japan, the two countries could ignore it. But if it headed for land, the United States would probably use its missile interception technology, including on Aegis-equipped ships off the Korean coast. If there were to be a more direct attack, like the torpedo that sank a South Korean warship in 2010, it is likely that both the United States and South Korea would respond. China would be less likely to take action.
Q. What was the global response to previous North Korean rocket launchings?
A. As the North’s missile technology has become more sophisticated, the launching of longer-range missiles has evoked more international concern.
In 1998, when the North launched a Taepodong that flew over Japan, Japan temporarily cut off its contribution toward a North Korean energy project. But in July 2006, when the North launched another long-range missile, various countries began imposing sanctions, while the Security Council began adding to economic sanctions.
In April 2009, when the North’s efforts to launch a three-stage Unha-2 rocket failed, the Security Council said it would strengthen punitive measures. It did so after the North conducted a nuclear test the next month.
In April 2012, the United States canceled planned food aid when the North tried to launch a more advanced missile, the Unha-3. That launching failed, but another in December succeeded in lifting a small satellite into orbit. The Security Council tightened sanctions yet again. After the North’s nuclear test in February 2013, China, the North’s longtime protector, participated in writing painful new sanctions aimed at North Korean banking, trade and travel.
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Q. What is the Obama administration’s policy on North Korea?
Kim Il-sung, left, the founder of North Korea, with his son, Kim Jong-il, in Pyongyang in 1983 at a mass rally celebrating the country's beginnings. Credit Reuters
A. The Obama administration adopted a policy of “strategic patience” in 2009, under which direct negotiations or offers of aid to Pyongyang are withheld unless the North Korea leadership shows “positive, constructive behavior” and willingness to negotiate over the dismantling of its nuclear weapons program.
The policy is a response to the American belief that the United States had unwisely offered aid, often in the wake of Pyongyang’s provocations, or struck agreements with the North on which the North later reneged. Strategic patience, in the words of Robert M. Gates, the former defense secretary, grew out of a desire not “to buy the same horse twice.”
Critics say that while the policy has allowed the United States to weather multiple rounds of belligerence by Kim Jong-il and his son, Kim Jong-un, without making concessions, it has done little to curb the development of North Korea’s nuclear weapons program.
Q. What sanctions are currently in place?
A. The Security Council has passed four resolutions since 2006 aimed at penalizing North Korea for its nuclear weapons program. In addition, the United States, which remains in a technical state of war with North Korea, has imposed its own regimen of strict economic sanctions. The combined effects have severely squeezed, but not crippled, North Korea’s economy. The United Nations has prohibited the North from conducting nuclear tests or launching ballistic missiles, requested that it abandon all future efforts to pursue nuclear weapons and urged it to return to negotiations with China, Japan, Russia, South Korea and the United States, the so-called six-party talks.
The resolutions have also imposed embargoes on large-scale arms, weapons-related research and development materials, and luxury goods; banned many types of financial transactions including transfers of cash; placed new restrictions on diplomats; and created monitoring mechanisms for enforcement.
The American sanctions freeze all North Korean property interests in the United States, ban most imports of goods and services from the North, and prohibit American dealings with any names on a blacklist of North Korean businesses and individuals suspected of illicit activities including money laundering, counterfeiting, currency smuggling and narcotics trafficking.
Nothing in the American sanctions prohibits American travel to North Korea or the export of food and other types of humanitarian aid, although there are some restrictions.
President Park Geun-hye of South Korea, center, during an urgent meeting of her top national security aides in Seoul, South Korea, on Wednesday. Credit Jeon Jin-Hwan/Newsis, via Associated Press
The sanctions leave room for considerable trade in many types of goods and services. China, which supplies much of North Korea’s basic needs, is not in any violation of the United Nations resolutions.
Q. How is the South Korean government responding to the North’s threats?
A. President Park Geun-hye, who took office in 2013, is the daughter of a former president who ran South Korea as a dictator during the Cold War. She once promised that if the North mounted a nuclear attack, its government would be “erased from the earth.” She has largely held a firm line on North Korea, after a more conciliatory stance in the 1990s.
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From 1998 to 2008, the South pursued a “sunshine policy” of reconciliation and economic cooperation that sent billions of dollars in business investments, goods and humanitarian aid to the North. Ms. Park’s immediate predecessor, Lee Myung-bak, said the North would need to give up its nuclear weapons to receive any more aid. But he was criticized for what many saw as a weak response after the North shelled a South Korean island in 2010, killing four people.
Q. Why hasn’t China stopped North Korea from its campaign of threats? Is there any other country that has enough influence on North Korea to stop it?
A. China, the North’s patron, has long feared that a collapse of the North Korean government could lead to a unified Korea allied with the United States.
China helped write and backed the most recent round of United Nations sanctions, but it has been loath to push the North too hard. Its patience with the North may be running out, but even China may have only limited information about the machinations within the Pyongyang government.
Q. Why are relations so bad between North and South Korea?
A. After the United States and the Soviet Union divided the Korean Peninsula at the end of World War II in 1945, they helped install rival governments in Seoul and Pyongyang. Each asserted claims to the whole of Korea. The two fought the 1950-53 Korean War, which ended not in a peace treaty but a truce. Mutual mistrust runs deep, although there have been intermittent attempts at political reconciliation and economic cooperation.
Q. How did the North get nuclear weapons?
A. The project started under Kim Il-sung, the country’s founder and the grandfather of the current leader. Mr. Kim knew that Gen. Douglas MacArthur wanted Washington to allow the use of nuclear weapons against Chinese and North Korean troops during the Korean War.
By the 1980s, American intelligence satellites were watching the nuclear complex at Yongbyon come together. Relations between the United States and the North grew especially tense over the issue in 1994, and some in the White House feared a war could break out. A pact was eventually hammered out that year, the Agreed Framework, but it fell apart in 2002, during the George W. Bush administration, partly over allegations the North was cheating on its agreements and developing another path to a bomb. In 2006, the North conducted its first nuclear test, a partial fizzle. But the subsequent tests were more successful.
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