The announcement Jan. 6, 2016 from North Korea that it had carried out a nuclear test brought to the front lines of global attention a phrase not often heard since the Cold War -- "the H-bomb."
As opposed to the atomic bomb, the kind dropped on Japan in the closing days of World War II, the hydrogen bomb, or so-called "superbomb" can be far more powerful -- experts say, by 1,000 times or more. A staggering difference in the destructive capability that could be unleashed.
The first hydrogen bomb, "Mike," erupts in the Pacific over the Eniwetok Atoll, Marshall Islands, Nov. 1, 1952. The 82-ton bomb, housed in a large aluminum building, was the first test of a thermonuclear weapon. The largest bomb ever made wiped out Elugelab island and vaporized everything within three miles.
Officers from the Korea Meteorological Administration point at the epicenter of seismic waves in North Korea, at the National Earthquake and Volcano Center of the Korea Meteorological Administration in Seoul, South Korea, Jan. 6, 2016. A Magnitude 5.1 earthquake was registered.
While seismic data supported the claim of a large explosion, there was no immediate way to confirm whether an H-bomb was detonated in North Korea.
People walk by a screen showing a news report on an earthquake near North Korea's nuclear facility, in Seoul, Jan. 6, 2016.
South Korean officials detected an "artificial earthquake" near North Korea's main nuclear test site, a strong indication that nuclear-armed Pyongyang had conducted its fourth atomic test. North Korea said it planned an "important announcement" later in the day.
North Korea's first three nuclear tests, in 2006, 2009 and 2013, were A-bombs on roughly the same scale as the ones used on Hiroshima and Nagasaki.
North Korea's nuclear ambitions
In this image from KRT, the North Korean state news agency, a news anchor announces the country conducted a hydrogen bomb test, Jan. 6, 2016.
The announcement sent shockwaves around the globe.
The fireball of a hydrogen bomb lights the Pacific sky a few seconds after the bomb was released by the U.S. over Bikini Atoll, Marshall Islands on May 21, 1956. Streamers at right are trails of rockets fired just before the blast for testing purposes.
The U.S. tested a series of 23 nuclear bombs on the atoll from 1946 to 1958. Nuclear testing left the atoll uninhabitable.
Atomic bombs rely on fission, or atom-splitting, just as nuclear power plants do. The hydrogen bomb, also called the thermonuclear bomb, uses fusion, or atomic nuclei coming together, to produce explosive energy. Stars also produce energy through fusion.
The stem of a hydrogen bomb, the first such nuclear device dropped from a U.S. aircraft, moves upward through a heavy cloud and comes through the top of the cloud, after the bomb was detonated over Namu Island in the Bikini Atoll, Marshall Islands, May 21, 1956.
The hydrogen bomb has never dropped on any targets. It was first successfully tested by the U.S., in bombs called "Mike" in 1952 and "Bravo" in 1954. The Soviets exploded their first h-bomb in 1955,
Health impact from nuclear tests
A man holding a portrait of Aikichi Kuboyama, chief radioman of Japanese fishing boat Daigo Fukuryumaru, who died of acute organ malfunction, leads anti-nuclear protesters during a march in Yaizu, Shizuoka prefecture, west of Tokyo on March 1, 2014.
The crew of a Japanese fishing boat that unknowingly went into the waters near the nuclear testing of "Bravo" in the 1950s got acute radiation sickness. Since the 1960s, nuclear tests have gone underground to reduce radioactive fallout.
The devastated city of Hiroshima after the first atomic bomb was dropped by a U.S. Air Force B-29 on Aug. 6, 1945. The atom bomb attack on Hiroshima was followed three days later by the Nagasaki bombing.
The U.S. atomic attacks killed 140,000 people in Hiroshima and more than 70,000 in Nagasaki, either instantly or later through the horrific effects of burns from the white-hot nuclear blast and radiation sickness.
The A-bombs and nuclear plants use a process, nuclear fission, which is the splitting of an atom such as uranium to create a burst of energy.
Kim Jong Un
North Korean leader Kim Jung Un is shown reportedly guiding the test fire of a tactical rocket in this undated photo released by country's Korean Central News Agency (KCNA) in Pyongyang, August 15, 2014.
North Korea has long seen having nuclear weapons as key to its security and its global stature.
The skyline (top) at Bikini Atoll in the Pacific is a study in black and white with the airplane-dropped H-bomb silhouettes clouds on May 21, 1956 (Bikini Atoll time). Below, in another picture taken about 50 miles from the target, the cloud forms the shape of a man-made sun.
Nuclear fusion joins, or fuses light atoms such as hydrogen in a reaction that creates a third, heavier atom and creates a powerful blast of energy as a byproduct. This is how the sun generates light and heat.
"Think what's going on inside the sun," explained Takao Takahara, professor of international politics and peace research at Meiji Gakuin University in Tokyo to the Washington Post. "In theory, the process is potentially infinite. The amount of energy is huge."
North Korean nuclear ambitions
North Korea's nuclear program has been a source of concern for a long time. The country withdrew from the Non-Proliferation Treaty in March 1993 and makes no bones about its intentions.
In this photo, thousands of North Koreans turn colored cards to form the symbol for the atom as gymnasts perform on the field below during a "mass games" performance at a stadium in Pyongyang, North Korea Friday, Sept. 19, 2008.
North Korea's Unha-3 rocket
A screen at the General Satellite Control and Command Center shows the moment North Korea's Unha-3 rocket is launched in Pyongyang, North Korea, Dec. 12, 2012, despite international warnings. Thought the test failed, it was a clear sign of the country's nuclear ambitions.
The technology of the hydrogen bomb is more sophisticated than an A-bomb, and once attained, it is a greater threat.
The rogue nation said that it tested a "miniaturized" hydrogen bomb, which, if confirmed, would represent a major improvement to their nuclear arsenal. They can be made small enough to fit on a head of an intercontinental missile.
"Fat Man" atomic bomb
The "Fat Man" atomic bomb, seen here, was dropped on Nagasaki, Japan. The bomb, carrying Plutonium, weighed 10,300 pounds and yielded 20 kilotons of explosive power. "Little Boy," which was dropped over Hiroshima weighed 9,700 pounds and carried Uranium-235. Its yield was 15 kilotons. The U.S. dry fuel hydrogen bomb in 1954 yielded 15 megatons -- 1,000 times more powerful than "Little Boy."
To trigger fusion requires an enormous amount of energy to begin with. For that reason, an H-bomb has an atomic bomb inside its core that acts as a trigger, explains Kim Du-yeon of the Carnegie Endowment for International Peace.
North Korean missile
A North Korean vehicle carries a missile during a mass military parade in Pyongyang's Kim Il Sung Square, April 15, 2012.
The H-bomb requires more technology in control and accuracy because of the greater amount of energy involved. Missile technology development has been a key part of the North Korean program.
French nuclear test
This picture taken in 1970 shows a French nuclear test at Mururoa, French Polynesia. Researchers have established a link between France's nuclear tests over the Pacific ocean in the late 1960s and the high incidence of thyroid cancer in Polynesia.
France carried out some 40 atmospheric atomic tests in Polynesia from 1966 to 1974. After that its tests were carried out underground.
A victim of the first atomic bomb ever used in warfare is seen in September 1945, at the Ujina Branch of the First Army Hospital in Hiroshima. The thermic rays emitted by the explosion burned the pattern of this woman's kimono upon her back.
The destructive power of the atomic bombs used on Japan was considered unprecedented at the time, incinerating buildings and people. They left lifelong scars on survivors, both physical and psychological, and on the cities themselves. Since then, countries have acquired even greater destructive power with the thermonuclear hydrogen bomb.
China and North Korea
In this photo, North Korean leader Kim Jong Un, right, joins hands and waves with visiting Chinese official Liu Yunshan, the Communist Party's fifth-ranking leader, during a parade in Pyongyang, North Korea, Oct. 10, 2015. China sees North Korea's claim to have conducted its first hydrogen bomb test on Jan. 6, 2016, as yet another act of defiance, boding ill for a relationship already under strain.
The hydrogen bomb is in fact already the global standard for the five nations with the greatest nuclear capabilities: the U.S., Russia, France, the U.K. and China. Other nations may also either have it or may be working on it, despite a worldwide effort to contain such proliferation.
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