Golez: Global security in extremis.
North Korea Hydrogen-Bomb Test Would Signal Dangerous New Phase
Questions remain, however, over what exactly North Korea tested and its significance
In this May 21, 1956 file photo, 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. PHOTO: /ASSOCIATED PRESS
BEIJING--A successful test of a North Korean hydrogen bomb, if confirmed, would be a milestone in the country’s nuclear program, potentially yielding a more powerful weapon than the kind it has tested in the past.
Questions remain, however, over what exactly North Korea tested Wednesday and its significance. The power of the blast undergroundnear a known test site in the country’s northeast appears to have been roughly the same as that of a device tested in 2013 that was thought to be weaker than a hydrogen bomb.
A hydrogen bomb—technically known as a “thermonuclear weapon”—usually uses a smaller, primary atomic explosion to ignite a secondary, much larger blast. The first stage is based on nuclear fission—the splitting of atoms --and the second on nuclear fusion, which combines atoms, smashing them together and unleashing more energy. Additional stages can be added to increase its destructive force.
North Korean state media announced that the country carried out a successful test of a thermonuclear weapon on Wednesday. Photo: AP
That makes the H-bomb far more powerful than early nuclear weapons that typically used a single-stage blast based only on nuclear fission. Those weapons are known as “pure fission” devices and are thought to have been used in all of North Korea’s three previous nuclear tests, which it said involved atomic bombs.
The first atomic weapons built and the only ones used in war—in the U.S. attacks on Japan in 1945—were all of the “pure fission” variety. During the Cold War, the power of such weapons rapidly increased.
The U.S. tested the first hydrogen bomb—called “Ivy Mike”--on Enewetak Atoll in the Pacific in 1952. The Soviet Union followed with an experimental blast in Siberia the next year.
Britain joined the H-bomb club in 1957, dropping one over Malden Island in the central Pacific. China detonated its first hydrogen bomb at its Lop Nur testing site in 1967 and France did the same in the South Pacific a year later.
Most nuclear weapons deployed today by the five permanent members of the U.N. Security Council—the U.S., the U.K., France, Russia and China -- are thought to be thermonuclear ones, according to nuclear experts.
In 1998, India conducted a series of nuclear tests, including one that may have been a hydrogen bomb. Some experts say it wasn’t powerful enough to count as one.
Similar questions surround North Korea’s latest test, which produced an earthquake tremor estimated by the U.S. Geological Survey at a magnitude of 5.1.
“There are contradictory signals,” said Yang Xiyu, an expert on North Korea at the China Institute of International Studies, a think tank connected to the Chinese foreign ministry. “They said it was a hydrogen bomb, but the explosion should have been much larger, and logically a larger explosion should trigger a larger quake. So it’s really a mystery.”
At the time North Korea said it detonated a hydrogen bomb, a magnitude 5.1 earthquake was detected near its nuclear-test site.
200 miles
RUSSIA
200 km
Vladivostok
CHINA
Earthquake
epicenter
NORTH
KOREA
Pyongyang
Sea of Japan
(East Sea)
Seoul
SOUTH
KOREA
Yellow Sea
Hiroshima
JAPAN
Source: U.S. Geological Survey
Still, he said, North Korea had been developing a hydrogen bomb for some time and “no matter if it’s an exaggeration or not, their statement indicates that they are marching in that direction.”
Many foreign experts have been skeptical of a statement made by North Korean leader Kim Jong Un in December that his country had developed a hydrogen bomb. Some experts have said they thought North Korea was preparing to test a “boosted fission” bomb, a more powerful version of those it has tested before.
North Korea has yet to say what the power of Wednesday’s blast was. The power, or “yield,” of a nuclear explosion is usually expressed as the tonnage of the explosive trinitrotoluene, or TNT, that would produce the same effect.
North Korea’s first nuclear test in 2006 was estimated at less than one kiloton and produced a seismic tremor of 4.1. The second test in 2009 was estimated at two to six kilotons and produced a seismic tremor of 4.7.
The third in 2013 was measured at around seven kilotons and produced a tremor of 5.1.
By comparison, the U.S. nuclear bomb dropped on Hiroshima in August 1945 had an explosive force of about 15 kilotons, while the first U.S. hydrogen bomb that was tested had a yield of 10,400 kilotons, or 10.4 megatons.
The largest hydrogen bomb ever tested was the Soviet Union’s Tsar Bomba in 1961. It had an estimated yield of 50 megatons.
Write to Jeremy Page at jeremy.page@wsj.com
