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NUCLEAR WEAPONS

A-bombs vs. H-bombs: What's the difference?

The announcement Wednesday 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.' Here's a primer on what that means, and why it worries people

North Korea says it has tested hydrogen bomb

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WHAT MAKES HYDROGEN BOMBS DIFFERENT?

More explosive: Compared with the atomic bomb (the kind dropped on Japan in the closing days of the Second World War), the hydrogen bomb can be far more powerful – by 1,000 times or more, experts say. 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. "Think what's going on inside the sun," Takao Takahara, professor of international politics and peace research at Meiji Gakuin University in Tokyo, told Associated Press. "In theory, the process is potentially infinite. The amount of energy is huge."

More compact: Hydrogen bombs can be made small enough to fit on a head of an intercontinental missile. "That the bomb can become compact is the characteristic, and so this means North Korea has the U.S. in mind in making this H-bomb announcement," Tatsujiro Suzuki, professor at the Research Center for Nuclear Weapons Abolition at Nagasaki University, told Associated Press.

What's the same: Both the A-bomb and H-bomb use radioactive material like uranium and plutonium for the explosive material.

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EXAMPLE: WHAT WOULD HAPPEN IF OTTAWA WERE BOMBED

Here's an example of how much more destructive a hydrogen bomb can be than an atomic one, as calculated by the NUKEMAP tool designed by nuclear-weapons historian Alex Wellerstein. (The largest orange circle denotes the thermal radiation radius, the area where the explosion can cause third-degree burns; the green is the radiation radius, where radiation doses are high enough to kill most people over the course of hours or weeks.)

If any of North Korea's previous atomic bombs had been dropped on Parliament Hill, the immediate devastation would be confined mostly to Ottawa's downtown core and neighbouring Gatineau. But a blast as powerful as the first-ever hydrogen bomb – codenamed Ivy Mike, detonated by the United States in 1952 – would devastate the greater Ottawa area and many neighbouring communities as well.

In an undated file photo, North Korean leader Kim Jong-un looks through a pair of binoculars during an inspection at a forward post off the east coast of the Korean peninsula.

In an undated file photo, North Korean leader Kim Jong-un looks through a pair of binoculars during an inspection at a forward post off the east coast of the Korean peninsula.

KCNA/REUTERS

WHAT TYPE OF BOMBS DOES NORTH KOREA ACTUALLY HAVE?

North Korea claims Wednesday's test was of a a miniaturized hydrogen nuclear bomb. While seismic data supported the claim of a large explosion, there was no immediate way to verify the type of weapon.

North Korea's first three nuclear tests, from 2006 to 2013, were A-bombs on roughly the same scale as the ones used on Hiroshima and Nagasaki, which together killed more than 200,000 people.

South Korean lawmaker Lee Cheol Woo says the country's spy agency told him in a private briefing that Pyongyang may not have conducted a hydrogen bomb test at all, given the relatively small size of the seismic wave reported. Lee says the National Intelligence Service told him that an estimated explosive yield of six kilotons and a quake with a magnitude of 4.8 were detected Wednesday. According to him, that's smaller than the estimated explosive yield that were reported after the 2013 nuclear test, and only a fraction of a typical successful hydrogen bomb test's explosive yield of hundreds of kilotons.

WHO ELSE HAS H-BOMBS NOW?

The hydrogen bomb is already the global standard for the five nations with the greatest nuclear capabilities: the United States, Russia, France, Britain and China. Other nations may also either have it or may be working on it, despite a worldwide effort to contain such proliferation.

Canada does not have nuclear weapons, though in the 1960s and early 1970s it housed American nuclear missiles on its soil, including BOMARC surface-to-air guided missiles. These were so-called boosted fission weapons: nuclear bombs that used a small amount of fusion-based fuel to speed up a fission reaction. Hybrid weapons like these fall short of being true hydrogen bombs like the one North Korea claims to have tested. Canada ended its involvement with nuclear weapons in 1984.

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The hydrogen bomb

The hydrogen bomb “Cherokee” explodes over Bikini Atoll in the Marshall Islands on May 21, 1956.

ASSOCIATED PRESS

HAVE H-BOMBS EVER BEEN USED BEFORE?

The hydrogen bomb was never dropped on any targets. It was first successfully tested in the 1950s by the U.S., in bombs called Mike and Bravo. Soviet tests soon followed. The crew of a Japanese fishing boat that unknowingly went into the waters near the nuclear testing of Bravo got acute radiation sickness. Since the 1960s, nuclear tests have gone underground to reduce radioactive fallout.

WHY HAVEN'T H-BOMBS BEEN BANNED?

The Treaty on the Non-Proliferation of Nuclear Weapons, introduced in 1970, has been ratified by 189 countries. The treaty forbids non-nuclear states, such as Canada, from importing or building their own nuclear weapons. But three nuclear-equipped nations – Israel, India and Pakistan – have never agreed to it, and North Korea withdrew from the treaty in 2003, three years before its first nuclear test.

More than 160 countries have ratified the Comprehensive Nuclear Test Ban Treaty since 1996. India and Pakistan have also conducted nuclear tests since then and are among eight countries including the United States and China preventing the treaty coming into force.

Terumi Tanaka, head of Nihon Hidankyo, or the Japan Federation of A- and H-Bomb Sufferers Organizations, has been working to ban nuclear weapons for years and was stunned by reports of the H-bomb test. "It defies hopes for progress," he said. "I am outraged."

With reports from Reuters and Evan Annett

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