Does North Korea have 'H' or 'A' bomb?

North Korea said Wednesday it had successfully tested a miniature hydrogen bomb, which if confirmed, would place it among a small group of countries with such dangerous weapons.

The world's nuclear arsenals have typically comprised two types of warheads: atomic bombs (A-bombs) such as those dropped on Hiroshima and Nagasaki, and much more powerful hydrogen, or thermonuclear bombs (H-bombs).

A third category of "enhanced radiation" (ER) warheads, once dubbed "neutron bombs" was developed using the thermonuclear principle, but they are not considered to be widely deployed at present.

- Who has nuclear weapons? -

Britain, France, China, Russia and the United States, the five permanent members of the UN Security Council, officially have nuclear weapons.

India and Pakistan also have nuclear weapons along with Israel, which maintains a policy of nuclear ambiguity, and North Korea is known to have carried out tests. If North Korea masters the technology needed to produce miniature warheads, it could conceivably use them to arm ballistic missiles able to reach neighbours in Asia and possibly the United States.

- The A-bomb -

Atomic bombs work on the principle of nuclear fission, where energy is released by splitting atoms of enriched uranium or plutonium encased in the warhead. The first test of an A-bomb took place in July 1945 in New Mexico, United States, and immediately demonstrated the new weapon's awesome power. Hiroshima was destroyed by one A-bomb with a uranium-fuel warhead that had the power of 15 kilotons (0.015 megaton). Nagasaki was destroyed three days later by a plutonium A-bomb of similar power, 17 kilotons, or the equivalent of 17,000 tons of TNT. The Soviet Union tested its first atomic bomb in August 1949 in the desert of Kazakhstan.

- The H-bomb -

The hydrogen, or thermonuclear bomb works on the principle of fusion of two nuclei, and generates temperatures similar to those found at the sun's core. When an H-bomb is detonated, chemical, nuclear and thermonuclear explosions succeed each other within milliseconds. The nuclear explosion triggers a huge increase in temperature that in turn provokes the nuclear fusion. The first US test of an H-bomb was on November 1, 1952 in the Marshall Islands, a chain in the Pacific Ocean. A year later the Soviet Union tested its own H-bomb, and the largest blast to date took place on October 30, 1961, when the Soviet "Tsar Bomba" exploded in the Arctic with a force of 57 megatons.

No H-bomb has been used in a conflict so far, but the world's nuclear arsenals are comprised for the most part of such weapons.

"Most of the thermonuclear warheads in service today have so-called 'dial-a-yield' options that allow for low explosive yields (less than 10 kilotons) with a considerable fraction of that yield derived from fusion reactions, that effectively make them enhanced radiation warheads," notes Shannon Kile of the Stockholm International Peace Research Institute (SIPRI).

- ER warheads -

They are based on the thermonuclear principle of the H-bomb, but are designed to generate more radiation than energy, thus targeting people while limiting damage to buildings, bridges, and other infrastructures. The warhead was developed to stop tanks and other armoured vehicles by killing or incapacitating their crews.