A five-minute primer on an issue making headlines Nuclear weapons have only ever been used twice in war. Sixty years ago the Japanese cities of Hiroshima and Nagasaki were bombed. What is a nuclear bomb? It derives its power from nuclear fission, fusion or a combination of both. The bombs dropped on Japan were fission-based and called atomic bombs or A-bombs. Those derived from fusion - where light nuclei such as helium and hydrogen combine into heavier elements to release energy - are known as hydrogen or H-bombs. Modern nuclear weapons use fission and fusion. How does it work? Fission bombs are powered by chemical explosives to compress either uranium-235 or plutonium into a dense supercritical mass which is exposed to neutrons. The energy released from their collisions splits them into more neutrons that continue the process and sets off a uncontrollable nuclear chain reaction. As the material heats and liquefies, a massive amount of energy is released. What are the effects? There is the blast or shock wave, intense temperatures hotter than the interior of the sun, ionising radiation (X-rays or gamma rays) and a mushroom cloud of debris, smoke or flame. There are also simultaneous spot fires. The delayed effect is radiation fallout causing prolonged health problems including cancers and deformities in fetuses. Were the bombs used in Japan the same? No. The gun-assembly Mark I, codenamed 'Little Boy' was 3 metres long, 71cm wide, weighed 4 tonnes and contained 60kg of uranium-235. It exploded 600 metres above a hospital in Hiroshima, on western Honshu Island with a force equivalent to about 13 kilotonnes of TNT. 'Fat Man' exploded 500 metres above a tennis court in Nagasaki. Who invented the bomb? No one can claim to be sole inventor of the A-bomb because it was a collaboration between the US, Britain and Canada in the Manhattan Project, or Manhattan Engineering District, founded in September 1942. American physicist Robert Oppenheimer and General Leslie Groves directed the project involving more than 30 production and test sites. Are modern nuclear weapons the same? No. They are far more sophisticated than 'Little Boy' and 'Fat Man'. Aren't dirty bombs nuclear weapons? No. A dirty bomb combines a conventional explosive, such as dynamite, with radioactive material. In most cases, the explosive is deadlier than the radioactive material. But the 'fallout' could cause long-term health problems in thousands of people.