Tokyo - North Korea says it successfully detonated a hydrogen bomb in its latest nuclear test on Sunday. Outside experts haven't been able to verify that claim, but say it's plausible. If true, it would represent a major step forward in North Korea's effort to develop a nuclear weapon capable of reaching the United States.MORE POWERFUL THAN AN A-BOMBA hydrogen bomb can be far more powerful than the atomic bombs the US dropped on Japan in World War II. The US conducted the first successful tests of hydrogen bombs in the 1950s.Their yields of 10 000 kilotons and more were several hundred times larger than the bombs that leveled Hiroshima and Nagasaki.This April 27, 1956, file photo shows the area in which the United States hydrogen bomb tests will take place in the Pacific Ocean. (AP Photo/Ed Gunder, File) Experts believe the yield of North Korea's latest test was at least 140 kilotons, which would make it some seven to eight times as powerful as Hiroshima (15 kilotons) and Nagasaki (about 20).USES THE POWER OF THE SUNAtomic bombs rely on fission, or the splitting of the nucleus of an atom, just as nuclear power plants do. The hydrogen bomb uses both fission and fusion — the fusing together of atomic nuclei — to produce more explosive energy. It's the same process that keeps the sun and other stars burning. H-bombs are also known as thermonuclear bombs, because of the extremely high temperature needed to induce fusion. A typical hydrogen bomb is two-stage: First, an atomic fission bomb detonates, and that in turn starts the fusion of a hydrogen isotope in a second section.SMALL ENOUGH TO FIT ON A MISSILEThe atomic bombs that hit Japan were huge and had to be dropped from planes flying overhead.This 1960 image released by the US Department of Defense shows the Little Boy atomic bomb, the type detonated over Hiroshima on Aug. 6, 1945. (File, AP)With its higher power, a hydrogen bomb can be made small enough to fit on the head of an intercontinental missile. The hydrogen bomb is the standard for the five nations with the greatest nuclear weapons capability: Russia, the US, France, China and the U.K.Other nations may either have it or be working on it, despite a worldwide effort to contain such proliferation.