The immediate energy release per atom is about 180 million electron volts (MeV) i.e., 74 TJ/kg. The following equation shows one possible split, namely into strontium-95 ( 95Sr), xenon-139 ( 139Xe), and two neutrons (n), plus energy: 235 U + n ⟶ 95 S r + 139 X e + 2 n + 180 M e V The U-235 nucleus can split in many ways, provided the atomic numbers add up to 92 and the atomic masses add to 236 (uranium plus the extra neutron). Most of these have the speed (kinetic energy) required to cause new fissions in neighboring uranium nuclei. The fission chain reaction in a supercritical mass of fuel can be self-sustaining because it produces enough surplus neutrons to offset losses of neutrons escaping the supercritical assembly. When a free neutron hits the nucleus of a fissile atom like uranium-235 ( 235U), the uranium nucleus splits into two smaller nuclei called fission fragments, plus more neutrons (for 235U three as often as two an average of 2.5 per fission). India Israel (undeclared) Pakistan North Korea Former United States Russia United Kingdom France China Others Effects and estimated megadeaths of explosions. Practitioners of nuclear policy, however, favor the terms nuclear and thermonuclear, respectively. In early news accounts, pure fission weapons were called atomic bombs or A-bombs and weapons involving fusion were called hydrogen bombs or H-bombs. Most known innovations in nuclear weapon design originated in the United States, although some were later developed independently by other states. Large industrial states with well-developed nuclear arsenals have two-stage thermonuclear weapons, which are the most compact, scalable, and cost effective option once the necessary technical base and industrial infrastructure are built. Pure fission weapons historically have been the first type to be built by new nuclear powers. Such weapons would produce far fewer radioactive byproducts than current designs, although they would release huge numbers of neutrons. Ī fourth type, pure fusion weapons, are a theoretical possibility. This process affords potential yields up to hundreds of times those of fission weapons. This sets in motion a sequence of events which results in a thermonuclear, or fusion, burn. Its detonation causes it to shine intensely with x-radiation, which illuminates and implodes the second stage filled with a large quantity of fusion fuel. The first stage is normally a boosted fission weapon as above (except for the earliest thermonuclear weapons, which used a pure fission weapon instead).
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