The reflector is used to bounce neutrons produced by fission back into the core to fission additional nuclei and increase the yield. Thus, the initiator provides a burst of neutrons to quickly start the chain reaction and maximize fission. The alpha particles then hit the beryllium and produce a reaction that releases neutrons. The polonium in the “pellet” type initiator releases alpha particles, a form of radiation, that are blocked by gold foil until the foil is broken by the implosive shockwave. The initiator and reflector also act to prevent fizzling and increase the yield. The fission taking place in the core exerts pressure on the tamper, which responds by pushing back on the core by virtue of the tamper’s inertia. The purpose of the tamper is to hold the core together long enough to allow the necessary fission generations to occur, otherwise the weapon will “fizzle” and not release the expected energy yield. For example, 99.9% of the energy released in a 100 kiloton (1 kiloton = 1,000 tons of TNT) nuclear explosion is released in the last 7 generations, out of a total of over 50 generations, and occurs in approximately 0.07 microseconds. The key is to create and sustain a chain reaction long enough to produce the desired explosive energy before the fissile core rips itself apart due to the internal pressure created by the energy release. Thus, supercriticality is when the fission of one nucleus in the chain reaction leads to the fission of more than one other nucleus.Įach fission event releases a large amount of energy in the form of light, heat, and radiation, so successive generations of fission events in the chain reaction will produce exponentially increasing amounts of energy. When the fissile material is arranged in such a manner that the fission of one nucleus leads to the fission of one other nucleus, the chain reaction is self-sustaining and the material is said to have reached its critical mass. These newly freed neutrons can then strike and fission other nuclei, which produces a chain reaction. When a fissile nucleus is struck by a neutron, the nucleus splits and emits additional neutrons and a large amount of energy. The physical basis of a nuclear weapon lies in creating this supercritical state. The shock waves compress the fissile core of uranium or plutonium into what is known as a supercritical state. These high explosives are finely machined in a lens configuration that sends shock waves into the center of the weapon. Regardless of the way the weapon is delivered (missile, bomb, artillery shell) the detonators fire simultaneously to set off a charge of high explosives that ring the outer surface of the tamper. In a so-called “implosion” weapon, which is the most common design used today, the weapon is armed with detonators that initiate the explosion. Initiator: a source of neutrons that can be a pellet (made of a sandwich of polonium-210 and beryllium separated by a layer of gold foil) placed in the center of the core, or that can be a tubular device mounted on the outside of the bomb that shoots a burst of neutrons into the core at the moment of detonation. Reflector: a material such as beryllium that bounces neutrons back into the core to increase fission.Ĭore: made of metallic plutonium-239 or uranium-235, the most widely used “fissile” isotopes, so-called because of their natural property to split, or fission, when struck by a low energy (or “thermal”) neutron. Tamper: a dense metal such as natural uranium that holds the core together by inertia. High explosive: shaped charges made from materials such as HMX, RDX, and TATB. Implosion design Figure 1 – A schematic diagram of an implosion bomb similar to the one designed by Iraqįiring Set: a wiring system that sends a large electrical impulse to set off the detonators.ĭetonators: devices used to ignite the high explosive section of the weapon. Table 2 - Five psi radii for various yield nuclear weaponsġ. Table 1 - Effective lethal radii for nuclear weapons The results is good agreement with theoretical predictions.Learn about the steps and materials needed to make a nuclear weapon, a description of weapon designs, and a history of nuclear weapon tests. = -induced transitions between specific nuclear states.
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