Nuclear weapons safety devices in gun-type and implosion-type weapon designs American nuclear weapons safety in accidents (a comment on a fashionable deception): "... one simple, dynamo-technology, low voltage switch stood between the United States and a major catastrophe ... It would have been bad news -- in spades." This kind of claim is a popular deception which is often used to "support" propaganda claiming the need for unilateral nuclear disarmament, while keeping us vulnerable to enemy terrorists! Even if a nuclear explosion occurs, it's likely to be less powerful than the charge of conventional explosive in it. The reasons are technical and due to the very difficult electronics problems in firing a lot of detonators simultaneously with a battery whose internal resistance is always too high to provide enough current under the load (even though the battery contains enough energy to fire all the detonators, it can't do so at the same time, directly). No practical battery pack in a deliverable nuclear weapon with 32 or more point implosion nuclear weapon has ever been capable of setting off the nuclear weapon directly because of the tremendous current needed. This has nothing to do with the energy in the battery, which is of course adequate. The problem is that batteries have an internal resistance which limits the amount of current (which limits the rate at which they deliver energy). To overcome this, implosion weapons from Trinity onward have used an X-unit containing a bank of high-capacitance capacitors. Energy is first transferred from the battery to the capacitors during a charging taking about a minute or so, then - at detonation time - the capacitor bank is discharged by a very special, high-current switch which can cleanly deliver an abrupt pulse with a rapid rise time (either a special krytron cold-cathode gas filled tube, or an explosive switch which fires a pointed metal stud through two metal conductors which are separated by a plastic insulator). This charging of the X-unit means a very severe limitation on the yield of accidental nuclear weapons. If you drop a nuclear bomb, even if switches are triggered in the impact, the X-unit is not going to be charged up at the right time to cause a full yield nuclear detonation. Nuclear weapons have "one-point safety", so if you were to close the switches and fire the weapon before the X-unit is adequately charged, which takes many seconds, inadequate current will be delivered to the detonators (which are connected in a parallel circuit, requiring an immense current to fire simultaneously). So most likely, all the detonators will just warm slightly, without firing. Even if there is enough current and one does fire (the one with the least resistance, the weakest link in the circuit, which gets hottest), it is a one-point explosion with the nuclear fission equivalent of just 4 pounds of TNT or about 2 kg of TNT. This nuclear explosion will be dwarfed by the explosion of the conventional explosives in the weapon, which in implosion devices greatly exceed 2 kg of TNT equivalent. To get a higher-yield nuclear explosion, modern warheads need not only simultaneous detonators to be fired by a recently charged-up capacitor bank, they also need the neutron gun to be fired about the time of maximum core compression, a matter of very careful timing, roughly millisecond or so after the X-unit is discharged by the special high-current switch to cause the implosion! Here, again, the electronics timing circuit is fully in control, and makes the weapon safe. The American bombs also contain disabling devices that prevent unauthorized or accidental nuclear explosions due to impact or theft, so the probability of random switch closures occurring at the right times for charging the capacitor bank, firing the detonators, then firing the neutron gun, is so astronomically insignificant you should worry more about the finite probability of everyone on earth being killed by a car accident on the same day, so we go extinct. For modern thermonuclear weapons, there is an additional switch which needs to be set off at a suitable time during the preparation in advance of the explosion: the fission stage boosting from deuterium and tritium gas mixture, which is injected into the hollow core prior to the explosion. (Some older weapons had the gas in sealed pits which avoided this, but since tritium has a half-life of only about 12 years, this meant that the pits had to be regularly taken out, unsealed, and re-gassed. A separate gas reservoir in the weapon allows tritium gas to replenished more easily after it decays.) http://archive.org/stream/TheEffectsOfNuclearWeapons/enw64a#page/n283/mode/2up

• published: 18 Oct 2013
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