The danger of an accidental nuclear detonation was given full attention by design engineers before the first atomic bomb was dropped on Japan in 1945. War machines and materials have never been safe to handle or to stand near, and accidents happen all the time, whether in the chaos of battle or on a quiet Thursday morning. Chemical munitions can accidentally blow up entire towns, but the introduction of nuclear explosives kicked the danger level up by a factor of a million, and serious engineering went into ensuring that it would never happen.[193]
Any nuclear explosion begins with a chemical detonation, used to assemble or compress fissile material, and there is only so much you can do to prevent it from going off accidentally. The explosive fission and nuclear fusion, however, are different. Conditions for the nuclear part to happen are subtle and precise, and prevention of an unplanned destruction of lives and property by nuclear means has been remarkably successful. In the early 1950s, the one-of-a-kind bomb dropped on Nagasaki was hastily modified to make it safe to handle. The round, plutonium ball at the center, or the “pit,” in the original bomb had to be placed at the center of the shells of explosive material in the complex assembly procedure. When loaded into the aircraft bomb bay, there were few reasons why it should not explode. A couple of SAFE plug-ins had to be manually replaced with ARMED plug-ins to enable the electrons inside, and it had to actually drop free of the airplane, which would jerk out a set of enabling wires on top of the bomb casing. These were not considered adequate measures, and there was no way to fly a “safe” bomb in this configuration.
This problem was solved by opening a round hole in the front of the big ball of chemical explosive. There was a removable piece of explosive, which was solid and looked like dull brown plastic, made to fit in this hole. For normal transport or just cruising around with a device hanging in the bomb bay, the center of the bomb was left empty, with no pit installed. The pit was stored separately as a “capsule,” to be installed on demand with 30 minutes of handiwork in the bomb bay. The flight crew member turned designated armorer would remove the cover on the nose or tail of the weapon, unhook the explosive segment, pull it out, and push the plutonium sphere into place. Replace the explosive segment, button up the hole, and she’s ready to rock and roll.
The capsule was carried in an open, rectangular metal box made of aluminum tubing, called the “birdcage.” It was designed to prevent anyone from storing two or more pits close together, as they were quite capable of becoming an out-of-control mini-reactor if within a foot of each other. The capsule was stuffed inside a metal tube at the center of the birdcage, closely resembling an old-fashioned soda-acid fire extinguisher, including the handling hoop, looking like a halo atop the canister.
The arming operation was not quite as easy as it sounds. The work area was not pressurized, and it was a cramped space. The special T-handle wrench was conveniently clipped to the wall of the bomb bay, but tools were easy to drop as hands froze in the high-altitude environment. It was a good first step. This procedure was improved soon by introduction of the Automated In-Flight Insertion (AIFI) mechanism. The capsule tube was mounted with the bomb in the bomb bay, with the explosive hole left open. Nothing that could happen to the airplane, from an electrical fire in the bomb bay to a vertical dive into the ground, could cause a nuclear explosion, because the pit could not be uniformly compressed without being at the center of the sealed explosive sphere. On a moment’s notice, a crew member could arm the device by pushing a button on the bomb control panel. An electric-motor-driven screw pushed the capsule into the open hole in the bomb and closed it. In a minute, the red light came on indicating that the device was armed and ready to drop.
The first weapon using manual in-flight capsule insertion was the MK-4 Mod 0, made available on March 19, 1949, after two years of development. The last device to use manual insertion was the MK-7, used in the atomic depth charge named “Betty.” It was finally retired in June of 1967 after 15 years on call as an anti-submarine weapon. The last bomb to use AIFI was probably the MK-39 mod 1, a big, ugly, gravity-dropped thermonuclear device. It was taken off the inventory in September 1965.
In every case of the airborne loss of one of these early bombs, the capsule was separated from the device, and in some instances it was not even in the airplane. In 1955 there was a radical change in bomb-core design, in which the traditional solid plutonium sphere was replaced with a thin, round shell of plutonium, collapsed by implosion onto a void. With this interesting design came a new paradigm for bomb safety. No longer would anything have to come apart and travel as two separate pieces, and the device would be a sealed, integral unit, proclaimed “one point safe.”
In this new design, when the hollow shell of fissile material is explosively collapsed, the bomb does not have the hyper-criticality characteristic of a nuclear explosive. In fact, it reaches a point just short of simple criticality, in which as many neutrons are being produced by fission as are being lost. Explosive criticality requires enough fissile material concentrated to be more than two critical masses in the same spot. The only way to simulate hyper-criticality in the new bomb is to introduce a heavy burst of high-energy neutrons at the center, as if the mass were fissioning way out of control. This is accomplished using “boosted fission” after fission is forced to start using an electrical neutron initiator. To achieve boosted fission, a mixture of tritium and deuterium gases, held in a small pressure tank, is vented into the void at the center of the plutonium shell right before detonation.[194] The extreme temperature and pressure exerted on this gas mixture as the bomb implodes causes a percentage of the deuterium and the tritium, both heavy isotopes of hydrogen, to fuse into helium-4. Left over from each reaction is a neutron, clocking out at an impressive 14.1 MeV and very likely to cause fission in the collapsing metal ball. Just before the fusion starts, a sudden blast of neutrons comes forth from the initiator, an electrically driven deuteron accelerator, and sets off fission artificially, adding energy to the implosion event.
If the exact sequence of events, the implosion start, the initiator start, and the hydrogen fusion at the center, do not occur with perfect timing, then the device will not explode in a nuclear fashion. An accidental ignition of the chemical explosive used for the implosion caused by fire, shock, or an errant electrical signal will blow the bomb to pieces and it may even crush the fissile ball into a tiny kernel, but it will not cause a nuclear event. Furthermore, the fusion components in a thermonuclear device, the most powerful explosive ever made, require the explosion of an imploding fission bomb just to set them off. Without the fission device working, a hydrogen bomb is little more than dead weight.
A further advantage to this improved design is that the yield of the bomb can be dialed in from the control panel. A 5-megaton device can be scaled back to a 1-megaton explosion just by turning a knob and pushing a YIELD SET button. This action sets the open interval on the gas-release valve, controlling the amount of deuterium-tritium mixture that is injected into the void at the center of the imploding sphere. More gas means more fusions in the explosion and thus more generated neutrons and a larger bomb yield. The energy release by the fusions that takes place in the center of the fission bomb does not add much to the explosion, but the dense cloud of high-speed neutrons produced by fusions determines how much of the fissile material burns before the explosive disassembly of the device. Full use of the boost gas also means that the fissile material, which can be plutonium, uranium, or both, is used more efficiently and less is needed than in the old-style bombs. The new bombs are lighter and more compact.
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On the morning of December 6, 1917, two ships collided in the Narrows of the Halifax, Nova Scotia harbor. The SS
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There are at least five different boost tank designs. The most common one looks like a bratwurst — a small cylinder, rounded at both ends, with the electrically actuated release valve built into the center of the tank. One looks like a pipe elbow, and three designs look like squat cylinders with truncated conical ends, all with the valve attached at one end. In released documents they are never referred to as deuterium-tritium tanks, but only as tritium tanks or tritium cylinders. With only tritium in the tank, the fusion scheme would not work. There were cases in which tritium and deuterium were kept in separate cylinders, to be mixed upon injection. This allowed the tritium, which would deteriorate with age, to be replenished on a schedule.