THREE SHAKES
The timer just outside the bombcase reached 5:00:00, and things began to happen.
First, high-voltage capacitors began to charge, and small pyrotechnics adjacent to the tritium reservoirs at both ends of the bomb fired. These drove pistons, forcing the tritium down narrow metal tubes. One tube led into the Primary, the other into the Secondary. There was no hurry here, and the objective was to mix the various collections of lithium-deuteride with the fusion-friendly tritium atoms. Elapsed time was ten seconds.
At 5:00:10, the timer sent out a second signal.
Time Zero.
The capacitors discharged, sending an impulse down a wire into a divider network. The length of the first wire was 50 centimeters. This took one and two-thirds nanoseconds. The impulse entered a dividing network using krytron switches— each of them a small and exceedingly fast device using selfionized and radioactive krypton gas to time its discharges with remarkable precision. Using pulse-compression to build their amperage, the dividing network split the impulse into seventy different wires, each of which was exactly one meter in length. The relayed impulses required three-tenths of a shake (three nanoseconds) to transit this distance. The wires all had to be of the same length, of course, because all of the seventy explosive blocks were supposed to detonate at the same instant. With the krytrons and the simple expedient of cutting each wire to the same length, this was easy to achieve.
The impulses reached the detonators simultaneously. Each explosive block had three separate detonators, and none of them failed to function. The detonators were small wire filaments, sufficiently thin that the arriving current exploded each. The impulse was transferred into the explosive blocks, and the physical detonation process began 4.4 nanoseconds after the signal was transmitted by the timer. The result was not an explosion, but an implosion, since the explosive force was mainly focused inward.
The high-explosives blocks were actually very sophisticated laminates of two materials, each laced with dust from light and heavy metals. The outer layer in each case was a relatively slow explosive with a detonation speed of just over seven thousand meters per second. The explosive wave in each expanded radially from the detonator, quickly reaching the edge of the block. Since the blocks were detonated from the outside-in, the blast front traveled inward through the blocks. The border between the slow and fast explosives contained bubbles—called voids— which began to change the shockwave from spherical-shaped to a planar, or flat wave, which was focused again to match exactly its metallic target, called "drivers."
The "driver" in each case was a piece of carefully shaped tungsten-rhenium. These were hit by a force wave traveling at more than nine thousand eight hundred meters (six miles) per second. Inside the tungsten-rhenium was a one-centimeter layer of beryllium. Beyond that was a one-millimeter thickness of uranium 235, which though thin weighed almost as much as the far thicker beryllium. The entire metallic mass was driving across a vacuum, and since the explosion was focused on a central point, the actual closing speed of opposite segments of the bomb was 18,600 meters (or 11.5 miles) per second.
The central aiming point of the explosives and the metallic projectiles was a ten-kilogram (22-pounds) mass of radioactive plutonium 239. It was shaped like a glass tumbler whose top had been bent outwards and down toward the bottom, creating two parallel walls of metal. Ordinarily denser than lead, the plutonium was compressed further by the million-atmospheres pressure of the implosion. This had to be done very quickly. The plutonium 239 mass also included a small but troublesome quantity of plutonium 240, which was even less stable and prone to pre-ignition. The outer and inner surfaces were slammed together and driven in turn toward the geometric center of the weapon.
The final external act came from a device called a "zipper." Operating off the third signal from the still-intact electronic timer, the zipper was a miniature particle accelerator, a very compact minicyclotron that looked remarkably like a handheld hair-dryer. This fired deuterium atoms at a beryllium target. Neutrons traveling ten percent of the speed of light were generated in vast numbers and traveled down a metal tube into the center of the Primary, called the Pit. The neutrons were timed to arrive just as the plutonium reached half of its peak density. Ordinarily a material weighing roughly twice an equivalent mass of lead, the plutonium was already ten times denser than that and still accelerating inward. The bombardment of neutrons entered a mass of still-compressing plutonium.
Fission.
The plutonium atom has an atomic weight of 239, that being the combined number of neutrons and protons in the atomic nucleus. What began happened at literally millions of places at once, but each event was precisely the same. An invading "slow" neutron passed close enough to a plutonium nucleus to fall under the Strong Nuclear Force that holds atomic nuclei together. The neutron was pulled into the atom's center, changing the energy state of the host nucleus and kicking it into an unstable state. The once symmetrical atomic nucleus began gyrating wildly and was torn apart by force fluctuations. In most cases a neutron or proton disappeared entirely, converted to energy in homage to Einstein's law E = MC2. The energy that resulted from the disappearance of the particles was released in the form of gamma- and X-radiation, or any of thirty or so other but less important routes. Finally, the atomic nucleus released two or three additional neutrons. This was the important part. The process that had required only one neutron to start released two or three more, each traveling at over ten percent of the speed of light—20,000 miles per second— through space occupied by a plutonium mass two hundred times the density of water. The majority of the newly liberated atomic particles found targets to hit.
A chain reaction merely means that the process builds on itself, that the energy released is sufficient to continue the process without outside assistance. The fission of the plutonium proceeded in steps called "doublings." The energy liberated by each step was double that of the preceding one, and that of each subsequent step was doubled again. What began as a trivial amount of energy and just a handful of freed particles doubled and redoubled, and the interval between steps was measured in fractions of nanoseconds. The rate of increase—that is, the acceleration of the chain reaction—is called the "Alpha," and is the most important variable in the fission process. An Alpha of 1,000 means that the number of doublings per microsecond is a vast number, 2'°°°—the number 2 multiplied by itself one thousand times. At peak fission— between 250 and 253—the bomb would be generating 10 billion billion watts of power, one hundred thousand times the electrical-generating capacity of the entire world. Fromm had designed the bomb to do just that—and that was only ten percent of the weapon's total designed output. The Secondary had yet to be affected. No part of it had yet been touched by the forces only a few inches away.
But the fission process had scarcely begun.
Some of the gamma rays, traveling at the speed of light, were outside the bombcase while the plutonium was still being compressed by the explosives. Even nuclear reactions take time. Other gamma rays started to impact on the Secondary. The majority of the gammas streaked through a gas cloud that only a few microseconds earlier had been the chemical explosive blocks, heating it far beyond the temperatures chemicals alone could achieve. Made up of very light atoms like carbon and oxygen, this cloud emitted a vast quantity of low-frequency "soft" X-rays. To this point, the device was functioning exactly as Fromm and Ghosn had planned.
The fission process was seven nanoseconds 0.7 shakes— old when something went wrong.
Radiation from the fissioning plutonium blazed in on the tritium-impregnated lithium-deuteride that occupied the geometric center of the Pit. The reason Manfred Fromm had left the tritium extraction to last lay in his basic engineer's conservatism. Tritium is an unstable gas, with a half-life of 12.3 years, meaning that a quantity of pure tritium will, after that time, be composed half of tritium and half of 3He. Called "heliumthree," 3He is a form of that second-lightest of elements whose nucleus lacks an extra neutron, and craves another. By filtering the gas through a thin block of palladium, the 3He would have been easily separated out, but Ghosn hadn't known about that. As a result, more than a fifth of the tritium was the wrong material. It could hardly have been a worse material.
The intense bombardment from the adjacent fission reaction seared the lithium compound. Normally a material half the density of salt, it was compressed to a metallic state that exceeded the density of earth's core. What began was actually a fusion reaction, though a small one, releasing huge quantities of new neutrons, and also changing many of the lithium atoms into more tritium, which broke down—"fused"—under the intense pressure to release yet more neutrons. The additional neutrons generated were supposed to invade the plutonium mass, boosting the alpha and causing at least a doubling of the weapon's unboosted fission yield. This had been the first method of increasing the power of the second-generation nuclear weapons. But the presence of 3He poisoned the reaction trapping nearly a quarter of the high-energy neutrons in uselessly stable helium atoms.
For several more nanoseconds, this did not matter. The plutonium was still increasing its reaction rate, still doubling, still increasing its Alpha at a rate only expressable numerically.
Energy was now flooding into the Secondary. The metallically coated straws flashed to plasma, pressing inward on the Secondary. Radiant energy in quantities not found on the surface of the sun vaporized but also reflected off elliptical surfaces, delivering yet more energy to the Secondary assembly, called the Holraum. The plasma from the immolated straws pounded inward toward the second reservoir of lithium compounds. The dense uranium 238 fins just outside the Secondary pit also flashed to dense plasma, driving inward through the vacuum, then striking and compressing the tubular containment of more 238U around the central container which held the largest quantity of lithium-deuteride/tritium. The forces were immense, and the structure was pounded with a degree of pressure greater than that of a healthy stellar core.
But not enough.
The Primary's reaction had already slackened. Starved of neutrons by the presence of the 3He poison, the bomb's explosive force began to blow apart the reaction mass as soon as the physical forces reached their balance. The chain reaction reached a moment of stability, at last unable to sustain its geometric rate of growth; the last two chain-reaction doublings were lost entirely, and what should have been a total Primary yield of seventy thousand tons of TNT was halved, halved again, and in fact ended with a total yield of eleven thousand two hundred tons of high explosive.
Fromm's design had been as perfect as the circumstances and materials allowed. An equivalent weapon less than a quarter the size was possible, but his specifications were more than adequate. A massive safety factor in the energy budget had been planned for. Even a thirty-kiloton yield would have been enough to ignite the "spark plug" in the Secondary to start a massive fusion "burn," but thirty-KT was not reached. The bomb was technically called a "fizzle."
But it was a fizzle equivalent to eleven thousand two hundred tons of TN1. That could be represented by a cube of high explosives seventy-five feet high, seventy-five feet long, and seventy-five feet thick, as much as could be carried by nearly four hundred trucks, or one medium-sized ship—but conventional explosives could never have detonated with anything approaching this deadly efficiency; in fact, a conventional explosion of this magnitude is a practical impossibility. For all that, it was still a fizzle.
As yet no perceptible physical effects had even left the bombcase, much less the truck. The steel case remained largely intact, though that would rapidly change. Gamma radiation had already escaped, along with X-rays, but these were invisible. Visible light had not yet emerged from the plasma cloud that had only three "shakes" before been over a thousand pounds of exquisitely designed hardware . . . and yet, everything that was to happen had already taken place. All that remained now was the distribution of the energy already released by natural laws which neither knew nor cared about the purposes of their manipulators.
Sum of All Fears
-- Tom Clancey
THE FIREBALL
2.03 As already seen, the fission of uranium (or plutonium) or the fusion of the isotopes of hydrogen in a nuclear weapon leads to the liberation of a large amount of energy in a very small period of time within a limited quantity of matter. As a result, the fission products, bomb casing, and other weapon parts are raised to extremely high temperatures, similar to those in the center of the sun. The maximum temperature attained by the fission weapon residues is several tens of million degrees, which may be compared with a maximum of 5,0000C (or 9,0000F) in a conventional high-explosive weapon. Because of the great heat produced by the nuclear explosion, all the materials are converted into the gaseous form. Since the gases, at the instant of explosion, are restricted to the region occupied by the original constituents in the weapon, tremendous pressures will be produced. These pressures are probably over a million times the atmospheric pressure, i.e., of the order of many millions of pounds per square inch.
2.04 Within less than a millionth of a second of the detonation of the weapon, the extremely hot weapon residues radiate large amounts of energy, mainly as invisible X rays, which are absorbed within a few feet in the surrounding (sea-level) atmosphere (§ 1.78). This leads to the formation of an extremely hot and highly luminous (incandescent) spherical mass of air and gaseous weapon residues which is the fireball referred to in § 1.32; a typical fireball accompanying an air burst is shown in Fig. 2.04. The surface brightness decreases with time, but after about a millisecond,[1] the fireball from a 1 megaton nuclear weapon would appear to an observer 50 miles away to be many times more brilliant than the sun at noon. In several of the nuclear tests made in the atmosphere at low altitude at the Nevada Test Site, in all of which the energy yields were less than 100 kilotons, the glare in the sky, in the early hours of the dawn, was visible 400 (or more) miles away. This was not the result of direct (line-of-sight) transmission, but rather of scattering and diffraction, i.e., bending, of the light rays by particles of dust and possibly by moisture in the atmosphere. However, high-altitude bursts in the megaton range have been seen directly as far a 700 miles away.
2.05 The surface temperatures of the fireball, upon which the brightness (or luminance) depends, do not vary greatly with the total energy yield of the weapon. Consequently, the observed brightness of the fireball in an air burst is roughly the same, regardless of the amount of energy released in the explosion. Immediately after its formation the fireball begins to grow in size, engulfing the surrounding air. This growth is accompanied by a decrease in temperature because of the accompanying increase in mass. At the same time, the fireball rises, like a hot-air balloon. Within seven-tenths of a millisecond from the detonation, the fireball from a 1-megaton weapon is about 440 feet across, and this increases to a maximum value of about 5,700 feet in 10 seconds. It is then rising at a rate of 250 to 350 feet per second. After a minute, the fireball has cooled to such an extent that it no longer emits visible radiation. It has then risen roughly 4.5 miles from the point of burst.
THE RADIOACTIVE CLOUD
2.06 While the fireball is still luminous, the temperature, in the interior at least, is so high that all the weapon materials are in the form of vapor. This includes the radioactive fission products, uranium (or plutonium) that has escaped fission, and the weapon casing (and other) materials. As the fireball increases in size and cools, the vapors condense to form a cloud containing solid particles of the weapon debris, as well as many small drops of water derived from the air sucked into the rising fireball.
2.07 Quite early in the ascent of the fireball, cooling of the outside by radiation and the drag of the air through which it rises frequently bring about a change in shape. The roughly spherical form becomes a toroid (or doughnut), although this shape and its associated motion are often soon hidden by the radioactive cloud and debris. As it ascends, the toroid undergoes a violent, internal circulatory motion as shown in Fig. 2.07a. The formation of the torrid is usually observed in the lower part of the visible cloud, as may be seen in the lighter, i.e., more luminous, portion of Fig. 2.07b. The circulation entrains more air through the bottom of the toroid, thereby cooling the cloud and dissipating the energy contained in the fireball. As a result, the toroidal motion slows and may stop completely as the cloud rises toward its maximum height.
2.08 The color of the radioactive cloud is initially red or reddish brown, due to the presence of various colored compounds (nitrous acid and oxides of nitrogen) at the surface of the fireball. These result from chemical interaction of nitrogen, oxygen, and water vapor in the air at the existing high temperatures and under the influence of the nuclear radiation. As the fireball cools and condensation occurs, the color of the cloud changes to white, mainly due to the water droplets as in an ordinary cloud.
2.09 Depending on the height of burst of the nuclear weapon and the nature of the terrain below, a strong updraft with inflowing winds, called "afterwinds," is produced in the immediate vicinity. These afterwinds can cause varying amounts of dirt and debris to be sucked up from the earth's surface into the radioactive cloud (Fig. 2.07b).
2.10 In an air burst with a moderate (or small) amount of dirt and debris drawn up into the cloud, only a relatively small proportion of the dirt particles become contaminated with radioactivity. This is because the particles do not mix intimately with the weapon residues in the cloud at the time when the fission products are still vaporized and about to condense. For a burst near the land surface, however, large quantities of dirt and other debris are drawn into the cloud at early times. Good mixing then occurs during the initial phases of cloud formation and growth. Consequently, when the vaporized fission products condense they do so on the foreign matter, thus forming highly radioactive particles (§ 2.23).