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∂2ω=0
Sep 2002
República de Califo
22×2,939 Posts
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Quote:
Originally Posted by tServo
"Atomic Times: My H-Bomb Year..." by Michael Harris.
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One of my all-time favorite bits of science nonfiction writing is the following passage by Richard Rhodes in his H-bomb memoir Dark Sun on the setting-off by the U.S. of Ivy Mike, the first modern-style two-stage thermonuclear device (in which a flood of X-rays from a primary fission device channeled and used to radiation-implode a heavy-hydrogen-based secondary) in November 1952 at Eniwetok atoll:
Quote:
When the radio signal from the [USS] Estes control room reached Mike, the capacitors in the Mike primary, already charged by the primary battery, discharged into a harness of electrical cables around the primary that carried the high-voltage current simultaneously to the ninety-two electric detonators inserted into the primary's high-explosive shell. (The increased number of detonators in the Mike primary made it possible to shape an implosion without using bulky high-explosive lenses, one way the TX-V device was made smaller and more transparent to radiation.) All ninety-two detonators fired with microsecond simultaneity; a detonation wave spread from each detonator, met other spreading donation waves moving inward and concentrating, emerged from the explosives as a shock wave, crossed to the aluminum pusher shell vaporizing as it passed, rocketed the pusher inward, crossed next to the primary's heavy uranium tamper, liquefied and vaporized the tamper, moved the material to the uranium shell of the core, hammered the uranium shell inward across an air gap to the plutonium ball levitated within, hammered the plutonium ball and crushed the Urchin initiator levitated at the center of the assembly. At that moment of maximum compression, with the vaporizing mass of uranium and plutonium supercritical, the shock wave shaped by the Munroe-effect grooves in the beryllium shell of the Urchin sliced through the shell and mixed beryllium with the polonium plated onto the ball of beryllium inside; alpha particles from the radioactive polonium knocked half a dozen neutrons from the beryllium; the neutrons ejected into the surrounding supercritical mass of uranium and plutonium and a chain reaction began.
Eighty generations later - a few millionths of a second - X-radiation from the furiously heating fission fireball hotter than the center of the sun escaped the primary mass entirely, began to ablate the blast shield over the Mike secondary and flooded down the cylindrical radiation channel inside the Mike casing. Instantly the radiation penetrated the thick polyethylene lining of the casing and heated it to a plasma. The plasma reradiated X rays that shone simultaneously from all sides inward onto the surface of the heavy uranium pusher, heating it instantly to ablation. The ablating surface of the pusher drove it explosively inward even as it liquefied and vaporized. The intense pulse of pressure concentrated as it moved inward, closed the first vacuum gap, compressed the floating thermal shield, closed the next vacuum gap, compressed the outer and inner dewars, encountered the deep, cold mass of liquid deuterium, compressed the deuterium inward and started to heat it. As the pressure pulse that was heating the deuterium to thermonuclear temperatures converged upon itself down the long axis of the secondary, it encountered the [Plutonium-rod] fission sparkplug, imploded that cylindrical system and activated a second fission explosion boosted with high-energy neutrons from fusion reactions in the tritium gas the sparkplug compressed.
All these processes, proceeding through microseconds, prepared Mike for thermonuclear burning. Now the escaping X-radiation of the fissioning sparkplug heated the compressed deuterium at its boundaries; the increasing thermal motion of the deuterium nuclei pushed them together until they passed the barrier of electrostatic repulsion between them and came within range of the nuclear strong force, at which point they began to fuse. Some fused to form a helium nucleus - an alpha particle - with the release of a neutron, the alpha and the neutron sharing an energy of 3.27 MeV. The neutron passed through the electrified mass of fusing deuterons and escaped, but the positively charged alpha dumped its energy into the heating deuterium mass and helped heat it further.
Other deuterium nuclei fused to form a tritium nucleus with the release of a proton, the triton and the proton sharing 4.03 MeV. The positively charged proton dumped more energy into the deuterium mass. The tritium nucleus fused in turn with another deuterium nucleus to form an alpha particle and a high-energy neutron that shared 17.59 MeV. The 14-MeV neutrons from this reaction began to escape the hot, compressed deuterium plasma and encountered the U238 nuclei of the vaporized uranium pusher. U238 fissions when it captures neutrons with energies above 1 MeV; so the U238 of the uranium pusher began to fission then under the intense neutron bombardment, flooding more X rays back into the deuterium mass from the outside just as the sparkplug fission reaction was radiating them from the inside, trapping the deuterium between two violent walls of heat and pressure. Deuterium-bred tritium fused with tritium as well, producing a helium nucleus and two neutrons that shared 11.27 MeV of energy. At lower orders of probability, deuterium captured a neutron and bred tritium; deuterium-bred helium fused with deuterium and made heavy helium plus a highly energetic proton, or captured a neutron and bred tritium plus a proton. All these reactions contributed to the force of the Mike explosion.
Moving outward from the cauldron of the secondary as gamma and X-radiation and as escaping high-energy neutrons, that explosion swelled back across the path the radiation-driven implosion had taken. Just as the big uranium pusher had served as a tamper for the secondary, so the thick, lead-lined Mike casing served as a tamper for the entire complex explosion, holding it together a few microseconds longer to give the fuel more time to react, but massive as the casing was, bomblight from its outer surface revealed the breakthrough of the developing explosion before the mass had time even to swell, much less to move.
Once the explosion broke through the casing, it expanded in seconds to a blinding white fireball more than three miles across (the Hiroshima fireball had measured little more than one-tenth of a mile) and rose over the horizon like a dark sun; the crews of the task force, thirty miles away, felt a swell of heat as if someone had opened a hot oven, heat that persisted long enough to seem menacing. "You would swear that the whole world was on fire," one sailor wrote home who turned around like Lot's wife to look. For a moment the fireball seemed to hover; then it began to rise. Los Alamos radiochemist George Cowan, a precise man whose ingenious tests would help measure Mike's yield, was there that day:
I was stunned. I mean, it was big. I'd been trying to visualize what it was going to be like, and I'd worked out a way to calibrate the shot. The initial fireball I guess I calibrated by holding up a quarter. If the quarter would cover the fireball then the yield would be less than something; if the fireball were bigger than the quarter, then it would be more than something. The question was, looking through my dark glasses, could I cover the fireball with a quarter. And I couldn't, so I knew it was big. As soon as I dared, I whipped off my dark glasses and the thing was enormous, bigger than I'd ever imagined it would be. It looked as though it blotted out the whole horizon, and I was standing on the deck of the Estes, thirty miles away.
Momentarily, the huge Mike fireball created every element that the universe had ever assembled and bred artificial elements as well. "In nanoseconds," writes the physicist Philip Morrison, "uranium nuclei captured neutron upon neutron to form isotopes in measurable amounts all the way from 239U up to mass number 255. Those quickly decayed, to produce a swath of transuranic species from uranium up to element 100, first isolated from that bomb debris and named fermium."
Swirling and boiling, glowing purplish with gamma-ionized light, the expanding fireball began to rise, becoming a burning mushroom cloud balanced on a wide, dirty stem with a curtain of water around its base that slowly fell back into the sea. The wings of the B-36 orbiting fifteen miles from ground zero at forty thousand feet heated ninety-three degrees almost instandy. In a minute and a half, the enlarging fireball cloud reached 57,000 feet; in two and a half minutes, when the shock wave arrived at the Estes, the cloud passed 100,000 feet. The shock wave announced itself with a sharp report followed by a long thunder of broken rumbling. After five minutes, the cloud splashed against the stratopause and began to spread out, its top cresting at twenty-seven miles, its stem eight miles across. "It really filled up the sky," notes Raemer Schreiber, who had seen shots before and was not easily impressed. "It was awesome. It just went on and on." At its farthest extent, the Mike cloud billowed out above a thirty-mile stem to form a huge canopy more than one hundred miles wide that loomed over the atoll. Radioactive mud fell out, followed by heavy rain.
Down below, Elugelab had vanished. The fireball had vaporized the entire island, leaving behind a circular crater two hundred feet deep and more than a mile across filled with seawater, a dark blue hole punched into the paler blue of the shallow atoll lagoon. The explosion vaporized and lifted into the air some eighty million tons of solid material that would fall out around the world. ... It stripped animals and vegetation from the surrounding islands and flashed birds to cinders in midair.
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