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August 6, 1945 -* 0000, final briefing, the target of choice is Hiroshima. Tibbets is pilot, Lewis is copilot. * 0245, Enola Gay begins takeoff roll. * 0730, the bomb is armed. * 0850, Flying at 31,000 ft, Enola Gay crosses Shikoku due east of Hiroshima. * Bombing conditions are good, the aimpoint is easily visible, no opposition is encountered. * 0916:02 (8:16:02 Hiroshima time) Little Boy explodes at an altitude of 1900±50 feet (580 m), 550 feet from the aim point (the Aioi Bridge) with a yield of 12-18 kt (the yield is uncertain due partly from the absence of any instrumented test with this weapon design). A state-of-the-art, 6-year study ending in 1987 which used all available evidence set the yield at 15 kt (±20%). Due partly from the absence of any instrumented test with this weapon design, the yield of this explosion has been subject to a wide range of estimates over the years, ranging from a low of 12.5 kT up to 18 kT. The most thorough analyses have typically placed the yield at 15-to-16 kT. Finally a definitive determination was arrived at in 2002 after nearly 60 years of investigation from a meticulous study of the radiation field produced by the bomb. The revised burst altitude is 20 meters higher that previously believed and is now precisely 600 meters. The yield of the explosion is now confirmed at 16 kT. The yield of Little Boy had been predicted before delivery at 13.4 kt, and the burst height was set at 1,850 ft. Using the 15-kt figure, the actual burst height was optimum for a blast pressure of about 12 psi (i.e., it maximized the area subjected to a 12 psi or greater overpressure). To inflict damage on a city, a blast pressure of 5 psi is sufficient. So greater damage would have resulted from an optimum burst height of 2,700 ft. Due to the uncertainty in predicting yield and the fact that bursting too high causes a rapid deterioration in effects, the burst height had been set conservatively low in case a low yield explosion occurred. The 1,900 foot burst height is optimal for a 5-kt weapon. The burst height was sufficient to prevent any significant fallout over Japan. 8.1.4 Fat Man The combat configuration for the implosion bomb (the Model 1561) basically consisted of the Gadget device encapsulated in a steel armor egg. The 2 steel half-ellipsoids were bolted to the dural equatorial band of the explosive assembly with the necessary X-Unit, batteries, and fuzing and firing electronics located in the front and aft shell. For use in combat, each Fat Man bomb required assembly almost from scratch -- a demanding and time consuming job. Assembly of a Fat Man bomb was (and may still be) the most complex field preparation operation for any weapon ever made. Like Little Boy, Fat Man was fuzed by 4 radar units called "Archies", the antennas for which were mounted on the tail of the bomb. Developed originally as fighter tail warning systems, these units measured the bomb's height above the ground and were set to detonate at a pre-calculated altitude (set to 1,850 ±100 ft). A barometric switch acted as a "fail-safe", preventing detonation until the bomb had fallen below 7,000'. Fat Man was 60 inches in diameter, 12 feet long, and weighed 10,300 lb. The Fat Man Plutonium core and its initiator left Kirtland Air Force Base for Tinian Island on July 26, 1945 in a C-54 transport plane. It arrived on Tinian on July 28. Also on July 28, 3 specially-modified B-29s flew from Kirtland Field carrying 3 Fat Man bomb assemblies including units F-31 and F-32, each encased in an outer ballistic shell. These arrived at Tinian on August 2 -- the first Fat Man units to do so. The bombing date was set for August 11 at this time with Kokura as the target. Assembly of practice (non-nuclear) weapons began shortly afterward with the first completed bomb (Fat Man unit F33) ready on Aug. 5. On August 7, a forecast of 5 days of bad weather around the 11th moved the bombing date up to August 10, then to August 9. This compressed the bomb assembly schedule so much that many check-out procedures had to be skipped during assembly. On August 8, the assembly of Fat Man unit F31 with the Plutonium core was completed in the early morning. At 2200, 'Fat Man' was loaded on the B-29 "Bock's Car". August 9, 1945 -* 0347, Bock's Car takes off from Tinian, the target of choice is Kokura Arsenal. Charles Sweeney is pilot. Soon after takeoff, he discovers that the fuel system will not pump from the 600 gallon reserve tank. * 1044, Bock's Car arrives at Kokura but finds it covered by haze; the aimpoint cannot be seen. Flak and fighters appear, forcing the plane to stop searching. Sweeney turns toward Nagasaki -- the only secondary target in range. * Upon arriving at Nagasaki, Bock's Car has enough fuel for only one pass over the city even with an emergency landing at Okinawa. Nagasaki is covered with clouds. But one gap allows a drop several miles from the intended aimpoint. * 11:02 (Nagasaki time) Fat Man explodes at 1650 ± 33 feet (503 m) near the perimeter of the city with a yield of 22±2 kt. Due to the hilly terrain around ground zero, 5 shock waves were felt in the aircraft (the initial shock and 4 reflections). Although Fat Man fell on the border of an uninhabited area, the eventual casualties still exceeded 70,000. Also, ground zero turned out to be the Mitsubishi Arms Manufacturing Plant -- the major military target in Nagasaki. It was utterly destroyed. The 1987 reassessment of the Japanese bombings placed the yield at 21-kt. At the extreme estimate ranges for Little Boy and Fat Man (low for Little Boy, high for Fat Man), a ratio of nearly 2-to-1 has been implied. The 1987 best estimate figures make Fat Man only about 40% larger than Little Boy (and possibly as little as 15% more). Using the 21-kt figure, the optimal burst height for Fat Man would have been about 3,100 feet. The actual burst height was optimal for 15 psi overpressure. The burst height was sufficient to prevent any fallout over Japan. 8.1.5 Availability of Additional Bombs The date that a 3rd weapon could have been used against Japan was no later than August 20. The core was prepared by August 13 and Fat Man assemblies were already on Tinian Island. It would have required less than a week to ship the core and prepare a bomb for combat. By mid-1945, the production of atomic weapons was a problem for industrial engineering rather than scientific research although scientific work continued -- primarily toward improving the bomb designs. The 3 reactors (B and D which went started up for production in December 1944 and F which started up February 1945) at Hanford had a combined design thermal output of 750 megawatts and were theoretically capable of producing 19.4 kg of Plutonium a month (6.5 kg/reactor) -- enough for over 3 'Fat Man' bombs. Monthly or annual production figures are unavailable for 1945 and 1946. But by the end of FY 1947 (June 30, 1947), 493 kg of Plutonium had been produced. Neglecting the startup month of each reactor, this indicates an average Plutonium production 5.6 kg/reactor even though they were operated at reduced power or even shut down intermittently beginning in 1946. Enriched-Uranium production is more difficult to summarize since there were 3 different enrichment processes in use that had interconnected production. The Y-12 plant calutrons also had reached maximum output early in 1945. But the amount of weapon-grade Uranium this translates into varies depending on the enrichment of the feedstock. Initially, this was natural-Uranium giving a production of weapon-grade Uranium of some 6 kg/month. But soon, the S-50 thermal diffusion plant began feeding 0.89% enriched-Uranium, followed by 1.1% enriched feed from the K-25 gaseous diffusion plant. The established production process was then thermal diffusion (to 0.89%) → gaseous diffusion (to 1.1%) → alpha calutron (to 20%) → beta calutron (up to 89%). Of these 3 plants, the K-25 plant had by far the greatest separation capacity and as it progressively came on line throughout 1945 the importance of the other plants decreased. When enough stages had been added to K-25 to allow 20% enrichment, the alpha calutrons were slated to be shut down even if the War continued. After Japan's surrender in August 1945, S-50 was shut down. The alpha calutrons followed in September. But K-25 was complete on August 15 and these shutdowns would have occurred in any case. At this point, gaseous diffusion was incapable of producing weapon-grade Uranium. A planned "top plant" had been cancelled in favor of more beta calutrons. An expansion of K-25 (called K-27) to produce a larger flow of 20% enriched feed was under construction and due to go in full operation by February 1, 1946. In October, production had increased to 32 kg of U235 per month. In November and December, additional beta tracks went on line and the percentage of downtime for all beta tracks fell, boosting production further. Between October 1945 and June 1946, these improvements led to a 117% increase in output at Oak Ridge to about 69 kg of U235 per month. It is very unlikely any more Little Boy-type bombs would have been used even if the War continued. Little Boy was very inefficient, and it required a large critical mass. If the U235 were used in a Fat Man-type bomb, the efficiency would have been increased by more than an order of magnitude. The smaller critical mass (15 kg) meant more bombs could be built. Oppenheimer suggested to Gen. Groves on July 19, 1945 (immediately after the Trinity test) that the U235 from Little Boy be reworked into Uranium/Plutonium composite cores for making more implosion bombs (4 implosion bombs could be made from 'Little Boy's pit). Groves rejected the idea since it would delay combat use. The improved composite core weapon was in full development at Los Alamos when the War ended. It combined 2 innovations: (a) a composite pit containing both U235 and Pu239 and (b) core levitation which allowed the imploding tamper to accelerate across an air gap before striking the pit, creating shock waves that propagated inward and outward simultaneously for more rapid and even compression. The composite pit had several advantages over using the materials separately: ● A single design could be used employing both of the available weapon materials. ● Using U235 with Plutonium reduced the amount of Plutonium and thus the neutron background while requiring a smaller critical mass than U235 alone. The levitated pit design achieved greater compression densities. This permitted using 25% less than fissile material for the same yield (or a doubled yield with the same amount of material). Production estimates given to Sec. Stimson in July 1945 projected a second Plutonium bomb would be ready by Aug. 24; that 3 bombs should be available in September and more each month reaching 7-or-more in December. Improvements in bomb design being prepared at the end of the War would have permitted one bomb to be produced for every 5 kg of Plutonium or 12 kg of Uranium in output. These improvements were apparently taken into account in this estimate. Assuming these bomb improvements were used, the October capacity would have permitted up to 6 bombs a month. Note that with the peak monthly Plutonium and HEU production figures (19.4 kg and 69 kg respectively), production of close to 10 bombs a month was possible. When the War ended on August 15, 1945, there was an abrupt change in priorities so that a wartime development and production schedule did not continue. Development of the levitated pit/composite core bomb ground to a halt immediately. It did not enter the U.S. arsenal until the late 1940s. Plans to increase initiator production to 10 times the July 1945 level were abandoned. Fissile material production continued unabated after the S-50 and alpha calutron shutdowns though the fall. But Plutonium shipments from Hanford were halted, and Plutonium nitrate concentrates were stockpiled there. In early 1946, K-25 and K-27 were reconfigured to produce weapon-grade Uranium directly. But the extremely costly Y-12 beta tracks continued to operate until the end of 1946. By that time, Y-12 had separated about 1,000 kg of weapon-grade Uranium. From this point on, gaseous diffusion enriched-Uranium was the mainstay of weapon-grade fissile material production in the U.S., dwarfing Plutonium production until highly-enriched Uranium production for weapons use was halted in 1964. The Hanford reactors accumulated unexpected neutron irradiation damage (the Wigner effect). In 1946, they were shut down or operated at reduced power. If war had continued, they both would have been pushed to continue full production regardless of cost or risk. The effects of these priority changes can be seen in the post war stockpile. Although Los Alamos had 60 'Fat Man' units (that is, the non-nuclear components to assemble complete Fat Man bombs) on hand in October 1945, the U.S. arsenal after had only 9 actual Fat Man-type bombs in July 1946 with initiators for only 7 of them. In July 1947, the arsenal had increased to 13 bombs. There was probably sufficient fissile material on hand for over 100 bombs, though. 8.2 - The First Hydrogen Bombs The discovery of fusion reactions arose early in the 20th Century out of the growing understanding of atomic physics. By the early 1920s, it was realized that Hydrogen fusion was the source of the Sun's power output although the details were still obscure. This work culminated in the paper published by Hans Bethe in Physical Review in 1939 describing the role of fusion reactions in the Sun, for which he received the Nobel Prize in Physics in 1967. 8.2.1 Early Research on Fusion Weapons The possibility of creating weapons employing fusion reactions was not seriously considered until the discovery of fission. Almost immediately, physicists around the World realized that fission explosions generating high temperatures might be possible. But a few years passed before the idea of using these temperatures to ignite fusion reactions was suggested. Tokutaro Hagiwara at the University of Kyoto proposed this idea in a speech in May 1941, apparently the first such mention. While working on atomic bomb research a few months later in September 1941, Enrico Fermi muses to Edward Teller ("out of the blue") whether a fission explosion could ignite a fusion reaction in Deuterium. After some study, Teller concluded that it is impossible and although no further work on the subject followed for a while, this conversation began Teller's eventual obsession with fusion bombs. [In The Making of the Atomic Bomb (1986), Richard Rhodes reported that Tokutaro Hagiwara at the University of Kyoto was the first person to conceive of igniting a thermonuclear reaction based on a single statement translated from a hand-written summary of a speech given by Hagiwara on May 123, 941. The statement read: "If by any chance U235 could be manufactured in a large quantity and of proper concentration, U235 has a great possibility of becoming useful as the initiating matter for a quantity of Hydrogen." Hagiwara himself never claimed any credit for conceiving this and in June 1999, a copy of the original speech was obtained from Hagiwara's daughter. Upon examination, it was discovered that a kanji character substitution had occurred in hand-copying the original speech text, changing "super" to "initiating". The actual text stated: "If in some way it becomes possible to manufacture a fairly large amount of U235 and mix it with a suitably concentrated Hydrogen on an appropriate scale, the U235 is expected to have a high probability of causing a super explosion." This refers to the incorrect idea that an atomic explosion requires a moderator, which was held by many early investigators of atomic weapons. -- Shuji Fukui, Tetsuji Imanaka, James C. Warf, The Bulletin of the Atomic Scientists, Vol. 56, No. 4, July/August 2000, p. 5, 65. Historical footnote: During World War II, the idea occurred in Germany that convergent shock waves and collapsing shells might focus enough energy to allow conventional high explosives to ignite limited fusion reactions. This idea was probably inspired by Gudderly's work in converging shock waves and certainly by the Allied attempts to destroy the heavy-water plant at Vemork, Norway. Since German physicists considered fission weapons to be beyond reach during the current war, they concluded that the Allied interest in heavy-water must be due its application in high-explosive weapons. The Germans actually checked craters left by the British "Grand Slam" (the largest conventional bomb dropped during the War) to discover whether its unusual power was due to fusion boosting. Polish researchers in the 1960s and 1970s reported actually generating fusion neutrons through convergent shock waves. Although the theoretical possibility remains, no one has apparently ever released significant amounts of energy this way.] Research into the possibility of fusion weapons took an irregular and halting journey from the time of Fermi and Teller's conversation until bombs were actually built in the early 1950s. During WW II, there was an initial surge of interest once fission bomb physics was fairly well understood. After preliminary theoretical investigation, it was realized that much better experimental data was needed and a fusion research program was included in the Manhattan Project at Los Alamos. Continuing theoretical investigations took repeated turns towards optimism … then pessimism … and back again. As the difficulty of the enterprise came clear, its priority was steadily downgraded. Teller on the other hand grew so captivated by the problem that he became unable to fulfill his duties at Los Alamos. He was relieved of all technical leadership responsibilities and was eventually transferred to a separate study group to prevent him from interfering in the work of others on the atomic bomb. During July-September 1942, Oppenheimer's theoretical study group (Oppenheimer, Bethe, Teller, John Van Vleck, Felix Bloch, Robert Serber, and Emil Konopinski) in Berkeley examined the principles of atomic bomb design and also considered the feasibility of fusion bombs. Megaton-range fusion bombs were considered highly likely. April, 1943 - During the initial organization effort at Los Alamos, Bethe is selected over Teller to head the Theoretical Division. Teller is soon placed in charge of lower priority research on fusion weapon design (designated the 'Super') but remains responsible for much theoretical work on the fission weapon as well. February, 1944 - The Los Alamos Governing Board reevaluates Deuterium fusion research and determines that Tritium would be necessary to make an explosive reaction. Priority of fusion bomb work is further downgraded. May, 1944 - Teller is removed entirely from the Theoretical Division to prevent his interference with fission bomb work. He is placed in charge of a small independent group for fusion research. At the end of the War, most of Los Alamos' scientific and technical talent -- and virtually all of its leadership -- left for civilian careers. Teller was among those who left. For a period of time, very little progress on weapon research of any kind occurred. A conference chaired by Teller was held in April 1946 to review the wartime progress on the Super. The design at that time was for a gun-type Uranium fission bomb to be surrounded by about a cubic meter of liquid Deuterium with the whole assembly being encased in a heavy tamper. A large-but-undetermined amount of Tritium would be required to ignite the reaction. If the amount of Tritium required was too large, then the bomb would be impractical. Since the fusion of one T atom releases 8% the energy of the fission of a Pu239 atom with which it competes for neutrons in production reactors, the energy boost from D+D fusion must be considerably more than a factor of 10 greater than that released by the Tritium starter fuel before the Super could be worthwhile. The assessment at the time of the conference was that the Super was basically sound, but that more detailed calculations would be required verify it. Also present at the conference was Klaus Fuchs who was spying for the Soviet Union. The Soviets thus were well informed about American interest and optimism about fusion weapons. In mid-1946, Teller developed an idea he called the Alarm Clock. This involved the use of fusion fuel (specifically Lithium6 Deuteride) inside a Uranium tamper of an implosion fission bomb. The idea was that the fission neutrons would breed Tritium form the Lithium. And fission energy would compress and heat the fusion fuel and ignite a reaction. A fusion-fission chain reaction would then proceed between the fusion fuel and tamper until the bomb disassembled. By the end of 1946, Teller thought the Alarm Clock idea unpromising. In his September 1947 memorandum "On the Development of Thermonuclear Bombs", he was pessimistic about Alarm Clock's potential but felt that it -- like the Super -- were possible and required further study. Due to limitations in computing devices then available, he proposed delaying further work on both approaches for 2 years. If work had proceeded on the Alarm Clock design at this time, the U.S. could probably have tested a device similar to Joe-4 (see below) before the end of 1949. In the 4 years following the end of the War, about 50% of the Los Alamos Theoretical Division's effort went into studying the 'Super' although its size and talent were much reduced from wartime levels. The absence of good calculating machines hampered the massive numerical computations that were required and greatly slowed progress. By 1949, the Cold War was in full swing with the Berlin blockade and Communist governments seizing control throughout Eastern Europe. This included Teller's homeland of Hungary where much of his family still lived. Early in the summer of 1949, he thus rejoined Los Alamos to pursue the 'Super'. On August 29, the first Soviet atomic bomb (codenamed "RDS-1" and called Joe-1 by U.S. Intelligence) was exploded, thus breaking the U.S. nuclear weapon monopoly. Up to this time, the more detailed work on the classical Super design had showed that it was marginal at best. The large amounts of Tritium required made it extremely expensive for the yield produced. And it was not even certain that the design would work at all. Teller remained optimistic, however. During the next few months, Robert Oppenheimer -- as head of the Atomic Energy Commission's General Advisory Council (GAC) -- consistently opposed accelerating work on the Super due to its demonstrated shortcomings. Despite this, on January 31, 1950 Pres. Truman announced that the U.S. would proceed to develop Hydrogen bombs. A couple of weeks after Truman's announcement, Teller issued a 72-page update of "On the Development of Thermonuclear Bombs". In this paper, he again regarded both the Super and Alarm Clock as viable candidates for weapons development. But he again proposed delaying decision on full scale development of either for another 2 years. At this time, Soviet research on the subject was already well underway, focusing on the Sakharov-Ginsberg version of the Alarm Clock concept which they called the Layer Cake. A special department was set up in March 1950 to proceed with actual Layer Cake weapon development. By February 1950, immediately after Truman's decision, Stanislaw Ulam had discovered by hand calculation that even more immense amounts of Tritium than previously believed would be necessary for the Super to have any chance of success. When Ulam and Cornelius Everett completed more detailed computations on June 16, the design even these huge amounts of Tritium appeared to be inadequate. Additional analysis by Ulam and Enrico Fermi nailed the coffin shut on the classical Super. When John Von Neumann's newly-invented ENIAC computer began doing extensive calculations on the problem later in the year, the negative results were simply piling more dirt on the grave. Until early 1951, real progress on Hydrogen bomb development was impossible because no one knew how to proceed. 8.2.2 Design and Testing of the First Fusion Weapons In January 1951, Ulam broke the barrier to progress by inventing the idea of staging -- i.e., using the energy released by an atomic bomb primary to compress an external fuel capsule. He initially developed the idea as a means to create improved fission bombs, the second stage being a mass of fissionable material. By late in the month, he realized that the powerful compression that was possible would overcome the obstacles to efficient large scale fusion reactions. By multiple staging, bombs of virtually unlimited size could be created. This key idea was not sufficient by itself. Before a workable design could be developed, a scheme was needed for generating efficient compression using this energy flux as was a means for igniting the fuel once it was compressed. Ulam's idea was to use the neutron flux or the hydrodynamic shock wave of the expanding bomb core to achieve compression. Working with Ulam, Teller added additional refinements to this insight during the month of February. Teller's principal contribution during this period was realizing that the thermal radiation flux from the primary was a more promising means of generating the necessary implosive forces. On March 9, 1951, Ulam and Teller jointly wrote a report "On Heterocatalytic Detonations I. Hydrodynamic Lenses and Radiation Mirrors" that summarized these ideas. From this point on, Teller increasingly began to claim exclusive credit for the breakthrough and eventually came to deny that Ulam had made any original or significant contribution. Later in March, Teller added an important additional element to the radiation implosion scheme. Adapting Ulam's idea to use staged implosion to trigger a fission reaction, Teller suggested placing a fissile mass in the center of the fusion fuel. The convergent shock wave would compress this to supercriticality upon arriving at the center, making it act as a "spark plug" to ignite the fusion reaction. This idea is perhaps not strictly necessary. The convergent shock wave will generate very high temperatures in the center any way and might suffice to initiate fusion as it does in modern laboratory inertial confinement fusion experiments. Since the continuing compression on the fusion fuel would act to confine the fission spark plug, this final combined design concept was termed the "equilibrium thermonuclear". Teller wrote this idea up in a report on April 4, 1951. It was only in April 1951 that the necessary physical principles were in hand to allow the development and testing of an actual Hydrogen bomb to go forward. More computations were required to design the device than for any other project in human History up to this point (made possible by the recent invention of the programmable computer). The elapsed time from this point until the detonation of the 'Mik' device was less than 19 months. An achievement as remarkable in its own way as the Manhattan Project. In April 1951, experiments with fusion reactions and atomic bombs were already being prepared by the U.S. as part of the Greenhouse test series including a test of the idea of fusion boosting. The Greenhouse George test in particular provided a valuable opportunity to evaluate the Teller-Ulam ideas by allowing the observation of radiation effects in heating and compressing (although not imploding) an external mass of fusion fuel. Since there are several known designs for incorporating fusion reactions into weapons, we come to a question that is largely a matter of definition: Which design qualifies as a *true* Hydrogen bomb? I will not try to debate this issue here (see Section 11 - Questions and Answers). Instead, I am including descriptions of all of the significant tests that lead to the development and deployment of early thermonuclear weapons. The tests are listed in chronological order. Each is followed by a brief discussion of its significance to weapons development. Greenhouse George Detonated 5/9/51 at 0930 (local time) on a 200 ft tower on Ebireru/Ruby island at Eniwetok atoll. Total yield: 225 kt
Detonated 5/25/51 at 0617 (local time) on a 300 ft tower on Engebi/Janet island at Eniwetok atoll. Total yield: 45.5 kt First test of a boosted fission device. A Deuterium-Tritium mixture in the U235 bomb core boosted fission yield by 100% over its expected unboosted yield. This innovation was eventually incorporated into most or all strategic weapons. But the fusion yield was negligible and overall yield was still limited by the capabilities of fission designs. Ivy Mike Detonated 11/1/52 at 0714:59.4 +/- 0.2 sec (local time) at ground level on Elugelab/Flora island at Enewetak atoll. Total yield: 10.4 Megatons. This was the first test of the Teller-Ulam (or Ulam-Teller) configuration. The Mike device used liquid Deuterium as the fusion fuel. It was a massive laboratory apparatus installed on Elugelab Island in the Enewetak Atoll consisting of a cylinder about 20 feet high (more exactly 243.625 inches or 6.19 m), 6 ft 8 in wide, and weighing 164,000 lb (including attached diagnostic instruments); also said to weigh 140,000 lb without "the cryogenic unit" (this may mean the casing by itself). It was housed in an open hanger-like structure 88-ft x 46-ft and 61-ft high, where assembly started in September of 1952. The Mike device consisted of a massive steel cylinder with rounded ends, a TX-5 implosion bomb at one end acted as the primary, and a giant stainless steel dewar (thermos) flask holding several hundred liters of liquid Deuterium surrounded by a massive natural Uranium pusher/tamper constituted the secondary fusion stage (known as the "Sausage"). The welded steel casing was lined with a layer of Lead. A layer of polyethylene several centimeters thick was attached to the lead with copper nails. This layer of plastic generated plasma pressure during the implosion. The "Sausage" consisted of a triple-walled stainless steel dewar. The inner most wall contained the liquid Deuterium. Between this wall and the middle wall was a vacuum to prevent heat conduction. Between the middle wall and the outer wall was another vacuum and a liquid nitrogen-cooled thermal radiation shield made of copper. To reduce thermal radiation leakage even further, the Uranium pusher (which was oxidized to a purple-black color, making it an excellent thermal radiator) was lined with Gold leaf. Down the axis of the dewar suspended in the liquid Deuterium was a Plutonium rod that acted as the "spark plug" to ignite the fusion reaction once the compression shock wave arrived at the center. It did not run the entire length of the dewar but was supported at each end by axial columns. The spark plug was a boosted fission device. It was hollow and was charged with a few grams of Tritium/Deuterium gas (which of course liquified once the dewar was charged with liquid Deuterium). The Mike device had a conservative design. The external casing was made of steel and was extraordinarily thick (usually described as "a foot thick", but more likely 10 inches to be consistent with the weight) to maximize the confinement of the radiation induced pressure inside. The interior diameter was thus about 60 inches. A very wide radiation channel was provided around the secondary stage to minimize thermal gradients and to make success less dependent on sophisticated analysis. Due to the low density of liquid Deuterium and the necessity of thermal insulation, the secondary itself was quite voluminous which -- when combined with the wide channel between the secondary and the casing -- led to the 80 inch diameter. The massive casing accounted for most of Mike's weight (about 85%). The TX-5 device was an experimental version of the implosion system that was also deployed as the Mk-5 fission bomb. It used a 92 point ignition system. That is, 92 detonators and explosive lenses were used to make the spherical imploding shock wave. This allows the formation of the implosion shock wave with a thinner layer of explosive than earlier designs. The TX-5 was designed to use different fission pits to allow variable yields. The highest reported yield for a TX-5 test was Greenhouse Easy at 47-kt on 20 April 1951 with a 2,700 lb device. The smaller mass compared with earlier designs kept the temperature higher and allowed thermal radiation to escape more quickly from the primary, thus enhancing the radiation implosion process. If the Easy configuration was used in Mike, then the secondary fusion/primary yield ratio was 50:1. The deployed Mk-5 had an external diameter of 43.75 inches. The TX-5 would have been substantially smaller since it lacked the Mk-5 bomb casing. 3 fuels were considered for Mike -- liquid Deuterium, Deuterated Ammonia (ND3), and Lithium Deuteride. The reason for choosing liquid Deuterium for this test was primarily due to 2 factors. The physics was simpler to study and analyze and extensive studies had already been conducted over the previous decade on pure Deuterium fuel. The desirability of Lithium6 Deuteride as a fuel was known. But sufficient Li6 could not be produced in time to make the November 1952 target date (in fact, construction of the first Lithium enrichment plant had just begun at the time of the test). Liquid Deuterium produces energy through 4 reactions: (1) D + T → He4 + n + 17.588 MeV (2) D + D → He3 + n + 3.268 MeV(3) D + D → T + p + 4.03 MeV (4) He3 + D → He4 + p + 18.34 MeV For Mike to function successfully, densities and temperatures in the secondary sufficient to ignite reactions (2) and (3) were required. This requires densities hundreds of times normal and temperatures in the tens of millions of degrees K (say, 75 g/cm3 and 3x107 oK). Since the reaction cross section of (1) is some 100 times higher than the combined value of (2) and (3), the Tritium is burned as fast as it is produced, contributing most of the energy early in the reaction. Reaction (4) on the other hand requires temperatures exceeding 200 million oK before its cross-section becomes large enough to contribute significantly. Whether sufficient temperatures are reached and quantities of He3 are produced to make (4) a major contributor depends on the combustion efficiency (percentage of fuel burned). If only reactions (1)-(3) contribute significantly, corresponding to the combustion of 25% of the Deuterium fuel or less, then the energy output is 57 kt/kg. If reaction (4) contributes to the maximum extent, the output is 82.4 kt/kg. The maximum temperature generated by an efficient burn reaches 350 million K. The fission fraction for Mike was quite high -- 77%. The total fusion yield was thus 2.4 Megatons which corresponds to the efficient thermonuclear combustion of 29.1 kg of Deuterium (172 liters) or the inefficient combustion of 41.6 kg (249 liters). The total fission yield was 7.9 Megatons -- the fission of 465 kg of Uranium. All but some 50 kt of this was due to fast fission of the Uranium secondary stage tamper by fusion neutrons (a 3.3-fold boost). The amount of Deuterium actually present in Mike was no more than 1,000 liters. Which is the amount of liquid Deuterium handled by Operation Ivy. In fact, it was probably substantially less than this since excess LD2 was undoubtedly brought along in case leakage or other losses occurred. Prior to test, Mike's yield was estimated at 1-10 Megatons with a most likely yield of 5 MT but with a remote possibility of yields in the range of 50-90 MT. The principal uncertainties here would have been the efficiency of the fusion burn and the efficiency with which the tamper captured neutrons. Both of these factors are strongly influenced by the success of the compression process. The fusion efficiency involved novel and complex physics which could not be calculated reliably even if the degree of compression were known. The physics for determining the efficiency of neutron capture, on the other hand, were well understood and could be calculated if the conditions could be predicted. The upper limit estimate provides some insight into the mass of the Uranium fusion tamper. Presumably the 90 MT figure was calculated by assuming complete fusion and fission of all materials in the secondary. If 1000 liters of Deuterium were burned with complete efficiency, the yield would be 13.9 MT. Fission must account for 76.1 MT, corresponding to a Uranium tamper mass of 4475 kg. Lower amounts of Deuterium would lead to higher tamper estimates (a ratio of 0.82 kg of U for each liter of LD2). The detonation of Mike completely obliterated Elugelab, leaving an underwater crater a 6240 feet wide and 164 ft deep in the atoll where an island had once been. Mike created a fireball 3 miles wide. The "mushroom" cloud rose to 57,000 ft in 90 seconds and topped out in 5 minutes at 135,000 ft (the top of the stratosphere) with a stem 8 miles across. The cloud eventually spread to 1000 miles wide with a stem 30 miles across. 80 million tons of soil were lifted into the air by the blast. TX-16/EC-16 The Mike design was actually converted into a deliverable weapon, demonstrating that Lithium Deuteride is not essential to making a usable weapon. The weaponized design (designated the TX-16) went into engineering development in June 1952 (5 months before the Ivy Mike test). The design eliminated the cryogenic refrigerator, reduced the weight of the tamper, drastically reduced the dimensions and mass of the casing, used a lighter and less powerful primary, and pared the weight in other areas. The expected yield was reduced to 7-MT. The device was about 60 inches in diameter, 25 ft long, and weighed 30,000 lb. This weapon design would have been filled with liquid Deuterium at a cryogenic filling station before take-off -- a reservoir in the weapon held sufficient liquid Hydrogen to replace boil-off losses during flight. Components for about 5 of these bombs were built in late 1953 and had reached deployment by the time of the Castle tests. A unit of the TX-16 (codenamed Jughead) was slated for proof test detonation on March 22, 1954 as part of the Castle series prior to its expected deployment as the EC-16 (Emergency Capability) gravity bomb in May 1954. The excellent results with the solid-fueled Shrimp device in the Castle Bravo test on March 1(see below) resulted in the cancellation of this test, and then of the entire EC-16 program on 2 April 2 1954. Soviet Test: Joe-4/RDS-6s Detonated: August 12, 1953, on a tower at Semipalatinsk in Kazakhstan Total yield: 400 kilotons
Detonated: March 1, 1954 (0645 local time) on reef 2,950 ft off of Nam/Charlie island, Bikini Atoll Total yield: 15 Megatons
Detonated 27 March 1954 on barge in Bikini atoll lagoon near Bravo test site at 0630:00.4 (local time). Total yield: 11 Megatons
Detonated 26 March 1954 (0610:00.7 local time) on barge in Bikini atoll lagoon off Yurochi Island. Total yield: 6.9 Megatons
Detonated May 5, 1954 (0610:00.1 local time) on barge in Bikini atoll lagoon above the Union crater. Total yield: 13.5 Megatons
Detonated 11/22/55 Total yield: 1.6 Megatons The first Soviet test of a Teller-Ulam/Sakharov 'Third Idea' bomb. It used radiation implosion to detonate a Lithium Deuteride-fueled capsule. This was the World's first air-dropped fusion bomb test. After this test, the Soviet Union used radiation implosion bombs as the basis for their strategic arsenal. Exploded underneath an inversion layer, the refracted shock did unexpected collateral damage, killing 3 people. if on the Internet, Press |