Thermonuclear weapon

The adjectives "thermonuclear," "fusion," and "hydrogen" are used mainly to describe multi-stage nuclear weapons, which allow large fusion yields. These operate on the radiation implosion principle, and are synonymous with the Teller-Ulam design, independently developed by at least five countries. "Thermonuclear" refers to thermonuclear fusion, where nuclei are fused via their high collision speeds at high temperatures. Unlike fission weapons, whose detonations are mediated via neutron transport, thermonuclear yield is more directly dependent on the temperatures and pressures achieved during compression of the secondary. These are in contrast to boosted fission devices, which employ thermonuclear fusion, but detonate a single stage design theoretically limited to around one megaton. Despite their name, the simplest and most common thermonuclear weapons derive most of their yield (>80% for US weapons) from fast fission of a natural or depleted uranium tamper. Clean thermonuclear weapons (<10% fission) have also been tested and possibly deployed. in 1958 == Basic principle ==

Primary and secondary stages The basic principle of the Teller–Ulam configuration is the idea that different parts of a thermonuclear weapon can be chained together in stages, with the detonation of each stage providing the energy to ignite the next stage. At a minimum, this implies a primary section that consists of an implosion-type fission bomb (a "trigger"), and a secondary section that consists of fusion fuel. The energy released by the primary compresses the secondary through the process of radiation implosion, at which point it is heated and undergoes nuclear fusion. This process could be continued, with energy from the secondary igniting a third fusion stage; the Soviet Union's AN602 "Tsar Bomba" is thought to have been a three-stage fission-fusion-fusion device. Theoretically by continuing this process thermonuclear weapons with arbitrarily high yield could be constructed. Fission weapons are limited in yield because only so much fission fuel can be amassed in one place before the danger of its accidentally becoming supercritical becomes too great. Surrounding the other components is a hohlraum or radiation case, a container that traps the first stage or primary's energy inside temporarily. The outside of this radiation case, which is also normally the outside casing of the bomb, is the only direct visual evidence publicly available of any thermonuclear bomb component's configuration. Numerous photographs of various thermonuclear bomb exteriors have been declassified. The primary is a standard implosion method fission bomb, though likely with a core boosted by small amounts of fusion fuel (usually 1:1 deuterium:tritium gas) for extra efficiency; the fusion fuel releases excess neutrons when heated and compressed, inducing additional fission. When fired, the or core would be compressed to a smaller sphere by special layers of conventional high explosives arranged around it in an explosive lens pattern, initiating the nuclear chain reaction that powers the conventional "atomic bomb". The secondary is usually shown as a column of fusion fuel and other components wrapped in many layers. Around the column is first a "pusher-tamper", a heavy layer of uranium-238 () or lead that helps compress the fusion fuel (and, in the case of uranium, may eventually undergo fission itself). Inside this is the fusion fuel, usually a form of lithium deuteride, which is used because it is easier to weaponize than liquefied tritium/deuterium gas. This dry fuel, when bombarded by neutrons, produces tritium, a heavy isotope of hydrogen that can undergo nuclear fusion, along with the deuterium present in the mixture. Inside the layer of fuel is the "spark plug", a hollow column of fissile material ( or ) often boosted by deuterium gas. The spark plug, when compressed, can undergo nuclear fission (because of the shape, it is not a critical mass without compression). The tertiary, if one is present, would be set below the secondary and probably be made of the same materials. The major components and their arrangement are in the diagram, though details are almost absent; what scattered details it does include likely have intentional omissions or inaccuracies. They are labeled "End-cap and Neutron Focus Lens" and "Reflector Wrap"; the former channels neutrons to the / spark plug while the latter refers to an X-ray reflector; typically a cylinder made of an X-ray opaque material such as uranium with the primary and secondary at either end. It does not reflect like a mirror; instead, it gets heated to a high temperature by the X-ray flux from the primary, then it emits more evenly spread X-rays that travel to the secondary, causing what is known as radiation implosion. In Ivy Mike, gold was used as a coating over the uranium to enhance the blackbody effect. The "toxic, brittle material" is widely assumed to be beryllium, which fits that description and would also moderate the neutron flux from the primary. Some material to absorb and re-radiate the X-rays in a particular manner may also be used. Candidates for the "special material" are polystyrene and a substance called "Fogbank", an unclassified codename. Fogbank's composition is classified, though aerogel has been suggested as a possibility. It was first used in thermonuclear weapons with the W76 thermonuclear warhead and produced at a plant in the Y-12 Complex at Oak Ridge, Tennessee, for use in the W76. Production of Fogbank lapsed after the W76 production run ended. The W76 Life Extension Program required more Fogbank to be made. This was complicated by the fact that the original Fogbank's properties were not fully documented, so a massive effort was mounted to re-invent the process. An impurity crucial to the properties of the old Fogbank was omitted during the new process. Only close analysis of new and old batches revealed the nature of that impurity. The manufacturing process used acetonitrile as a solvent, which led to at least three evacuations of the Fogbank plant in 2006. Widely used in the petroleum and pharmaceutical industries, acetonitrile is flammable and toxic. Y-12 is the sole producer of Fogbank. Summary A simplified summary of the above explanation is: • A (relatively) small fission bomb known as the "primary" explodes. • Energy released in the primary is transferred to the "secondary" (or fusion) stage. This energy compresses the fusion fuel and sparkplug; the compressed sparkplug becomes supercritical and undergoes a fission chain reaction, further heating the compressed fusion fuel to a high enough temperature to induce fusion. • Energy released by the fusion events continues heating the fuel, keeping the reaction going. • The fusion fuel of the secondary stage may be surrounded by a layer of additional fuel that undergoes fission when hit by the neutrons from the reactions within. These fission events account for about half of the total energy released in typical designs. == Compression of the secondary ==

How exactly the energy is "transported" from the primary to the secondary has been the subject of some disagreement in the open press but is thought to be transmitted through the X-rays and gamma rays that are emitted from the fissioning primary. This energy is then used to compress the secondary. The crucial detail of how the X-rays create the pressure is the main remaining disputed point in the unclassified press. There are three proposed theories: • Radiation pressure exerted by the X-rays. This was the first idea put forth by Howard Morland in an article in The Progressive. • Plasma pressure exerted by the X-ray-ionized channel filler (a polystyrene or plastic foam or "Fogbank"). This was a second idea put forward by Chuck Hansen and later by Howard Morland. • Ablation pressure exerted by the tamper/pusher. This is the concept best supported by physical analysis. Radiation pressure The radiation pressure exerted by the large quantity of X-ray photons inside the closed casing might be enough to compress the secondary. Electromagnetic radiation such as X-rays or light carries momentum and exerts a force on any surface it strikes. The pressure of radiation at the intensities seen in everyday life, such as sunlight striking a surface, is usually imperceptible, but at the extreme intensities found in a thermonuclear bomb the pressure is enormous. For two thermonuclear bombs for which the general size and primary characteristics are well understood, the Ivy Mike test bomb and the modern W-80 cruise missile warhead variant of the W-61 design, the radiation pressure was calculated to be for the Ivy Mike design and for the W-80. Foam plasma pressure Foam plasma pressure is the concept that Chuck Hansen introduced and published independently during the course of the Progressive case, based on research that located declassified documents listing special foams as liner components within the radiation case of thermonuclear weapons. The sequence of firing the weapon (with the foam) would be as follows: • The high explosives surrounding the core of the primary fire, compressing the fissile material into a supercritical state and beginning the fission chain reaction. • The fissioning primary emits thermal X-rays, which "reflect" along the inside of the casing, irradiating the polystyrene foam. Richard Rhodes' book Dark Sun stated that a layer of plastic foam was fixed to the lead liner of the inside of the Ivy Mike steel casing using copper nails. Rhodes quotes several designers of that bomb explaining that the plastic foam layer inside the outer case is to delay ablation and thus recoil of the outer case: if the foam were not there, metal would ablate from the inside of the outer case with a large impulse, causing the casing to recoil outwards rapidly. The purpose of the casing is to contain the explosion for as long as possible, allowing as much X-ray ablation of the metallic surface of the secondary stage as possible, so it compresses the secondary efficiently, maximizing the fusion yield. Plastic foam has a low density, so causes a smaller impulse when it ablates than metal does. == Design variations ==

Possible variations to the weapon design have been proposed: • Either the tamper or the casing have been proposed to be made of (highly enriched uranium) in the final fission jacket. The far more expensive is also fissionable with fast neutrons like the in depleted or natural uranium, but its fission-efficiency is higher. This is because nuclei also undergo fission by slow neutrons ( nuclei require a minimum energy of about ) and because these slower neutrons are produced by other fissioning nuclei in the jacket (in other words, supports the nuclear chain reaction whereas does not). Furthermore, a jacket fosters neutron multiplication, whereas nuclei consume fusion neutrons in the fast-fission process. Using a final fissionable/fissile jacket of would thus increase the yield of a Teller–Ulam bomb above a depleted uranium or natural uranium jacket. This has been proposed specifically for the W87 warheads retrofitted to currently deployed LGM-30 Minuteman III ICBMs. • In some descriptions, additional internal structures exist to protect the secondary from receiving excessive neutrons from the primary. • The inside of the casing may or may not be specially machined to "reflect" the X-rays. X-ray "reflection" is not like light reflecting off a mirror, but rather the reflector material is heated by the X-rays, causing the material to emit X-rays, which then travel to the secondary. Most bombs do not apparently have tertiary "stages"—that is, third compression stage(s), which are additional fusion stages compressed by a previous fusion stage. The fissioning of the last blanket of uranium, which provides about half the yield in large bombs, does not count as a "stage" in this terminology. The US tested three-stage bombs in several explosions during Operation Redwing but is thought to have fielded only one such tertiary model, i.e., a bomb in which a fission stage, followed by a fusion stage, finally compresses yet another fusion stage. This US design was the heavy but highly efficient (i.e., nuclear weapon yield per unit bomb weight) B41 nuclear bomb. The Soviet Union is thought to have used multiple stages (including more than one tertiary fusion stage) in their Tsar Bomba, which was in intended use. The fissionable jacket could be replaced with lead, as was done with the Tsar Bomba. If any hydrogen bombs have been made from configurations other than those based on the Teller–Ulam design, the fact of it is not publicly known. A possible exception to this is the Soviet early Sloika design. In essence, the Teller–Ulam configuration relies on at least two instances of implosion occurring: first, the conventional (chemical) explosives in the primary would compress the fissile core, resulting in a fission explosion many times more powerful than that which chemical explosives could achieve alone (first stage). Second, the radiation from the fissioning of the primary would be used to compress and ignite the secondary fusion stage, resulting in a fusion explosion many times more powerful than the fission explosion alone. This chain of compression could conceivably be continued with an arbitrary number of tertiary fusion stages, each igniting more fusion fuel in the next stage The smaller warhead allows more of them to fit onto a single missile and improves basic flight properties such as speed and range. == History ==

First tests United States , first full-scale thermonuclear test, Enewetak Atoll, November 1, 1952 The idea of a thermonuclear fusion bomb ignited by a smaller fission bomb was first proposed by Enrico Fermi to his colleague Edward Teller when they were talking at Columbia University in September 1941, The first atomic bomb test by the Soviet Union in August 1949 came earlier than expected by Americans, and over the next several months there was an intense debate within the US government, military, and scientific communities regarding whether to proceed with development of the far more powerful Super. Despite the objections raised, on 31 January 1950, President Harry S. Truman made the decision to go forward with the development of the new weapon. it raised expectations to a near certainty that the concept would work. On 1 November 1952, the Teller–Ulam configuration was tested at full scale in the Mike shot of Operation Ivy, at an island in the Enewetak Atoll, with a yield of (over 450 times more powerful than the bomb dropped on Nagasaki during World War II). The device, dubbed the Sausage, was created by Richard Garwin, assigned this task by Edward Teller. This was not widely known until 2001, as his involvement was kept secret. It used an extra-large fission bomb as a "trigger" and liquid deuterium—kept in its liquid state by of cryogenic equipment—as its fusion fuel, and weighed around altogether. The liquid deuterium fuel of Ivy Mike was impractical for a deployable weapon, and the next advance was to use a solid lithium deuteride fusion fuel instead. In 1954 this was tested in the "Castle Bravo" shot (the device was code-named Shrimp), which had a yield of , 2.5 times what was expected, and is the largest US bomb ever tested. Efforts shifted towards developing miniaturized Teller–Ulam weapons that could fit into intercontinental ballistic missiles and submarine-launched ballistic missiles. By 1960, with the W47 warhead deployed on Polaris ballistic missile submarines, megaton-class warheads were as small as in diameter and in weight. Further innovation in miniaturizing warheads was accomplished by the mid-1970s, when versions of the Teller–Ulam design were created that could fit ten or more warheads on the end of a small MIRVed missile. United Kingdom on Christmas Island was the first British hydrogen bomb test. In 1954 work began at Aldermaston to develop the British fusion bomb, with Sir William Penney in charge of the project. British knowledge on how to make a thermonuclear fusion bomb was rudimentary, and at the time the United States was not exchanging any nuclear knowledge because of the Atomic Energy Act of 1946. The United Kingdom had worked closely with the Americans on the Manhattan Project. British access to nuclear weapons information was cut off by the United States at one point due to concerns about Soviet espionage. Full cooperation was not reestablished until an agreement governing the handling of secret information and other issues was signed. The Soviet Union assisted the Chinese nuclear program from 1957, but this was abruptly ended by the Sino-Soviet split in 1959. For thermonuclear weapons, China had received a lithium deuteride production plant, and limited knowledge of the Soviet layer cake design. Unlike the US and USSR, at the time of their hydrogen bomb program, China operated no production facilities for plutonium or tritium. Plutonium production reactor in Jiuquan became operational only in 1967, and plutonium separation began in September 1968. During 1963, Chinese scientists led by Peng Huanwu extensively investigated this design, but knew it was too inefficient to be the hydrogen bomb possessed by other countries. Nonetheless, plans were made to test a small layer cake designs in 1966 and "658", a three-staged layer cake design capable of reaching one megaton (similar to the British backup design Orange Herald Large), in October 1967. In September and October 1965, a theoretical research crash project ran in Shanghai led by Yu Min, using digital computers and manual calculation. Yu held a lecture series on the layer cake bomb, and in doing so realized its flaw was its slow production of tritium from lithium deuteride i.e. the Jetter cycle. This resulted in a Teller-Ulam analogue design for compression of a thermonuclear secondary by a fission primary. In December 1965, this design was selected as the focus of thermonuclear development. Yu later stated this rapid development prevented the hydrogen bomb research program from crumbling during the ten-year Cultural Revolution beginning in May 1966 (such as occurred to China's first crewed space program). The differences are unclear, as the Chinese design also channels energy from a nuclear fission primary to compress a thermonuclear secondary. Like the initial Soviet and British hydrogen bombs, the secondary is spherical, unlike the first cylindrical secondaries used in the US. reported that in 1995, a supposed Chinese double agent delivered information indicating that China knew secret details of the US W88 warhead, supposedly through espionage. (This line of investigation eventually resulted in the abortive trial of Wen Ho Lee.) France Following their first nuclear test in 1960, France prioritized fission weapon development and deliverability by Mirage IV bombers. In 1966, de Gaulle felt pressure that China would become the fourth thermonuclear country, and set a deadline of 1968 for the first hydrogen bomb test. A participating scientist, Pierre Billaud, wrote of French thermonuclear knowledge in 1965: Early tests "closely fitted Li6D [lithium deuteride] to the fissile core", implying a layer cake design. France began testing thermonuclear principles in the 1966–70 French nuclear tests, beginning with the 125 kt Rigel boosted fission shot in September 1966. In April 1967, physicist outlined the radiation implosion idea central to the Teller-Ulam design, but the weapons scientists were not immediately convinced it was the solution. In June, France lost the hydrogen bomb race to China's three-megaton Project 639 test. By mid-1967, like their Chinese counterparts, French scientists had identified an extremely high, almost twenty-fold density increase of the lithium deuteride fuel, to be crucial to megaton success, but planned a test of Carayol's correct Teller-Ulam analogue as only one of three designs for summer of 1968. France's hydrogen bomb development path was crucially influenced by the British scientist William Richard Joseph Cook, who led the successful British hydrogen bomb programme a decade prior. Unlike France, the UK, as well as the US and USSR, had aerial reconnaissance capabilities to collect nuclear fallout from testing and make deductions, including France's lack of progress in thermonuclear weapons. In September 1967, Cook provided limited thermonuclear development information to the military attache at the French Embassy in London, specifically that their current designs would not succeed and that the solution was more simple. This allowed the French scientists to identify and proceed with only Carayol's proposal for the ultimately successful 1968 thermonuclear tests. It is believed this was done on the instruction of Prime Minister Harold Wilson, aimed as an overture to de Gaulle, who was currently blocking the accession of the United Kingdom to the European Communities due to its closer relationship to the United States. However, de Gaulle again vetoed UK accession in November 1967, and was very shocked when made aware of the British contribution. The first DT-boosted warhead, the MR 41, was tested in the Castor and Pollux shots of July and August 1968, successfully yielding in the former. The "Canopus" test in the Fangataufa atoll in French Polynesia on 24 August 1968 was the country's first multistage thermonuclear weapon test. The bomb was detonated from a balloon at a height of . The result of this test was significant atmospheric contamination. France is currently believed to have nuclear weapons equal in sophistication to the other major nuclear powers. France has about 60 air-launched missiles tipped with TN 80/TN 81 warheads with a yield of about each. France's nuclear program has been carefully designed to ensure that these weapons remain usable decades into the future. India On 11 May 1998, India announced that it had detonated a thermonuclear bomb in its Operation Shakti tests ("Shakti-I", specifically, in Hindi the word 'Shakti' means power). However, Harold M. Agnew, former director of the Los Alamos National Laboratory, said that India's assertion of having detonated a staged thermonuclear bomb was believable. The yield of India's hydrogen bomb test remains highly debatable among the Indian science community and the international scholars. The question of politicisation and disputes between Indian scientists further complicated the matter. In an interview in August 2009, the director for the 1998 test site preparations, K. Santhanam claimed that the yield of the thermonuclear explosion was lower than expected and that India should therefore not rush into signing the Comprehensive Nuclear-Test-Ban Treaty. Other Indian scientists involved in the test have disputed Santhanam's claim, arguing that his claims are unscientific. British seismologist Roger Clarke argued that the magnitudes suggested a combined yield of up to , consistent with the Indian announced total yield of . US seismologist Jack Evernden has argued that for correct estimation of yields, one should 'account properly for geological and seismological differences between test sites'. Israel Israel is alleged to possess thermonuclear weapons of the Teller–Ulam design, but it is not known to have tested any nuclear devices, although it is widely speculated that the Vela incident of 1979 may have been a joint Israeli–South African nuclear test. It took him a year to convince the CIA about Israel's capability and finally in 1976, Carl Duckett of the CIA testified to the US Congress, after receiving credible information from an "American scientist" (Teller), on Israel's nuclear capability. After he conveyed the matter to the higher level of the US government, Teller reportedly said: "They [Israel] have it, and they were clever enough to trust their research and not to test, they know that to test would get them into trouble." In January 2016, North Korea claimed to have successfully tested a hydrogen bomb, although only a magnitude 5.1 seismic event was detected at the time of the test, a similar magnitude to the 2013 test of a atomic bomb. These seismic recordings cast doubt upon North Korea's claim that a hydrogen bomb was tested and suggest it was a non-fusion nuclear test. On 3 September 2017, the country's state media reported that a hydrogen bomb test was conducted that resulted in "perfect success". According to the US Geological Survey (USGS), the blast released energy equivalent to an earthquake with a seismic magnitude of 6.3, 10 times more powerful than previous nuclear tests conducted by North Korea. US Intelligence released an early assessment that the yield estimate was , with an uncertainty range of . On 12 September, NORSAR revised its estimate of the explosion magnitude upward to 6.1, matching that of the CTBTO but less powerful than the USGS estimate of 6.3. Its yield estimate was revised to , while noting the estimate had some uncertainty and an undisclosed margin of error. On 13 September, an analysis of before and after synthetic-aperture radar satellite imagery of the test site was published suggesting the test occurred under of rock, and the yield "could have been in excess of 300 kilotons". == Public knowledge ==

Classification Detailed knowledge of fission and fusion weapons is considered classified information to some degree in virtually every industrialized country. In the United States, such knowledge can by default be classified as "Restricted Data", even if it is created by people who are not government employees or associated with weapons programs. Material that is automatically considered classified as soon as it is created is termed born secret, a legal doctrine that has been disputed in cases such as United States v. Progressive, Inc.. The constitutionality of born secret is rarely invoked for cases of private speculation. The official policy of the United States Department of Energy has been not to acknowledge the leaking of design information, as such acknowledgment would potentially validate the information as accurate. In a small number of prior cases, the US government has attempted to censor weapons information in the public press, with limited success. According to the New York Times, physicist Kenneth W. Ford defied government orders to remove classified information from his book Building the H Bomb: A Personal History. Ford claims he used only pre-existing information and even submitted a manuscript to the government, which wanted to remove entire sections of the book for concern that foreign states could use the information. The Teller–Ulam design was for many years considered one of the top nuclear secrets, and even today it is not discussed in any detail by official publications with origins "behind the fence" of classification. United States Department of Energy (DOE) policy has been, and continues to be, that they do not acknowledge when "leaks" occur, because doing so would acknowledge the accuracy of the supposed leaked information. Aside from images of the warhead casing, most information in the public domain about this design is relegated to a few terse statements by the DOE and the work of a few individual investigators. Unclassified knowledge Though large quantities of vague data have been officially released—and larger quantities of vague data have been unofficially leaked by former bomb designers—most public descriptions of nuclear weapon design details rely to some degree on speculation, reverse engineering from known information, or comparison with similar fields of physics (inertial confinement fusion is the primary example). Such processes have resulted in a body of unclassified knowledge about nuclear bombs that is generally consistent with official unclassified information releases and related physics and is thought to be internally consistent, though there are some points of interpretation that are still considered open. The state of public knowledge about the Teller–Ulam design has been mostly shaped from a few specific incidents outlined in a section below. US Department of Energy statements nuclear warhead, allow for some speculation as to the relative size and shapes of the primaries and secondaries in US thermonuclear weapons. In 1972 the United States government declassified a document stating "[I]n thermonuclear (TN) weapons, a fission 'primary' is used to trigger a TN reaction in thermonuclear fuel referred to as a 'secondary, and in 1979 added, "[I]n thermonuclear weapons, radiation from a fission explosive can be contained and used to transfer energy to compress and ignite a physically separate component containing thermonuclear fuel." To this latter sentence the US government specified that "Any elaboration of this statement will be classified." [emphasis in original] The only information that may pertain to the spark plug or tamper was declassified in 1991: "Fact that fissile or fissionable materials are present in some secondaries, material unidentified, location unspecified, use unspecified, and weapons undesignated." In 1998 the DOE declassified the statement that "The fact that materials may be present in channels and the term 'channel filler', with no elaboration", which may refer to the polystyrene foam (or an analogous substance). Whether these statements vindicate some or all of the models presented above is up for interpretation, and official US government releases about the technical details of nuclear weapons have been purposely equivocating in the past (e.g., Smyth Report). Other information, such as the types of fuel used in some of the early weapons, has been declassified, though precise technical information has not been. United States v. The Progressive Most of the current ideas on the workings of the Teller–Ulam design came into public awareness after the DOE attempted to censor a magazine article by US anti-weapons activist Howard Morland in 1979 on the "secret of the hydrogen bomb". In 1978, Morland had decided that discovering and exposing this "last remaining secret" would focus attention onto the arms race and allow citizens to feel empowered to question official statements on the importance of nuclear weapons and nuclear secrecy. Most of Morland's ideas about how the weapon worked were compiled from accessible sources: the drawings that most inspired his approach came from the Encyclopedia Americana. Morland also interviewed (often informally) many former Los Alamos scientists (including Teller and Ulam, though neither gave him any useful information), and he used a variety of interpersonal strategies to encourage informative responses from them (i.e., asking questions such as "Do they still use spark plugs?" even if he was not aware what the latter term specifically referred to). Morland eventually concluded that the "secret" was that the primary and secondary were kept separate and that radiation pressure from the primary compressed the secondary before igniting it. When an early draft of the article, to be published in The Progressive magazine, was sent to the DOE after falling into the hands of a professor who was opposed to Morland's goal, the DOE requested that the article not be published and pressed for a temporary injunction. The DOE argued that Morland's information was (1) likely derived from classified sources, (2) if not derived from classified sources, itself counted as "secret" information under the "born secret" clause of the 1954 Atomic Energy Act, and (3) was dangerous and would encourage nuclear proliferation. Morland and his lawyers disagreed on all points, but the injunction was granted, as the judge in the case felt that it was safer to grant the injunction and allow Morland, et al., to appeal. Through a variety of more complicated circumstances, the DOE case began to wane as it became clear that some of the data they were attempting to claim as "secret" had been published in a students' encyclopedia a few years earlier. After another H-bomb speculator, Chuck Hansen, had his own ideas about the "secret" (quite different from Morland's) published in a Wisconsin newspaper, the DOE claimed that The Progressive case was moot, dropped its suit, and allowed the magazine to publish its article, which it did in November 1979. Morland had by then, however, changed his opinion of how the bomb worked, suggesting that a foam medium (the polystyrene) rather than radiation pressure was used to compress the secondary, and that in the secondary there was a spark plug of fissile material as well. He published these changes, based in part on the proceedings of the appeals trial, as a short erratum in The Progressive a month later. In 1981, Morland published a book about his experience, describing in detail the train of thought that led him to his conclusions about the "secret". Morland's work is interpreted as being at least partially correct because the DOE had sought to censor it, one of the few times they violated their usual approach of not acknowledging "secret" material that had been released; however, to what degree it lacks information, or has incorrect information, is not known with any confidence. The difficulty that other countries had in developing the Teller–Ulam design (even when they apparently understood the design, such as with the United Kingdom) makes it somewhat unlikely that this simple information alone is what provides the ability to manufacture thermonuclear weapons. Nevertheless, the ideas put forward by Morland in 1979 have been the basis for all the current speculation on the Teller–Ulam design. == Notable accidents ==

On 5 February 1958, during a training mission flown by a B-47, a Mark 15 nuclear bomb, also known as the Tybee Bomb, was lost off the coast of Tybee Island near Savannah, Georgia. The US Air Force maintains that the bomb was unarmed and did not contain the live fissile core necessary to initiate a nuclear explosion. The bomb was thought by the Department of Energy to lie buried under several feet of silt at the bottom of Wassaw Sound. On 17 January 1966, a fatal collision occurred between a B-52G and a KC-135 Stratotanker over Palomares, Spain. The conventional explosives in two of the Mk28-type hydrogen bombs detonated upon impact with the ground, dispersing plutonium over nearby farms. A third bomb landed intact near Palomares while the fourth fell off the coast into the Mediterranean sea and was recovered a few months later. On 21 January 1968, a B-52G, with four B28FI thermonuclear bombs aboard as part of Operation Chrome Dome, crashed on the ice of the North Star Bay while attempting an emergency landing at Thule Air Base in Greenland. The resulting fire caused extensive radioactive contamination. Personnel involved in the cleanup failed to recover all the debris from three of the bombs, and one bomb was not recovered. == See also ==