Weapon design laboratories
Berkeley
The first systematic exploration of nuclear weapon design concepts took place in the summer of 1942 at the
University of California, Berkeley. Important early discoveries had been made at the adjacent
Lawrence Berkeley Laboratory, such as the 1940 cyclotron made production and isolation of plutonium. A Berkeley professor,
J. Robert Oppenheimer, had just been hired to run the nation's secret bomb design effort. His first act was to convene the 1942 summer conference.
By the time he moved his operation to the new secret town of Los Alamos, New Mexico, in the spring of 1943, the accumulated wisdom on nuclear weapon design consisted of five lectures by Berkeley professor
Robert Serber, transcribed and distributed as the
Los Alamos Primer. The Primer addressed fission energy,
neutron production and
capture,
nuclear chain reactions,
critical mass, tampers, predetonation, and three methods of assembling a bomb: gun assembly, implosion, and "autocatalytic methods," the one approach that turned out to be a dead end.
Los Alamos
At
Los Alamos, it was found in April 1944 by
Emilio G. Segrè that the proposed
Thin Man Gun assembly type bomb would not work for plutonium because of predetonation problems caused by Pu-240 impurities. So
Fat Man, the implosion-type bomb, was given high priority as the only option for plutonium. The Berkeley discussions had generated theoretical estimates of critical mass, but nothing precise. The main wartime job at Los Alamos was the experimental determination of critical mass, which had to wait until sufficient amounts of fissile material arrived from the production plants: uranium from
Oak Ridge, Tennessee, and plutonium from the
Hanford site in Washington.
In 1945, using the results of critical mass experiments, Los Alamos technicians fabricated and assembled components for four bombs: the
Trinity Gadget,
Little Boy,
Fat Man, and an unused spare Fat Man. After the war, those who could, including Oppenheimer, returned to university teaching positions. Those who remained worked on levitated and hollow pits and conducted weapon effects tests such as
Crossroads Able and Baker at
Bikini Atoll in 1946.
All of the essential ideas for incorporating fusion into nuclear weapons originated at Los Alamos between 1946 and 1952. After the
Teller-Ulam radiation implosion breakthrough of 1951, the technical implications and possibilities were fully explored, but ideas not directly relevant to making the largest possible bombs for long-range Air Force bombers were shelved.
Because of Oppenheimer's initial position in the H-bomb debate, in opposition to large thermonuclear weapons, and the assumption that he still had influence over Los Alamos despite his departure, political allies of
Edward Teller decided he needed his own laboratory in order to pursue H-bombs. By the time it was opened in 1952, in
Livermore, California, Los Alamos had finished the job Livermore was designed to do.
Livermore
With its original mission no longer available, the Livermore lab tried radical new designs, that failed. Its first three nuclear tests were fizzles: in 1953, two single-stage fission devices with uranium hydride pits, and in 1954, a two-stage thermonuclear device in which the secondary heated up prematurely, too fast for radiation implosion to work properly.
Shifting gears, Livermore settled for taking ideas Los Alamos had shelved and developing them for the Army and Navy. This led Livermore to specialize in small-diameter tactical weapons, particularly ones using two-point implosion systems, such as the Swan. Small-diameter tactical weapons became primaries for small-diameter secondaries. Around 1960, when the superpower arms race became a ballistic missile race, Livermore warheads were more useful than the large, heavy Los Alamos warheads. Los Alamos warheads were used on the first
intermediate-range ballistic missiles, IRBMs, but smaller Livermore warheads were used on the first
intercontinental ballistic missiles, ICBMs, and
submarine-launched ballistic missiles, SLBMs, as well as on the first
multiple warhead systems on such missiles.
[34]
In 1957 and 1958 both labs built and tested as many designs as possible, in anticipation that a planned 1958 test ban might become permanent. By the time testing resumed in 1961 the two labs had become duplicates of each other, and design jobs were assigned more on workload considerations than lab specialty. Some designs were horse-traded. For example, the
W38 warhead for the
Titan I missile started out as a Livermore project, was given to Los Alamos when it became the
Atlas missile warhead, and in 1959 was given back to Livermore, in trade for the
W54 Davy Crockett warhead, which went from Livermore to Los Alamos.
The period of real innovation was ending by then, anyway. Warhead designs after 1960 took on the character of model changes, with every new missile getting a new warhead for marketing reasons. The chief substantive change involved packing more fissile uranium into the secondary, as it became available with continued
uranium enrichment and the dismantlement of the large high-yield bombs.
Explosive testing
Nuclear weapons are in large part designed by trial and error. The trial often involves test explosion of a prototype.
In a nuclear explosion, a large number of discrete events, with various probabilities, aggregate into short-lived, chaotic energy flows inside the device casing. Complex mathematical models are required to approximate the processes, and in the 1950s there were no computers powerful enough to run them properly. Even today's computers and their codes are not fully adequate.
[35]
It was easy enough to design reliable weapons for the stockpile. If the prototype worked, it could be weaponized and mass produced.
It was much more difficult to understand how it worked or why it failed. Designers gathered as much data as possible during the explosion, before the device destroyed itself, and used the data to calibrate their models, often by inserting
fudge factors into equations to make the simulations match experimental results. They also analyzed the weapon debris in fallout to see how much of a potential nuclear reaction had taken place.
Light pipes
An important tool for test analysis was the diagnostic light pipe. A probe inside a test device could transmit information by heating a plate of metal to incandescence, an event that could be recorded at the far end of a long, very straight pipe.
The picture below shows the Shrimp device, detonated on March 1, 1954 at Bikini, as the
Castle Bravo test. Its 15-megaton explosion was the largest ever by the United States. The silhouette of a man is shown for scale. The device is supported from below, at the ends. The pipes going into the shot cab ceiling, which appear to be supports, are diagnostic light pipes. The eight pipes at the right end (1) sent information about the detonation of the primary. Two in the middle (2) marked the time when x-radiation from the primary reached the radiation channel around the secondary. The last two pipes (3) noted the time radiation reached the far end of the radiation channel, the difference between (2) and (3) being the radiation transit time for the channel.
[36]
From the shot cab, the pipes turned horizontal and traveled 7500 ft (2.3 km), along a causeway built on the Bikini reef, to a remote-controlled data collection bunker on Namu Island.
While x-rays would normally travel at the speed of light through a low density material like the plastic foam channel filler between (2) and (3), the intensity of radiation from the exploding primary created a relatively opaque radiation front in the channel filler which acted like a slow-moving logjam to retard the passage of radiant energy. Behind this moving front was a fully-ionized, low-z (low atomic number) plasma heated to 20,000 °C, soaking up energy like a black box, and eventually driving the implosion of the secondary.
[37]
The radiation transit time, on the order of half a microsecond, is the time it takes the entire radiation channel to reach thermal equilibrium as the radiation front moves down its length. The implosion of the secondary is based on the temperature difference between the hot channel and the cool interior of the secondary. Its timing is important because the interior of the secondary is subject to neutron preheat.
While the radiation channel is heating and starting the implosion, neutrons from the primary catch up with the x-rays, penetrate into the secondary and start breeding tritium with the third reaction noted in the first section above. This Li-6 + n reaction is exothermic, producing 5 MeV per event. The spark plug is not yet compressed and thus is not critical, so there won't be significant fission or fusion. But if enough neutrons arrive before implosion of the secondary is complete, the crucial temperature difference will be degraded. This is the reported cause of failure for Livermore's first thermonuclear design, the Morgenstern device, tested as
Castle Koon, April 7, 1954.
These timing issues are measured by light-pipe data. The mathematical simulations which they calibrate are called radiation flow hydrodynamics codes, or channel codes. They are used to predict the effect of future design modifications.
It is not clear from the public record how successful the Shrimp light pipes were. The data bunker was far enough back to remain outside the mile-wide crater, but the 15-megaton blast, two and a half times greater than expected, breached the bunker by blowing its 20-ton door off the hinges and across the inside of the bunker. (The nearest people were twenty miles (32 km) farther away, in a bunker that survived intact.)
[38]
Fallout analysis
The most interesting data from Castle Bravo came from radio-chemical analysis of weapon debris in fallout. Because of a shortage of enriched lithium-6, 60% of the lithium in the Shrimp secondary was ordinary lithium-7, which doesn't breed tritium as easily as lithium-6 does. But it does breed lithium-6 as the product of an (n, 2n) reaction (one neutron in, two neutrons out), a known fact, but with unknown probability. The probability turned out to be high.
Fallout analysis revealed to designers that, with the (n, 2n) reaction, the Shrimp secondary effectively had two and half times as much lithium-6 as expected. The tritium, the fusion yield, the neutrons, and the fission yield were all increased accordingly.
[39]
As noted above, Bravo's fallout analysis also told the outside world, for the first time, that thermonuclear bombs are more fission devices than fusion devices. A Japanese fishing boat, the
Lucky Dragon, sailed home with enough fallout on its decks to allow scientists in Japan and elsewhere to determine, and announce, that most of the fallout had come from the fission of U-238 by fusion-produced 14 MeV neutrons.
Underground testing
Subsidence Craters at Yucca Flat, Nevada Test Site.
The global alarm over radioactive fallout, which began with the Castle Bravo event, eventually drove nuclear testing underground. The last U.S. above-ground test took place at
Johnston Island on November 4, 1962. During the next three decades, until September 23, 1992, the U.S. conducted an average of 2.4 underground nuclear explosions per month, all but a few at the
Nevada Test Site (NTS) northwest of Las Vegas.
The
Yucca Flat section of the NTS is covered with subsidence craters resulting from the collapse of terrain over radioactive underground caverns created by nuclear explosions (see photo).
After the 1974
Threshold Test Ban Treaty (TTBT), which limited underground explosions to 150 kilotons or less, warheads like the half-megaton W88 had to be tested at less than full yield. Since the primary must be detonated at full yield in order to generate data about the implosion of the secondary, the reduction in yield had to come from the secondary. Replacing much of the lithium-6 deuteride fusion fuel with lithium-7 hydride limited the deuterium available for fusion, and thus the overall yield, without changing the dynamics of the implosion. The functioning of the device could be evaluated using light pipes, other sensing devices, and analysis of trapped weapon debris. The full yield of the stockpiled weapon could be calculated by extrapolation.
Production facilities
When two-stage weapons became standard in the early 1950s, weapon design determined the layout of America's new, widely dispersed production facilities, and vice versa.
Because primaries tend to be bulky, especially in diameter, plutonium is the fissile material of choice for pits, with beryllium reflectors. It has a smaller critical mass than uranium. The
Rocky Flats plant in Boulder, Colorado, was built in 1952 for pit production and consequently became the plutonium and beryllium fabrication facility.
The Y-12 plant in
Oak Ridge,
Tennessee, where
mass spectrometers called
Calutrons had enriched uranium for the
Manhattan Project, was redesigned to make secondaries. Fissile U-235 makes the best spark plugs because its critical mass is larger, especially in the cylindrical shape of early thermonuclear secondaries. Early experiments used the two fissile materials in combination, as composite Pu-Oy pits and spark plugs, but for mass production, it was easier to let the factories specialize: plutonium pits in primaries, uranium spark plugs and pushers in secondaries.
Y-12 made lithium-6 deuteride fusion fuel and U-238 parts, the other two ingredients of secondaries.
The
Savannah River plant in
Aiken,
South Carolina, also built in 1952, operated
nuclear reactors which converted U-238 into Pu-239 for pits, and lithium-6 (produced at Y-12) into tritium for booster gas. Since its reactors were moderated with heavy water, deuterium oxide, it also made deuterium for booster gas and for Y-12 to use in making lithium-6 deuteride.
Warhead design safety
A diagram of the
Green Grass warhead's steel ball-bearing safety device, shown left, filled (safe) and right, empty (live). The steel balls were emptied into a hopper underneath the aircraft before flight, the steel balls could be re-inserted using a funnel by rotating the bomb on its trolley and raising the hopper.
It is inherently dangerous to have a weapon containing a quantity and shape of fissile material which can form a critical mass through a relatively simple accident. Because of this danger, the high explosives in
Little Boy (four bags of
Cordite powder) were inserted into the bomb in flight, shortly after takeoff on August 6, 1945. It was the first time a gun-type nuclear weapon had ever been fully assembled.
Gun-type weapons have always been inherently unsafe.
Neither of these effects is likely with implosion weapons since there is normally insufficient fissile material to form a critical mass without the correct detonation of the lenses. However, the earliest implosion weapons had pits so close to criticality that accidental detonation with some nuclear yield was a concern.
On August 9, 1945,
Fat Man was loaded onto its airplane fully assembled, but later, when levitated pits made a space between the pit and the tamper, it was feasible to utilize in-flight pit insertion. The bomber would take off with no fissile material in the bomb. Some older implosion-type weapons, such as the US
Mark 4 and
Mark 5, used this system.
In-flight pit insertion will not work with a hollow pit in contact with its tamper.
As shown in the diagram, one method used to decrease the likelihood of accidental detonation used metal balls. The balls were emptied into the pit; this would prevent detonation by increasing density of the hollowed pit. This design was used in the Green Grass weapon, also known as the
Interim Megaton Weapon and was also used in
Violet Club and the
Yellow Sun Mk.1 bombs.
Alternatively, the pit can be "safed" by having its normally-hollow core filled with an inert material such as a fine metal chain, possibly made of cadmium to absorb neutrons. While the chain is in the center of the pit, the pit can not be compressed into an appropriate shape to fission; when the weapon is to be armed, the chain is removed. Similarly, although a serious fire could detonate the explosives, destroying the pit and spreading plutonium to contaminate the surroundings as has happened in
several weapons accidents, it could not however, cause a nuclear explosion.
The US
W47 warhead used in
Polaris A1 and
Polaris A2 had a safety device consisting of a boron-coated-wire inserted into the hollow pit at manufacture. The warhead was armed by withdrawing the wire onto a spool driven by an electric motor. However, once withdrawn the wire could not be re-inserted.
[40]While the firing of one detonator out of many will not cause a hollow pit to go critical, especially a low-mass hollow pit that requires boosting, the introduction of two-point implosion systems made that possibility a real concern.
In a two-point system, if one detonator fires, one entire hemisphere of the pit will implode as designed. The high-explosive charge surrounding the other hemisphere will explode progressively, from the equator toward the opposite pole. Ideally, this will pinch the equator and squeeze the second hemisphere away from the first, like toothpaste in a tube. By the time the explosion envelops it, its implosion will be separated both in time and space from the implosion of the first hemisphere. The resulting dumbbell shape, with each end reaching maximum density at a different time, may not become critical.
Unfortunately, it is not possible to tell on the drawing board how this will play out. Nor is it possible using a dummy pit of U-238 and high-speed x-ray cameras, although such tests are helpful. For final determination, a test needs to be made with real fissile material. Consequently, starting in 1957, a year after Swan, both labs began one-point safety tests.
Out of 25 one-point safety tests conducted in 1957 and 1958, seven had zero or slight nuclear yield (success), three had high yields of 300 t to 500 t (severe failure), and the rest had unacceptable yields between those extremes.
Of particular concern was Livermore's W47 warhead for the Polaris submarine missile. The last test before the 1958 moratorium was a one-point test of the W47 primary, which had an unacceptably high nuclear yield of 400 lb (180 kg) of TNT equivalent (Hardtack II Titania). With the test moratorium in force, there was no way to refine the design and make it inherently one-point safe. Los Alamos had a suitable primary that was one-point safe, but rather than share with Los Alamos the credit for designing the first SLBM warhead, Livermore chose to use mechanical safing on its own inherently unsafe primary. The wire safety scheme described above was the result.
[41]
It turns out that the
W47 may have been safer than anticipated. The wire-safety system may have rendered most of the warheads "duds," unable to fire when detonated.
[41]
When testing resumed in 1961, and continued for three decades, there was sufficient time to make all warhead designs inherently one-point safe, without need for mechanical safing.
In addition to the above steps to reduce the probability of a nuclear detonation arrising from a single fault, locking mechanisms referred to by NATO states as Permissive Action Links are sometimes attached to the control mechanisms for nuclear warheads. Permissive Action Links act solely to prevent an unauthorised use of a nuclear weapon.
References:
- ^ DoE Fact Sheet: Reliable Replacement Warhead Program
- ^ Sybil Francis, Warhead Politics: Livermore and the Competitive System of Nuclear Warhead Design, UCRL-LR-124754, June 1995, Ph.D. Dissertation, Massachusetts Institute of Technology, available from National Technical Information Service. This 233-page thesis was written by a weapons-lab outsider for public distribution. The author had access to all the classified information at Livermore that was relevant to her research on warhead design; consequently, she was required to use non-descriptive code words for certain innovations.
- ^ Walter Goad, Declaration for the Wen Ho Lee case, May 17, 2000. Goad began thermonuclear weapon design work at Los Alamos in 1950. In his Declaration, he mentions "basic scientific problems of computability which cannot be solved by more computing power alone. These are typified by the problem of long range predictions of weather and climate, and extend to predictions of nuclear weapons behavior. This accounts for the fact that, after the enormous investment of effort over many years, weapons codes can still not be relied on for significantly new designs."
- ^ Chuck Hansen, The Swords of Armageddon, Volume IV, pp. 211-212, 284.
- ^ The public literature mentions three different force mechanism for this implosion: radiation pressure, plasma pressure, and explosive ablation of the outer surface of the secondary pusher. All three forces are present; and the relative contribution of each is one of the things the computer simulations try to explain. See Teller-Ulam design.
- ^ Dr. John C. Clark, as told to Robert Cahn, "We Were Trapped by Radioactive Fallout," The Saturday Evening Post, July 20, 1957, pp. 17-19, 69-71.[1]
- ^ Richard Rhodes, Dark Sun; the Making of the Hydrogen Bomb, Simon and Schuster, 1995, p. 541.
- ^ Chuck Hansen, The Swords of Armageddon, Volume VII, pp. 396-397.
- ^ a b Sybil Francis, Warhead Politics, pp. 141, 160.
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