Fusion-boosted fission weapons
The next step in miniaturization was to speed up the fissioning of the pit to reduce the amount of time inertial confinement needed. The hollow pit provided an ideal location to introduce fusion for the boosting of fission. A 50-50 mixture of tritium and deuterium gas, pumped into the pit during arming, will fuse into helium and release free neutrons soon after fission begins. The neutrons will start a large number of new chain reactions while the pit is still critical.
Once the hollow pit is perfected, there is little reason not to boost.
The concept of fusion-boosted fission was first tested on May 25, 1951, in the Item shot of Operation Greenhouse, Eniwetok, yield 45.5 kilotons.
Boosting reduces diameter in three ways, all the result of faster fission:
- Since the compressed pit does not need to be held together as long, the massive U-238 tamper can be replaced by a light-weight beryllium shell (to reflect escaping neutrons back into the pit). The diameter is reduced.
- The mass of the pit can be reduced by half, without reducing yield. Diameter is reduced again.
- Since the mass of the metal being imploded (tamper plus pit) is reduced, a smaller charge of high explosive is needed, reducing diameter even further.
Since boosting is required to attain full design yield, any reduction in boosting reduces yield. Boosted weapons are thus variable-yield weapons. Yield can be reduced any time before detonation, simply by putting less than the full amount of tritium into the pit during the arming procedure.
The first device whose dimensions suggest employment of all these features (two-point, hollow-pit, fusion-boosted implosion) was the Swan device, tested June 22, 1956, as the Inca shot of Operation Redwing, at Eniwetok. Its yield was 15 kilotons, about the same as Little Boy, the Hiroshima bomb. It weighed 105 lb (47.6 kg) and was cylindrical in shape, 11.6 inches (29.5 cm) in diameter and 22.9 inches (58 cm) long. The above schematic illustrates what were probably its essential features.
Eleven days later, July 3, 1956, the Swan was test-fired again at Eniwetok, as the Mohawk shot of Redwing. This time it served as the primary, or first stage, of a two-stage thermonuclear device, a role it played in a dozen such tests during the 1950s. Swan was the first off-the-shelf, multi-use primary, and the prototype for all that followed.
After the success of Swan, 11 or 12 inches (300 mm) seemed to become the standard diameter of boosted single-stage devices tested during the 1950s. Length was usually twice the diameter, but one such device, which became the W54 warhead, was closer to a sphere, only 15 inches (380 mm) long. It was tested two dozen times in the 1957-62 period before being deployed. No other design had such a long string of test failures. Since the longer devices tended to work correctly on the first try, there must have been some difficulty in flattening the two high explosive lenses enough to achieve the desired length-to-width ratio.
One of the applications of the W54 was the Davy Crockett XM-388 recoilless rifle projectile, shown here in comparison to its Fat Man predecessor, dimensions in inches.
Another benefit of boosting, in addition to making weapons smaller, lighter, and with less fissile material for a given yield, is that it renders weapons immune to radiation interference (RI). It was discovered in the mid-1950s that plutonium pits would be particularly susceptible to partial pre-detonation if exposed to the intense radiation of a nearby nuclear explosion (electronics might also be damaged, but this was a separate issue). RI was a particular problem before effective early warning radar systems because a first strike attack might make retaliatory weapons useless. Boosting reduces the amount of plutonium needed in a weapon to below the quantity which would be vulnerable to this effect.
Two-stage thermonuclear weapons
Pure fission or fusion-boosted fission weapons can be made to yield hundreds of kilotons, at great expense in fissile material and tritium, but by far the most efficient way to increase nuclear weapon yield beyond ten or so kilotons is to tack on a second independent stage, called a secondary.
In the 1940s, bomb designers at Los Alamos thought the secondary would be a canister of deuterium in liquified or hydride form. The fusion reaction would be D-D, harder to achieve than D-T, but more affordable. A fission bomb at one end would shock-compress and heat the near end, and fusion would propagate through the canister to the far end. Mathematical simulations showed it wouldn't work, even with large amounts of prohibitively expensive tritium added in.
The entire fusion fuel canister would need to be enveloped by fission energy, to both compress and heat it, as with the booster charge in a boosted primary. The design breakthrough came in January 1951, when Edward Teller and Stanisław Ulam invented radiation implosion - for nearly three decades known publicly only as the Teller-Ulam H-bomb secret.
The concept of radiation implosion was first tested on May 9, 1951, in the George shot of Operation Greenhouse, Eniwetok, yield 225 kilotons. The first full test was on November 1, 1952, the Mike shot of Operation Ivy, Eniwetok, yield 10.4 megatons.
In radiation implosion, the burst of x-ray energy coming from an exploding primary is captured and contained within an opaque-walled radiation channel which surrounds the nuclear energy components of the secondary. For a millionth of a second, most of the energy of several kilotons of TNT is absorbed by a plasma (superheated gas) generated from plastic foam in the radiation channel. With energy going in and not coming out, the plasma rises to solar core temperatures and expands with solar core pressures. Nearby objects which are still cool are crushed by the temperature difference.
The cool nuclear materials surrounded by the radiation channel are imploded much like the pit of the primary, except with vastly more force. This greater pressure enables the secondary to be significantly more powerful than the primary, without being much larger.
A. Warhead before firing; primary (fission bomb) at top, secondary (fusion fuel) at bottom, all suspended in polystyrene foam. B. High-explosive fires in primary, compressing plutonium core into supercriticality and beginning a fission reaction. C. Fission primary emits X-rays which reflects along the inside of the casing, irradiating the polystyrene foam. D. Polystyrene foam becomes plasma, compressing secondary, and fissile uranium (U-235) sparkplug begins to fission. E. Compressed and heated, lithium-6 deuteride fuel begins fusion reaction, neutron flux causes tamper to fission. A fireball is starting to form.
For example, for the Redwing Mohawk test on July 3, 1956, a secondary called the Flute was attached to the Swan primary. The Flute was 15 inches (38 cm) in diameter and 23.4 inches (59 cm) long, about the size of the Swan. But it weighed ten times as much and yielded 24 times as much energy (355 kilotons, vs 15 kilotons).
Equally important, the active ingredients in the Flute probably cost no more than those in the Swan. Most of the fission came from cheap U-238, and the tritium was manufactured in place during the explosion. Only the spark plug at the axis of the secondary needed to be fissile.
A spherical secondary can achieve higher implosion densities than a cylindrical secondary, because spherical implosion pushes in from all directions toward the same spot. However, in warheads yielding more than one megaton, the diameter of a spherical secondary would be too large for most applications. A cylindrical secondary is necessary in such cases. The small, cone-shaped re-entry vehicles in multiple-warhead ballistic missiles after 1970 tended to have warheads with spherical secondaries, and yields of a few hundred kilotons.
As with boosting, the advantages of the two-stage thermonuclear design are so great that there is little incentive not to use it, once a nation has mastered the technology.
In engineering terms, radiation implosion allows for the exploitation of several known features of nuclear bomb materials which heretofore had eluded practical application. For example:
- The best way to store deuterium in a reasonably dense state is to chemically bond it with lithium, as lithium deuteride. But the lithium-6 isotope is also the raw material for tritium production, and an exploding bomb is a nuclear reactor. Radiation implosion will hold everything together long enough to permit the complete conversion of lithium-6 into tritium, while the bomb explodes. So the bonding agent for deuterium permits use of the D-T fusion reaction without any pre-manufactured tritium being stored in the secondary. The tritium production constraint disappears.
- For the secondary to be imploded by the hot, radiation-induced plasma surrounding it, it must remain cool for the first microsecond, i.e., it must be encased in a massive radiation (heat) shield. The shield's massiveness allows it to double as a tamper, adding momentum and duration to the implosion. No material is better suited for both of these jobs than ordinary, cheap uranium-238, which happens, also, to undergo fission when struck by the neutrons produced by D-T fusion. This casing, called the pusher, thus has three jobs: to keep the secondary cool, to hold it, inertially, in a highly compressed state, and, finally, to serve as the chief energy source for the entire bomb. The consumable pusher makes the bomb more a uranium fission bomb than a hydrogen fusion bomb. It is noteworthy that insiders never used the term hydrogen bomb.[17]
- Finally, the heat for fusion ignition comes not from the primary but from a second fission bomb called the spark plug, imbedded in the heart of the secondary. The implosion of the secondary implodes this spark plug, detonating it and igniting fusion in the material around it, but the spark plug then continues to fission in the neutron-rich environment until it is fully consumed, adding significantly to the yield.[18]
The initial impetus behind the two-stage weapon was President Truman's 1950 promise to build a 10-megaton hydrogen superbomb as America's response to the 1949 test of the first Soviet fission bomb. But the resulting invention turned out to be the cheapest and most compact way to build small nuclear bombs as well as large ones, erasing any meaningful distinction between A-bombs and H-bombs, and between boosters and supers. All the best techniques for fission and fusion explosions are incorporated into one all-encompassing, fully-scalable design principle. Even six-inch (152 mm) diameter nuclear artillery shells can be two-stage thermonuclears.
In the ensuing fifty years, nobody has come up with a better way to build a nuclear bomb. It is the design of choice for the U.S., Russia, Britain, France, and China, the five thermonuclear powers. The other nuclear-armed nations, Israel, India, Pakistan, and North Korea, probably have single-stage weapons, possibly boosted.[18]
Interstage
In a two-stage thermonuclear weapon, three types of energy emerge from the primary to impact the secondary: the expanding hot gases from high explosive charges which implode the primary, plus the electromagnetic radiation and the neutrons from the primary's nuclear detonation. An essential energy transfer modulator called the interstage, between the primary and the secondary, protects the secondary from the hot gases and channels the electromagnetic radiation and neutrons toward the right place at the right time.
There is very little information in the open literature about the mechanism of the interstage. Its first mention in a U.S. government document formally released to the public appears to be a caption in a recent graphic promoting the Reliable Replacement Warhead Program. If built, this new design would replace "toxic, brittle material" and "expensive 'special' material" in the interstage.[19] This statement suggests the interstage may contain beryllium to moderate the flux of neutrons from the primary, and perhaps something to absorb and re-radiate the x-rays in a particular manner.[20]
The interstage and the secondary are encased together inside a stainless steel membrane to form the canned subassembly (CSA), an arrangement which has never been depicted in any open-source drawing.[21] The most detailed illustration of an interstage shows a British thermonuclear weapon with a cluster of items between its primary and a cylindrical secondary. They are labeled "end-cap and neutron focus lens," "reflector/neutron gun carriage," and "reflector wrap." The origin of the drawing, posted on the internet by Greenpeace, is uncertain, and there is no accompanying explanation.[22]
References:
References:
- ^ "Restricted Data Declassification Decisions from 1946 until Present"
- ^ Fissionable Materials section of the Nuclear Weapons FAQ, Carey Sublette, accessed Sept 23, 2006
- ^ All information on nuclear weapon tests comes from Chuck Hansen, The Swords of Armageddon: U.S. Nuclear Weapons Development since 1945, October 1995, Chucklea Productions, Volume VIII, p. 154, Table A-1, "U.S. Nuclear Detonations and Tests, 1945-1962."
- ^ Nuclear Weapons FAQ: 4.1.6.3 Hybrid Assembly Techniques, accessed December 1, 2007. Drawing adapted from the same source.
- ^ Nuclear Weapons FAQ: 4.1.6.2.2.4 Cylindrical and Planar Shock Techniques, accessed December 1, 2007.
- ^ "Restricted Data Declassification Decisions from 1946 until Present", Section V.B.2.k "The fact of use in high explosive assembled (HEA) weapons of spherical shells of fissile materials, sealed pits; air and ring HE lenses," declassified November 1972.
- ^ Until a reliable design was worked out in the early 1950s, the hydrogen bomb (public name) was called the superbomb by insiders. After that, insiders used a more descriptive name: two-stage thermonuclear. Two examples. From Herb York, The Advisors, 1976, "This book is about . . . the development of the H-bomb, or the superbomb as it was then called." p. ix, and "The rapid and successful development of the superbomb (or super as it came to be called) . . ." p. 5. From National Public Radio Talk of the Nation, November 8, 2005, Siegfried Hecker of Los Alamos, "the hydrogen bomb – that is, a two-stage thermonuclear device, as we referred to it – is indeed the principal part of the US arsenal, as it is of the Russian arsenal."
- ^ a b Howard Morland, "Born Secret," Cardozo Law Review, March 2005, pp. 1401-1408.
- ^ "Improved Security, Safety & Manufacturability of the Reliable Replacement Warhead," NNSA March 2007.
- ^ A 1976 drawing which depicts an interstage that absorbs and re-radiates x-rays. From Howard Morland, "The Article," Cardozo Law Review, March 2005, p 1374.
- ^ "SAND8.8 - 1151 Nuclear Weapon Data -- Sigma I," Sandia Laboratories, September 1988.
- ^ The Greenpeace drawing. From Morland, Cardozo Law Review, March 2005, p 1378.
- ^ Herbert York, The Advisors: Oppenheimer, Teller and the Superbomb (1976).
- ^ "The ‘Alarm Clock' . . . became practical only by the inclusion of Li6 (in 1950) and its combination with the radiation implosion." Hans A. Bethe, Memorandum on the History of Thermonuclear Program, May 28, 1952.
- ^ See map.
- ^ Samuel Glasstone, The Effects of Nuclear Weapons, 1962, Revised 1964, U.S. Dept of Defense and U.S. Dept of Energy, pp.464-5. This section was removed from later editions, but, according to Glasstone in 1978, not because it was inaccurate or because the weapons had changed.
- ^ "Nuclear Weapons FAQ: 1.6".
- ^ "Neutron bomb: Why 'clean' is deadly".
