While every nuclear weapon design falls into one of these categories, specific designs have occasionally become the subject of news accounts and public discussion, often with incorrect descriptions about how they work and what they do. Examples:
Single stage Minor actinide fission weapons Some isotopes of
protactinium,
neptunium,
americium,
curium,
californium,
berkelium, and
einsteinium have calculated
critical mass values, ranging in the kilograms to tens of kilograms. Few possess an adequate combination of high
fission cross section (for detonation), low
spontaneous fission rate (to limit
predetonation), low alpha or gamma decay rate (to allow handling). All suffer from a far higher cost of production compared to standard
fissile material. This is due to both production of the quantity required, often in
high flux reactors, and complex chemical separation procedures. For elements with atomic number Z > 96 (curium and above), total global production has never exceeded a single critical mass of separated material.
Neptunium-237 is considered the most immediately concerning minor actinide isotope for weaponization. Comprising ~0.05% of
spent nuclear fuel, ~5 tons are produced annually worldwide. The
International Atomic Energy Agency has established monitoring for facilities capable of separation of the isotope, but is yet to classify it as a "special fissionable material", alongside
plutonium-239,
uranium-233, and
enriched uranium. In September 2002, researchers at the
Los Alamos National Laboratory briefly produced the first known nuclear
critical mass involving a significant quantity of neptunium, in combination with shells of
enriched uranium (
uranium-235), discovering that the critical mass of a bare sphere of neptunium-237 "ranges from kilogram weights in the high fifties to low sixties," showing that it "is about as good a bomb material as [uranium-235]." The United States federal government made plans in March 2004 to move America's supply of separated neptunium to a nuclear-waste disposal site in
Nevada. Certain isotopes of
americium are also considered weaponizable, despite considerable challenge, based on the testimony of nuclear weapons physicists.
Layer cake concept: The layer cake was an early design for a weapon using thermonuclear reactions, involving an spherical implosion bomb design that contained alternating layers of fission and fusion fuel. As a single-stage device, it was only capable of generating a limited amount of fusion reactions and could not be scaled up indefinitely, and has been equated with
boosted fission weapons. It could, however, provide interim capability for high yields, mostly from the low-cost materials of lithium deuteride and
natural or
depleted uranium, as opposed to expensive fissile material. Layer cake devices were researched by at least the United States, Soviet Union, United Kingdom, and China. The Soviet Union and China constructed and tested layer cake nuclear weapons, while the others did not. The U.S. name for the design, "Alarm Clock," came from Teller: he called it that because it might "wake up the world" to the possibility of the potential of the Super. The Russian name for the same design was more descriptive of its design: Sloika (), a layered pastry cake. The United States never developed or tested the design in this form, because its inherent limitations made it unappealing compared to the "Classical Super" design, despite it being a fairly straightforward development compared to the "Classical Super." In the Soviet Union, however, the Sloika was tested as
RDS-6s on August 12, 1953, with a yield of 400 kilotons of TNT, of which 15-20% was from thermonuclear fusion reactions. The test used lithium-6 deuteride and mixed with a small quantity of lithium-6 tritide. It was the first nuclear test to ignite a solid thermonuclear fuel; previous US tests in
Operation Greenhouse and Ivy had used cryogenic or gaseous deuterium and tritium. Because the Soviet Sloika test was an air drop, it was sometimes claimed that the USSR won the race to make the first deliverable hydrogen bomb, as the first U.S. thermonuclear test (Ivy Mike, 1952) was of an undeliverably large "device." Those who push back against some claims make a distinction between "true," staged thermonuclear weapons and "boosted" weapons, and include the "Sloika" with the latter. The first Soviet test of a staged thermonuclear weapon design,
RDS-37, was not until 1955. It has been argued that the Sloika, rather than a dead-end, was integral to the Soviet development of staged thermonuclear weapons, as efforts to better implode Sloika-style designs were the Soviet path towards radiation implosion. The Soviet Union later tested
RDS-27, a modification of RDS-6s to use only lithium deuteride, without the expensive tritium, allowing mass production. The device was tested in the
1955 nuclear test series and demonstrated an expected lower yield of 250 kilotons. In its pursuit of thermonuclear weapons, the third
Chinese nuclear weapons test was also a layer-cake design. Chinese nuclear scientists had acquired some details of the sloika from the Soviets at some point during their period of nuclear cooperation. It was codenamed "
596L", as it was based on
China's first nuclear device, "596", a fission implosion bomb, but with an extra layer of
lithium deuteride represented by the "L". The weapon was tested on 9 May 1966, dropped from a
Xi'an H-6 bomber over
Lop Nur, and yielded approximately 220 kt.
Multi-stage Multi-stage fission weapons The idea of using
radiation implosion channelled from one fission bomb to effectively compress a second fission bomb was considered as part of the
British hydrogen bomb programme, which mistakenly believed this would be necessary before igniting a third thermonuclear stage. This original configuration was nicknamed "
Tom, Dick, and Harry", although British weapon designers soon focused on more conventional two-stage weapons and used "Dick" to refer to their thermonuclear secondaries. Nonetheless, components for such a "triple bomb" were constructed as the "Halliard 1" option of the
Operation Grapple test series. A small radiation casing contained the primary and secondary fission bombs, and sat inside a large radiation casing alongside the thermonuclear tertiary. Despite previous test successes making any Halliard test unnecessary, the weapon was fired at the request of the United States, to whom the concept was "novel and of deep interest". It was fired as the Grapple Z3 shot on 11 September 1958, yielding 800 kilotons.
Clean bombs Designs with lead tampers ''; see below for elaboration. On March 1, 1954, the largest-ever U.S. nuclear test explosion, the 15-megaton
Castle Bravo shot of
Operation Castle at Bikini Atoll, delivered a promptly lethal dose of fission-product fallout to more than of Pacific Ocean surface. Radiation injuries to
Marshall Islanders and
Japanese fishermen made that fact public and revealed the role of fission in hydrogen bombs. In response to the public alarm over fallout, an effort was made to design a clean multi-megaton weapon, relying almost entirely on fusion. The energy produced by the fissioning of
unenriched natural uranium, when used as the tamper material in the secondary and subsequent stages in the Teller-Ulam design, can far exceed the energy released by fusion, as was the case in the Castle Bravo test. Replacing the
fissionable material in the tamper with another
high-Z material (
lead) is essential to producing a "clean" bomb. In such a device, the tamper no longer contributes energy, so for any given weight, a clean bomb will have less yield. This was called the "materials substitution method". the 9.3-megaton
Hardtack Poplar test at 95%, and the 4.5-megaton
Redwing Navajo test at 95% fusion.
Designs with no tampers shot Housatonic, the cleanest and highest yield-to-weight ratio test ever, testing the Ripple design. The Ripple concept, which used ablation to achieve fusion using very little fission, was and still is by far the cleanest design. Unlike previous clean bombs, which were clean simply by replacing the uranium-238 tamper with lead, Ripple was inherently clean. The fission sparkplug was replaced by a large deuterium-tritium gas core, surrounded by a tamper-like lithium deuteride shell. It is assumed that thin concentric shells of a high-Z material like lead, driven by the small
Kinglet primary allowed propagated sustained shockwaves to the core, sustaining the thermonuclear burn and giving the device its name. The design was influenced by the nascent field of
inertial confinement fusion. Ripple was also extremely efficient; plans for a 15 kt/kg were made during
Operation Dominic. Shot Androscoggin featured a proof-of-concept Ripple design, resulting in a 63-kiloton fizzle (significantly lower than the predicted 15 megatons). It was repeated in shot Housatonic, which featured a 9.96 megaton explosion that was reportedly >99.9% fusion. Beginning with the
1958 Soviet nuclear tests, physicists
Yuri Trutnev and
Yuri Babayev developed very lightweight thermonuclear weapons with no use of fissile material in the secondary stage. These weapons were adapted into the majority of Soviet and modern Russian weapons. It is possible this design involves a deuterium-tritium mixture ignition, similar to the Soviet peaceful nuclear explosion devices. In the Soviet
peaceful nuclear explosion program "Nuclear Explosions for the National Economy", "clean" bombs were used for a 1971 triple salvo test related to the
Pechora–Kama Canal project. It was reported that about 250 nuclear devices might be used to get the final goal. The
Taiga test was to demonstrate the feasibility of the project. Three of these devices of 15 kiloton yield each were placed in separate boreholes, simultaneously detonated, catapulting a radioactive plume into the air that was carried eastward by wind. The resulting trench was around long and wide, with an unimpressive depth of just . Despite their "clean" nature, the area still exhibits a noticeably higher (albeit mostly harmless) concentration of
fission products, the intense
neutron bombardment of the soil, the device itself and the support structures also activated their stable elements to create a significant amount of man-made radioactive elements like
60Co. A larger scale project as was envisioned, however, would have had significant consequences both from the fallout of radioactive plume and the radioactive elements created by the neutron bombardment.
Third generation First and second generation nuclear weapons release energy as omnidirectional blasts. Third generation nuclear weapons are experimental special effect warheads and devices that can release energy in a directed manner, some of which were tested during the
Cold War but were never deployed. These include: • Project Prometheus, also known as "Nuclear Shotgun", which would have used a nuclear explosion to accelerate kinetic penetrators against ICBMs. •
Project Excalibur, a nuclear-pumped X-ray laser to
destroy ballistic missiles. •
Nuclear shaped charges that focus their energy in particular directions. •
Project Orion explored the use of nuclear explosives for rocket propulsion.
Fourth generation and pure fusion weapons The idea of "4th-generation" nuclear weapons has been proposed as a possible successor to the examples of weapons designs listed above. These methods tend to revolve around using non-nuclear primaries to set off further fission or fusion reactions. For example, if
antimatter were usable and controllable in macroscopic quantities, a reaction between a small amount of antimatter and an equivalent amount of matter could release energy comparable to a small fission weapon, and could in turn be used as the first stage of a very compact thermonuclear weapon. Extremely-powerful lasers could also potentially be used this way, if they could be made powerful-enough, and compact-enough, to be viable as a weapon. Most of these ideas are versions of
pure fusion weapons, and share the common property that they involve hitherto unrealized technologies as their "primary" stages. While many nations have invested significantly in
inertial confinement fusion research programs, since the 1970s it has not been considered promising for direct weapons use, but rather as a tool for weapons- and energy-related research that can be used in the absence of full-scale nuclear testing. Whether any nations are aggressively pursuing "4th-generation" weapons is not clear. In many cases (as with antimatter) the underlying technology is presently thought to be very far from being viable, and if it was viable would be a powerful weapon in and of itself, outside of a nuclear weapons context, and without providing any significant advantages above existing nuclear weapons designs Since the 1950s, the United States and Soviet Union investigated the possibility of releasing significant amounts of nuclear fusion energy without the use of a fission primary. Such "pure fusion weapons" were primarily imagined as low-yield, tactical nuclear weapons whose advantage would be their ability to be used without producing fallout on the scale of weapons that release fission products. In 1998, the
United States Department of Energy declassified the following: Scientists such as
Arjun Makhijani have argued that ICF programs, including the United States'
stockpile stewardship components of the
National Ignition Facility,
Z Pulsed Power Facility, and Los Alamos
magnetized target fusion could contribute to or be primarily pursued for eventual use in pure fusion weapons. Such experiments have also been contested as
violations of the Comprehensive Nuclear-Test Ban Treaty.
Red mercury, a likely hoax substance, has been hyped as a catalyst for a pure fusion weapon.
Arbitrarily large multi-staged devices The idea of a device which has an arbitrarily large number of Teller-Ulam stages, with each driving a larger radiation-driven implosion than the preceding stage, is frequently suggested, but technically disputed. There are "well-known sketches and some reasonable-looking calculations in the open literature about two-stage weapons, but no similarly accurate descriptions of true three stage concepts." and
Edward Teller announced the design of a 10,000-megaton weapon code-named
SUNDIAL at a meeting of the General Advisory Committee of the Atomic Energy Commission. Much of the information about these efforts remains classified, but such "gigaton" range weapons do not appear to have made it beyond theoretical investigations. While both the US and Soviet Union investigated (and in the case of the Soviets, tested) "very high yield" (e.g. 50 to 100-megaton) weapons designs in the 1950s and early 1960s, these appear to represent the upper-limit of Cold War weapon yields pursued seriously, and were so physically heavy and massive that they could not be carried entirely within the bomb bays of the largest bombers. Cold War warhead development trends from the mid-1960s onward, and especially after the
Limited Test Ban Treaty, instead resulted in highly-compact warheads with yields in the range from hundreds of kilotons to the low megatons that gave greater options for deliverability. Following the concern caused by the estimated gigaton scale of the 1994
Comet Shoemaker-Levy 9 impacts on the planet
Jupiter, in a 1995 meeting at
Lawrence Livermore National Laboratory (LLNL),
Edward Teller proposed to a collective of U.S. and Russian ex-
Cold War weapons designers that they collaborate on designing a 1,000-megaton
nuclear explosive device for diverting extinction-class asteroids (10+ km in diameter), which would be employed in the event that one of these asteroids were on an impact trajectory with Earth.
Specific effect Salted bombs . A salted bomb is designed to disperse radioisotopes which remain harmful for longer, similar to Chernobyl. A salted bomb is a nuclear weapon which intentionally disperses a large quantity of one or more selected
radioisotopes, typically produced in-situ from irradiation by the weapon detonation, and designed to make the blast area inhospitable to humans for many years. This is distinct from most nuclear weapons, which also produce and disperse deadly radioisotopes as the
fission products of uranium and plutonium, but this fission is part of the device's yield, and fission product radioactivity drops off more rapidly. A commonly selected radioisotope is
cobalt-60 (), which could be formed from the weapon's neutron irradiation of a tamper or jacket of natural cobalt (almost entirely
cobalt-59). The table below gives relative values for gamma radiation from standard nuclear weapon fission product fallout, with a range of short-, medium-, and long-lived half-lives, versus from cobalt-60, which has a half-life of 5.27 years. Cobalt-60 has a higher relative intensity from six months after detonation to 75 years after detonation, at which point
long-lived fission product radiation overtakes cobalt again: Salted weapons were investigated by U.S. Department of Defense. Such a weapon was tested at least once in the
Operation Redwing series as shot
Flathead. The device was a
TX-28S variant of the
B28 nuclear bomb, where the "S" stood for "Salted". The triple "taiga" nuclear
salvo test, as part of the preliminary March 1971
Pechora–Kama Canal project, produced a small amount of fission products and therefore a comparatively large amount of case material activated products are responsible for most of the residual activity at the site today, namely .
fusion generated neutron activation was responsible for about half of the gamma dose at the test site. That dose is too small to cause deleterious effects, and normal green vegetation exists all around the lake that was formed. The concept of salted bombs as "doomsday weapons" was made popular by
Nevil Shute's 1957
novel, and subsequent 1959 movie,
On the Beach, and the 1964 film
Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb; the material added to the bombs is referred to in the film as 'cobalt-thorium G'.
Neutron bombs A neutron bomb, technically referred to as an enhanced radiation weapon (ERW), is a type of tactical nuclear weapon designed specifically to release a large portion of its energy as energetic neutron radiation. This contrasts with standard thermonuclear weapons, which are designed to capture this intense neutron radiation to increase its overall explosive yield. In terms of yield, ERWs typically produce about one-tenth that of a fission-type atomic weapon. Even with their significantly lower explosive power, ERWs are still capable of much greater destruction than any conventional bomb. Meanwhile, relative to other nuclear weapons, damage is more focused on biological material than on material infrastructure (though extreme blast and heat effects are not eliminated). ERWs are more accurately described as suppressed yield weapons. When the yield of a nuclear weapon is less than one kiloton, its lethal radius from blast, , is less than that from its neutron radiation. However, the blast is more than potent enough to destroy most structures, which are less resistant to blast effects than even unprotected human beings. Blast pressures of upwards of are survivable, whereas most buildings will collapse with a pressure of only . Commonly misconceived as a weapon designed to kill populations and leave infrastructure intact, these bombs (as mentioned above) are still very capable of leveling buildings over a large radius. The intent of their design was to kill tank crews – tanks giving excellent protection against blast and heat, surviving (relatively) very close to a detonation. Given the Soviets' vast tank forces during the Cold War, this was the perfect weapon to counter them. The neutron radiation could instantly incapacitate a tank crew out to roughly the same distance that the heat and blast would incapacitate an unprotected human (depending on design). The tank chassis would also be rendered highly radioactive, temporarily preventing its re-use by a fresh crew. Neutron weapons were also intended for use in other applications, however. For example, they are effective in anti-nuclear defenses – the neutron flux being capable of neutralising an incoming warhead at a greater range than heat or blast. Nuclear warheads are very resistant to physical damage, but are very difficult to harden against extreme neutron flux. ERWs were two-stage thermonuclears with all non-essential uranium removed to minimize fission yield. Fusion provided the neutrons. Developed in the 1950s, they were first deployed in the 1970s, by U.S. forces in Europe. The last ones were retired in the 1990s. A neutron bomb is only feasible if the yield is sufficiently high that efficient fusion stage ignition is possible, and if the yield is low enough that the case thickness will not absorb too many neutrons. This means that neutron bombs have a yield range of 1–10 kilotons, with fission proportion varying from 50% at 1 kiloton to 25% at 10 kilotons (all of which comes from the primary stage). The neutron output per kiloton is then 10 to 15 times greater than for a pure fission implosion weapon or for a strategic warhead like a
W87 or
W88. ==Weapon design laboratories==