Nuclear Weapon Design

American nuclear technology evolved rapidly between 1944 and 1950, moving from the primitive Fat Man and Little Boy to more sophisticated, lighter, more powerful, and more efficient designs. Much design effort shifted from fission to thermonuclear weapons after President Truman decided that the United States should proceed to develop a hydrogen bomb, a task which occupied the Los Alamos Laboratory from 1950 through 1952. The "George" shot of Operation Greenhouse (May 9, 1951) confirmed for the first time that a fission device could produce the conditions needed to ignite a thermonuclear reaction. The "Mike" test of Operation Ivy, 1 November, 1952, was the first explosion of a true two-stage thermonuclear device.

From 1952 until the early years of the ICBM era [roughly to the development of the first multiple independently targeted reentry vehicles (MIRVs) in the late 1960’s], new concepts in both fission primary and fusion secondary design were developed rapidly. However, after the introduction of the principal families of weapons in the modern stockpile (approximately the mid 1970’s), the rate of design innovations and truly new concepts slowed as nuclear weapon technology became a mature science. It is believed that other nations’ experiences have been roughly similar, although the United States probably has the greatest breadth of experience with innovative designs simply because of the more than 1,100 nuclear detonations it has conducted. The number of useful variations on the themes of primary and secondary design is finite, and designers’ final choices are frequently constrained by considerations of weapon size, weight, safety, and the availability of special materials.

Nuclear weaponry has advanced considerably since 1945, as can be seen at an unclassified level by comparing the size and weight of “Fat Man” with the far smaller, lighter, and more powerful weapons carried by modern ballistic missiles. Most nations of the world, including those of proliferation interest, have subscribed to the 1963 Limited Test Ban Treaty, which requires that nuclear explosions only take place underground. Underground testing can be detected by seismic means and by observing radioactive effluent in the atmosphere. It is probably easier to detect and identify a small nuclear test in the atmosphere than it is to detect and identify a similarly sized underground test. In either case, highly specialized instrumentation is required if a nuclear test explosion is to yield useful data to the nation carrying out the experiment.

US nuclear weapons technology is mature and might not have shown many more qualitative advances over the long haul, even absent a test ban. The same is roughly true for Russia, the UK, and possibly for France. The design of the nuclear device for a specific nuclear weapon is constrained by several factors. The most important of these are the weight the delivery vehicle can carry plus the size of the space available in which to carry the weapon (e.g., the diameter and length of a nosecone or the length and width of a bomb bay). The required yield of the device is established by the target vulnerability. The possible yield is set by the state of nuclear weapon technology and by the availability of special materials. Finally, the choices of specific design details of the device are determined by the taste of its designers, who will be influenced by their experience and the traditions of their organization.

Fission Weapons

An ordinary “atomic” bomb of the kinds used in World War II uses the process of nuclear fission to release the binding energy in certain nuclei. The energy release is rapid and, because of the large amounts of energy locked in nuclei, violent. The principal materials used for fission weapons are U-235 and Pu-239, which are termed fissile because they can be split into two roughly equal-mass fragments when struck by a neutron of even low energies. When a large enough mass of either material is assembled, a self-sustaining chain reaction results after the first fission is produced.

The minimum mass of fissile material that can sustain a nuclear chain reaction is called a critical mass and depends on the density, shape, and type of fissile material, as well as the effectiveness of any surrounding material (called a reflector or tamper) at reflecting neutrons back into the fissioning mass. Critical masses in spherical geometry for weapon-grade materials are as follows:
			   Uranium-235      Plutonium-239

	Bare sphere:		56 kg	 	11 kg
	Thick Tamper:		15 kg		 5 kg
The critical mass of compressed fissile material decreases as the inverse square of the density achieved. Since critical mass decreases rapidly as density increases, the implosion technique can make do with substantially less nuclear material than the gun-assembly method. The "Fat Man" atomic bomb that destroyed Nagasaki in 1945 used 6.2 kilograms of plutonium and produced an explosive yield of 21-23 kilotons [a 1987 reassessment of the Japanese bombings placed the yield at 21 Kt]. Until January 1994, the Department of Energy (DOE) estimated that 8 kilograms would typically be needed to make a small nuclear weapon. Subsequently, however, DOE reduced the estimate of the amount of plutonium needed to 4 kilograms. Some US scientists believe that 1 kilogram of plutonium will suffice.

If any more material is added to a critical mass a condition of supercriticality results. The chain reaction in a supercritical mass increases rapidly in intensity until the heat generated by the nuclear reactions causes the mass to expand so greatly that the assembly is no longer critical.

Fission weapons require a system to assemble a supercritical mass from a sub-critical mass in a very short time. Two classic assembly systems have been used, gun and implosion. In the simpler gun-type device, two subcritical masses are brought together by using a mechanism similar to an artillery gun to shoot one mass (the projectile) at the other mass (the target). The Hiroshima weapon was gun-assembled and used 235 U as a fuel. Gun-assembled weapons using highly enriched uranium are considered the easiest of all nuclear devices to construct and the most foolproof.

The Nagasaki Bomb

Compared to one used on Hiroshima, the Nagasaki bomb was rounder and fatter, so it was called "Fat Man." The fissile material was plutonium 239. The plutonium was divided into subcritical portions and packed into a spherical case. To cause the chain reaction, gunpowder around the periphery of the case was used to force the units to the center. Thus, it was called an "implosion-type" bomb.
The fission of slightly more than one kilogram of plutonium 239 is thought to have released destructive energy equivalent to about 21,000 tons of TNT.

<The Nagasaki Bomb (Fat Man)>

<Nagasaki Bomb Diagram>

Length: Approx. 3.2 meters (128 inches)
Weight: Approx. 4.5 tons (10,000 pounds)
Diameter: Approx. 1.5 meters (60 inches)
Element: Plutonium 239

Gun-Device

In the gun device, two pieces of fissionable material, each less than a critical mass, are brought together very rapidly to forma single supercritical one. This gun-type assembly may be achieved in a tubular device in which a high explosive is used to blow one subcritical piece of fissionable material from one end of the tube into another subcritical piece held at the opposite end of the tube.

Manhattan Project scientists were so confident in the performance of the “Little Boy” uranium bomb that the device was not even tested before it was used. This 15-kt weapon was airdropped on 06 August 1945 at Hiroshima, Japan. The device contained 64.1 kg of highly enriched uranium, with an average enrichment of 80%. The six bombs built by the Republic of South Africa were gun-assembled and used 50kg of uranium enriched to between 80 percent and 93 percent in the isotope U-235.

Compared with the implosion approach, this method assembles the masses relatively slowly and at normal densities; it is practical only with highly enriched uranium. If plutonium -— even weapon-grade -- were used in a gun-assembly design, neutrons released from spontaneous fission of its even-numbered isotopes would likely trigger the nuclear chain reaction too soon, resulting in a "fizzle" of dramatically reduced yield.

Implosion-Device

Because of the short time interval between spontaneous neutron emissions (and, therefore, the large number of background neutrons) found in plutonium because of the decay by spontaneous fission of the isotope Pu-240, Manhattan Project scientists devised the implosion method of assembly in which high explosives are arranged to form an imploding shock wave which compresses the fissile material to supercriticality.

The core of fissile material that is formed into a super-critical mass by chemical high explosives (HE) or propellants. When the high explosive is detonated, an inwardly directed implosion wave is produced. This wave compresses the sphere of fissionable material. The decrease in surface to volume ratio of this compressed mass plus its increased density is then such as to make the mass supercritical. The HE is exploded by detonators timed electronically by a fuzing system, which may use altitude sensors or other means of control.

The nuclear chain-reaction is normally started by an initiator that injects a burst of neutrons into the fissile core at an appropriate moment. The timing of the initiation of the chain reaction is important and must be carefully designed for the weapon to have a predictable yield. A neutron generator emits a burst of neutrons to initiate the chain reaction at the proper moment —- near the point of maximum compression in an implosion design or of full assembly in the gun-barrel design.

A surrounding tamper may help keep the nuclear material assembled for a longer time before it blows itself apart, thus increasing the yield. The tamper often doubles as a neutron reflector.

Implosion systems can be built using either Pu-239 or U-235 but the gun assembly only works for uranium. Implosion weapons are more difficult to build than gun weapons, but they are also more efficient, requiring less SNM and producing larger yields. Iraq attempted to build an implosion bomb using U-235. In contrast, North Korea chose to use 239 Pu produced in a nuclear reactor.

Boosted Weapons

To fission more of a given amount of fissile material, a small amount of material that can undergo fusion, deuterium and tritium (D-T) gas, can be placed inside the core of a fission device. Here, just as the fission chain reaction gets underway, the D-T gas undergoes fusion, releasing an intense burst of high-energy neutrons (along with a small amount of fusion energy as well) that fissions the surrounding material more completely. This approach, called boosting, is used in most modem nuclear weapons to maintain their yields while greatly decreas-ing their overall size and weight.

Enhanced Radiation Weapons

An enhanced radiation (ER) weapon, by special design techniques, has an output in which neutrons and x-rays are made to constitute a substantial portion of the total energy released. For example, a standard fission weapon's total energy output would be partitioned as follows: 50% as blast; 35% as thermal energy; and 15% as nuclear radiation. An ER weapon's total energy would be partitioned as follows: 30% as blast; 20% as thermal; and 50% as nuclear radiation. Thus, a 3-kiloton ER weapon will produce the nuclear radiation of a 10-kiloton fission weapon and the blast and thermal radiation of a 1-kiloton fission device. However, the energy distribution percentages of nuclear weapons are a function of yield.

Fusion Weapons

A more powerful but more complex weapon uses the fusion of heavy isotopes of hydrogen, deuterium, and tritium to release large numbers of neutrons when the fusile (sometimes termed "fusionable") material is compressed by the energy released by a fission device called a primary. Fusion (or ‘‘thermonuclear’ weapons derive a significant amount of their total energy from fusion reactions. The intense temperatures and pressures generated by a fission explosion overcome the strong electrical repulsion that would otherwise keep the positively charged nuclei of the fusion fuel from reacting. The fusion part of the weapon is called a secondary.In general, the x-rays from a fission primary heat and compress material surrounding a secondary fusion stage.

It is inconvenient to carry deuterium and tritium as gases in a thermonuclear weapon, and certainly impractical to carry them as liquefied gases, which requires high pressures and cryogenic temperatures. Instead, one can make a “dry” device in which ⁶Li is combined with deuterium to form the compound ⁶Li D (lithium-6 deuteride). Neutrons from a fission “primary” device bombard the ⁶ Li in the compound, liberating tritium, which quickly fuses with the nearby deuterium. The a particles, being electrically charged and at high temperatures, contribute directly to forming the nuclear fireball. The neutrons can bombard additional ⁶Li nuclei or cause the remaining uranium and plutonium in the weapon to undergo fission. This two-stage thermonuclear weapon has explosive yields far greater than can be achieved with one point safe designs of pure fission weapons, and thermonuclear fusion stages can be ignited in sequence to deliver any desired yield. Such bombs, in theory, can be designed with arbitrarily large yields: the Soviet Union once tested a device with a yield of about 59 megatons.

In a relatively crude sense, 6 Li can be thought of as consisting of an alpha particle ( ⁴He) and a deuteron ( ²H) bound together. When bombarded by neutrons, ⁶ Li disintegrates into a triton ( ³ H) and an alpha:

6 Li + Neutron = 3 H + 3 He + Energy.

This is the key to its importance in nuclear weapons physics. The nuclear fusion reaction which ignites most readily is

2 H + 3 H =
4 He + n + 17.6 MeV,

or, phrased in other terms, deuterium plus tritium produces ⁴He plus a neutron plus 17.6 MeV of free energy:

D + T = 4 He + n + 17.6 MeV.

Lithium-7 also contributes to the production of tritium in a thermonuclear secondary, albeit at a lower rate than⁶Li. The fusion reactions derived from tritium produced from ⁷ Li contributed many unexpected neutrons (and hence far more energy release than planned) to the final stage of the infamous 1953 Castle/BRAVO atmospheric test, nearly doubling its expected yield.

Safing, Arming, Fuzing, and Firing (SAFF)

The ability to make effective use of a nuclear weapon is limited unless the device can be handled safely, taken safely from storage when required, delivered to its intended target, and then detonated at the correct point in space and time to achieve the desired goal. Although the intended scenarios for use of its weapons will strongly influence specific weaponization concepts and approaches, functional capabilities for safing, arming, fuzing, and firing (SAFF) will be fundamental.

Nuclear weapons are particularly destructive, with immediate effects including blast and thermal radiation and delayed effects produced by ionizing radiation, neutrons, and radioactive fallout. They are expensive to build, maintain, and employ, requiring a significant fraction of the total defense resources of a small nation. In a totalitarian state the leader must always worry that they will be used against the government; in a democracy the possibility of an unauthorized or accidental use must never be discounted. A nuclear detonation as the result of an accident would be a local catastrophe.

Because of their destructiveness, nuclear weapons require precautions to prevent accidental detonation during any part of their manufacture and lifetime. And because of their value, the weapons require reliable arming and fuzing mechanisms to ensure that they explode when delivered to target. Therefore, any nuclear power is likely to pay some attention to the issues of safing and safety, arming, fuzing, and firing of its nuclear weapons. The solutions adopted depend upon the level of technology in the proliferant state, the number of weapons in its stockpile, and the political consequences of an accidental detonation.

Whether to protect their investment in nuclear arms or to deny potential access to and use of the weapons by unauthorized persons, proliferators or subnational groups will almost certainly seek special measures to ensure security and operational control of nuclear weapons. These are likely to include physical security and access control technologies at minimum and may include use control. The techniques used today by the existing western nuclear weapon states represent the culmination of a half-century of evolution in highly classified military programs, and proliferators may well choose simpler solutions, perhaps by adapting physical security, access, and operational controls used in the commercial sector for high-value/high-risk assets.

From the very first nuclear weapons built, safety was a consideration. The two bombs used in the war drops on Hiroshima and Nagasaki posed significant risk of accidental detonation if the B-29 strike aircraft had crashed on takeoff. As a result, critical components were removed from each bomb and installed only after takeoff and initial climb to altitude were completed. Both weapons used similar arming and fuzing components. Arming could be accomplished by removing a safety connector plug and replacing it with a distinctively colored arming connector. Fuzing used redundant systems including a primitive radar and a barometric switch. No provision was incorporated in the weapons themselves to prevent unauthorized use or to protect against misappropriation or theft.

In later years, the United States developed mechanical safing devices. These were later replaced with weapons designed to a goal of less than a 1 in a 1 million chance of the weapon delivering more than 4 pounds of nuclear yield if the high explosives were detonated at the single most critical possible point. Other nations have adopted different safety criteria and have achieved their safety goals in other ways.

In the 1950’s, to prevent unauthorized use of U.S. weapons stored abroad, permissive action links (PALs) were developed. These began as simple combination locks and evolved into the modern systems which allow only a few tries to arm the weapon and before disabling the physics package should an intruder persist in attempts to defeat the PAL.

Safing To ensure that the nuclear warhead can be stored, handled, deployed, and employed in a wide spectrum of intended and unintended environmental and threat conditions, with assurance that it will not experience a nuclear detonation. In U.S. practice, safing generally involves multiple mechanical interruptions of both power sources and pyrotechnic/explosive firing trains. The nuclear components may be designed so that an accidental detonation of the high explosives is intrinsically unable to produce a significant (>4 pounds TNT equivalent) nuclear yield; it is simpler to insert mechanical devices into the pit to prevent the assembly of a critical mass into the pit or to remove a portion of the fissile material from inside the high explosives. Mechanical safing of a gun-assembled weapon is fairly straightforward; one can simply insert a hardened steel or tungsten rod across a diameter of the gun barrel, disrupting the projectile. All U.S. weapons have been designed to be intrinsically one-point safe in the event of accidental detonation of the high explosives, but it is not anticipated that a new proliferator would take such care.

Arming Placing the nuclear warhead in a ready operational state, such that it can be initiated under specified firing conditions. Arming generally involves mechanical restoration of the safing interrupts in response to conditions that are unique to the operational environment (launch or deployment) of the system. A further feature is that the environment typically provides the energy source to drive the arming action. If a weapon is safed by inserting mechanical devices into the pit (e.g., chains, coils of wire, bearing balls) to prevent complete implosion, arming involves removal of those devices. It may not always be possible to safe a mechanically armed device once the physical barrier to implosion has been removed.

Fuzing To ensure optimum weapon effectiveness by detecting that the desired conditions for warhead detonation have been met and to provide an appropriate command signal to the firing set to initiate nuclear detonation. Fuzing generally involves devices to detect the location of the warhead with respect to the target, signal processing and logic, and an output circuit to initiate firing.

Firing To ensure nuclear detonation by delivering a precise level of precisely timed electrical or pyrotechnic energy to one or more warhead detonating devices. A variety of techniques are used, depending on the warhead design and type of detonation devices.

Depending on the specific military operations to be carried out and the specific delivery system chosen, nuclear weapons pose special technological problems in terms of primary power and power-conditioning, overall weapon integration, and operational control and security.

Not all weapons possessors will face the same problems or opt for the same levels of confidence, particularly in the inherent security of their weapons. The operational objectives will in turn dictate the technological requirements for the SAFF subsystems. Minimal requirements could be met by surface burst (including impact fuzing of relatively slow moving warhead) or crude preset height of burst based on simple timer or barometric switch or simple radar altimeter. Modest requirements could be met by more precise HOB (height of burst) based on improved radar triggering or other methods of measuring distance above ground to maxmize radius of selected weapons effects, with point-contact salvage fuzing. Parachute delivery of bombs to allow deliberate laydown and surface burst. Substantial requirements could be met by variable HOB, including low-altitude for ensured destruction of protected strategic targets, along with possible underwater or exoatmospheric capabilities.

Virtually any country or extranational group with the resources to construct a nuclear device has sufficient capability to attain the minimum SAFF capability that would be needed to meet terrorist or minimal national aims. The requirements to achieve a “modest” or “substantial” capability level are much more demanding. Both safety and protection of investment demand very low probability of failure of safing and arming mechanisms, with very high probability of proper initiation of the warhead. All of the recognized nuclear weapons states and many other countries have (or have ready access to) both the design know-how and components required to implement a significant capability.

In terms of sophistication, safety, and reliability of design, past U.S. weapons programs provide a legacy of world leadership in SAFF and related technology. France and the UK follow closely in overall SAFF design and may actually hold slight leads in specific component technologies. SAFF technologies of other nuclear powers —- notably Russia and China -— do not compare. Japan and Germany have technological capabilities roughly on a par with the United States, UK, and France, and doubtless have the capability to design and build nuclear SAFF subsystems.

Reliable fuzing and firing systems suitable for nuclear use have been built since 1945 and do not need to incorporate any modern technology. Many kinds of mechanical safing systems have been employed, and several of these require nothing more complex than removable wires or chains or the exchanging of arming/ safing connector plugs. Safing a gun-assembled system is especially simple. Arming systems range from hand insertion of critical components in flight to extremely sophisticated instruments which detect specific events in the stockpile to target sequence (STS). Fuzing and firing systems span an equally great range of technical complexity.

Any country with the electronics capability to build aircraft radar altimeter equipment should have access to the capability for building a reasonably adequate, simple HOB fuze. China, India, Israel, Taiwan, South Korea, Brazil, Singapore, the Russian Federation and the Ukraine, and South Africa all have built conventional weapons with design features that could be adapted to more sophisticated designs, providing variable burst height and rudimentary Electronic Counter Counter Measure (ECCM) features. With regard to physical security measures and use control, the rapid growth in the availability and performance of low-cost, highly reliable microprocessing equipment has led to a proliferation of electronic lock and security devices suitable for protecting and controlling high-value/at-risk assets. Such technology may likely meet the needs of most proliferant organizations.

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