Nuclear Weapons : Are they easy to design?

Weapons

Nuclear weapons are small, light, and inexpensive compared to the conventional ordnance needed to destroy large area targets. Although the infrastructure for a nuclear enterprise is complex, the weapons themselves use relatively straightforward designs. Nuclear explosives enable a single missile or aircraft to destroy an entire city, giving great leverage to a state or subnational group with even a small stockpile of such devices. Nuclear weapons were first developed more than a half century ago with technology and knowledge of physics far less than available today.

Identifying some of the key technologies needed to acquire nuclear weapons capability may allow effective intervention and/or identification of trends of concern. Although a great deal of information, much of which is not correct, on the principles of nuclear explosives is available in the public domain, development of nuclear weapons, even in the early stages, requires an understanding and mastery of the relevant physical principles. Such an understanding, which is necessary even to plan a program to achieve a nuclear weapon capability, contains elements from fields not generally familiar to today’s scientists. A number of steps are necessary to develop nuclear weapons, and if these steps are not well understood, false starts will be made, and valuable resources will be allocated to inappropriate tasks. In the worst case, skilled personnel may be lost to radiation or to other accidents. Misallocation of resources can delay, and in some cases prevent, achievement of the goals of a weapons program.

The nuclear weapons publicly known to have been fielded use only two fundamental principles for releasing nuclear energy: fission and fusion. Under these major categories, "boosting," "staging," and the use of either high explosive- driven implosion or a propellant-powered gun mechanism to assemble a supercritical mass constitute the major elements of the known nuclear weapons types. The various systems may be combined in many different ways, with the single requirement that a fission chain reaction is needed to ignite nuclear fusion in a weapon.

Knowledge required for designing a Nuclear Weapon

This section describes the general process and the capabilities required for understanding and designing nuclear weapons. The paths a proliferator might take can be quite different than the paths that the nuclear powers have taken in the past.

The following are the technologies a proliferator must master to be able to convert a supply of special nuclear material into actual nuclear explosives:
• Fast-fission chain reaction theory and practice,
• Fast assembly of critical and supercritical masses of fissile material,
• High explosive (HE) and propellant characteristics and design,
• HE initiation,
• Firing sets for HE initiation,
• Thermonuclear boosting of fission primary, and
• Thermonuclear/second stage of nuclear weapons.

The fission reactions commonly studied in nuclear reactor physics use thermal neutrons, and the cross sections usually tabulated are those for low-energy particles.

In a nuclear weapon, the time scales dealt with do not allow full thermalization of the neutrons, hence "fast" neutrons, that is, the neutrons emitted and interacting at higher energies must be considered. Thus, the important neutron interactions for the weapons designer are those which occur at roughly MeV energies. In addition, reactor neutron transport codes need to be modified to fully account for the different physical regimes.

A comprehensive understanding of the similarities and differences between nuclear reactor physics and nuclear weapon physics is essential to make progress in nuclear weapon design.

For a nuclear weapon to release its energy in a time which is short compared to the hydrodynamic disassembly time, rapid assembly to form a supercritical mass is essential. This assembly can be accomplished in a linear fashion, as in a gun-assembled weapon, or it can be accomplished in a spherical fashion, as in an implosion weapon.

In the first case, two subcritical masses of the fissile material are rapidly assembled into a supercritical mass, one mass being fired by the gun at the other mass. In the second case, the fissile material is initially in a subcritical configuration, and then energy contributed by conventional explosives is concentrated on the fissile material to achieve a supercritical mass. The fissile materials will be driven to high pressure/ high energy conditions by the high-explosive energy. This will require calculations of initial, intermediate, and final configurations, using hydrodynamic programs and appropriate equations of state for these regimes of temperature and pressure.

High Explosives (HE) or propellants are the means of choice for assembly of most nuclear weapons. Given this, the potential proliferator must understand and master the data and design of systems to accomplish such assembly. Propellants are used to assemble gun-type weapons, and are usually relatively slow burning. Much useful data from conventional artillery tube-fired weapons development is generally available. Much data concerning implosion is also available from the development of modern conventional HE weapons including shaped charges.

Special considerations applicable to nuclear weapons development involve shock wave propagation and focusing. Such considerations go beyond much of conventional explosive design work, and would require specialized programs, equations of state in HE pressure and temperature regimes, and data on detonation velocities and strengths.

Initiation of the main charge of a nuclear explosive in such a way as to provide the desired final configuration of the fissile material often proves to be a major design challenge. Traditionally, this challenge has been met by initiating the charge at a number of discrete points, and then tailoring the converging shock wave through the use of lenses consisting of slower and faster burning explosives. Such initiation can be accomplished either by electrical signals or by fuse trains, both ending at a detonator, which initiates the shock wave at the lens charge.

Firing sets for nuclear devices, the means for activating the initiation of the main charge of HE for a nuclear weapon, can also have performance characteristics, which lie outside the range of conventional engineering. If the proliferator is relying on initiation at a discrete number of points, then these points must be activated nearly simultaneously to have a smooth implosion. The simultaneity required depends on the internal design of the explosive, but it is common to require a higher degree of simultaneity than is usually the case for conventional explosives. Thus, high energy must be delivered to all the detonators at nearly the same time. This will require high-energy, low-impedance capacitors, and high-current, high-speed switches.

Once the potential proliferator has begun to understand the operation of a simple fission weapon, he may well want to increase the yield to make more efficient use of his special nuclear material. One way to do this is to boost the fission yield by incorporating thermonuclear reactions into the design of the weapon. Introduction of the neutrons from thermonuclear reactions at the time of super criticality of the fissile material can have a dramatic effect on the yield. The usual fusion material used for this purpose is a mixture of deuterium and tritium gas.

When the proliferator begins to think in terms of greatly increasing the yield of his nuclear weapons, he may consider design and development of thermonuclear and/or second stages. To do this, he would have to obtain and master hydrodynamic computer programs, which correctly describe regimes of extremely high temperatures and pressures. He would show interest in equations of state of special nuclear materials under these conditions. He would also be interested in neutron and reaction cross sections for both fissionable materials and thermonuclear materials at these high temperatures and pressures. Finally, he would attempt to obtain lithium (and/or lithium deuteride), tritium and deuterium.

Finally, the actual coupling of the nuclear weapon primary with a thermonuclear/ boosted-fission secondary will require mastery of a complex set of physical principles. The proliferator will not only have to understand hydrodynamic calculations under extreme physical conditions, he will have to obtain and understand the flow of energy from the primary to and around the secondary. Energy flow and the behavior of materials under these extreme conditions of temperature and pressure comprise a complex set of problems, well beyond the experience of most of today’s physicists.

Neutron Initiator Design

One of the key elements in the proper operation of a nuclear weapon is initiation of the fission chain reaction at the proper time. To obtain a significant nuclear yield of the nuclear explosive, sufficient neutrons must be present within the supercritical core at the right time. If the chain reaction starts too soon, the result will be only a "fizzle yield," much below the design specification; if it occurs too late, there may be no yield whatever. Several ways to produce neutrons at the appropriate moment have been developed.

In a gun-assembled weapon, the assembly speed is relatively slow. This requires a strong source of alpha particles. An implosion weapon may require a source which can produce a precisely timed burst of neutrons. This requires a strong source of alpha particles, something of the order of 10 curies of 210Po or a similarly active alpha emitter. This isotope of polonium has a half-life of almost 140 days, and a neutron initiator using this material needs to have the polonium replaced frequently. Since the 210Po is made in a nuclear reactor, this means that potential proliferators need either to have a nuclear reactor of their own, or to have access to one. To supply the initiation pulse of neutrons at the right time, the polonium and the beryllium need to be kept apart until the appropriate moment and then thoroughly and rapidly mixed.

One of the ways to make an external neutron generator is by using an electronically controlled particle accelerator called a pulse neutron tube. Such a system might use the deuterium-deuterium or deuterium-tritium fusion reactions to produce large amounts of neutrons. Typically, deuterium nuclei are accelerated to an energy sufficient to cause a fusion reaction when they strike a deuterium- or tritium-rich target.

This impact can result in a short pulse of neutrons sufficient to initiate the fission chain reaction. The timing of the pulse can be precisely controlled. Similar devices are used in oil well logging.

Types of Nuclear Design Useful for a Terrorist

1. Uranium Gun-Assembled Devices

A terrorist with access to >50 kg of Highly Enriched Uranium (HEU) would almost certainly opt for a gun assembled weapon despite the inherent inefficiencies of such a device, both because of its simplicity and the perceived lack of a need to test a gun assembly.

The disadvantage of a gun design is that it needs significantly more fissile material than an efficient implosion device of similar yield. This may be important to a subnational group intending to explode a series of devices, but would be of much less importance if only one blast were contemplated.

2. Implosion assembly

If the subnational group had only 239Pu or needed to be economical with a limited supply of HEU, then it would likely turn to an implosion assembly. The simplest design of an implosion weapon places a solid plutonium (or HEU) pit at the center of a sphere, surrounded by a certain amount of tamper material such as 238U, to be compressed by the large amount of high explosive filling the sphere. It is generally asserted in the open literature that 32 lens charges were used for the Fatman device, the charges arranged in much the same way as the segments on a soccer ball.

This summarizes briefly, the technologies required for a proliferator to make a nuclear bomb. It is of course not an easy task, but with the advent of un-scrutinized communications, the personnel required top make one can be easily assembled making it harder to safeguard national security interests.

References:-
Atomic and Nuclear Physics ; S.N.Ghosal
Nuclear Physics; S.B.Patel
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