Carrying a box with the plutonium for the first nuclear bomb.

Elements, Isotopes, and Nuclides

Each element in the periodic table is uniquely characterized by the number of protons that an atom of the element contains in its nucleus—called its atomic number. The number of protons determines the number and configuration of electrons surrounding the nucleus, which in turn govern the chemical properties of  the element (for example, the chemical compounds it will form with other elements).

Most elements occur in multiple forms, called isotopes of the element, which differ  in the number of neutrons that each atom contains in its nucleus. The different isotopes of an element all have the same number of protons and hence the same chemical properties, but their differing numbers of neutrons give them different nuclear properties.[2]

As an illustration of these concepts, the element uranium (denoted U) has the atomic number 92, meaning all uranium nuclei contain 92 protons. The isotopes of uranium are U-232, U-233, U-234, U-235, U-236, U-237, U-238, and U-239. These contain, respectively, 140, 141, 142, 143, 144, 145, 146, and 147 neutrons. (The designations 232 through 239—the mass numbers of the isotopes—represent the sum of the numbers of protons and neutrons in the nucleus.) All of these isotopes behave chemically as uranium, but they differ in their nuclear behavior.

Nuclide is the general term for a species of atom as characterized by the number of protons and the number of neutrons in its nucleus—that is, by its atomic number and its mass number. Thus, all of the isotopes of all of the elements constitute the set of nuclides.

Radioactivity and Ionizing Radiation

Each species of nuclide is either stable or unstable. Unstable ones undergo spontaneous nuclear transformations that result in the emission of ionizing radiation and the replacement of the original nucleus with one of a different type; stable ones do not. The phenomenon of spontaneous nuclear transformation in unstable nuclides is called radioactivity or radioactive decay, and such nuclides are called, correspondingly, radionuclides.

Ionizing radiation consists of electromagnetic waves and subatomic particles that are sufficiently energetic to rip electrons from atoms or molecules they encounter—i.e., to ionize matter—and to break chemical bonds. This property makes such radiation dangerous to living cells and to the organisms composed of them.

Ionizing radiation cannot be seen or heard or felt, but it—and therefore the radioactive substances that emanate it—can be detected by instruments designed for the purpose. On the other hand, both the dangers and the detectability of radioactivity can be reduced through the use of shielding (for example, layers of water or steel or lead, in thicknesses that depend on the types and quantities of the radioactive substances present and the fraction by which the resulting radiation is to be attenuated).

The spontaneous nuclear transformations in a population of a given radionuclide proceed in such a way that, while the time of transformation of a particular nucleus cannot be predicted, the number of transformations per second is directly proportional at any given moment to the number of such nuclei present. This means, in mathematical terms, that the population of any radionuclide will decline exponentially with time, unless it is being augmented by creation of additional nuclei of the species by some other process.

Exponential decline is characterized by a fixed half-life—the time for any given initial number of radioactive atoms to fall by half. Each species of radionuclide has its own characteristic and invariant half-life, as well as its own characteristic form of nuclear transformation and corresponding form(s) of emission of ionizing radiation. [3]

The nuclei newly created by radioactive decay are called decay products or daughters of the original radionuclide. The daughter species may themselves be stable or unstable. Some heavy nuclides have unstable (radioactive) daughters, that in turn have radioactive daughters, and so on through several "generations". These are called decay chains. In the case of radionuclides with radioactive progeny, determining the hazardousness and detectability of a given quantity of the radionuclide generally requires taking into account the presence of the progeny as well.

Radioactivity and radiation are important to the topic of nuclear weapons and their acquisition and use by countries or terrorists for three sets of reasons.

• First, all nuclear-explosive nuclides are radioactive.[4] Their radioactivity has no beneficial effect on the explosive process, but the types and intensities of the ionizing radiation emitted by these radionuclides emit determine the radiation hazards that people who handle or transport them, or weapons made from them, must either experience or take steps to avoid. The half-lives, energy releases, and decay products of nuclear-explosive nuclides are germane, in some instances, to the problems of designing nuclear weapons using those nuclides and to the shelf-life of those weapons. And the types and intensities of the ionizing radiation from nuclear-explosive nuclides and their decay products also determine the ease or difficulty of detecting nuclear-explosive materials, or weapons made from them (or, conversely, the ease or difficulty of shielding these from detection).
• Second, a substantial part of the global inventory of nuclear-explosive materials exists in the form of spent fuel from nuclear reactors, wherein the nuclear-explosive nuclides are intimately mixed with fission products. The fission products are far more radioactive (and hence more dangerous to handle and easier to detect) than the nuclear-explosive nuclides themselves, and, in addition, the fission products must be removed (by rather complicated chemical means discussed below) in order for an explosive chain reaction to be possible. In the cases where nuclear-explosive materials are mixed with fission products, then, the presence of the latter is a key determinant of the difficulty of stealing and smuggling the materials, of handling them, and of processing them to extract the nuclear-explosive nuclides for use in weapons.
• Third, the damaging effects of any nuclear explosion include both a very powerful burst of "prompt" ionizing radiation from the nuclear explosion itself and the continuing, residual radiation from fission products and neutron-activation products from the explosion in fallout;[5] the forms and intensities of these radiations are, along with other factors, important determinants of the casualties that the use of a nuclear weapon would cause.

Some of further information about radioactivity and radiation needed to assess their effects in these contexts is provided in Box 1: Radioactivity and Radiation: How They’re Measured and How They Work, and Box 2: Radiation Doses and Consequences.

Nuclear Fission, Nuclear-Explosive Materials, and
Nuclear Weapons

All nuclear weapons rely on the process of nuclear fission.[6] In the simplest nuclear weapons, fission is the only source of the nuclear energy that is released. In more advanced nuclear weapons—such as "boosted" fission weapons and thermonuclear weapons—some of the energy is generated by fusion reactions that are ignited by energy from the fission explosion.[7] But boosted and thermonuclear weapons are beyond the technical grasp of terrorists, and even countries that aspire to make them cannot do so without mastering simpler fission weapons first. So it is mastery of fission that governs who can make nuclear weapons, and fission that we must examine in order to understand what this entails.

The Fission Process and Fission Chain Reactions

In fission, a heavy nucleus—one with an atomic number of 92 or higher—splits into two lighter nuclei (called fission products) plus two to four free neutrons, accompanied by the release of energy. With some heavy nuclides, nuclear fission occurs spontaneously from time to time – this is one of the forms of spontaneous nuclear transformation falling under the heading of radioactivity – but the rate at which this occurs is too slow for it to generate, in itself, very much energy.

The potential for the large energy releases from fission that are exploited in nuclear weapons (and in nuclear reactors) comes from induced fission, a process in which a heavy nucleus is induced to split by absorption of a free neutron. In the case of heavy nuclides with an odd number of neutrons to start with – such as uranium-235 or plutonium-239 – the absorption of a neutron with the very low energy associated with thermal motion at room temperature is sufficient to induce fission. For heavy nuclei with an even number of neutrons to start with, fission can only be induced if the absorbed neutron carries a much higher energy.

Because each instance of fission releases neutrons that, under the right circumstances, can induce further fissions, the possibility exists of a chain reaction—a situation in which each fission event leads to at least one more. The circumstances that govern whether or not a chain reaction occurs include the kinds of heavy nuclides that are present, the geometry and density in which they are arranged, and the presence (and densities and geometries) of other materials that act to reflect, slow, or absorb neutrons.

Depending on these circumstances, it may happen that, for each and every nuclei that fissions, exactly one of the resulting neutrons induces yet another fission. This situation corresponds to a chain reaction that is just "critical", whereby the fission rate and thus the rate of nuclear energy release do not change with time. (This would be the case, for example, in a nuclear reactor operating at constant power level).

If the circumstances are such, on the other hand, that the neutrons released by each fission succeed in inducing more than one additional fission, the chain reaction is "supercritical", and the fission rate and rate of nuclear-energy release grow with time. This growth can be gradual, as in a nuclear reactor during the startup phase, when its power is being increased from zero up to the reactor's rated output, or it can be extremely rapid, as in a nuclear bomb.[8]

Fissionable, Fissile, and Nuclear-Explosive Nuclides

Nuclides that are capable of undergoing induced fission are called fissionable. The term fissile is applied to the subset of fissionable nuclides that are capable of sustaining a fission chain reaction under circumstances in which emitted neutrons are "thermalized"—slowed down to velocities characteristic of the temperature of the surroundings – before inducing further fissions. Nuclear-explosive nuclides are the subset of fissionable ones that are capable, in their pure form, of sustaining a chain reaction carried by "fast" neutrons (a chain reaction in which, that is, the emitted neutrons induce further fissions before slowing down).

All fissile nuclides are also nuclear-explosive ones, but not all nuclear-explosive nuclides are fissile. For example, the even-numbered isotopes of plutonium—such as Pu-238, Pu-240, and Pu-242—are not fissile, inasmuch as they cannot sustain a chain reaction in circumstances where the emitted neutrons slow down before they can induce fissions; in the absence of materials to slow down the neutrons, however, these isotopes can sustain a chain reaction carried by fast ones.[9]

There are some fissionable nuclides, finally, that are neither fissile nor nuclear-explosive. An example is uranium-238, which constitutes 99.3 percent of natural uranium. Fission can be induced in U-238 only by very energetic neutrons, and this uranium isotope cannot sustain a chain reaction because, on the average, fewer than one of the neutrons produced by the fission of a U-238 nucleus retains enough of its energy for long enough to induce another such nucleus to fission.

Reactivity, Critical Mass, and Explosive Yield

The nuclear reactivity of any nuclear-explosive nuclide or mixture of such nuclides depends partly on their nuclear properties—such as their "cross sections" (meaning, effectively, reaction probabilities) for induced fission by incident neutrons of various energies and, alternatively, for absorbing such neutrons without fissioning. Reactivity also depends on the densities and chemical forms in which the nuclear-explosive nuclide or nuclides are present (where chemical form refers to whether they exist as metals or as compounds such as oxides), and whether and to what extent the elements or compounds containing the nuclear-explosive isotopes are diluted or contaminated with other nuclides and compounds that can slow or absorb neutrons.

For any circumstance in which an explosive chain reaction is possible, one can calculate the critical mass for the material as determined by its composition, density, geometry, and environment (e.g., surrounding material that can reflect neutrons that would otherwise escape). The critical mass is the smallest amount of material that, under the indicated circumstances, can support a chain reaction. Assembling even modestly more than a critical mass makes the configuration supercritical and allows the possibility of explosive growth of the chain reaction.

The reactivities of different nuclear-explosive nuclides—or mixtures and compounds containing such nuclides—are typically compared by referring to their "bare-sphere" critical masses. The bare-sphere critical mass is the smallest amount of the material than can sustain a chain reaction when arranged as a solid sphere, at the material’s normal density, without any surrounding neutron reflector.[10]

The critical mass can be made smaller than the bare-sphere value by surrounding the nuclear-explosive material with another material or materials that reflect neutrons and/or by compressing the nuclear-explosive material to higher than normal density. The reduction available from use of a reflector is in the range of factor of two or so. It is easy to show that the critical mass decreases with the square of the material density, which means that if it were possible to compress the nuclear-explosive material to twice its normal density the critical mass would decrease by a factor of four.

Thus, a nuclear-fission weapon might use a considerably smaller amount of nuclear-explosive material than the bare-sphere critical mass. Alternatively, if a large explosive yield is desired, a weapon might use considerably more than a bare-sphere critical mass (which of course would need to be maintained in subcritical pieces prior to detonation of the weapon).

The explosive yield—that is, the release of energy—from a nuclear weapon is measured, by convention, in terms of the corresponding quantity of the chemical high explosive, TNT. The explosion of one metric ton (1,000 kilograms) of TNT releases approximately 1 billion calories of energy, and the corresponding unit of measure—"one ton of TNT equivalent"—is defined as exactly 1 billion calories.[11]

The first three nuclear weapons (the one tested at Alamogordo, New Mexico in July 1945 and those dropped on Hiroshima and Nagasaki the following month) had yields in the range of 10 to 20 kilotons (10,000 to 20,000 tons) of TNT.[12] Early efforts by proliferating states are likely to aim for the same range, as would the sorts of designs likely to be tried by terrorists. Pure fission weapons of more advanced design have explored a range of yields from a fraction of a kiloton to about 500 kilotons.[13] (Thermonuclear weapons may have yields extending into the multi-megaton range, that is, multiple thousands of kilotons.) The destructive effects of nuclear explosions of various yields are treated in the next section, entitled Nuclear-Weapon Effects.

next: weapons designs & materials

FOOTNOTES

[1] All of the information in this section is widely available in public sources. It falls far short of the level of detail that could be helpful to someone actually trying to make a nuclear weapon. For an excellent summary of the technical facts surrounding nuclear weapons and nuclear energy, and some of the resulting policy issues, see Richard L. Garwin and Georges Charpak, Megawatts and Megatons: A Turning Point in the Nuclear Age (New York, NY: Knopf, 2001).

[2] Nuclear properties include whether the nucleus of the isotope is stable or radioactive, what its half-life and emissions are if it is radioactive, and how susceptible it is to being split—fissioned—if struck by a free neutron. These properties are defined and discussed further in subsequent sections.

[3] A complication (which is only rarely of interest in the practical matters of concern here) is that some radionuclides display more than one isomer, where this term refers nuclei that have the same numbers of protons and neutrons but differ in their energy state in a manner that affects their radioactive properties.

[4] Only a very few of the many radioactive isotopes are nuclear explosives, however. See below.

[5] Neutron activation products are radioactive substances formed from the elements in ordinary materials—such as steel and concrete and air—when these elements absorb neutrons generated in fission or fusion reactions in a nuclear explosion.

[6] For the classic technical introduction to nuclear weapons, once secret, see Robert Serber, The Los Alamos Primer: The First Lectures on How to Build An Atomic Bomb (Berkeley, CA: University of California Press, 1992).

[7] In boosted fission weapons, the energy directly added by the fusion reactions is very modest, but the high-energy neutrons emitted by these reactions lead to a large increase in the amount of fission that takes place.

[8] The time for the energy release rate to double in a nuclear reactor in startup typically would be measured in seconds, minutes, or even hours; the doubling time of the energy release rate in a nuclear bomb is a small fraction of a millionth of a second.

[9] Considerable confusion has been generated over the years by the erroneous assumption that only fissile nuclides are nuclear explosives. Even today the literature contains references to fissile materials where it is clear from the context that the larger category of nuclear-explosive materials was meant.

[10] A sphere has the smallest ratio of surface to volume of any shape, hence the smallest area through which neutrons can escape for a given volume of fissioning material. Thus no other shape will have as small a critical mass.

[11] A calorie is about 4.2 joules, so 1 billion calories—a ton of TNT equivalent—is about 4.2 billion joules or 4.2 gigajoules. A kiloton is then 4,200 gigajoules.

[12] Specifically, the Hiroshima bomb (an HEU gun-type bomb known as "Little Boy") is believed to have had a yield of approximately 15 kilotons, and the Nagasaki bomb (a plutonium implosion bomb known as "Fat Man," for its nearly spherical shape) roughly 21 kilotons.

[13] Releasing a kiloton of nuclear-explosive energy requires fissioning about 150,000 billion billion heavy nuclei, which is about 57 grams of uranium or plutonium. A 20-kiloton explosion would thus entail fissioning about 1.2 kilograms of material. Clearly, only a fraction of the material in the critical mass needed for an explosion actually fissions before the weapon blows itself apart.