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Appendix C:  Basic Nuclear Physics and Weapons Effects

C.1   Overview
This appendix offers a basic overview of nuclear physics, proliferation considerations, the effects of nuclear detonations, nuclear targeting, and the physics of countering nuclear threats. It is information useful in understanding the basic technical aspects of the U.S. nuclear stockpile and efforts to counter nuclear threats.

C.2   Nuclear Physics

C.2.1   Atomic Structure

Matter is the material substance in the universe that occupies space and has mass. All matter in the observable universe is made up of various combinations of separate and distinct particles. When these particles are combined to form atoms, they are called elements. An element is one of more than 110 known chemical substances, each of which cannot be broken down further without changing its chemical properties. The number of protons in an atom’s nucleus identifies the atomic element. The smallest unit of a given amount of an element is called an atom. Atoms are composed of electrons, protons, and neutrons.

Figure C.1 Diagram of an Atom

Nuclear weapons depend on the potential energy that can be released from the nuclei of atoms. In the atoms of heavy elements, which serve as fissile material in nuclear weapons, the positively charged protons and electrically neutral neutrons, collectively known as nucleons, form the enormously dense nucleus of the atom that is located at the center of a group of shells of orbiting, negatively charged electrons. See Figure C.1 for an illustration of the structure of an atom.

Electron interactions determine the chemical characteristics of atoms whereas nuclear activities depend on the characteristics of the nucleus. Examples of chemical characteristics include the tendency of elements to combine with other elements (e.g., hydrogen and oxygen combine to form water), the ability to conduct electricity, and the ability to undergo chemical reactions, such as oxidation (e.g., iron and oxygen combine to form iron oxide or rust). Examples of nuclear characteristics include the tendency of a nucleus to split apart, the ability of a nucleus to absorb a neutron, and radioactive decay where the nucleus emits a particle from the nucleus. An important difference between chemical and nuclear reactions is there can neither be a loss nor a gain of mass during a chemical reaction, but mass can be converted to energy in a reaction at the nuclear level. This change of mass into energy is what is responsible for the tremendous release of energy during a nuclear detonation.

Isotopes are atoms that have identical atomic numbers (same number of protons) but a different atomic mass (different number of neutrons). Different isotopes of the same element have different nuclear characteristics, for example uranium-235 (U-235) has significantly different nuclear characteristics than U-238. Figure C.2 is an illustration of the two primordial isotopes of uranium. There are currently 23 known isotopes of uranium.

Figure C.2 Isotopes of Uranium

C.2.2   Radioactive Decay
Radioactive decay is the process of nucleus change and particle and/or energy release as the nucleus attempts to reach a more stable configuration. The nuclei of many isotopes are unstable and have statistically predictable timelines for radioactive decay. These unstable isotopes are known as radioisotopes. Radioisotopes have several decay modes, including alpha, beta, and gamma decay and spontaneous fission. The rate of decay is characterized in terms of “half-life,” or the amount of time required for half of a given amount of the radioisotope to decay. Half-lives of different isotopes range from a very small fraction of a second to billions of years. The rate of decay is also characterized as activity, or the number of decay events or disintegrations that occur in a given time.

These unstable isotopes are known as radioisotopes. Radioisotopes have several decay modes, including alpha, beta, and gamma decay and spontaneous fission. The rate of decay is characterized in terms of “half-life,” or the amount of time required for half of a given amount of the radioisotope to decay. Half-lives of different isotopes range from a very small fraction of a second to billions of years. The rate of decay is also characterized as activity, or the number of decay events or disintegrations that occur in a given time.

C.2.3   Nuclear Reactions

Fission, the splitting apart of nuclei, and fusion the fusing together of nuclei, are key examples of nuclear reactions that can be induced in the nucleus. Fission occurs when a large nucleus, such as in a plutonium atom, is split into smaller fragments. Fusion occurs when the nuclei of two light atoms, each with a small nucleus, such as hydrogen, collide with enough energy to fuse two nuclei into a single larger nucleus.

Fission may occur spontaneously or when a sub-atomic particle, such as a neutron, collides with the nucleus and imparts sufficient energy to cause the nucleus to split into two or more fission fragments, which become the nuclei of newly created lighter atoms and are almost always radioactive. Fission releases millions of times more energy than the chemical reactions that cause conventional explosions. The fission that powers both nuclear reactors and weapons is typically the neutron-induced fission of certain isotopes of uranium or plutonium. The neutrons produced by fission events (Figure C.3) can interact with the nuclei of other fissile atoms and produce other fission events, referred to as a chain reaction.

Figure C.3 Fission Event

Criticality describes whether the rate of fission is increasing (supercritical), remaining constant (critical), or decreasing (subcritical). See Figure C.4 for an illustration of a sustained chain reaction of fission events. In a highly supercritical configuration, the number of fission events increases very quickly, which results in the release of tremendous amounts of energy in a very short time, causing a nuclear detonation.

Figure C.4 Chain Reaction of Fission Events

Fissile material is called a subcritical mass, or subcritical component, when the amount is so small and the configuration with atoms is so spread-out that any fission event caused by a random neutron does not cause a sustained chain reaction of fission events. This is because almost all neutrons produced escape without producing a subsequent fission event.

Different types of fissile isotopes have different probabilities of fission when nuclei are struck with a neutron. Each fissile isotope produces a different average number of neutrons per fission event. These are the two primary factors in determining the material’s fissile efficiency. Only fissile isotopes can undergo a multiplying chain reaction of fission events to produce a nuclear detonation. Generally, the larger the amount of fissile material in one mass, the closer it is to approaching criticality if it is subcritical and the more effectively it can sustain a multiplying chain reaction if it is supercritical.

Figure C.5 Fusion Event

Nuclear fusion is the combining of two light nuclei to form a heavier nucleus. For the fusion process to take place, two nuclei must be forced together by sufficient energy so the strong, attractive, short-range, nuclear forces overcome the electrostatic forces of repulsion. Because the positively charged protons in the colliding nuclei repel each other, it takes a huge amount of energy to get the nuclei close enough to fuse. It is, therefore, easiest for nuclei with smaller numbers of protons to achieve fusion. In almost all cases, a fusion event produces one high-energy free neutron, which can be used in a nuclear weapon to cause another fission event. Fusion also releases significantly more energy than a chemical reaction does. Figure C.5 is an illustration of a fusion event.

C.2.4   Basic Nuclear Weapon Designs
All current nuclear weapons use the basic approach of producing a very large number of fission events through a multiplying chain reaction and releasing a huge amount of nuclear energy in a very short period of time. Typically dozens of generations of fission events in a nuclear detonation will take only approximately one millionth of a second.

The earliest name for a nuclear weapon was atomic bomb or A-bomb. This term has been criticized as a misnomer because all conventional explosives generate energy from reactions between atoms (i.e., the release of binding energy that had been holding atoms together as a molecule). However, the name is still associated with current nuclear weapons and is accepted by historians, the public, and even by some of the scientists who created the first nuclear weapons. A fission weapon is a nuclear weapon due to the primary energy release coming from the nuclei of fissile atoms. Fusion weapons are called hydrogen bombs or H-bombs because isotopes of hydrogen are used to achieve fusion events that increase the yield of the detonation. Fusion weapons are also called thermonuclear weapons, due to the high temperatures and pressure required for the fusion reactions to occur.1

Achieving Supercritical Mass
To produce a nuclear detonation, a weapon must contain enough fissile material to achieve a supercritical mass and a multiplying chain reaction of fission events. A supercritical mass can be achieved in two different ways. The first way is to have two subcritical components positioned far enough apart so any stray neutrons that cause a fission event in one subcritical component cannot begin a sustained chain reaction of fission events between the two components. At the same time, the components must be configured in such a way that when the detonation is desired, one component can be driven toward the other to form a supercritical mass when they are positioned together.

The second approach is to have one subcritical fissile component surrounded with high explosives (HE). When the detonation is desired, the HE is exploded, with force pushing inward to compress the fissile component to a point where it goes from subcritical to supercritical, because the fissile nuclei become closer to each other, with less space between them for neutrons to escape. This causes most of the neutrons produced to cause subsequent fission events and achieve a multiplying chain reaction. Both of these approaches can be enhanced by using a proper casing as a tamper to hold in the explosive force. By using a neutron reflecting material around the supercritical mass, and by using a neutron generator to produce a large number of neutrons at the moment the fissile material reaches its designed super-criticality, the first generation of fission events in the multiplying chain reaction is a larger number of fission events.

Currently, nuclear weapons use one of four basic design approaches: gun assembly, implosion, boosted, or staged.

Gun Assembly Weapons
Gun assembly (GA) weapons (Figure C.6) rapidly assemble two subcritical fissile components into one supercritical mass. This assembly is structured in a tubular device in which a propellant is used to drive one subcritical mass into another, forming one supercritical mass and a nuclear detonation. In general, the GA design is less technically complex than other designs and is also the least efficient.2

Figure C.6 Unclassified Illustration of a GA Weapon
Source: Joint DOE/DoD Topical Classification Guide for Nuclear Assembly Systems (TCG-NAS-2), March 1997)

Implosion Weapon
Implosion weapons (Figure C.7) use the method of imploding one subcritical fissile component to achieve greater density and a supercritical mass. This compression is achieved by using high explosives surrounding a subcritical sphere of fissile material to drive the fissile material inward, thereby compressing it. The increased density achieves super-criticality since the fissile nuclei are closer together, increasing the probability that any given neutron causes a subsequent fission event. In general, the implosion design is more technically complex than the GA design and more efficient.

Figure C.7 Unclassified Illustration of an Implosion Weapon
(Source: TCG-NAS-2, March 1997)

Boosted Weapons
A boosted weapon increases the efficiency and yield for a weapon of the same volume and weight when a small amount of fusionable material, such as deuterium or tritium gas, is placed inside the core of a fission device. The immediate fireball, produced by the supercritical mass, has a temperature of tens of millions of degrees and creates enough heat and pressure to cause the nuclei of the light atoms to fuse together.

In this environment a small amount of fusion gas, measured in grams, can produce a huge number of fusion events. Generally, for each fusion event, there is one high-energy neutron produced. These high-energy neutrons then interact with the fissile material, before the weapon breaks apart in the nuclear detonation, to cause additional fission events that would not occur if the fusion gas were not present. This approach to increasing yield is called “boosting” and is used in most modern nuclear weapons to meet yield requirements within size and weight limits. In general, the boosted weapon design is more technically complex than the implosion design and also more efficient.

Figure C.8 Unclassified Illustration of a Staged Weapon
(Source: TCG-NAS-2, March 1997)

Staged Weapons
A staged weapon (Figure C.8) normally uses a boosted primary stage and a secondary stage to produce a significantly increased yield. In the first stage, a boosted fission device releases the energy of a boosted weapon, which includes a large number of X-rays. The X-rays transfer energy to the secondary stage, causing fusionable material in the secondary to undergo fusion, which releases large numbers of high-energy neutrons. These neutrons, in turn, interact with fissionable material in the secondary to cause a huge number of fission events, thereby significantly increasing the yield of the whole weapon. The two-stage weapon design is more technically complex than any other weapon design. For a given size, it can produce a much larger yield than any other design. 

C.3   Proliferation Considerations
Generally, the smaller the size (e.g., volume, dimensions, and weight) of the warhead, the more difficult it is to get the nuclear package to function to produce a nuclear detonation and the harder it is to achieve a higher yield. The simplest and easiest design is the GA design followed by the implosion design. Since the boosted and staged designs are significantly more difficult, they are not practical candidates for any nation’s first generation of nuclear weapons.

Most proliferating nations have focused on the implosion design for a number of reasons. The GA design is the least efficient producing yield in a weapon-sized device and has inherent operational disadvantages not associated with the other designs. Highly enriched uranium (HEU), which can be used in either a GA or implosion design, is very expensive due to the cost of the enrichment process. Since plutonium is produced in a reactor that can also be used for the simultaneous production of electrical power, the cost is partially or completely offset by the value of the electricity produced. However, in a GA design, plutonium is susceptible to preinitiation, a significantly reduced yield due to the early initiation of fission events that destroys the weapon before it reaches its designed super-criticality. For this reason, a GA design cannot use plutonium.

Up to this time, nations that have pursued a nuclear weapons capability have been motivated to design warheads small enough to be delivered using missiles or high-performance jet aircraft.3 This is probably because, unlike the situation in the early 1940s, many nations today, and even some non-government actors, possess some type of effective air-defense system, which render non-stealth, large cargo, or passenger aircraft ineffective at penetrating a potential adversary’s target. Due to this size limit, it is very likely that the first generation weapons developed by proliferating nations are low-yield weapons, typically between one4 and 10 kilotons (kt).5

Figure C.9 Energy Distribution for a Typical
Nuclear Detonation

Figure C.10 Hiroshima After the Nuclear Detonation

C.4   The Effects of Nuclear Detonations
A nuclear detonation produces effects overwhelmingly more significant than those produced by a conventional explosive, even if the nuclear yield is relatively low. A nuclear detonation differs from a conventional explosion in several ways. A typical nuclear detonation6 produces energy that, weight for weight, is millions of times more powerful than that produced by a conventional explosion. It also produces an immediate large, hot nuclear fireball, electromagnetic pulse (EMP), thermal radiation, prompt nuclear radiation, air blast wave, residual nuclear radiation, interference with communications signals, and, if the fireball interacts with the terrain, ground shock. Figure C.9 depicts the overarching energy distribution for a typical nuclear detonation.

C.4.1   Ground Zero
Nuclear detonations can occur on, below, or above the Earth’s surface. Ground zero (GZ) is the point on the Earth’s surface closest to the detonation. The effects of a nuclear detonation can destroy structures and systems and can injure or kill exposed personnel at great distances from GZ. Figure C.10 shows Hiroshima after being attacked with a nuclear weapon on August 6, 1945.

Nuclear detonation effects for people or objects close to GZ are devastating. However, the distances that effects can travel away from GZ are limited.

C.4.2   Overall Effects
The yield of the weapon is one of the most important factors in determining the level of casualties and damage. Other factors include the type and density of target elements near GZ, HOB, terrain, or objects in the area that could interfere with various effects moving away from GZ and the weather in the target area.

If properly employed,7 any one nuclear weapon should defeat any one military target. However, a few nuclear weapons with relatively low yields, such as the yields of any nation’s first generation of nuclear weapons, would not defeat a large military force, such as the allied force in the first Gulf War. A single, low-yield nuclear weapon employed in a major metropolitan area produces total devastation in an area large enough to produce tens of thousands, and possibly more than 300,000 fatalities. Yet, it does not wipe-out the entire major metropolitan area. The survival of thousands of people who are seriously injured or exposed to a moderate level of nuclear radiation depends on the response of various federal, state, and local government agencies and non-governmental organizations.

C.4.3   Casualty and Damage Distances for Populated Areas
A very low-yield, 1-kt detonation produces severe damage effects approximately one quarter of a mile from GZ. Within the severe damage zone, almost all buildings collapse and 99 percent of persons become fatalities quickly. Moderate damage extends approximately one half mile and includes structural damage to buildings, many prompt fatalities, severe injuries, overturned cars and trucks, component damage to electronic devices, downed cellphone towers, and induced radiation at ground level that could remain hazardous for several days. Light damage would extend out approximately 1.5 miles and includes some prompt fatalities, some persons with severe injuries, and the effects on infrastructure as stated for medium damage. Some fatalities or injuries may occur beyond the light damage zone.

A low-yield, 10-kt detonation produces severe damage effects approximately one half mile from GZ. Moderate damage extends approximately one mile and light damage ranges approximately three miles.

A high-yield, strategic 1-megaton (MT) detonation8 produces severe damage effects slightly beyond two miles from GZ. Moderate damage extends out beyond four miles and light damage encompasses beyond 12 miles.

Figure C.11 Approximate Immediate Fireball Size

C.4.4   Nuclear Fireball
A typical nuclear weapon detonation produces a huge number of X-rays, which heat the air around the detonation to extremely high temperatures, causing the heated air to expand and form a large fireball within a small fraction of a second. The size of the immediate fireball is a function of yield and the surrounding environment. Figure C.11 shows the size of the immediate fireball for selected yields and environments.

The immediate fireball is tens of millions of degrees (i.e., as hot as the interior of the sun). Inside the fireball, the temperature and pressure cause a complete disintegration of molecules and atoms. Current targeting procedures do not consider the fireball to be one of the primary weapon effects but a nuclear fireball can be used to incinerate chemical or biological agents.

C.4.5   Thermal Radiation
Thermal radiation is electromagnetic radiation in the visible light spectrum that can be sensed as heat and light. Thermal radiation is maximized with a low-air burst and the optimum HOB increases with yield. Thermal radiation can ignite wood frame buildings, vegetation, and other combustible materials at significant distances from GZ. It can also cause burns to exposed skin directly or indirectly, if clothing ignites or the individual is caught in a fire ignited by the heat. Anything that casts a shadow or reduces light, including buildings, trees, dust from the blast wave, heavy rain, and dense fog, provides some protection against thermal burns or the ignition of objects. Figure C.12 shows types of burns and approximate maximum distances for selected yields.9

Figure C.12 Thermal Radiation Burns

Flash blindness, or dazzle, is a temporary loss of vision caused when eyes are overwhelmed with intense thermal light. On a clear night, dazzle may last for up to 30 minutes and may affect people at distances beyond 10 miles. On a clear day, dazzle can affect people at distances beyond those for first degree burns; albeit it lasts for a shorter period of time.

Since thermal radiation can be scattered and reflected in the air, flash blindness can occur regardless of whether an individual is looking toward the detonation. At distances where it can produce a first degree burn, thermal radiation is intense enough to penetrate through the back of the skull to overwhelm the eyes. Retinal burns can occur at great distances for individuals looking directly at the fireball at the moment of the nuclear detonation. Normally, retinal burns cause a permanent blindness to a small portion of the eye in the center of the normal field of vision.

Because thermal radiation can start fires and cause burns at such great distances, if a nuclear weapon is employed against a populated area on a clear day, with an air burst at approximately the optimum HOB, it is likely the thermal effects account for more casualties than any other effect. With a surface burst or if rain or fog are in the area, the thermal radiation effects would be reduced.

The effects of thermal radiation can be reduced with protective enclosures, thermal protective coatings, and the use of non-flammable clothing, tools, and equipment. Thermal protective coatings include materials that swell when exposed to flame, thus absorbing the heat rather than allowing it to penetrate through the material and ablative paints, which act like a melting heat shield. Materials like stainless steel, as opposed to temperature-sensitive metals like aluminum, are used to protect against thermal radiation. In order to reduce the amount of absorbed energy, light colors and reflective paints are also used. For effective thermal hardening, the use of combustible materials is minimized. Finally, to mitigate the effects of thermal radiation, it is important to protect items prone to melting, such as rubber gaskets, O-rings, and seals.

C.4.6   Air Blast
In the case of surface and low-air bursts, the fireball expands, immediately pushing air away from the point of the detonation, causing a dense wall of air to travel at great speed away from the detonation. Initially, this blast wave moves at several times the speed of sound, but quickly slows to a point at which the leading edge of the blast wave is traveling at the speed of sound and continues at this speed as it moves farther away from GZ. Shortly after breaking away from the fireball, the wall of air reaches its maximum density of overpressure, or over the nominal air pressure.10 As the blast wave travels away from this point, the wall of air becomes wider, loses density, and the overpressure continues to decrease.

At significant distances from ground zero, overpressure can have a crushing effect on objects as they are engulfed by the blast wave. In addition to overpressure, the blast wave has an associated wind speed as it passes any object. This can be quantified as dynamic pressure that can move, rather than crush, objects. The blast wave has a positive phase and a negative phase for both overpressure and dynamic pressure.

As the blast wave hits a target object, the positive overpressure initially produces a crushing effect. If the overpressure is great enough, it can cause instant fatality to an exposed person. Less overpressure can collapse the lungs and, at lower levels, can rupture the ear drums. Overpressure can implode a building. Immediately after the positive overpressure has begun to affect the object, dynamic pressure exerts a force that can move people or objects laterally at high speed, causing injury or damage. Dynamic pressure can also strip a building from its foundation, blowing it to pieces.

As the positive phase of the blast wave passes an object, it is followed by a vacuum effect (i.e., the negative pressure caused by the lack of air in the space behind the blast wave). This is the beginning of the negative phase of dynamic pressure. The vacuum effect, or negative overpressure, can cause a wood frame building to explode, especially if the positive phase has increased the air pressure inside the building by forcing air in through broken windows. The vacuum effect then causes the winds in the trailing portion of the blast wave to be pulled back into the vacuum. This produces a strong wind moving back toward GZ. While the negative phase of the blast wave is not as strong as the positive phase, it may move objects back toward ground zero, especially if trees or buildings are severely weakened by the positive phase. Figure C.13 shows the overpressure in pounds per square inch (psi) and the approximate distances associated with various types of structural damage.11  

Figure C.13 Air-Blast Damage to Structures

If the detonation occurs at ground level, the expanding fireball pushes into the air in all directions, creating an ever-expanding hemispherical blast wave, called the incident wave. As the blast wave travels away, its density continues to decrease. After some significant distance, it loses destructive potential and becomes a mere gust of wind. Yet, if the detonation is a low-air burst, a portion of the blast wave travels toward the ground and is then reflected off the ground. This reflected wave travels up and out in all directions, reinforcing the incident wave traveling along the ground. Because of this, air blast is maximized with a low-air burst rather than a surface burst.

If the terrain is composed of a surface that absorbs more thermal radiation than grass or soil, the thermal radiation leads to a greater than normal heating of that surface. The surface produces heat before the arrival of the blast wave. This creates a “non-ideal” condition that causes the blast wave to become distorted when it reaches the heated surface, resulting in an abnormal reduction in the blast wave density and psi. Extremely cold weather (minus 50o Fahrenheit or colder) can lead to increased air-blast damage distances. If a surface burst occurs in a populated area or if there is rain and/or fog at the time of burst, the blast effect would probably account for more casualties than any other effect.

Structures and equipment can be reinforced to become less vulnerable to air blast. Nevertheless, any structure or piece of equipment is destroyed if it is close enough to the detonation. High priority facilities that must survive a close nuclear strike are usually constructed underground, making it much harder to defeat.

Individuals who sense a blinding white flash and intense heat coming from one direction should immediately fall to the ground and cover their heads with their arms. This provides the highest probability the air blast passes overhead, without moving them laterally, and debris in the blast wave does not cause impact or puncture injuries. Exposed individuals who are very close to the detonation have no chance of survival. At distances at which a wood frame building can survive, however, exposed individuals significantly increase their chance of survival if they are on the ground when the blast wave arrives and remain on the ground until after the negative phase blast wave has moved back toward ground zero.

C.4.7   Ground Shock
Given surface or near-surface detonations, the fireball’s expansion and interaction with the ground causes a significant shock wave to move into the ground in all directions. This causes an underground fracture or “rupture” zone. The intensity and significance of the shock wave and the fracture zone decrease with distance from the detonation. A surface burst produces significantly more ground shock than a near-surface burst in which the fireball barely touches the ground.

Underground structures, especially ones deep underground, are not vulnerable to the direct primary effects of a low-air burst. However, the shock produced by a surface burst may damage or destroy an underground target, depending on the yield of the detonation, soil or rock type, depth of the target, and its structure. It is possible for a surface detonation to fail to crush a deep underground structure but have an effective shock wave that crushes or buries entrance or exit routes and destroys connecting communications lines. This could cause the target to be “cut-off” and render it, at least temporarily, incapable of performing its intended function. Normally, a surface burst or shallow sub-surface burst is used to attack deeply buried targets. As a rule of thumb, a 1-kt surface detonation can destroy an underground facility as deep as a few tens of meters. A 1-MT surface detonation can destroy the same target as deep as a few hundred meters.

Deeply buried underground targets can be attacked through the employment of an earth-penetrating warhead to produce a shallow sub-surface burst. Only a few meters of penetration into the earth is required to achieve a “coupling” effect, in which most of the energy that would have gone up into the air with a surface burst is trapped by the material near the surface and reflected downward to reinforce the original shock wave. This reinforced shock wave is significantly stronger and can destroy deep underground targets to distances usually two to five times deeper than those destroyed through the employment of a surface burst.12 Ground shock is the governing effect for damage estimation against any underground target.

Underground facilities and structures can be buried deeper to reduce vulnerability to damage from a surface or shallow sub-surface detonation. Facilities and equipment can be built with structural reinforcement or other designs to decrease their vulnerability to ground shock. For functional survivability, entrance and exit routes, as well as communications lines connected to ground-level equipment, can be hardened or made redundant.

C.4.8   Surface Crater
In the case of near-surface, surface, and shallow sub-surface bursts, the fireball’s interaction with the ground causes it to engulf much of the soil and rock within its radius and remove the material as it moves upward. This removal of material results in the formation of a crater. A near-surface burst would produce a small, shallow crater. The crater from a surface burst, with the same yield, is larger and deeper while the crater size is maximized with a shallow sub-surface burst at the optimum depth.13 The size of the crater is a function of the yield of the detonation, depth of burial, and type of soil or rock.

For deeply buried detonations, such as those created with underground nuclear testing, the expanding fireball creates a spherical volume of hot radioactive gases. As the radioactive gas cools and contracts, the spherical volume of space becomes an empty cavity with a vacuum effect. The weight of the heavy earth above the cavity and the vacuum effect within the cavity cause a downward pressure for the earth to fall in the cavity. This can occur unpredictably at any time from minutes to months after the detonation. When it occurs, the cylindrical mass of earth collapsing down into the cavity forms a crater on the surface, called a subsidence crater.

A crater produced by a recent detonation near the ground surface is probably radioactive. Individuals required to enter or cross such a crater could be exposed to significant levels of ionizing radiation, possibly enough to cause casualties or fatalities. If a deep underground detonation has not yet formed the subsidence crater, it is very dangerous to enter the area on the surface directly above the detonation.

Normally, the wartime employment of nuclear weapons does not use crater formation to attack targets. Though at the height of the Cold War, the North Atlantic Treaty Organization (NATO) forces had contingency plans to use craters from nuclear detonations to channel, contain, or block enemy ground forces. The size of the crater and its radioactivity for the first several days produces an obstacle extremely difficult, if not impossible, for a military unit to cross.

A crater by itself does not present a hazard to people or equipment, unless an individual tries to drive or climb into the crater. In the case of deep underground detonations, the rule is to keep away from the area where the subsidence crater could be formed until after the collapse occurs.

C.4.9   Underwater Shock
An underwater nuclear detonation generates a shock wave in a manner similar to that in which a blast wave is formed in the air. The expanding fireball pushes water away from the point of detonation, creating a rapidly moving dense wall of water. In the deep ocean, this underwater shock wave moves out in all directions, gradually losing its intensity. In shallow water, it can be distorted by surface and bottom reflections. Shallow bottom interactions may reinforce the shock effect.

If the yield is large enough and the depth of detonation is shallow enough, the shock wave ruptures the water’s surface. This can produce a large surface wave that moves away in all directions. It may also produce a “spray dome” of radioactive water above the surface.

If a submarine is close enough to the detonation, the underwater shock wave is strong enough to rapidly move the vessel. This near-instantaneous movement could force the ship against the surrounding water with a force beyond its design capability, causing a structural rupture of the vessel. The damage to the submarine is a function of weapon yield, depth of detonation, depth of the water under the detonation, bottom conditions, and the distance and orientation of the submarine. People inside the submarine are at risk if the boat’s structure fails. Even if the submarine structure remains intact, the lateral movement may cause injuries or fatalities to those inside the submarine.

Surface ships may be vulnerable to the underwater shock wave striking their hull. If the detonation produces a significant surface wave, it can damage surface ships at greater distances. If ships move into the radioactive spray dome, the dome could present a radioactive hazard to people on the ship. Normally, nuclear weapons are not used to target enemy naval forces.

Both surface ships and submarines can be designed to be less vulnerable to the effects of underwater nuclear detonations. Yet, any ship or submarine can be damaged or destroyed if it is close enough to a nuclear detonation.

C.4.10   Initial Nuclear Radiation

Nuclear radiation is ionizing radiation emitted by nuclear activity consisting of neutrons, alpha and beta particles, and electromagnetic energy in the form of gamma rays.14 Gamma rays are high-energy photons of electromagnetic radiation with frequencies higher than visible light or ultraviolet rays.15 Gamma rays and neutrons are produced from fission events. Alpha and beta particles and gamma rays are produced by the radioactive decay of fission fragments. Alpha and beta particles are absorbed by atoms and molecules in the air at short distances and are insignificant compared with other effects. Gamma rays and neutrons travel great distances through the air in a general direction away from ground zero.16

Since neutrons are produced almost exclusively by fission events, they are produced in a fraction of a second, and no significant number of neutrons is produced after that. Conversely, gamma rays are produced by the decay of radioactive materials and produced for years after the detonation. Initially, these radioactive materials are in the fireball. For surface and low-air bursts, the fireball rises quickly and, within approximately one minute, is at an altitude high enough that none of the gamma radiation produced inside the fireball has any impact to people or equipment on the ground. For this reason, initial nuclear radiation is defined as the nuclear radiation produced within one minute post-detonation. Initial nuclear radiation is also called “prompt nuclear radiation.”

The huge number of gamma rays and neutrons produced by a surface, near-surface, or low-air burst may cause casualties or fatalities to people at significant distances. For a description of the biological damage mechanisms, see section C.4.12 on the biological/medical effects of ionizing radiation. The unit of measurement for radiation exposure is the Centi-Gray (cGy).17 The 450 cGy exposure dose level is considered to be the lethal dose for 50 percent of the population (LD50) with medical assistance. People who survive at this dose level would have a significantly increased risk of contracting mid-term and long-term cancers. Figure C.14 shows selected levels of exposure, the associated near-term effects on humans, and the distances by yield.18

Figure C.14 Near-Term Effects of Initial Nuclear Radiation

Low levels of exposure can increase an individual’s risk for contracting long-term cancers. For example, in healthy male adults ages 20 to 40, an exposure of 100 cGy increases this risk by approximately 10 to 15 percent and lethal cancer by approximately six to eight percent.19

The ground absorbs more gamma rays and neutrons than the air. Almost half of the initial nuclear radiation resulting from a surface burst is quickly absorbed by the earth. In the aftermath of a low-air burst, half of the nuclear radiation travels in a downward direction. Much of that radiation is scattered and reflected by atoms in the air, adding to the amount of radiation traveling away from GZ. Because of this, initial nuclear radiation is maximized with a low-air burst.

Initial nuclear radiation effects can be predicted with reasonable accuracy. Some non-strategic or terrorist targets may include people as a primary target element. In this case, initial nuclear radiation is considered with air blast to determine the governing effect. Initial nuclear radiation is always considered for safety (if safety of populated areas or friendly troop personnel is a factor) and safety distances are calculated based on a “worst-case” assumption (i.e., there is a maximum initial radiation effect and objects in the target area will not shield or attenuate the radiation).

Individuals can do very little to protect themselves against initial nuclear radiation after a detonation has occurred since initial radiation is emitted and absorbed in less than one minute. The DoD has developed an oral chemical prophylactic to reduce the effects of ionizing radiation exposure, however, the drug does not reduce the hazard to zero. Just as with most of the other effects, it is fatal if an individual is very close to the detonation.

Initial nuclear radiation can also damage the electrical components in certain equipment. Equipment can be hardened to make electronic components less vulnerable to initial nuclear radiation. Generally structures are not vulnerable to initial nuclear radiation.

C.4.11   Residual Nuclear Radiation
Residual nuclear radiation consists of alpha and beta particles as well as gamma rays emitted from radioactive nuclei. There are types of residual nuclear radiation that result from a typical detonation. Residual radiation also results from a deep underground detonation, but the radiation remains underground unless radioactive gases vent from the fireball or residual radiation escapes by another means. An exo-atmospheric detonation creates a cloud in orbit that could remain significantly radioactive for many months.

Induced Radiation on the Ground is radioactivity caused by neutron absorption. With a detonation near the ground, neutrons are captured by light metals in the soil or rock near the ground surface.20 These atoms become radioactive isotopes capable of emitting, among other things, gamma radiation. The induced radiation is generally created in a circular pattern, most intense at GZ immediately after the detonation. The intensity decreases over time and with distance from GZ. In normal soil, it takes approximately five to seven days for induced radiation to decay to a safe level. In a populated area, the induced radiation could extend beyond building collapse, especially with a low-yield detonation. This could cause first responders who are not trained to understand induced radiation to move into an area still radioactively hot because, without radiation detectors, they would not be aware of the radioactive hazard.

Induced Radiation in the Air is caused by the production of carbon-14 by nitrogen absorbing neutrons. Carbon-14 atoms can remain suspended in the air, are beta particle emitters, and have a long half-life (5,715 years). During the 1950s and 1960s, when four nuclear nations conducted aboveground nuclear testing, a two to three percent increase occurred in total carbon-14 levels worldwide. Gradually, the carbon-14 is returning to pre-testing levels. There are no known casualties attributed to the increase but any increase in carbon-14 levels could be an additional risk.

Fallout is the release of small radioactive particles that drop from the fireball to the ground. In most technical jargon, fallout is defined as the fission fragments from the nuclear detonation. The fireball contains other types of radioactive particles as well that fall to the ground and contribute to the total radioactive hazard. These include the radioactive fissile material that did not undergo fission, as no weapon fissions 100 percent of the fissile material, and material from warhead components induced with neutrons that have become radioactive. Residual gamma radiation is colorless, odorless, and tasteless and cannot be detected with the five senses, unless an extremely high level of radiation exists.

If the detonation is a true air burst in which the fireball does not interact with the ground or any significant structure, the size and heat of the fireball causes it to retain almost all of the weapon debris, usually one or at most a few tons of material, as it moves upward in altitude and downwind. In this case, very few particles fall to the ground at any moment and no significant radioactive hot-spot on the ground is caused by the fallout. The fireball rises to become a long-term radioactive cloud. The cloud travels with the upper atmospheric winds and circles the hemisphere several times, over a period of months, before it dissipates completely. Most of the radioactive particles decay to stable isotopes before falling to the ground. The particles that reach the ground are distributed around the hemisphere at the latitudes of the cloud travel route. Even though there would be no location receiving a hazardous amount of fallout radiation, certain locations on the other side of the hemisphere could receive more fallout, which is measurable with radiation detectors, than the area near the detonation. This phenomenon is called worldwide fallout.

If the fireball interacts with the ground or any significant structure (e.g., a large bridge or a building), the fireball has different properties. In addition to the three types of radioactive material, the fireball would also include radioactive material from the ground or structure induced with neutrons. The amount of material in the fireball would be much greater than the amount with an air burst. For a true surface burst, a 1-kt detonation would extract thousands of tons of earth up into the fireball, although only a small portion would be radioactive. This material would disintegrate and mix with the radioactive particles. As large and hot as the fireball is (1-kt detonation produces almost 200 feet in diameter and tens of millions of degrees), it has no potential to carry thousands of tons of material. Thus, as the fireball rises, it begins to release a significant amount of radioactive dust, which falls to the ground and produces a radioactive fallout pattern around GZ and in areas downwind. The intensity of radioactivity in this fallout area would be hazardous for weeks. This is called early fallout, caused primarily by a surface-burst detonation regardless of the weapon design. Early fallout would be a concern in the case of the employment of a nuclear threat device during a terrorist attack.

Normally, fallout should not be a hazardous problem for a detonation that is a true air burst. Yet, if rain and/or snow occurs in the target area, radioactive particles could be “washed-out” of the fireball, creating a hazardous area of early fallout. If a detonation is a surface or near-surface burst, early fallout would be a significant radiation hazard around GZ and downwind.

Generally, a deep underground detonation presents no residual radiation hazard to people or objects on the surface. If there is an accidental venting or some other unintended escape of radioactivity, however, it could become a radioactive hazard to people in the affected area. The residual nuclear cloud from an exo-atmospheric detonation could damage electronic components in some satellites over a period of time, usually months or years, depending on how close a satellite gets to the radioactive cloud, the frequency of the satellite passing near the cloud, and its exposure time.

There are four actions that provide protection against residual radiation. First, personnel with a response mission should enter the area with at least one radiation detector, and all personnel should employ personal protective equipment (PPE).21 While the PPE does not stop the penetration of gamma rays, it will prevent the responder personnel from breathing any airborne radioactive particles. Second, personnel should only be exposed to radioactivity the minimum time possible to accomplish a given task. Third, personnel should remain at a safe distance from radioactive areas. Finally, personnel should use shielding when possible to further reduce the amount of radiation received. It is essential for first responder personnel to follow the PPE principles of time, distance, and shielding.

C.4.12   Biological/Medical Effects of Ionizing Radiation
Ionizing radiation is any particle or photon that produces an ionizing event (i.e., strip an electron away from an atom), including alpha and beta particles, gamma and cosmic rays, and X-rays. Ionizing events cause biological damage to humans and other mammals. The greater the exposure dose, the greater the biological problems caused by the ionizing radiation. At medium and high levels of exposure, there are near-term consequences, including impaired performance that can cause casualties and death. Figure C.15 lists the types of biological damage associated with ionizing events.

Figure C.15 Biological Damage from Ionization

At low levels of exposure, ionizing radiation does not cause any near-term medical problems. However, at the 75 cGy level, approximately five percent of healthy adults experience mild threshold symptoms (i.e., transient mild headaches and mild nausea). At the 100 cGy level, approximately 10 to 15 percent of healthy adults experience threshold symptoms and a smaller percentage experience some vomiting. Low levels of ionizing radiation exposure also result in a higher probability of contracting mid- and long-term cancers. Figure C.16 shows healthy adults’ increased risk of contracting cancer after ionizing radiation exposure, by gender.

Figure C.16 Increased Cancer Risk at Low Levels of Exposure to Ionizing Radiation

Protection from ionizing radiation can be achieved through shielding. Most materials shield from radiation, but some materials need to be present in significant amounts to reduce the penetrating radiation by half. Figure C.17 illustrates the widths required for selected types of material to stop half the gamma radiation, called “half-thickness,” and to stop 90 percent of the radiation, called “tenth-value thickness.”

Figure C.17 Radiation Shielding

C.4.13   Electromagnetic Pulse
EMP is a very short duration pulse of low-frequency, or long-wavelength, electromagnetic radiation (EMR). EMP is produced when a nuclear detonation occurs in a non-symmetrical environment, especially at or near the Earth’s surface or high altitudes.22 The interaction of gamma and X-rays with the atoms in the air generates an instantaneous flow of electrons. These electrons immediately change direction, primarily due to the Earth’s magnetic field and velocity, emitting a large number of low-frequency EMR photons. This entire process occurs almost instantaneously.

Any unprotected equipment with electronic components could be vulnerable to EMP. A large number of low-frequency photons can be absorbed by any antenna or any component acting as an antenna. This energy moves within the equipment to unprotected electrical wires or electronic components and generates a flow of electrons. The electron flow becomes voltage within the electronic component or system. Modern electronic equipment using low voltage components can be overloaded with a voltage beyond its designed capacity. At low levels of EMP, this can cause a processing disruption or a loss of data. At increased EMP levels, certain electronic components can be destroyed. EMP can damage unprotected electronic equipment, including computers, vehicles, aircraft, communications equipment, and radars. EMP does not result in structural damage to buildings or bridges, for example, and is not a direct hazard to humans. It is possible the effects of electronics failing instantaneously in items such as vehicles, aircraft, and life-sustaining equipment in hospitals could cause injuries or fatalities.

A high-altitude detonation, or an exo-atmospheric detonation within a certain altitude range, generates an EMP that could cover a very large region of the Earth’s surface, as large as one thousand kilometers across. A surface or low-air burst produces local EMP with severe intensity, traveling through the air out to distances of hundreds of meters. Generally, the lower the yield, the more significant the EMP is compared with air blast. Unprotected electronic components are vulnerable and electrical lines as well as telephone wires carry the pulse to much greater distances, possibly 10 kilometers, and could destroy any electronic device connected to the power lines.

Since electronic equipment can be hardened against the effects of EMP, it is not considered in traditional approaches for damage estimation. The primary objective of EMP hardening is to reduce the electrical pulse entering a system or piece of equipment to a level that does not cause component burnout or operational upset. It is always cheaper and more effective to design EMP protection into the system during design development. Potential hardening techniques include the use of certain materials as radio frequency shielding filters, internal enclosed protective “cages” around essential electronic components, and enhanced electrical grounding and shielded cables. Additionally, equipment can be hardened if it is kept in closed protective cases or in EMP-protected rooms or facilities. Normally, the hardening that permits equipment to operate in intense radar fields (e.g., helicopters operating in front of a ship’s radars) also provides a significant degree of EMP protection.

Because EMP is so fast, circuit breakers, surge protectors, and power strips are not effective against EMP since these are designed to protect against a lightning strike, whereas EMP is at least one thousand times faster than a bolt of lightning.

C.4.14    Transient Radiation Effects on Electronics
Transient radiation effects on electronics (TREE) is damage to electronic components exposed to initial nuclear radiation gamma rays and neutrons. Gamma rays and neutrons moving away from GZ can affect electronic components and associated circuitry by penetrating deep into materials and electronic devices. Gamma rays can induce stray currents of electrons that generate electromagnetic fields similar to EMP. Neutrons can collide with atoms in key electronic materials causing damage to the crystal (chemical) structure and changing electrical properties. All electronics are vulnerable to TREE but smaller, solid-state electronics such as transistors and integrated circuits are the most vulnerable. Although initial nuclear radiation passes through material and equipment in a matter of seconds, the damage is usually permanent.

In the case of a high-altitude or exo-atmospheric burst, prompt gamma rays and neutrons can reach satellites or other space systems. If these systems receive large doses of this initial nuclear radiation, their electrical components can be damaged or destroyed. If a nuclear detonation is a low-yield surface or low-air burst, the prompt gamma rays and neutrons could be intense enough to damage or destroy electronic components at distances beyond those affected by air blast. Since electronic equipment can be hardened against the effects of TREE, it is not considered in damage estimation.

Equipment designed to be protected against TREE is called “rad-hardened.”  Generally, special shielding designs can be effective, but TREE protection may include using shielded containers with a mix of heavy shielding for gamma rays and certain light materials to absorb neutrons. Just as with EMP hardening, it is always less expensive and more effective to design rad-hardening protection into the system during design and development.

C.4.15   Blackout
Blackout is the interference with radio and radar waves resulting from an ionized region of the atmosphere. Nuclear detonations in the atmosphere generate a flow of gamma rays and X-rays moving away from the detonation. These photons produce a large number of ionizing events in the atoms and molecules in the air, creating a large region of ions with more positively charged atoms closer to the detonation, which can interfere with communications transmissions. Blackout does not cause damage or injuries directly. However, the interference with communications or radar operations could cause accidents indirectly, for example, the loss of air traffic control (due to either loss of radar capability or the loss of communications).

A high-altitude or exo-atmospheric detonation produces a large ionized region of the upper atmosphere that could be as large as thousands of kilometers in diameter. This ionized region could interfere with communications signals to and from satellites and with AM radio transmissions relying on atmospheric reflection. Under normal circumstances, this ionized region interference continues for a period of time, up to several hours, after the detonation. The ionized region can affect different frequencies out to different distances and for different periods of time.

A surface or low-air burst produces a smaller ionized region of the lower atmosphere that could be as large as tens of kilometers in diameter. These bursts could interfere with Very High Frequency (VHF) and Ultra High Frequency (UHF) communications signals and radar waves that rely on line-of-sight transmissions. Normally, this low altitude ionized region interference would continue for a period of time, up to a few tens of minutes, after the detonation. There is no direct protection against the blackout effect.

C.5   Nuclear Weapons Targeting Process

C.5.1   Nuclear Weapons Targeting Overview
Nuclear weapons targeting is based on nuclear weapons effects and accounts for the characteristics of U.S. nuclear weapons, predictable effects of those weapons, and resulting damage expectancy. It is a process by which the United States calculates how well it meets damage requirements to defeat adversary targets. The nuclear weapons targeting process is cyclical, beginning with guidance and priorities issued by the President, the Secretary of Defense, and the Chairman of the Joint Chiefs of Staff (CJCS) in conjunction with appropriate combatant command guidance and priorities. These objectives direct joint force and component commanders and the targeting process continues through the combat assessment phase. Figure C.18 illustrates the U.S. nuclear targeting cycle.

Figure C.18 U.S. Nuclear Targeting Cycle

Objectives and Guidance:  Guidance and objectives are issued by the President and the CJCS while joint force and component commanders initiate the targeting cycle.

Target Development: Targets are developed focused on identifying and nominating critical elements of enemy military forces and their means of support for attack.

Weaponeering Assessment: Planners analyze each target nominated for a nuclear strike to determine the optimal means of nuclear attack. During this process, planners consider the employment characteristics of available weapons including yields, delivery accuracy, and fuzing. Damage prediction, consequences of execution, and collateral damage preclusion are additional factors considered in this analysis. Target analysts use target information including location, size, shape, target hardness, and damage criteria (moderate or severe) as inputs to nuclear targeting methodologies.

Force Application:  Information is integrated concerning the target, weapon system, and munitions types in addition to non-lethal force options to select specific weapons to attack specific targets.

Execution Planning and Force Execution: The final tasking order is prepared and transmitted; specific mission planning and material is received at the unit level; and presidential authorization for use is issued.

Combat Assessment:  A joint effort determines if the required target effects meet the military campaign objectives. Nuclear combat assessment is composed of two segments, battle damage assessment and a re-attack recommendation.

C.5.2   Nuclear Weapons Targeting Terminology
Damage criteria are standards identifying specific levels of destruction or material damage required for a particular target category. These criteria vary by the intensity of the damage and by the particular target category, class, or type and are based on the nature of the target including its size, hardness, and mobility as well as the target’s proximity to military or non-military assets. These criteria provide a means by which to determine how best to strike particular targets and, following the attack, evaluate whether the target or target sets were sufficiently damaged to meet operational objectives.

Radius of damage (RD) is the distance from the nuclear weapon burst at which the target elements have a 50 percent probability of receiving at least the specified (severe or moderate) degree of damage. In strategic targeting, this has been called the weapon radius. Because some target elements inside the RD will escape the specified degree of damage while some outside the RD are damaged, response variability results. The RD depends on the type of target, the yield of the weapon, the damage criteria, and HOB of the nuclear weapon.

Circular error probable (CEP) is a measurement of the delivery accuracy of a weapon system and is used as a factor in determining probable damage to a target. The CEP is the radius of a circle within which half of the weapons are expected to fall. A weapon has a 50 percent probability of landing within one CEP of an aim-point.

Probability of damage (PD) is the prospect of achieving at least the specified level of damage, assuming the weapon arrives and detonates on target. It is expressed as fractional coverage for an area target and probability of damage for a point target. The PD is a function of nuclear weapons effects and weapons system delivery data including yield, RD, CEP, and HOB.

Probability of arrival (PA) is the likelihood the weapon arrives and detonates in the target area, calculated as a product of weapon system reliability (WSR), pre-launch survivability (PLS), and probability to penetrate (PTP). The equation for planners is WSR x PLS x PTP = PA.

WSR: Compounded reliability based on test data for each warhead-type and each delivery system type.

PLS: Probability the weapon system will survive a strike by the enemy.

PTP: Probability the weapon system survives enemy air-defense measures and reaches the target.

Damage expectancy (DE) is calculated as the product of the PD and the PA, shown in the formula PA x PD = DE. DE accounts for both weapons effects and the probability of arrival in determining the probability of achieving at least the specified level of damage.

Nuclear collateral damage is undesired damage or casualties produced by the effects of nuclear weapons. Such damage includes danger to friendly forces, civilians, and non-military-related facilities, as well as the creation of obstacles and residual nuclear radiation contamination. Since the avoidance of casualties among friendly forces and non-combatants is a prime consideration when planning either strategic or theater nuclear operations, preclusion analyses must be performed to identify and limit the proximity of a nuclear strike to civilians and friendly forces. Specific techniques for reducing collateral damage include:

reducing weapon yield—the yield of the weapon needed to achieve the desired damage is weighed against the associated risks in the target area;

improving accuracy—accurate delivery systems are more likely to strike closer to the aim-point, reducing the required yield and the potential collateral damage;

employing multiple weapons—collateral damage can be reduced by dividing one large target into two or more smaller targets and by using more than one lower-yield weapon rather than one high-yield weapon;

adjusting the height of burst—HOB adjustments, including the use of air bursts to preclude any significant fallout, can help to minimize collateral damage; and

offsetting the desired ground zero (DGZ)
—moving the DGZ away from target center may achieve the desired effects while avoiding or minimizing collateral damage.

Counterforce targeting plans to destroy the military capabilities of an enemy force. Typical counterforce targets include bomber bases, ballistic missile submarine bases, intercontinental ballistic missile (ICBM) silos, air-defense installations, command and control centers, and weapons of mass destruction storage facilities. Since these types of targets are harder and more protected, the forces required to implement this strategy need to be numerous and accurate.

Countervalue targeting plans the destruction or neutralization of selected enemy military and military-related targets such as industries, resources, and/or institutions contributing to the enemy’s war effort. Since these targets tend to be softer and less protected, weapons required for this strategy need not be as numerous or as accurate as those required to implement a counterforce targeting strategy.

Layering is a technique that plans more than one weapon against a target. This method is used to either increase the probability of target destruction or improve the probability a weapon arrives and detonates on target to achieve a specific level of damage.

Cross-targeting incorporates the concept of “layering” and uses different delivery platforms for employment against one target to increase the probability of at least one weapon arriving at that target. Using different delivery platforms, such as ICBMs, SLBMs, or aircraft-delivered weapons, increases the probability of achieving the desired damage or target coverage.

C.6   Physics for Countering Nuclear Threats
While the technical challenges to building advanced designs such as staged nuclear weapons are significant, the relative simplicity of a GA design raises the possibility non-state actors with sufficient fissile material could assemble a supercritical mass and produce a nuclear detonation using an improvised nuclear device (IND). The physical effects of a nuclear detonation demonstrate the best protection from this threat is to prevent terrorists from acquiring nuclear materials for use in an IND. Maintaining close coordination between the science and the operations of countering nuclear threats (CNT) is paramount.

C.6.1   Fission Yield and Nuclear Forensics
The fission process produces isotopes with a wide range of atomic mass and atomic number, though some fission fragments are more likely to be produced than others. Atomic masses follow a characteristic twin-peaked distribution and most of the isotopes produced have atomic masses near 95 and 140. The detailed shape of this fission product yield curve depends on the specific nucleus undergoing fission and on the energy of the neutrons inducing fission. Figure C.19 compares fission yield curves for U-235 and plutonium-239 (Pu-239). Fission from Pu-239 results in relatively more heavy nuclei than from U-235, as well as higher yield in the atomic mass range 100-120.

Figure C.19 Fission Product Yield by Mass for
U-235 and Pu-239

These differences in yield can be used by nuclear forensic scientists to provide information about a nuclear device. By measuring the relative quantities of fission fragments after detonation, scientists can construct a yield curve and infer what the device used as fissile material.

C.6.2   Detection of Nuclear Material
The same principles of PPE, time, distance, and shielding, which protect personnel from radiation, complicate the detection of nuclear materials. Charged particles from radioactive decay (alpha and beta particles) are easily shielded in transport. In most cases, gamma rays and neutrons emitted from shielded sources are comparable with natural background readings at distances greater than 10 meters.

The penetrating power of radiation varies greatly depending on the type of radiation in question. In general, charged particles can be shielded more easily, while neutral particles penetrate matter more deeply. Alpha particles have the least penetrating power and can be stopped by a sheet of paper or human skin. Beta particles are lighter than alpha particles and permeate more deeply, penetrating skin and traveling several feet in air, but are stopped in a fraction of an inch of metal or plastic. Gamma rays are energetic photons that can transfuse matter deeply. These require a layer of dense material, such as lead, for shielding. Since neutrons are electrically neutral, they interact weakly with matter. Neutrons are absorbed by successively bouncing off light nuclei. As a result, shielding neutron radiation requires thick layers of materials rich in hydrogen, such as water or concrete. Figure C.20 compares the penetration of various types of radiation.

These differences in yield can be used by nuclear forensic scientists to provide information about a nuclear device. By measuring the relative quantities of fission fragments after detonation, scientists can construct a yield curve and infer what the device used as fissile material.

Figure C.20 Penetrating Power of Various Types of Radiation

1 The term thermonuclear is also used to refer to a two-stage nuclear weapon.

2 Technical efficiency is measured by the amount of energy produced for a given amount of fissile material. Less efficient devices require more material to produce the same energy yield.

3 Typically, the maximum weight for a warhead to be compatible with a high-performance jet aircraft would be approximately 1,000 to 1,500 kilograms (kg) (2,200 to 3,300 pounds) and approximately 750 to 1,000 kg (1,650 to 2,200 pounds) for the typical missile being proliferated (e.g., Nodong or SCUD-variant missiles).

4 A 1-kt detonation releases the energy equivalent to 1,000 tons of TNT.

5 The Fat Man and Little Boy weapons had respective yields of 21 and 15 kt but were almost 10,000 pounds each with dimensions much larger than today’s modern warheads.

6 For the purposes of this appendix, a typical nuclear detonation is one that occurs on the Earth’s surface or at a height of burst (HOB) low enough for the primary effects to cause damage to surface targets. Detonations that are exo-atmospheric, high altitude, or deeply buried underground have different effects.

7 Proper employment includes using the required yield at the required location with an effective HOB (e.g., a high-altitude detonation does not destroy a building or a bridge). Examples of single military targets include one or a group of structures in a relatively small area, special contents within a structure (e.g., biological agents), a missile silo or launcher position, a military unit (e.g., a single military ship, an air squadron, or even a ground-force battalion), a communications site, and a command post.

8 A 1-MT detonation releases the energy equivalent to one million tons of TNT.

9 The distances in Figure C.11 are based on scenarios in which the weather is clear, there are no obstacles to attenuate thermal radiation, and the weapon is detonated as a low-air burst at the optimum HOB to maximize the thermal effect.

10 At a short distance beyond the radius of the immediate fireball, the blast wave would reach a density pressure of thousands of pounds per square inch.

11 The distances in Figure C.12 are based on an optimum HOB to maximize the blast effect and the existence of no significant terrain that would stop the blast wave (e.g., the side of a mountain). For surface bursts, the distances shown are reduced by approximately 30 to 35 percent for the higher overpressures and by 40 to 50 percent for one psi.

12 The amount of increased depth of damage is primarily a function of the yield and the soil or rock type. For a 1-kt detonation, the maximum crater size would have a burial depth between 32 and 52 meters, depending on the type of soil or rock.

13 For a 1-kt detonation, the maximum crater size would have a burial depth between 32 and 52 meters, depending on the type of soil or rock.

14 Ionizing radiation is defined as electromagnetic radiation (gamma rays or X-rays) or particulate radiation (e.g., alpha particles, beta particles, neutrons) capable of producing ions directly or indirectly in its passage through or interaction with matter.

15 A photon is a unit of electromagnetic radiation consisting of pure energy and zero mass. The spectrum of photons include AM and FM radio waves, radarwaves, microwaves, infrared waves, visible light, ultraviolet waves, X-rays, and gamma or cosmic rays.

16 Both gamma rays and neutrons are scattered and reflected by atoms in the air, causing each gamma ray and neutron to travel a “zig-zag” path moving generally away from the detonation. Some neutrons and photons may be reflected so many times that, at a significant distance from GZ, travel back toward ground zero.

17 cGy represents the amount of energy deposited by ionizing radiation in a unit mass of material and is expressed in units of joules per kilogram (J/kg).

18 For the purposes of this appendix, all radiation doses are assumed to be acute (total radiation received within approximately 24 hours) and whole-body exposure. Exposures over a longer period of time (chronic), or exposures to an extremity (rather than to the whole body) could have less effect on a person’s health.

19 Calculated from data in Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII - Phase 2, National Academy of Sciences, Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation, 2006.

20 Neutrons induced into typical soil are captured primarily by sodium, manganese, silicon, and aluminum atoms.

21 PPE for first responders includes a sealed suit and self-contained breathing equipment with a supply of oxygen.

22 EMP can also be produced by using conventional methods.