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Nuclear power is unique. The ability to harness nuclear energy has changed the world. The peaceful applications of nuclear power for the developed and developing world have been an unprecedented game changer and have accelerated the development timeline of

The U.S. nuclear deterrent, with its unique attributes, is a central element of U.S. national security policy.
nations through increased access to energy resources and advanced technologies. Similarly, the ability to use nuclear energy for military purposes has fundamentally altered the international security environment since the employment of nuclear weapons by the United States during World War II.

The U.S. nuclear deterrent, with its unique attributes, is a central element of U.S. national security policy for several reasons. First, the U.S. nuclear deterrent reduces the probability that a nuclear peer or nuclear-armed adversary might engage the United States in a strategic nuclear exchange. Second, U.S. nuclear forces provide a nuclear “umbrella” of protection for many allied nations so that these nations do not need to develop and field their own nuclear weapons. This helps to minimize nuclear proliferation. Third, the U.S. nuclear arsenal deters nuclear or radiological attack against the United States, its allies, and its partners by state-sponsored terrorist organizations or proliferant nations. These U.S. nuclear weapons programs also provide the scientific, technological, and engineering foundation for the U.S. nuclear counterterrorism and counterproliferation programs. For these reasons, it is the current policy of the United States to retain and maintain its nuclear deterrent indefinitely until verifiable worldwide nuclear disarmament is achieved.

The Changing International Security Environment

During an April 5, 2009 visit to Prague, Czech Republic, President Barack Obama described his vision for a new direction for U.S. nuclear forces in the world: “I state clearly and with conviction America’s commitment to seek the peace and security of a world without nuclear weapons...As long as these weapons exist, the United States will maintain a safe, secure, and effective arsenal...” Concrete steps toward achieving this vision were described and outlined in the results of the 2010 Quadrennial Defense Review (QDR) and the 2010 Nuclear Posture Review (NPR). Both reviews acknowledge the United States is faced with a new security environment that has changed dramatically since the end of the Cold War. While the threat of global nuclear war has become remote, the risk of nuclear attack has increased. Both the QDR and the NPR Reports note that the most immediate and extreme danger for the United States are the dual threats of nuclear proliferation and nuclear terrorism. Additional countries—especially those that do not conform to international norms and structures—may acquire or seek to acquire nuclear weapons. Sub-state actors and terrorist organizations have also declared their intent to acquire nuclear threat devices.[1] While facing these increasingly urgent threats, the two reviews assert that it is important for the United States to continue addressing the more familiar challenge of ensuring strategic stability with existing nuclear powers—most notably Russia and China.

Russia remains America’s only peer in the area of nuclear weapons capabilities. The nature of the U.S.-Russia relationship has changed fundamentally since the days of the Cold War. While policy differences continue to arise between the two countries and Russia continues to modernize its still-formidable nuclear forces, Russia and the United States are no longer adversaries and prospects for military confrontation have declined dramatically. The two nations have increased their cooperation in areas of shared interest, including preventing nuclear terrorism and nuclear proliferation.

The United States and China increasingly share responsibilities for addressing global security threats, including weapons of mass destruction (WMD) proliferation and terrorism. At the same time, the United States and China’s Asian neighbors remain concerned about the pace and scope of China’s current military modernization efforts, including the qualitative modernization of its nuclear forces. China’s nuclear arsenal remains much smaller than the arsenals of Russia and the United States; however, the lack of transparency surrounding China’s nuclear programs and the strategy and doctrine that guide them raise questions about China’s future strategic intentions.

2010 Nuclear Posture Review

As a result of these changes in the international security environment, the United States has modified the role of U.S. nuclear weapons, retaining the benefits of the peaceful applications of nuclear power, while mitigating the concomitant risks. The 2010 Nuclear Posture Review Report outlines the Administration’s approach to implementing the president’s agenda for reducing nuclear dangers and pursuing the long-term goal of a world without nuclear weapons. The report also details how the United States will sustain a safe, secure, and effective nuclear deterrent as long as nuclear weapons exist.

The 2010 NPR is the third comprehensive review of U.S. nuclear policies and posture; the first two were conducted in 1994 and 2001 by the Clinton and Bush Administrations, respectively. The 2010 review was an interagency effort conducted by the Department of Defense in close consultation with the Departments of Energy and State and in direct engagement with the president. The NPR focused on five key objectives on the United States’ nuclear agenda, placing nonproliferation and nuclear counterterrorism as primary U.S. national security priorities for the first time.
The 2010 Nuclear Posture Review Report outlines the Administration’s approach to implementing the president’s agenda for reducing nuclear dangers and pursuing the long-term goal of a world without nuclear weapons.

1.3.1 Preventing Nuclear Proliferation and Nuclear Terrorism

The 2010 NPR stressed the prevention of nuclear proliferation and nuclear terrorism and outlined steps for the United States to lead expanded international efforts to strengthen the global nonproliferation regime. Specifically the NPR recommended:

  • Bolstering the nonproliferation regime, including increasing funding for DOE nonproliferation programs;
  • Accelerating efforts to implement the president’s initiative to lock down all vulnerable nuclear materials against theft or seizure, and increasing the United States’ ability to detect and interdict nuclear materials; and
  • Pursuing arms control to support the Treaty on the Nonproliferation of Nuclear Weapons (NPT) Article VI obligations, including the New Strategic Arms Reduction Treaty (START), the Comprehensive Nuclear Test Ban Treaty (CTBT), and a verifiable Fissile Material Cutoff Treaty (FMCT).

The NPR Report also addressed the renewed U.S. commitment to hold fully accountable any state, terrorist group, or other non-state actor that supports or enables terrorist efforts to obtain or use weapons of mass destruction, either by facilitating, financing, or providing expertise or safe haven for such efforts.

1.3.2 Reducing the Role of Nuclear Weapons

Since the end of the Cold War, the United States has reduced the role of nuclear weapons in deterring non-nuclear attacks on itself and its allies and partners. The United States is continuing to strengthen conventional military capabilities, missile defenses, and counter-WMD capabilities so that the role of U.S. nuclear weapons in deterring non-nuclear attacks—conventional, biological, or chemical—can continue to be reduced while strengthening deterrence. The NPR Report also explained changes in U.S. declaratory policy to include the strengthening of negative security assurances. Specifically, the United States declared that it will not use or threaten to use nuclear weapons against non-nuclear weapons states that are party to the NPT and in compliance with their nuclear nonproliferation obligations.[2]

1.3.3 Maintaining Strategic Deterrence and Stability at Reduced Nuclear Force Levels

As the United States and Russia reduce their nuclear forces, maintaining stability remains a priority. As a first step, the 2010 NPR analytically derived U.S. positions for New START negotiations with Russia:

  • The United States and Russia agreed to limits of 1,550 accountable strategic warheads, 700 deployed strategic delivery vehicles, and a combined limit of 800 deployed and non-deployed strategic delivery vehicles. The Treaty does not constrain U.S. missile defenses and allows the United States to pursue conventional global strike systems.
  • Under New START, the United States will retain a nuclear triad and “deMIRV”[3] its ICBMs to one warhead each.

The NPR also proposed pursuing high-level, bilateral dialogues with Russia and China aimed at promoting more stable and transparent strategic relationships. With Russia, this includes a discussion of future bilateral nuclear weapons reductions, including strategic and non-strategic, deployed and non-deployed. With China, the United States seeks to address each side’s concerns about the other’s strategic forces and policies. The goal of such a dialogue is to enhance confidence, improve transparency, and reduce mistrust.

1.3.4 Strengthening Regional Deterrence and Reassuring U.S. Allies
and Partners

The United States remains committed to strengthening bilateral and regional security architectures and to adapting these relationships to emerging twenty-first century requirements. The United States will continue the forward deployment of U.S. forces in key regions, the strengthening of U.S. and allied non-nuclear capabilities, and the provision of extended deterrence in order to deter potential threats, demonstrate to neighboring states that the pursuit of nuclear weapons will only undermine their goal of achieving military or political advantages, and reassure non-nuclear U.S. allies and partners that their security interests can be protected without their own nuclear deterrent capabilities. Security architectures in key regions will retain a nuclear dimension as long as nuclear threats to U.S. allies and partners remain. The United States will continue to be able to extend its nuclear umbrella through forward-deployable fighters and bombers and through U.S. intercontinental and submarine-launched ballistic missiles (ICBMs and SLBMs). The United States plans to retain the capability to forward deploy U.S. nuclear weapons on tactical fighters and heavy bombers and to proceed with a full-scope life extension of the B61 bomb, which will be able to be carried by these aircraft.

1.3.5 Sustaining a Safe, Secure, and Effective Nuclear Arsenal

The United States will sustain a safe, secure, and effective nuclear arsenal as long as nuclear weapons exist. The United States is modernizing its nuclear weapons infrastructure, sustaining the science, technology, and engineering base, investing in human capital, and ensuring senior leadership focus. The NPR Report outlined several principles guiding future U.S. stockpile management decisions:

  • The United States will not conduct nuclear testing and will seek ratification and entry into force of the Comprehensive Nuclear Test Ban Treaty.
  • The United States will make decisions on how to sustain specific warheads on a case-by-case basis.
  • The United States will not develop new nuclear warheads; life extension programs (LEPs) will be based on designs that are, or have been, in the U.S. stockpile and will not provide new military capabilities or support new military missions.
  • LEP decisions will be made on a case-by-case basis with strong preference given to refurbishment or reuse. Replacement of nuclear components with redesigned components will require presidential and congressional approval.
1.4 Nuclear Weapons from 1939-1945

An understanding of the unique status of nuclear weapons is integral to understanding their role. Nuclear weapons are distinct from other weapons; they are in a class by themselves. An early realization of their unrivaled destructive power necessitated the development of separate and unique systems and procedures to produce, field, maintain, deploy, employ, and dispose of these special weapons. From the dawn of the nuclear era, even a new vocabulary was required to talk about atomic warfare; among these terms was the ominous phrase “mutual assured destruction” (MAD), with its connotations of Armageddon and the culture of impending doom it produced. Because of their tremendous power, the U.S. military did not have peacetime physical custody of nuclear weapons until 1959, almost fifteen years after the first successful nuclear detonation.

The potential to release nuclear energy for military use was first described in a letter signed by Dr. Albert Einstein to President Franklin D. Roosevelt in August 1939. The letter, written by Einstein at the urging of Dr. Leó Szilárd, described the possibility of setting up a nuclear chain reaction in a large mass of uranium—a phenomenon that would lead to the construction of bombs—and concluded with the statement that experimental work grounded in these principles was being carried out by the Nazis in Berlin. Einstein’s statement that “such bombs might very well prove to be too heavy for transportation by air” did not diminish his estimate of the potential for a huge increase in the destructive capacity of a single bomb, which he thought could be carried or delivered to a target by ship.

In early 1940, two physicists, the Austrian Otto Frisch and the German Rudolph Peierls—both of whom had sought refuge from the Nazis and were working at Birmingham University in England—wrote a memorandum suggesting that if a five kilogram mass of uranium-235 (U-235) were made to fission, it would release an atomic explosion equivalent to thousands of tons of dynamite. Frisch and Peierls explained a method of separating the U-235 and detonating it in a bomb, discussed the radiological hazards the explosion would create, and examined the moral implications of the bomb’s use. The significance of Frisch and Peierls’ breakthrough—a massively powerful bomb, light enough to be carried by an aircraft—soon resonated through the government of the United Kingdom, and in the summer of 1941, the UK government-appointed Maud Committee presented its report endorsing Frisch and Peierls’ conclusions. The Maud Committee report described the facility and processes needed to build an atomic bomb and provided an estimate of the cost. Shortly thereafter, Prime Minister Winston Churchill authorized work to begin on Britain’s atomic bomb project, managed by the Nuclear Weapon Directorate, code named Tube Alloys.[4]

The first Maud Committee report was sent from Britain to the United States in March 1941, but no comment was received in return. Given the lack of response, a member of the committee flew secretly to the United States in August 1941 to discuss the findings. Subsequent to these discussions, the National Academy of Sciences proposed an all-out U.S. effort to build nuclear weapons.

In a meeting on October 9, 1941, President Roosevelt was impressed with the need for an accelerated program, and by November he had authorized the “all-out” effort recommended by the Academy and encouraged by the British. A new U.S. policy committee, the Top Policy Group, was created to inform the president of developments in the program. The first meeting of the group took place on December 6, 1941, one day before the Japanese attack on Pearl Harbor and the entry of the United States into World War II.

Eventually, these efforts led the United States to establish the Manhattan Engineering District, also known as the “Manhattan Project,” whose goal was to develop and produce nuclear bombs in time to affect the outcome of World War II. In 1943, as outlined in the Quebec Agreement, the team of scientists working on the British project was transferred to the Manhattan Project. Several scientists from Canada also joined the project. The U.S. Army Corps of Engineers and Major General Leslie Groves provided oversight management and control of the Manhattan Project, which eventually employed more than 130,000 people. Dr. J. Robert Oppenheimer served as the civilian director of the scientific and engineering research and development activities.

On July 16, 1945, the United States detonated its first nuclear explosive device called the “Gadget” at the Trinity Site, located within the current White Sands Missile Range, near the town of Alamagordo, New Mexico. Twenty-one days later, on August 6, President Harry S. Truman authorized a specially equipped B-29 bomber called Enola Gay (Figure 1.2) to drop a nuclear bomb, Little Boy (Figure 1.3), on Hiroshima, Japan. Soon after Hiroshima was attacked, President Truman called for Japan’s surrender. With no response from the Japanese after three days, on August 9, another B-29 bomber, Bockscar (Figure 1.4), dropped a second U.S. atomic weapon, Fat Man (Figure 1.5), on Nagasaki.

On August 14, 1945, Japan surrendered. The use of nuclear weapons had shortened the war and reduced the number of potential casualties on both sides by precluding a planned U.S. land invasion of Japan. The atomic bombs dropped on Hiroshima and Nagasaki remain the only nuclear weapons ever used in warfare. Their use permanently altered the global balance of power.

1.5 Nuclear Weapons from 1945-1992

The United States enjoyed a nuclear monopoly until the Soviet Union conducted its first nuclear test on August 29, 1949. On October 3, 1952, following the resumption of its independent nuclear weapons program in 1947, the United Kingdom detonated its first nuclear device, becoming the third nation to become nuclear weapons-capable. Less than a month later, on November 1, 1952, the United States detonated its first thermonuclear device, followed nine months later by the Soviet Union’s first thermonuclear test.[5] The arms race was on.

Both the United States and the Soviet Union increased their stockpile quantities until each possessed nuclear weapons in sufficient quantities to achieve a second-strike capability, meaning that both sides would be capable of massive retaliation even after absorbing an all-out first strike. In this way, the United States and the Soviet Union were certain of mutual assured destruction, which provided deterrence for both nations. These were the uneasy years of the nuclear “balance of terror,” when the potential for
For the first decade or so of the nuclear era, the U.S. nuclear weapons program was focused on producing sufficient nuclear material to build enough weapons to support a nuclear capability for almost every type of military delivery system available at the time.
.total devastation from a superpower nuclear exchange was the most urgent threat facing the nation, and the prospect of an attack against the North Atlantic Treaty Organization (NATO) in Western Europe was a very real possibility

For the first decade or so of the nuclear era, the U.S. nuclear weapons program was focused on producing sufficient nuclear material to build enough weapons to support a nuclear capability for almost every type of military delivery system available at the time. This was considered essential because of the possibility of Cold War escalation—the danger that a potential U.S.-Soviet conflict would escalate from a conventional confrontation to the limited use of battlefield and tactical nuclear weapons to an all-out strategic exchange. Throughout the late 1950s, the United States was committed to increasing its nuclear weapons quantities to enhance flexibility in the types of nuclear-capable military delivery vehicles and the bombs and warheads available for delivery.

By 1965, the U.S. nuclear weapons stockpile had grown to more than 31,000 warheads (see Chapter 3: U.S. Nuclear Forces, for a discussion of historical and current stockpile quantities). Most of these warheads had relatively low yields and were for short-range, non-strategic (then called “tactical”) systems. At the time, many weapons were forward deployed in Europe within the territories of NATO allies.

Beginning in the mid-1960s, the United States shifted its priorities from quantity to quality, and U.S. stockpile production established a recurring pattern of deployment, fielding, and then replacement by more modern weapons. Thus, from the mid-1960s until 1992, the U.S. nuclear weapons program was characterized by a continuous cycle of modernization programs that included building and subsequently replacing the weapons in the U.S. nuclear stockpile with newer, more modern designs. In addition to warheads that were simpler[6] for the military operator, modern characteristics included greater yield, smaller size,[7] better employment characteristics,[8] and more modern safety, security, and control features. A key part of this process was the use of nuclear testing for a wide variety of purposes,[9] including the ability to:

  • better understand nuclear physics and weapon design and functioning;
  • determine more accurately the nature and distances associated with nuclear detonation effects;
  • refine new designs in the development process;
  • test the yield of weapons;
  • confirm or define certain types of safety or yield problems found in nuclear components in weapons that were already fielded; and
  • certify the design modification required to correct those problem

Until 1992, the United States utilized a complementary combination of underground nuclear testing (UGT)[10] and non-nuclear testing and evaluation to refine designs in the development stage, certify weapon designs and production processes, validate safety, estimate reliability, detect defects, and confirm effective repairs. In order for a nuclear weapon to be fielded, it had to go through development, testing and evaluation, initial and subsequent full-scale production, and, finally, fielding for possible wartime employment. During and after fielding, stockpile activities included exchanging limited life components (LLCs),[11] detecting components with design or aging defects and replacing them, conducting periodic validations for safety, and updating reliability estimates. Eventually, as the weapon aged, and additional modern safety, security, and operational design features became available, the United States would begin development of a newer, better, and more sophisticated system to replace the fielded weapon. These modernization programs were usually timed to provide replacement weapons after the older warheads had been deployed for a period of 15-20 years, a period known as the “protected period.” During the protected period, required operational quantities of existing warheads were preserved, even though quality assurance testing would usually consume one weapon per year for each type of weapon. At the end of the protected period, the older weapon would begin the retirement process; at the same time, the replacement system would be in the production and fielding process. In this way, the U.S. nuclear arsenal was continually replenished by weapons with better safety and security features that met the required effectiveness with less collateral damage and fewer undesirable effects. This ensured that the United States had an extremely modern, sophisticated stockpile predicated on a substantial nuclear and non-nuclear component production capacity and the continuation of underground nuclear testing.

The End of Underground Nuclear Testing

Because of congressional pressure, the United States voluntarily suspended its program of nuclear testing in 1992. Public Law 102-377, the legislation that halted U.S. nuclear testing, had several key elements. The law included a provision for 15 additional nuclear tests to be conducted by the end of September 1996 for the primary purpose of modifying weapons in the established stockpile to include three modern safety features.[12] With a limit of 15 tests within less than four years, however, and without any real advance notice of the requirement, there was no technically credible way (at the time) to certify design modifications that would incorporate any of the desired safety features into existing warhead-types.[13] Therefore, the decision was made to forgo the 15 additional tests permitted under the new law, and no other tests were conducted.[14]

This nuclear test prohibition impacted the stockpile management process in several significant ways. First, the legislation was too restrictive to achieve the objective of improving the safety of those already-fielded warhead-types lacking all available modern safety features. Second, the moratorium on UGT also resulted in suspending production of weapons being developed with new, untested designs—including those with newer safety and security improvements beyond those specified in the legislation. These changes resulted in a shift toward a second paradigm for the U.S. nuclear weapons program. The modernization and production cycle, in which newer-design warheads replaced older warheads, was replaced by a new strategy of indefinitely retaining existing warheads without nuclear testing and with no plans for weapon replacement. Third, the UGT moratorium created an immediate concern for many senior stockpile managers that any weapon-type that developed a nuclear component problem might have to be retired because nuclear tests could no longer be used to define the specific problem and confirm that the correcting modification was acceptable. Without nuclear testing, there was a possibility that one weapon-type after another would be retired because of an inability to correct emerging problems, which might eventually lead to unintended, unilateral disarmament by the United States. (While this has not occurred, it was a projected issue in 1992.)

Stockpile Management Since 1992

In response to these new circumstances and the resulting paradigm shift, the Fiscal Year 1994 National Defense Authorization Act (Public Law 103-160) required the Department of Energy to “establish a stewardship program to ensure the preservation of the core intellectual and technical competencies of the United States in nuclear weapons.”

Public Law 103-160 required the Department of Energy to “establish a stewardship program to ensure the preservation of the core intellectual and technical competencies of the United States in nuclear weapons.”

In the absence of nuclear testing, the DOE Stockpile Stewardship Program was directed to: support a focused, multifaceted program to increase the understanding of the enduring stockpile; predict, detect, and evaluate potential problems due to the aging of the stockpile; refurbish and remanufacture weapons and components, as required; and maintain the science and engineering institutions needed to support the nation’s nuclear deterrent, now and in the future. In other words, the nuclear weapons establishment was called upon to determine how to ensure the continued safety, security, and effectiveness of the weapons in the U.S. nuclear stockpile without underground testing, and without any plan to replace aging weapons, even as they aged beyond any previously experienced lifespan.

This “science-based” approach, which has served as a substitute for nuclear testing since 1992, has developed and matured significantly since its inception, and now includes computer simulations, experiments, and the data from more than 1,000 previous nuclear tests. The capabilities of this integrated analytical computation system are maturing constantly with the expectation that, over time, the system will provide the same level of confidence that was achieved through nuclear testing in 1992. As U.S. weapons continue to age, however, innovative solutions to evolving problems must continue to be developed.

Since early 1993, the United States has maintained its nuclear stockpile through a newer, shortened process comparable to the previous cycle of development, production, retirement, and replacement. The process of modernize and replace became one of retain and maintain, consisting primarily of activities associated with the continuous assessment, maintenance and repair, and refurbishment of U.S. weapons, with periodic reductions in quantities corresponding with the U.S. reductions in strategic forces associated with strategic force reduction treaties.

As a result of the 2010 Nuclear Posture Review, plans are currently in place to refurbish and modernize the U.S. nuclear weapons infrastructure to continue to sustain a safe, secure, and effective nuclear deterrent. Additionally, the United States nuclear program, both independently and in cooperation with foreign partners, is actively engaged in nuclear threat reduction activities to enhance international stability and national security.


The Departments of Defense and Energy are cooperating as partners in the plan to retain and maintain the U.S. strategic deterrent with safe, secure, and reliable nuclear weapons now and in the future, as the nation cooperates with Russia and other nuclear weapons states to reduce nuclear forces and moves toward a verifiable global elimination of all nuclear weapons. The president has acknowledged that until such a world exists, however, the United States will maintain a safe, secure, and effective nuclear deterrent. The goal of this volume is to provide an understanding of the current U.S. actions associated with maintaining this safe, secure, and effective nuclear deterrent while effectively countering the nuclear threats of nuclear proliferation and nuclear terrorism.

[1] Nuclear threat devices include improvised nuclear devices (INDs), radiological dispersal devices (RDDs), radiological exposure devices (REDs), and any device that may produce nuclear yield, such as nuclear weapons that have fallen out of state control.

[2] In making this strengthened assurance, the United States affirmed that any state eligible for the assurance that uses chemical or biological weapons against the United States or its allies and partners would face the prospect of a devastating conventional military response and that any individuals responsible for the attack, whether national leaders or military commanders, would be held fully accountable. Given the catastrophic potential of biological weapons and the rapid pace of biotechnology development, the United States reserves the right to make any adjustment in the assurance that may be warranted by the evolution and proliferation of the biological weapons threat and U.S. capacities to counter that threat. In the case of countries not covered by this assurance—states that possess nuclear weapons and states not in compliance with their nuclear nonproliferation obligations—there remains a narrow range of contingencies in which U.S. nuclear weapons may still play a role in deterring a conventional or a chemical or biological weapon attack against the United States or its allies and partners.

[3] “MIRV” stands for multiple independently targetable reentry vehicle. Using a MIRV warhead, a single launched missile can strike several targets or fewer targets redundantly.

[4] Eventually, the term “tube alloy” was used as the code word for plutonium, whose existence was kept secret at that time. A few years later, scientists in the United States used the term “tuballoy” to refer to depleted uranium.

[5] A thermonuclear weapon uses both nuclear fission and nuclear fusion to produce a greatly increased yield in a device small enough to be delivered as a weapon.

[6] As a function of simplicity, the United States moved away from warheads requiring in-flight-insertion (IFI) of the nuclear component, to warheads that were self-contained “sealed-pit” devices (“wooden rounds”) without requiring the military operator to insert components, or “build” the warhead. While these warheads may have been more complex internally, this was transparent to the operator, and the pre-fire procedures were much simpler.

[7] Smaller warhead size allowed strategic missiles to carry a larger number of re-entry bodies/vehicles and made nuclear capability possible for a greater number of delivery methods, including the possibility for nuclear weapons to be human-portable or fired by cannon artillery.

[8] Some of the features that provided increased operational capability included selectable yields, better fuzing (for a more accurate height of burst), increased range (for cannon-fired warheads), and shorter response times.

[9] The United States conducted nuclear tests from 1945 until 1992. The United States, together with the United Kingdom, the Soviet Union, and France, observed a voluntary moratorium on testing from October 1958-1960. The moratorium was broken by France in 1960, and the United States and the Soviet Union resumed testing in 1961.

[10] The United States conducted above ground and undersea testing until 1963, when the Limited Threshold Test Ban Treaty entered into force, banning nuclear tests in the atmosphere, outer space, and under water. (For more information on the Threshold Test Ban Treaty, see Appendix B: International Nuclear Treaties and Agreements.)

[11] Some age-related changes affecting nuclear warhead components are predictable and well understood. LLCs in any given warhead-type might include power sources, neutron generators, tritium reservoirs, and gas transfer systems. LLCs are replaced at pre-determined times during scheduled limited life component exchanges (LLCEs). In a similar manner to that in which one replaces components of an automobile—such as oil filters, brake pads, and tires—so too must LLCs be replaced before their deterioration adversely affects warhead function or personnel safety.

[12] Public Law 102-377, the Fiscal Year 1993 Energy and Water Development Appropriations Act, specified three desired safety features for all U.S. nuclear weapons: enhanced nuclear detonation safety (ENDS), insensitive high explosive (IHE), and a fire-resistant pit (FRP).

[13] At the time the legislation was passed in 1992, scientists estimated that each modification to any given type of warhead would require at least five successful nuclear tests, which had to be done sequentially; one test was necessary to confirm that the modification did not corrupt the wartime yield, and four tests were needed to confirm nuclear detonation safety for four different peacetime abnormal environments.

[14] The 1992 legislation also stated that if, after September 30, 1996, any other nation conducted a nuclear test, then the restriction would be eliminated. Since October 1996, several nations have conducted nuclear tests. The current restriction is one of policy, not of law.