History of
Nuclear Explosive

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From 1945 to 1992, the United States conducted both nuclear explosive and non-nuclear testing. Since 1992, the United States has not conducted nuclear explosive testing. Instead, the United States has developed and relied upon certifying the continued safety, security, and effectiveness of nuclear weapons as well as evaluating the effects of nuclear weapons on systems without the use of nuclear explosive testing. Uncertainties and challenges associated with these approaches may make it necessary in the future to resume some level of nuclear explosive testing to certify the aging nuclear stockpile. The requirement to resume nuclear explosive testing is assessed on an annual basis by the directors of the national security laboratories and the Commander of U.S. Strategic Command (USSTRATCOM). These assessments are reported to Congress and the President.

U.S. Nuclear Testing Program

The U.S. nuclear testing program began with the Trinity test on July 16, 1945, at a location approximately 55 miles northwest of Alamogordo, New Mexico, now called the Trinity Site. The test confirmed that the implosion design weapon used in the Fat Man atomic bomb would function to produce a nuclear detonation and also gave the Manhattan Project scientists their first look at the effects of a nuclear detonation.

The United States conducted five additional nuclear tests between 1946 and 1948. By 1951, the United States had increased the ability to produce nuclear devices for testing and conducted 16 nuclear tests that year. Between 1951 and 1958, the United States conducted 188 nuclear tests. Increasing the knowledge and data associated with nuclear physics and weapon design was the main purpose of most of these tests. Some tests were designed to develop nuclear weapons effects data while a few were safety experiments. These tests were a mixture of underground, aboveground, high-altitude, underwater, and abovewater detonations.

In 1958, the United States instituted a self-imposed moratorium on nuclear tests. On October 31, 1958, the United States entered into a unilateral testing moratorium announced by President Eisenhower with the understanding that the former Soviet Union also would refrain from conducting tests. The Soviet Union resumed testing in September 1961 with a series of the largest number of tests ever conducted.

On September 15, 1961, the United States resumed testing at the Nevada Test Site (NTS) on a year-round basis and conducted an average of approximately 27 tests per year over the next three decades. These included 24 joint tests with the United Kingdom;1 35 tests for peaceful purposes as part of the Plowshare program;2 seven to increase the capability to detect, identify, and locate nuclear tests as part of the Vela Uniform3 program; four to study nuclear material dispersal in possible accident scenarios; and post-fielding tests of specific weapons. By 1992, the United States had conducted a total of 1,054 nuclear tests. In 1992, Congress passed legislation that prohibited the United States from conducting an underground nuclear test and led to the current policy restriction on nuclear explosive testing.

Early Years of U.S. Nuclear Testing

The first six nuclear tests represented the infancy stage of the U.S. nuclear testing program. The first test at the Trinity Site in New Mexico provided the confidence required for an identical weapon to be employed at Nagasaki. The second and third tests, both in 1946, used identical Fat Man design devices to evaluate the effects of airdrop and underwater detonations in the vicinity of Bikini Island, located in the Pacific. The next three tests were conducted in 1948 on towers on the Enewetak Atoll in the Pacific, testing three different weapon designs. These first six tests began with no previous data and, by today’s standards, had very crude test measurement equipment and computational capabilities. Because of this, only limited amounts of scientific data were generated by each of these events.

The 188 nuclear tests conducted between 1951 and 1958 included 20 detonations above one megaton (MT), one detonation between 500 kilotons (kt) and one MT, 13 detonations between 150 and 500 kt, and 17 tests that produced zero or near-zero-yields, primarily as safety experiments. Many of these tests produced aboveground detonations, which were routine at the time. The locations for these tests included the NTS and Nellis Air Force Range in Nevada, Enewetak Atoll, Bikini Island, and the Pacific Ocean. Some of the highest yield detonations were produced by test devices far too large to be used as deliverable weapons. For example, the Mike device, which produced a 10.4 MT detonation on November 1, 1952, at Enewetak, was almost seven feet in diameter, 20 feet long, and weighed 82 tons. On February 28, 1954, the Bravo test on Bikini Island produced a surface burst detonation of approximately 15 MT, the highest yield ever produced by the United States. The Bravo device was a two-stage design in a weapon-size device, using enriched lithium as fusion fuel in the secondary stage. Figure 14.1 shows the Bravo fireball shortly after detonation.

Figure 14.1
Figure 14.1 Bravo Fireball

During this period, as the base of scientific data grew and as sensor technology, test measurement, and diagnostic equipment became more sophisticated and more capable, the amount of data and scientific information gained from each test increased. The initial computer codes, used to model fissile material compression, fission events, and the like, were based on two-dimensional models. These computer models became more capable as the scientific database expanded and computing technology evolved.

Transition to Underground Nuclear Testing

Between October 31, 1958, and September 14, 1961, the United States conducted no nuclear tests because of a self-imposed testing moratorium. The United States resumed nuclear testing on September 15, 1961, and conducted 100 tests over the next 14 months to include underground, underwater, and aboveground detonations. These tests included nine detonations above one MT, eight detonations between 500 kt and one MT, and four detonations between 150 and 500 kt. The locations for these tests included the NTS, the vicinity of Christmas Island in the East Indian Ocean, the Pacific Ocean, Johnston Island in the Pacific, and Carlsbad, New Mexico. The last four tests of this group were conducted during a nine-day period between October 27 and November 4, 1962. These were the last U.S. nuclear tests that produced aboveground or surface burst detonations.

In compliance with the 1963 Limited Test Ban Treaty (LTBT), all subsequent U.S. nuclear test detonations were conducted deep underground. Initially, some thought this restriction would have a negative impact on the program to develop accurate data on the effects of nuclear weapons. The Atomic Energy Commission (AEC) and the Defense Atomic Support Agency (DASA)4 responded with innovative ways to minimize the impact of this restriction. Through the use of long and deep horizontal tunnels, and with the development of specialized sensors and diagnostic equipment to meet the need, the effects testing program continued successfully.

In the 30 years between November 9, 1962, and September 23, 1992, the United States conducted 760 deep underground nuclear tests (UGT).5 The locations for these tests included the NTS, Nellis Air Force Range in Nevada, and the vicinities of Fallon, Nevada; Hattiesburg, Mississippi; Amchitka, Alaska; Farmington, New Mexico; Grand Valley, Colorado; and Rifle, Colorado.6 The tests during the period between November 1962 and April 1976 included four detonations above one MT, 14 detonations between 500 kt and one MT, and 88 detonations between 150 and 500 kt.7 Of the 1,054 total U.S. nuclear tests, 63 had simultaneous detonations of two or more devices while 23 others had zero or near-zero yield.

Generally, a device for a weapons-related UGT (for physics research, to refine a warhead design in engineering development, or for a post-fielding test) was constructed at one of the two design laboratories (LANL or LLNL), as shown in Figure 14.2, and transported to the test site and positioned down a deep vertical shaft in one of the NTS test areas. Informally, this type of test was called a “vertical test.” Typically, a large instrumentation package would be lowered into the shaft and positioned relatively close to the device with electrical wires running back to aboveground recording instruments. The vertical shaft was covered with earth and structural support was added to prevent the weight of the earth from crushing the instrumentation package or the device. This closed the direct opening to the surface and precluded the fireball from pushing hot radioactive gases up the shaft into the atmosphere. When the detonation occurred, the hundreds or thousands of down-hole instruments momentarily transmitted data but were almost immediately consumed in the fireball.

Figure 14.2
Figure 14.2 LANL Rack Assembly and Alignment Complex (RAAC)

The preparation for a vertical UGT took months and included drilling the vertical shaft and preparation of the instrumentation package, which was constructed vertically, usually within 100 meters of the shaft. The instrumentation package was typically 40 to 80 feet high, several feet in diameter, and surrounded by a temporary wooden structure. The structure would have levels, approximately seven to eight feet apart, and a temporary elevator to take technicians to the various floors to place and prepare the instruments. The test device would be lowered into the shaft, followed by the cylindrical instrument package. After the test, the ground above the detonation would often collapse into the cavity left by the cooling fireball, forming a subsidence crater on the surface directly over the test location.8 See Figure 14.3 for a photograph of a preparation site for an underground nuclear test.

Figure 14.3
Figure 14.3 Underground Nuclear Test Preparation

Generally, a UGT device for an effects test was positioned in a long, horizontal tunnel deep in the side of one of the mountains in the Yucca Mountain Range, located at the north end of the NTS. Informally, this type of test was called a “horizontal test.” The tunnels were relatively large, usually more than 30 to 40 feet across, and ran several miles into the side of the mountain. Typically, the tunnel had a small-scale railroad track running from the entrance to the deepest part of the main tunnel, which included a train to support the logistics movement of workers and equipment. The main tunnel would have many long branches, called “side-drifts,” each of which could support a UGT. Instruments were positioned at various distances from the device and a huge blast door was constructed to permit the instantaneous effects of nuclear and thermal radiation, X-rays, and electromagnetic pulse to travel to instruments at greater distances but to close prior to the arrival of the blast wave. After the detonation, instruments outside the blast door would be recovered and the side-drift would be closed and sealed with a large volume of earth.

For both vertical and horizontal UGTs, the device would be prepared in a laboratory environment and transported to the test site, usually only a few days prior to the test date. On the test date, the NTS operations center would continuously monitor wind direction and speed to determine where any airborne radioactive particles would travel in the unlikely event of a “venting” incident.9 If the wind conditions could blow venting gases to a populated area, the test was delayed until the wind conditions changed. Frequently, UGTs were delayed hours or days.

In 1974, the Threshold Test Ban Treaty (TTBT) was signed by the United States. The treaty would not be ratified until 1990 but, in 1976, the United States announced it would observe the treaty pending ratification. The treaty limited all future tests to a maximum yield of 150 kt. This presented a unique problem because, at the time, each of the three legs of the nuclear triad required new warheads with yields exceeding 150 kt and this compelled the weapons design community to make two major changes to nuclear weapons development.

First, new warhead designs would now be limited to using tested and proven secondary stage components, which provide most of the yield in high-yield weapons. The rationale for this change was that if previous testing had already determined the output required from the primary stage to ignite or drive the secondary and if testing had also determined the output of the secondary, then all that would be needed was a test to determine if the new primary would produce a yield large enough to drive the secondary. Of the 1,054 U.S. nuclear tests, at least 82 had yields that exceeded 150 kt. Another 79 may have had yields exceeding 150 kt but are listed in unclassified source documents only as being between 20 to 200 kt. Many of these tests provided the data for scientists to determine the required information (e.g., ignition threshold, yield output) to certify several different secondary stage designs, which would produce yields greater than 150 kt. See Figure 14.4 for a summary of U.S. nuclear tests by yield.

Figure 14.4
Figure 14.4 U.S. Nuclear Tests by Yield

Second, in order to test any new warhead with a yield greater than 150 kt, the warhead would have to be reconfigured to ensure it would not produce a yield in excess of 150 kt. Thus, new strategic warheads capable of yields greater than 150 kt would not have the benefit of a nuclear test in the full-yield configuration.

By the 1980s, the U.S. nuclear testing program had evolved into a structure that categorized tests as physics research, effects, warhead development engineering, and post-fielding tests. Physics research tests contributed to the scientific knowledge and technical data associated with general weapons design principles. The effects tests contributed to the base of nuclear effects data and to testing the vulnerability of key weapons and systems to the effects of nuclear detonations. See Chapter 9: Nuclear Survivability and Effects Testing for more information.

Development tests were used to test or refine key aspects of specific designs to increase yield output or to improve certain nuclear detonation safety features. Post-fielding tests were conducted to provide stockpile confidence and ensure safety. For each warhead-type, a stockpile confidence test (SCT) was conducted between 6 and 12 months after fielding. This was intended to check the yield to ensure any final refinements in the design added after the last development test and any imperfections that may have resulted from the mass-production process did not corrupt the designed yield. Post-fielding tests were also used to confirm or repair safety or yield problems when non-nuclear testing, other surveillance, or computer simulation detected possible problems, especially unique abnormalities with the fissile components. If a problem was confirmed and a significant modification applied, a series of nuclear tests could be used to validate the modification to ensure that fixing one problem did not create a new issue.

Transition to 3-D Codes

By the early 1980s, the United States had conducted more than 970 nuclear tests, most of which had the basic purpose of increasing the scientific data associated with weapon design or refining specific designs. The national security laboratories had acquired the most capable computers of the time and were expanding the computer codes to analyze, for example, fissile material compression and fission events in a three-dimensional (3-D) model. By the mid-1980s, use of 3-D codes had become routine. The 3-D codes provided more accurate estimates of what would be achieved with new designs or what might happen, for nuclear detonation safety considerations, in an abnormal environment.

With the 3-D codes, the national security laboratories evaluated a broader range of abnormal environments for fielded warhead-types (e.g., the simultaneous impact of two high-velocity fragmentation pieces). This led to safety experiments and improvements that might not have otherwise occurred.10 The increased computational modeling capability with the 3-D codes also helped scientists to refine the near-term nuclear testing program to include tests that would enhance the base of scientific knowledge and data. Each year, the results of the nuclear testing program increased U.S. computational modeling capabilities.

End of Underground Nuclear Testing

Throughout the 20th century, most nations that developed nuclear weapons tested them to obtain information about how the weapons worked as well as how the weapons behaved under various conditions and how personnel, structures, and equipment behaved when subjected to nuclear explosions. In 1963, three of the four nuclear states (the United States, the United Kingdom, and the then Soviet Union) and many non-nuclear states signed the Limited Test Ban Treaty, pledging to refrain from testing nuclear weapons in the atmosphere, underwater, or in outer space. The Treaty, however, permitted underground nuclear testing. France continued atmospheric testing until 1974 and China continued until 1980. Then, in 1992, the United States voluntarily suspended its program of nuclear testing. Public Law (Pub. L.) 102-377, Fiscal Year 1993 Energy and Water Development Appropriations Act, 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.11 However, with a limit of 15 tests within less than four years 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.12 Therefore, the decision was made to forgo the 15 additional tests permitted under the new law and no other tests were conducted.

The 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. Second, the moratorium on underground nuclear testing also resulted in suspending production of weapons being developed with new, untested designs. These changes resulted in a shift toward a second era for the U.S. nuclear weapons program: the modernization and production cycle, in which newerdesign warheads replaced older warheads, was supplanted by a new strategy of indefinitely retaining existing warheads without nuclear testing and with no plans for weapon replacement. Third, the underground nuclear testing 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 the correcting modification was acceptable. There was a concern that without nuclear testing, there was a possibility that one weapontype after another would be retired because of an inability to fully diagnose and correct emerging problems, which might eventually lead to unintended, unilateral disarmament by the United States. This fear has not been realized in the years since 1992. However, as the legacy Cold War stockpile continues to be deployed, age-related issues, including those related to nuclear components, are an increasing concern. Eventually, all of the weapons in the legacy stockpile will need to be replaced by new warheads whose designs place a premium on yield margin so that they can be certified without the benefit of nuclear explosive testing. This is the third U.S. nuclear era. See Chapter 1: The U.S. Nuclear Deterrent: Past, Present, and Future for a more detailed description.

1 The United States and the United Kingdom were preparing to conduct a 25th test when President George H. W. Bush announced a moratorium on underground nuclear testing in 1992. Until that point, the nuclear relationship between the United States and the United Kingdom, as defined by the 1958 Mutual Defense Agreement, allowed for the conduct of joint tests between the two nations. This was helpful to the United Kingdom-especially following the atmospheric testing moratorium of 1958-because the UK did not have the same access to land that could be used for underground nuclear testing as the United States and the Soviet Union. Following the 1992 testing moratorium, the United Kingdom formally undertook to end nuclear testing in 1995 and they ratified the Comprehensive Nuclear-Test-Ban Treaty in April 1998. See Chapter 10: International Programs, for a more detailed discussion of the nuclear relationship between the United States and the United Kingdom.

2 The Plowshare program was primarily intended to evaluate the use of nuclear detonations for constructive purposes (e.g., to produce craters for the rapid and effective creation of canals).

3 Vela Uniform was an element of Project Vela undertaken by DoD to develop and implement methods to monitor compliance with the 1963 Partial Test Ban Treaty focused on monitoring seismic signals in order to detect underground and underwater nuclear testing. Vela Uniform performed seven underground nuclear tests in the continental U.S. and Alaska.

4 While the AEC was a forerunner organization to the current NNSA, the DASA served as a precursor to the current Defense Threat Reduction Agency (DTRA).

5 Four of these were surface experiments, without a nuclear detonation, to study plutonium scattering.

6 After May 17, 1973, all U.S. nuclear tests were conducted at the NTS.

7 81 of the 90 tests are listed in the unclassified record with a yield between 20 and 200 kt.

8 The collapse that caused the subsidence crater could occur at any time, from minutes to months, after the detonation, making the time of the collapse unpredictable.

9 Venting incidents occurred very few times during the history of U.S. underground nuclear testing. Venting occurs when a vertical UGT shaft is close enough to an unknown deep underground cave system that leads to the surface and permits the expanding fireball to push hot radioactive gases through the underground cave system to the surface and into the air. Instruments to determine geology thousands of feet underground were not precise enough to detect all possible underground caves or cavities. Venting can also occur if the blast door for a horizontal UGT is not strong enough to contain the blast wave.

10 For example, an interim fix for one of the Army warheads was fielding a “horse blanket” to be draped over the container to provide fragmentation/projectile shielding for transportation and storage; the ultimate fix put the shielding inside the container.

11 Pub. L. 102-377 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). 12 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, all of 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.

12 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, all of 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.