United States Nuclear Weapons
Nuclear warheads are identified by a W with a program number, e.g. W88. Nuclear bombs are identified by a B with a program number, e.g. B61. Modifications include Mod numbers, e.g., B61, Mod 11, or simply B61-11.
The atomic bomb, originated in 1939. At that time, Dr. Albert Einstein persuaded President Franklin Roosevelt to fund development of the bomb. Amid extreme secrecy, scientists from around the United States worked day and night to make the concept a reality. Finally, by late 1943, the scientists were confident enough to tell the Army Air Forces (AAF) to begin preparing for the bomb's use. On 06 August 1945 the B-29 "Enola Gay" flew from Tinian to strike Hiroshima, Japan, on the world's first atomic bombing mission. Three days later "Bockscar" dropped the plutonium bomb on Nagasaki. The Japanese surrendered in the following days thereby ending World War II.
At first the United States relied on US Strategic Air Command to defend Europe against Soviet attack, but in 1950 the start of the Korean War led to European re-equipping and consideration of the deployment of tactical nuclear weapons. By 1951, progress on a thermonuclear bomb of smaller dimensions revived interest in the long-range ballistic missile. Two months before President Truman announced that the United States would develop the thermonuclear bomb, the Air Force contracted with Consolidated Vultee Aircraft Corporation (later Convair) to resume study, and then to develop, the Atlas intercontinental ballistic missile, a project that had been dormant for four years.
Studies of the possibilities of using thermonuclear reactions to obtain very large explosions began in the summer of 1942-almost a year before the Los Alamos Laboratory was formed. Such studies continued here during the war, though at a necessarily modest rate partly because the Laboratory's primary mission was to develop a fission bomb as rapidly as possible, partly because a fission bomb appeared to be prerequisite to the initiation of any thermonuclear reaction, and partly because the theoretical investigation of the feasibility of achieving a large-scale thermonuclear re-action- at least the "Classical Super" form then considered-was enormously more difficult than that required in connection with obtaining an explosive fission reaction.
Studies of possible thermonuclear weapons continued here in the years immediately after the war, but these too were necessarily limited in scope. Only one of the small but capable group working on the Super during the war continued on the Los Alamos staff after the spring of 1946. In addition, the need for improvements in fission weapons was evident and pressing. And, for several years at least, the computing resources available here (or anywhere else in the country) were completely inadequate for a definitive handling of the problems posed by a thermo-nuclear weapon.
Nevertheless, in 1947 the pattern emerged for a possible "booster," that is, a device in which a small amount of thermonuclear fuel is ignited by a fission reaction and produces neutrons that in turn enhance the fission reaction. In 1948 it was decided to include a test of such a system in the series then planned for 1951. Following the first test of a fission bomb by the Soviets in August 1949, President Truman decided at the end of January 1950 that the United States should undertake a concerted effort to achieve a thermonuclear weapon even though no clear and persuasive pattern for such a device was available at that time. In May of 1951, as part of the Greenhouse test series, two experiments involving thermonuclear reactions were conducted. One, the George shot, the design of which resulted from the crash program on the H-bomb, confirmed that our understanding of means of initiating a small-scale thermonuclear reaction was adequate. The other, the Item shot, demonstrated that a booster could be made to work.
Quite fortuitously, in the period between one and two months preceding these experiments but much too late to have any effect on their designs, a new insight concerning thermonuclear weapons was realized. Almost immediately this insight gave promise of a feasible approach to thermonuclear weapons, provided only that the design work be done properly. This approach was the one of which Robert Oppenheimer was later (1954) to say, "The program we had in 1949 was a tortured thing that you could well argue did not make a great deal of technical sense . . . . The program in 1951 was technically so sweet that you could not argue about that." On this new basis and in an impressively short time, considering the amount and novelty of the design work and engineering required, the Mike shot, with a yield of about 10 megatons, was conducted in the Pacific on November 1, 1952.
As tested, Mike was not a usable weapon: it was quite large and heavy, and its thermonuclear fuel, liquid deuterium, required a refrigeration plant of great bulk and complexity. Nevertheless, its performance amply confirmed the validity of the new approach. In the spring of 1954, a number of devices using the new pattern were tested, including the largest nuclear explosion (about 15 megatons) ever conducted by the United States. Some of these devices were readily adaptable (and adopted) for use in the stockpile.
Since 1954 a large number of thermonuclear tests have been carried out combining and improving the features first demonstrated in the Item and Mike shots. The continuing objective has been weapons of smaller size and weight, of improved efficiency, more convenient and safe in handling and delivery, and more specifically adapted to the needs of new missiles and aircraft.
The early atomic bomb required polonium/beryllium detonator pits (or initiators) to generate the neutrons of the explosive sequence. Polonium-210 has a half-life of about 138 days, a fact that mandated the replacement of the pits periodically. In order to access the pits, personnel opened threaded couplings machined from fissile uranium-a process that produced radioactive waste items. With the phasing out of the atomic bomb, and the phasing in of the TN weapon, a sealed neutron initiator replaced the polonium/beryllium pit. These second generation capsules, brought into the inventory as of late 1954, still required periodic disassembly to verify the integrity of the fissile materials. As of 1962, capsules were completely phased out.
In 1953, Eisenhower introduced his new look strategy which came to mean that any aggression by USSR would be met by a massive retaliatory response using nuclear weapons. Between 1945 and 1962, during the atmospheric test series, the US Government conducted 235 nuclear weapons tests, principally in Nevada and the Pacific. Stockpiling of the atomic bomb began slowly, with only 13 in the entire arsenal in 1947; 56 in 1948; 298 in mid-1950. The leap came during the Korean war, between 1950 and the close of 1952, when stockpiles reached a total of 832 bombs. In 1955, the United States sustained an inventory of 2,280 nuclear (atomic and thermonuclear) bombs.
With President Kennedy in power the old "trip-wire" strategy which could have resulted in massive nuclear war was replaced in 1961 by "flexible response." The number of nuclear warheads in strategic alert forces increased from 850 on 30 June 30 1961 to 2700 estimated as of 30 June 1965.
The most modern safety features in U.S. nuclear weapons are incorporated in the Peacekeeper intercontinental ballistic missile warhead (W87), the ground-launched cruise missile warhead (W84), and a modern strategic bomb (the B83)-all first deployed in the 1980s. They include features such as high explosive that is virtually impossible to detonate inadvertently (developed by Los Alamos and Livermore in the 1970s) as well as creative features that enhance electrical nuclear detonation safety and make the weapons safe in the event of fire.
The Moscow Treaty was signed at Moscow on May 24, 2002, and entered into force on June 1, 2003. The Moscow Treaty both reflects and significantly contributes to the emergence of the new strategic relationship between the United States and Russia. The Treaty places upon the United States a legal obligation to implement fully its publicly announced plans to reduce to a level of 1700-2200 strategic nuclear warheads by December 31, 2012.
In using the term "operationally deployed strategic nuclear warheads" the United States means reentry vehicles on ICBMs in their launchers, reentry vehicles on SLBMs in their launchers on board submarines, and nuclear armaments loaded on heavy bombers or stored in weapons storage areas of heavy bomber bases. A small number of spare strategic nuclear warheads (including spare ICBM warheads) are located at heavy bomber bases and the United States does not consider these warheads to be operationally deployed strategic nuclear warheads.
The Treaty makes clear that the Parties need not implement their reductions in an identical manner. Russia, like the United States, may reduce its strategic nuclear warheads by any method it chooses. We do not yet know how Russia intends to count its reductions for purposes of the Moscow Treaty. It could use "operationally deployed strategic nuclear warheads" or some other method. Russia may or may not implement Moscow Treaty reductions in the same way it has implemented reductions under START. Moscow Treaty numbers are not comparable to START Treaty data due to the different counting approaches of the two treaties.
The US plans for achieving by December 31, 2012, the strategic offensive reductions required by Article I of the Treaty involve, as the first planned step in reducing U.S. operationally deployed strategic nuclear warheads,
- retiring 50 Peacekeeper ICBMs,
- removing four Trident submarines from strategic service, and
- no longer maintaining the ability to return the B-1B heavy bomber to nuclear service.
These steps are already underway (Peacekeeper deactivation and SSBN-to-SSGN modification) or completed (B-1B). At any given time, the United States will have two of the remaining 14 Trident SSBNs in overhaul. Those SSBNs in overhaul will not contain operationally deployed strategic nuclear warheads.
The United States plans to reduce its operationally deployed strategic nuclear warheads to 3500-4000 by 2007. Specific decisions about U.S. forces beyond 2007 have not been made. It is anticipated that reductions beyond 2007 will involve decreasing the number of operationally deployed strategic nuclear warheads on ballistic missiles and lowering the number of operationally deployed warheads at heavy bomber bases. These plans, however, will be periodically assessed, and will evolve over time.
Eight of the nine weapons in the current stockpile are not as safe and secure as they could be made. Only the W84 nuclear warhead is equipped with all of the safety and surety features available. The other eight designs do not incorporate all of the safety and surety features that are available. The W62, in fact, does not have any safety features.
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