Early USA Nuclear Power

Mark 3 – “Fat Man” plutonium implosion weapon (used against Nagasaki), effectively the same as the “Gadget” device used in the Trinity (nuclear test) with minor design differences. (21 kilotons, 1945–1950)

Mark 4 – Post-war “Fat Man” redesign. Bomb designed with weapon characteristics as the foremost criteria. (1949–1953)

Mark 5 – Significantly smaller high efficiency nuclear bomb. (1–120 kilotons, 1952–1963)

Mark 6 – Improved version of Mk-4. (8–160 kilotons, 1951–1962)

Mark 7 – Multi-purpose tactical bomb. (8–61 kilotons, 1952–1967)

Mark 8 – Gun-assembly, HEU weapon designed for penetrating hardened targets. (25–30 kilotons, 1951–1957)

At the end of World War II, the defeated Axis nations and many of the victorious allied countries were devastated, their economies and industrial capabilities unable to support even a peacetime society. In terms of war-fighting capability, the USSR possessed a large and well-armed ground force that could easily have overrun Western Europe, or possibly even China. The USSR was unable to sustain such a force, however, because of catastrophic conditions in the homeland, nor did it have a long-range air or naval capability to support an aggressive national strategy.

The prevalent strategic view in the Truman phase of the air “atomic age” (1945-53) was that long-range bombers carrying nuclear weapons against enemy cities or military forces could defeat any nation or force hostile to the United States and its interests. In this environment, the US Air Force was established as a separate service, and the Army and Navy were reduced essentially to “token” forces. These small ground and naval services would be required in future conflicts primarily to provide certain occupation and logistic forces to support the primary weapon: the long-range bomber.

On June 25, 1950, ground and air forces of Communist North Korea crossed the border into South Korea in an all-out assault to gain control over the entire Korean peninsula. The perceived US strategy was articulated by one Air Force Officer who, when told that US ground troops were to be committed to the war, is said to have remarked: “The old man [General MacArthur] must be off his rocker. When the Fifth Air Force gets to work on them, there will not be a North Korean left in North Korea.”

Only after three full years of conventional warfare, involving mostly US air, ground, and naval weapons of World War II vintage, was the Korean War finally brought to a conclusion. US forces had achieved their goal of maintaining the independence of South Korea without the employment of “tactical” or “strategic” nuclear weapons. Both uses were considered: tactical, in the sense of direct support to ground operations (against troop concentrations, bridges, and so forth); and strategic, against mainly factories and assembly areas in North Korea and Manchuria. President Truman apparently gave consideration to the use of nuclear weapons against the Soviet Union in this period. A journal kept in his own handwriting has an entry dated January 27, 1952, contemplating a threat of an “all-out war” against the Soviet Union as well as China: “It means that Moscow, St. Petersburg [Leningrad], Vladivostok, Peking, Shanghai, Port Arthur, Dairen, Odessa, Stalingrad, and every manufacturing plant in China and the Soviet Union will be eliminated.”

After the Korean War ended in stalemate in mid-1953 the war in Indochina, between French forces and the Communist Vietminh, continued in a deadly struggle. A large French force was surrounded by the Vietminh at Dienbienphu, with the end of the Korean War freeing guns, munitions, and technical advisors for the fight against the French. By the spring of 1954 the situation at Dienbienphu was critical and the French asked for American air strikes, first conventional and then nuclear (under the code name Vulture). B-29s flying from the Philippines or carrier aircraft could have carried out the strikes. Although President Eisenhower, the Secretary of State, and four of the five members of the U. S. Joint Chiefs of Staff favored direct intervention, British opposition and French reluctance to accept American direction led to an end of Operation Vulture. Dienbienphu fell to the Communists on May 7, 1954. It was a severe military defeat for the French and-more importantly-led to a political end of the conflict.

At the time the same nuclear weapons would have been used against tactical targets (Korea and Indochina) and strategic targets (the Soviet Union and China). Although American scientists during this period were developing relatively small nuclear weapons, at the time of Truman’s journal entry the atomic bombs available were few in number and large in size. The Mk 3 and Mk 4 weapons then in the inventory were more than ten feet long, five feet in diameter, and weighed over five tons. Under normal circumstances, the bombs were not assembled; to put them together required a crew of trained technicians and almost a day.

As the Cold War increased in intensity, President Truman ordered an increase in nuclear weapons production. The American arsenal grew from perhaps seven bombs-or, more accurately, the components for seven- in mid-1947, to about 25 a year later, and to some 50 in mid-1949. Indications are that by mid-1950 production had provided an arsenal of at least 300, with approximately another hundred being added each year during the Korean War, and even more after that-possibly even totalling as many as 2,000 by mid-1955. Under the Truman Administration there occurred not only an increase in numbers, but a diversification of types: the Mk 5, which weighed only 3,000 pounds, had retractable fins, and was the first nuclear weapon that could be carried externally on an aircraft; the Mk 6 (8,500 pounds), which was the first to be mass produced; the small, 1,600-pound Mk 7 that could be used as a missile warhead as well as a bomb; and the Mk 8 (3,300 pounds), which was intended to penetrate hardened structures. All four weapons were produced from 1951-52 onward. Thus, after an initial period of possessing virtually no useable nuclear capability, by the early 1950s the United States was becoming a true nuclear power.

Subsequently, the Eisenhower-Dulles Administration (1953-61) enunciated the strategy that became known as “massive retaliation.” According to this doctrine, aggression against the United States or its allies would be deterred with the threat of massive retaliatory nuclear strikes; if deterrence should fail, the US would prevail against the Soviet Union in a general war. The doctrine, however, also called for conventional forces to deter or contain localized aggression without resorting to nuclear weapons. While the mix and balance of conventional forces and nuclear weapons were not specified in the major policy documents, the Army and Navy both sought to modernize and maintain large conventional forces. This, in turn, further reinforced proponents of nuclear weapons as a means of controlling defense spending through the use of a relatively small Air Force and Navy nuclear attack forces. In this period, the ability of the Soviet Union to attack the United States with nuclear weapons consisted of a few long-range Soviet nuclear bombers that would have to survive both the long flight to the United States (no bases in the Western hemisphere being available) and the expanding US warning and air defense system. Also, in the 1950s an active US civil defense program was in existence.

3 thoughts on “Early USA Nuclear Power

  1. Pingback: Early USA Nuclear Power – faujibratsden

  2. I have always wondered and cannot find a satisfactory answer: after the prototype at Trinity; Fat Man & Little Boy were there ANY OTHERS ready to use?


  3. Availability of Additional Bombs

    The date that a third weapon could have been used against Japan was no later than August 20. The core was prepared by August 13, and Fat Man assemblies were already on Tinian Island. It would have required less than a week to ship the core and prepare a bomb for combat.

    By mid 1945 the production of atomic weapons was a problem for industrial engineering rather than scientific research, although scientific work continued – primarily toward improving the bomb designs.

    The three reactors (B and D which went started up for production in December 1944, and F which started up February 1945) at Hanford had a combined design thermal output of 750 megawatts and were theoretically capable of producing 19.4 kg of plutonium a month (6.5 kg/reactor), enough for over 3 Fat Man bombs. Monthly or annual production figures are unavailable for 1945 and 1946, but by the end of FY 1947 (30 June 1947) 493 kg of plutonium had been produced. Neglecting the startup month of each reactor, this indicates an average plutonium production 5.6 kg/reactor even though they were operated at reduced power or even shut down intermittently beginning in 1946.

    Enriched uranium production is more difficult to summarize since there were three different enrichment processes in use that had interconnected production. The Y-12 plant calutrons also had reached maximum output early in 1945, but the amount of weapon-grade uranium this translates into varies depending on the enrichment of the feedstock. Initially this was natural uranium giving a production of weapon-grade uranium of some 6 kg/month. But soon the S-50 thermal diffusion plant began feeding 0.89% enriched uranium, followed by 1.1% enriched feed from the K-25 gaseous diffusion plant. The established production process was then: thermal diffusion (to 0.89%) -> gaseous diffusion (to 1.1%) -> alpha calutron (to 20%) -> beta calutron (up to 89%). Of these three plants, the K-25 plant had by far the greatest separation capacity and as it progressively came on line throughout 1945 the importance of the other plants decreased. When enough stages had been added to K-25 to allow 20% enrichment, the alpha calutrons were slated to be shut down even if the war continued.

    After Japan’s surrender in August 1945, S-50 was shut down; the alpha calutrons followed in September. But K-25 was complete on August 15, and these shutdowns would have occurred in any case. At this point gaseous diffusion was incapable of producing weapon grade uranium, a planned “top plant” had been cancelled in favor of more beta calutrons. An expansion of K-25, called K-27, to produce a larger flow of 20% enriched feed was under construction and due to go in full operation by 1 February 1946. In October production had increased to 32 kg of U-235 per month.

    In November and December additional beta tracks went on line, and the percentage of downtime for all beta tracks fell, boosting production further. Between October 1945 and June 1946, these improvements led to a 117% increase in output at Oak Ridge, to about 69 kg of U-235 per month.

    It is very unlikely any more Little Boy-type bombs would have been used even if the war continued. Little Boy was very inefficient, and it required a large critical mass. If the U-235 were used in a Fat Man type bomb, the efficiency would have been increased by more than an order of magnitude. The smaller critical mass (15 kg) meant more bombs could be built. Oppenheimer suggested to Gen. Groves on July 19, 1945 (immediately after the Trinity test) that the U-235 from Little Boy be reworked into uranium/plutonium composite cores for making more implosion bombs (4 implosion bombs could be made from Little Boy’s pit). Groves rejected the idea since it would delay combat use.

    The improved composite core weapon was in full development at Los Alamos when the war ended. It combined two innovations: a composite pit containing both U-235 and Pu-239, and core levitation which allowed the imploding tamper to accelerate across an air gap before striking the pit, creating shock waves that propagated inward and outward simultaneously for more rapid and even compression.

    The composite pit had several advantages over using the materials separately:

    * A single design could be used employing both of the available weapon materials.
    * Using U-235 with plutonium reduced the amount of plutonium and thus the neutron background, while requiring a smaller critical mass than U-235 alone.

    The levitated pit design achieved greater compression densities. This permitted using 25% less than fissile material for the same yield, or a doubled yield with the same amount of material.

    Production estimates given to Sec. Stimson in July 1945 projected a second plutonium bomb would be ready by Aug. 24, that 3 bombs should be available in September, and more each month – reaching 7 or more in December. Improvements in bomb design being prepared at the end of the war would have permitted one bomb to be produced for every 5 kg of plutonium or 12 kg of uranium in output. These improvements were apparently taken into account in this estimate. Assuming these bomb improvements were used, the October capacity would have permitted up to 6 bombs a month. Note that with the peak monthly plutonium and HEU production figures (19.4 kg and 69 kg respectively), production of close to 10 bombs a month was possible.

    When the war ended on August 15 1945 there was an abrupt change in priorities, so a wartime development and production schedule did not continue. Development of the levitated pit/composite core bomb ground to a halt immediately. It did not enter the US arsenal until the late forties. Plans to increase initiator production to ten times the July 1945 level were abandoned.

    Fissile material production continued unabated after the S-50 and alpha calutron shutdowns though the fall, but plutonium shipments from Hanford were halted, and plutonium nitrate concentrates were stockpiled there.



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