Nuclear Weapons

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Alarmed by the military advances of Hitler’s army, and fearing that Germany was developing its own nuclear weapon, Einstein wrote a letter to President Roosevelt in 1939 informing him of possibility to “set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated.” (a) In response to Einstein’s letter, Roosevelt ordered a team of physicists to initiate research on uranium fission and whether a bomb could be built in a short time. The task proved to be difficult as researchers soon found that uranium-238 could not sustain a chain reaction; uranium-235 was a possibility, but only if it was enriched to very high concentrations; separating the two isotopes was extremely difficult as they were chemically identical, and their masses differed by a mere 1%.

Figure 1 The “Trinity”.
Figure 1 The “Trinity”.
Figure 2 The “Little Boy,” Hiroshima Japan, August 6, 1945. The bomb was fueled by U-235 and had a yield of only 13 kilotons.
Figure 2 The “Little Boy,” Hiroshima Japan, August 6, 1945. The bomb was fueled by U-235 and had a yield of only 13 kilotons.

As the work continued, a second possible path to building the bomb was suggested by scientists at Lawrence Radiation Laboratory at Berkeley who produced a new man-made element, first identified as element-94 and later named plutonium, that fissioned more easily and could be produced easily in large quantities; it is formed when uranium-238 captures a neutron, becoming neptunium-239, which is unstable and rapidly decays to plutonium-239.

On December 2, 1942, researchers headed by Italian-émigré Enrico Fermi of the University of Chicago achieved the first self-sustaining chain reaction in a graphite and uranium pile. Soon thereafter, Robert Oppenheimer, a professor of physics at the University of California at Berkeley led a team of nuclear physicists in what became to be known as the “Manhattan Project” to develop the atomic bomb. The effort eventually succeeded in the first nuclear explosion test named Trinity in Alamogordo Bombing range near Los Alamos, New Mexico. The test conducted on July 16, 1945 used plutonium and had a yield of 21 kilotons of TNT (Figure 1).

Less than a month later, at 8:15 on the morning of August 6, 1945, the United States detonated the first atomic bomb on Hiroshima, Japan, which leveled the city and killed 140,000 people instantly. Germany was not a target, as it surrendered earlier in May. In addition, many tens of thousands of people died from cancer in the years that followed (1). That bomb, nicknamed “The Little Boy,” (allegedly named after US President Roosevelt) used uranium-235 and had the destructive power of 13,000 tons of TNT (Figure 2) (b). Three days later a bigger bomb, nicknamed “The Fat Man,” (allegedly named after British Prime Minister, Winston Churchill) was dropped over Nagasaki, killing another 75,000. It was fueled by plutonium-239 and had the destructive power of 21,000 tons of TNT (c). Shortly thereafter, Japan surrendered and WWII was officially ended.

Following Trinity test, and bombing of Hiroshima and Nagasaki, the US conducted numerous other tests over Marshall Islands in the Pacific and underwater. In 1946, then US President Truman suggested establishment of an international agency that oversaw all nuclear activity. Not surprisingly, the Soviet Union, a non-nuclear power objected, arguing that all atomic weapons must be abolished before such agency would have the legitimacy. Three years later, in August 1949, the Soviets tested their first fission bomb. The Cold War had begun.

As the Cold War intensified, so did the demand for bigger and more deadlier nuclear bombs. The advent of the Soviet bomb had reduced the US absolute superiority to only a numerical advantage. The Hungarian émigré physicist Edward Teller promoted the development of thermonuclear (hydrogen) bomb. In 1950, Truman ordered accelerated development of the bomb. On November 1, 1952, US detonated “Mike,” the first H-bomb at Eniwetok in the Pacific. Three years later, the Soviet Union followed suit and detonated its first hydrogen bomb with a force equivalent to 1.6 megaton of TNT.



(1) “The Atomic Bombings of Hiroshima and Nagasaki: The Manhattan Engineer District, The Manhattan Engineer District,” 1946, The World Wide School, 1997. Copies can be downloaded from

(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Additional Comments

(a) Later in his life Einstein admitted that signing the letter to Mr. Roosevelt was the most tragic mistake in his life, and that he had never believed that the President would ever use the atomic weapon.

(b) Tri-nitro-toluene (TNT) is a chemical compound used commonly in road construction and for demolition. It is non-radioactive and its explosion does not result in the release of any nuclear products. Nuclear destructive power is often expressed in terms of kilograms or tons of TNT. One ton of TNT=4.18x109 J.

(c) In comparison, the destructive power of the planes that flew into and destroyed New York’s Twin Towers in 2001 was equivalent to only 1 kiloton of TNT.

Further Reading

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