Uranium Bomb
The issue that now faced the scientists was the design and the operation of the weapons. Many concepts were considered and after much testing, it was decided that a gun-barrel design would be the most effective for the uranium bomb. Development efforts at the time were all directed to the design and operation of a successful uranium bomb.
The design worked by shooting a sub-critical mass (an amount of an element that has not yet reached critical mass) of uranium-235 with conventional explosives down a gun barrel to impact with a target made from more sub-critical mass uranium-235. The collision of the two would rapidly cause the uranium to reach critical mass, resulting in an nuclear fission reaction and subsequent explosion.1 The design was relatively simple and the scientist were so confident of success and given the limited supply of refined uranium available, they decided the weapon did not require any testing. This design for the uranium bomb would later become the Little Boy bomb dropped on Hiroshima.
Plutonium Bomb
The design of the plutonium bomb was more complex. The gun-barrel design using plutonium was tested in a nuclear reactor, however, an issue arose, once the plutonium-239 was shot with a plutonium-239 bullet, it did not react like uranium. Instead, it gained another neutron to become plutonium-240.2 This plutonium isotope was found to split up and fission up to 40,000 times faster than P239, it was too spontaneously to control. This mean that the gun-barrel design was not suitable since the P240 isotope would detonate the bomb too early.
Fortunately, the scientists had prepared for this failure, at the time, another design was being worked on as well. This design worked on a implosion. In July 1944, Oppenheimer concluded that the plutonium bomb could not utilize the gun-barrel design. A month afterwards, Oppenheimer shifted the work focus of the personnel from the uranium bomb towards the plutonium bomb.
Fortunately, the scientists had prepared for this failure, at the time, another design was being worked on as well. This design worked on a implosion. In July 1944, Oppenheimer concluded that the plutonium bomb could not utilize the gun-barrel design. A month afterwards, Oppenheimer shifted the work focus of the personnel from the uranium bomb towards the plutonium bomb.
The implosion design worked by surrounding a sub-critical mass sphere of plutonium in the core of the bomb and placing conventional explosives surround this core. Multiple detonators were placed on the surface of the conventional explosive; when detonated, the explosives would apply great force towards the core, increasing the density which resulted in the plutonium core becoming super-critical (mass of element exceeds critical mass), and thus a nuclear reaction.
However, this seemingly simple design had an issue, it was difficult to design the bomb so the explosives would push on the plutonium core evenly. If it did not apply even pressure, the plutonium would be ejected, resulting in a dirty bomb effect, dispersing nuclear material, resulting in large amounts of unwanted fallout.3
The physicist Richard C. Tolman came up with an ingenious method of using fast and slow burning explosives, this way, the shock wave and thus the pressure exerted on the bomb would be spherical.
The physicist Richard C. Tolman came up with an ingenious method of using fast and slow burning explosives, this way, the shock wave and thus the pressure exerted on the bomb would be spherical.
Another issue that was also faced with the construction of the plutonium bomb was the scientists had great difficulty at trying to form the plutonium into a perfect sphere. It was discovered when the plutonium's density was attempted to be found, the results were not constant. It was originally believed that the plutonium was contaminated but it turned out that the plutonium contained many different allotropes (an element can have multiple different atomic structures, these different structures are known as allotropes). This meant that with the change of temperature, the characteristics and shape of the plutonium ball could literally change by itself. The job of shaping the plutonium was a highly hazardous and dangerous one, many chemists and metallurgists were removed from the job for having dangerously high levels of the element. Eventually, the metallurgists found a plutonium-gallium alloy which stabilized the plutonium's unpredictability allowing them to form it into a spherical shape. On the 2nd of July 1945, the first hemisphere for a plutonium core was produced.
Due to the difficulties of the development and the complexity of the design, the scientists decided that the plutonium implosion bomb should be tested.
Due to the difficulties of the development and the complexity of the design, the scientists decided that the plutonium implosion bomb should be tested.
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Trinity
The scientists had decided that because of the complexity of the design and even though it was a waste of material that a test of the bomb was necessary. Plans for the test were made before the first plutonium sphere for the core could be obtained. How this testing was going to be done was debated. Groves had approved for a test; however, he was concerned by the fact that it would waste the valuable material, thus he suggested that the test should be a fizzle. A fizzle meant that the test would use a smaller yield than the actual bomb that was going to be dropped.4 However, Oppenheimer disagreed with this suggestion, rather he preferred a full scale test. This test of the implosion weapon was code named 'Trinity'. In March 1944, it was decided that the test would occur at a site around 150km south of Albuquerque, New Mexico.
Groves was worried at the fact that if the bomb was to fail, he would have had the task of explaining to a committee the loss of billions of dollars worth of the scarce plutonium. So it was decided that a huge vessel was to be built to recover the plutonium in case of a failure. The construction of the vessel used a huge 217 tonnes of iron and steel. However, as the date of the drop came closer, it was later decided that the plutonium supply was great enough and the scientists were confident enough of success of the design that they decided to drop the bomb without using the metal vessel.
Groves was worried at the fact that if the bomb was to fail, he would have had the task of explaining to a committee the loss of billions of dollars worth of the scarce plutonium. So it was decided that a huge vessel was to be built to recover the plutonium in case of a failure. The construction of the vessel used a huge 217 tonnes of iron and steel. However, as the date of the drop came closer, it was later decided that the plutonium supply was great enough and the scientists were confident enough of success of the design that they decided to drop the bomb without using the metal vessel.
A few months before the full-scale test, it was decided a smaller scale explosion was to be conducted. A wooden tower was constructed around 700m away from the planned ground zero. It was loaded with over 100 tons of TNT spiked with nuclear fission products. This relatively small explosion was watched by Oppenheimer and Groves' deputy and was used to produce crucial data needed for the full-scale explosion.
For the final test, a 30m steel tower was built to use to detonate the bomb from. The bomb was detonated from an elevated position to give a better indication of how the bomb would have acted once dropped from a plane, also, it was done to emit maximum energy on target without spreading large amounts of fallout.
For the final test, a 30m steel tower was built to use to detonate the bomb from. The bomb was detonated from an elevated position to give a better indication of how the bomb would have acted once dropped from a plane, also, it was done to emit maximum energy on target without spreading large amounts of fallout.
'The Gadget' as the test bomb was codenamed, was assembled in just one day and the day after on the 14th of July 1945, the bomb was precariously hoisted up the 30m platform. All preparations were now complete, the next step was detonation.
Finally, on the 16th of July 1945 at 5:30am, the test was conducted. The explosion had an equivalent yield to 20kt of TNT which was over 200 times the yield of the smaller scale test. The explosion was observed by many of the scientists of the project such as Fermi, Chadwick, Oppenheimer, Tolman as well as many important scientists and military personnel. Most of the observers were given special dark glasses to protect against the dangerous radiating light from the explosion. Physicist Richard Feynman claims that he was the only one to not use the glasses. During the test, the 'Jumbo' vessel that was decided not to be used was instead placed a few hundred metres away from the bomb as a test to whether or not it would have worked.
The effects of the bomb were truly tremendous and like nothing ever seen before, the shock wave from the explosion reached a huge 160km away and the resulting mushroom cloud was over 12km high. The initial amazement blew away all the scientists. Blown away by the results of the bomb, Director of the test Kenneth Bainbridge was quoted as saying to Oppenheimer after the explosion, 'Now, we are all sons of bitches.'5
In an official report of the test General Farrell wrote, 'The lighting effects beggared description. The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray, and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined...'
Under the explosion, a crater was formed and due to the super hot temperatures of the bomb, it heated the desert sand into a green glass like substance named after the test, 'Trinitite'. Also, Jumbo was able to withstand the effects of the blast.
In an official report of the test General Farrell wrote, 'The lighting effects beggared description. The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray, and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined...'
Under the explosion, a crater was formed and due to the super hot temperatures of the bomb, it heated the desert sand into a green glass like substance named after the test, 'Trinitite'. Also, Jumbo was able to withstand the effects of the blast.
In 1965, in a television documentary, upon recalling the events of the Trinity test, Oppenheimer remarked that the test brought back the words of Bhagavad Gita:
"Now, I am become Death, the destroyer of worlds."6
Design, operation and testing were now all complete. Harry Truman, who had only assumed office 82 days after the death of Roosevelt was aware of the capabilities of the new weapon, the next issue was whether or not Truman was going to give authorisation for it's deployment.
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1. Hoddeson, Lillian; Henriksen, Paul W.; Meade, Roger A.; Westfall, Catherine L. Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943–1945. Pg. 119
2. ibid. Pg 226-229
3. http://en.wikipedia.org/wiki/Nuclear_weapon_design#Implosion-type_weapon4. http://en.wikipedia.org/wiki/Fizzle_(nuclear_test)
5. http://en.wikipedia.org/wiki/Kenneth_Bainbridge
6. http://www.atomicarchive.com/Movies/Movie8.shtml
2. ibid. Pg 226-229
3. http://en.wikipedia.org/wiki/Nuclear_weapon_design#Implosion-type_weapon4. http://en.wikipedia.org/wiki/Fizzle_(nuclear_test)
5. http://en.wikipedia.org/wiki/Kenneth_Bainbridge
6. http://www.atomicarchive.com/Movies/Movie8.shtml
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