construction, three men stood ready, prepared to sacrifice their lives. If the reaction went out of control, their job was to douse the carbon bricks in neutron-absorbing chemicals. But by the time they had done this, they would already have received fatal doses of radiation. It’s hardly surprising that their hopes for a safe test of the reactor were greater than those of any of the others present.
According to project mythology, Fermi insisted that the scientists and invited guests have a lunch break before proceeding with the first live test of the reactor. All but one of the control rods were withdrawn already. The final rod was pulled back and the Geiger counters monitoring radiation from the pile began to roar. The neutron count had shot up. The pile stayed critical—generating enough neutrons to make the reaction self-sustaining—for a little over four minutes before Fermi shut it down. The production of fuel that would eventually be at the heart of the first atomic bomb had been proved possible.
In March 1943, the most famous part of the complex Manhattan Project operation was brought online. Under newly appointed director Robert Oppenheimer, what had been a ranch school at Los Alamos in New Mexico was converted into a cross between a holiday camp, a university, and a factory, where arrays of mechanical calculators could churn through the complex arithmetic required to predict the behavior of a totally new kind of weapon, and where the raw materials being assembled elsewhere would be pulled together to build the first atomic bombs.
One of the difficulties that took up much of the time of the inhabitants of Los Alamos was working out just how to make a bomb based on nuclear fission explode, rather than generate energy in a slow and steady fashion. It is easier said than done to create a nuclear explosion. In a bomb, it’s necessary to get the materials over critical mass, the amount required for the process to run away with itself, very quickly. The production of neutrons sparking the chain reaction has to peak suddenly. Otherwise, parts of the radioactive substance will go critical at different times and it will blow apart, yielding a tiny amount of its potential and leaving most of the radioactive matter intact.
Two different techniques would eventually be employed to achieve critical mass quickly enough. The bomb based on separated uranium 235 used the relatively simple gun method. Here a cylinder of uranium was shot into a hole in another piece of uranium at high speed, taking the combined mass suddenly above critical. The plutonium bomb used an alternative approach, where a hollow sphere of plutonium segments was forced in on itself by multiple simultaneous explosive blasts from different directions.
This second approach was much more complex, needing lengthy experimentation with the conventional explosive charges that would be used to drive the plutonium together. The different segments all had to arrive at the same time, requiring exquisitely precise coordination of a sphere had to be forced inward, using special shaped charges called explosive lenses that focused the blast of the conventional explosive in a particular direction. But because of differences between plutonium and uranium, the gun approach could not have achieved criticality quickly enough for plutonium, and the imploding sphere had to be attempted.
As it happened—though no one on the Allied side knew it—the race to complete an atomic weapon was one-sided. In part thanks to a successful raid on a Norwegian plant that produced a substance called heavy water, used in the German experimental reactors to slow down neutrons to enable them to work with uranium 238, the Germans didn’t even succeed in getting a reactor working properly before the war came to an end. Some of the German scientists involved claimed after the war that they had intentionally worked as slowly as they could to prevent the Nazis from getting such a devastating
According to project mythology, Fermi insisted that the scientists and invited guests have a lunch break before proceeding with the first live test of the reactor. All but one of the control rods were withdrawn already. The final rod was pulled back and the Geiger counters monitoring radiation from the pile began to roar. The neutron count had shot up. The pile stayed critical—generating enough neutrons to make the reaction self-sustaining—for a little over four minutes before Fermi shut it down. The production of fuel that would eventually be at the heart of the first atomic bomb had been proved possible.
In March 1943, the most famous part of the complex Manhattan Project operation was brought online. Under newly appointed director Robert Oppenheimer, what had been a ranch school at Los Alamos in New Mexico was converted into a cross between a holiday camp, a university, and a factory, where arrays of mechanical calculators could churn through the complex arithmetic required to predict the behavior of a totally new kind of weapon, and where the raw materials being assembled elsewhere would be pulled together to build the first atomic bombs.
One of the difficulties that took up much of the time of the inhabitants of Los Alamos was working out just how to make a bomb based on nuclear fission explode, rather than generate energy in a slow and steady fashion. It is easier said than done to create a nuclear explosion. In a bomb, it’s necessary to get the materials over critical mass, the amount required for the process to run away with itself, very quickly. The production of neutrons sparking the chain reaction has to peak suddenly. Otherwise, parts of the radioactive substance will go critical at different times and it will blow apart, yielding a tiny amount of its potential and leaving most of the radioactive matter intact.
Two different techniques would eventually be employed to achieve critical mass quickly enough. The bomb based on separated uranium 235 used the relatively simple gun method. Here a cylinder of uranium was shot into a hole in another piece of uranium at high speed, taking the combined mass suddenly above critical. The plutonium bomb used an alternative approach, where a hollow sphere of plutonium segments was forced in on itself by multiple simultaneous explosive blasts from different directions.
This second approach was much more complex, needing lengthy experimentation with the conventional explosive charges that would be used to drive the plutonium together. The different segments all had to arrive at the same time, requiring exquisitely precise coordination of a sphere had to be forced inward, using special shaped charges called explosive lenses that focused the blast of the conventional explosive in a particular direction. But because of differences between plutonium and uranium, the gun approach could not have achieved criticality quickly enough for plutonium, and the imploding sphere had to be attempted.
As it happened—though no one on the Allied side knew it—the race to complete an atomic weapon was one-sided. In part thanks to a successful raid on a Norwegian plant that produced a substance called heavy water, used in the German experimental reactors to slow down neutrons to enable them to work with uranium 238, the Germans didn’t even succeed in getting a reactor working properly before the war came to an end. Some of the German scientists involved claimed after the war that they had intentionally worked as slowly as they could to prevent the Nazis from getting such a devastating
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