![]() ![]() 14 Documents show that every few months, US officials received sinister letters, like one to the head of the Atomic Energy Commission, David Lilienthal, presenting opportunities to purchase a quantity of cubes for hundreds of thousands of dollars each, lest they be sold to entities “not considered over-friendly to the United States.” 15 As the US was in no short supply of uranium ore by that time because of the work of the CDT, the US countered those offers with the going price of raw uranium metal, which was about six dollars per pound. 13 Since the Allied Control Commission prohibited German citizens from possessing any amount of uranium, the black-market dealers assumed the cubes were a rare commodity and took considerable personal risk in attempting to sell them. The cubes fueled a black market in uranium throughout Eastern Europe after the war, peddled by what intelligence officer Joseph Chase described in a 16 March 1951 communiqué as a “ghostly gang” of profit seekers. With each successive experiment, the slope of the line increases, showing that the German scientists were approaching, but never achieved, criticality. ![]() As the amount of uranium was increased with each experiment, the slope of the line will approach infinity. (Image courtesy of the AIP Niels Bohr Library and Archives.) Each line plots the subcritical multiplication factor for an experiment. The graph shown here was obtained by an Allied reconnaissance mission during World War II and shows the criticality calculations for each of the reactor experiments. Placing a neutron-generating radium–beryllium mixture at the center of their pile as the initial source of neutrons, the German scientists measured the neutron population near the periphery as they added increasing amounts of natural uranium, which contains approximately 0.7% fissile 235U. In their experiments, the German scientists were empirically searching for the optimal geometry and minimum quantity of uranium needed. Steady-state operation of a nuclear reactor at k eff = 1 requires continuous fine-tuning of the pile’s geometry, typically by inserting or withdrawing one or more of the neutron-absorbing control rods, analogous to pressing on a car’s accelerator or brake to maintain a constant speed. Finally, in a supercritical status, where k eff is greater than 1, an increasing number of neutrons is produced each cycle. In a critical pile, where k eff = 1, the population of neutrons remains constant from generation to generation. For a subcritical pile, k eff is less than 1, the number of neutrons lost is greater than the number produced by fission, and the neutron population decreases with time. The self-sustaining status of a pile can then be placed into three categories. To quantify that condition, physicists talk in terms of an overall neutron multiplication factor, k eff, which equals the number of neutrons in generation n + 1 divided by the number of neutrons in generation n. In practice, there are many other opportunities for neutrons to go unused, but with careful design, the neutron losses are surmountable, and self-sustaining reactors are possible. In a simplified model of the assembled pile of fissile uranium, two competing effects determine the neutron’s outcome: The neutrons released from fission can lead to new fission or can escape the surface of the uranium pile and not participate in further fission. The process is the fundamental operating principal of a critical nuclear reactor.Ī minimum quantity, the so-called critical mass, of fissile material is required to create a self-sustaining chain reaction. The self-sustaining cycle perpetuates until all the fissile material is consumed. Since a single neutron leads to fission that produces more neutrons, the newly generated neutrons generate subsequent fission reactions, producing the famous nuclear chain reaction. Along with those fission fragments, two or three neutrons are also ejected, and a large amount of energy is released that can be used for power generation. Because the nuclei of those fissile isotopes lie close to the edge of stability, the addition of a single neutron splits the nucleus into two smaller pieces called fission fragments, which are lighter elements such as barium and cesium. ![]() However, choosing the appropriate number and orientation of a reactor’s various components requires a detailed understanding of nuclear fission physics.įission readily occurs in a few isotopes of certain elements–for instance, uranium-235 and plutonium-239–when a neutron is absorbed into the nucleus. Once it is assembled, the only moving parts required are control rods that are moved in and out of the core to modulate its power output. A nuclear reactor is at once both simple and complex. ![]()
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