In Captain America: The First Avenger, the quasi-Nazi villain Red Skull wields a cosmic cube, and I must admit that’s what came to mind when I read about the two-inch uranium cubes at the center of Nazi Germany’s nuclear program:
Several German physicists were involved in that research program; perhaps the most widely recognized was Werner Heisenberg.
Rather than working together under central leadership the way the Manhattan Project scientists eventually would, the German nuclear researchers were divided into three groups that each ran a separate series of experiments. Each was code-named after the city in which the experiments took place: Berlin (B), Gottow (G), and Leipzig (L). Although the Germans began their work nearly two years before serious US efforts began, their progress toward creating a sustained nuclear reactor was extremely slow. The reasons for the delay were varied and complex and included fierce competition over finite resources, bitter interpersonal rivalries, and ineffectual scientific management.
In the winter of 1944, as the Allies began their invasion of Germany, the German nuclear researchers were trying desperately to build a reactor that could achieve criticality. Unaware of the immense progress the Manhattan Project had made, the Germans hoped that though they were almost certainly going to lose the war, they would be able to salvage the reputation of their physics community by being the first to achieve a self-sustaining nuclear reactor.
In holding out that hope, officials moved the Berlin reactor experiments headed by Heisenberg south ahead of the Allied invasion. They eventually landed in a cave underneath a castle, shown in figure 1, in the small town of Haigerloch in southwest Germany.
In that cave laboratory Heisenberg’s team built their last experiment: B-VIII, the eighth experiment of the Berlin-based group. Heisenberg described the setup of the reactor in his 1953 book Nuclear Physics. The experimental nuclear reactor comprised 664 uranium cubes, each weighing about five pounds. Aircraft cable was used to string the cubes together in long chains hanging from a lid, as shown in figure 2. The ominous uranium chandelier was submerged in a tank of heavy water surrounded by an annular wall of graphite. That configuration was the best design the German program had achieved thus far, but it was not sufficient to achieve a self-sustaining, critical reactor.
In 1944, as Allied forces began moving into German-occupied territory, Leslie Groves, commander of the Manhattan Project, ordered a covert mission code-named Alsos (Greek word for “groves”) to take a small number of military personnel and scientists to the front lines in Europe to gather information on the state of the German scientific program. The mission broadly aimed to gather information and potentially capture data and instrumentation from all scientific disciplines from microscopy to aeronautics. The most pressing task was to learn how far German physicists had gotten in their study of nuclear reactions. The initial leg of the Alsos mission began in Italy and moved to Germany as the Allied military forces swept south.6 Among the men involved in the mission was Samuel Goudsmit. After the war, he went on to be the American Physical Society’s first editor-in-chief and the founder of Physical Review Letters.
As the Allies closed in on southern Germany, Heisenberg’s scientists quickly disassembled B-VIII. The uranium cubes were buried in a nearby field, the heavy water was hidden in barrels, and some of the more significant documentation was hidden in a latrine. (Goudsmit had the dubious honor of retrieving those documents.) When the Alsos team arrived in Haigerloch in late April 1945, the scientists working on the experiment were arrested and interrogated to reveal the location of the reactor materials. Heisenberg had escaped earlier by absconding east on a bicycle under cover of night with uranium cubes in his backpack.
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Many scholars have long thought that the German scientists could not have possibly created a working nuclear reactor because they did not have enough uranium to make the B-VIII reactor work. In Heisenberg’s own words, “The apparatus was still a little too small to sustain a fission reaction independently, but a slight increase in its size would have been sufficient to start off the process of energy production.” That statement was recently confirmed using Monte Carlo N-particle modeling of the B-VIII reactor core. The model showed that the rough analyses completed by the Germans in 1945 were correct: The reactor core as designed would not have been able to achieve a self-sustaining nuclear chain reaction given the amount of uranium and its configuration. But the design might have worked if the Germans had put 50% more uranium cubes in the core.
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