Nuclear security

Tracking the journey of a uranium cube

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 (see the box on page 41 for a description of the physics of nuclear reactors). 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.2

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.