In the late 1930s and early 1940s, both the United States and Germany undertook projects to see if an atomic bomb was feasible. Their first step was to construct a reactor in which controlled fission (splitting of an atom) could be sustained. The United States effort was located at the University of Chicago, led by the physicist Enrico Fermi. The German effort was led by the Nobel prize winning physicist Werner Heisenberg and Army physicist Kurt Diner. Both experiments utilized natural uranium as a source of radioactivity.
Natural uranium is a metal which can be refined and machined into cubes or other shapes. It contains two types of uranium, called isotopes. U235 is strongly radioactive and constitutes less than one percent of natural uranium. U238 is weakly radioactive and constitutes over 99 percent. The combination in nature is therefore weakly radioactive and not hazardous to humans. Like other metals such as iron, uranium is present in natural waters in trace amounts and adsorbs onto the surfaces of clay. This makes layers of clay (called shales) in the earth more radioactive than layers of sandstone or limestone, a feature which is the basis of the gamma ray tool used in logging oil wells. Shales can be distinguished from sandstone or limestone layers, allowing a geologic understanding of the subsurface. The radioactivity can also bae measured on shales and other rocks exposed on the earth's surface with a Geiger Counter.
Radioactivity measured by the Gamma Ray log in an oil well. Intervals with higher radioactivity (to the right) are shales. Intervals with lower radioactivity (to the left) are sandstones.
Atoms of natural uranium emit a particle called a neutron, which if it impacts another uranium atom can cause it to split and in turn emit more neutrons. Although the amount of neutrons emitted by natural uranium is lower than that of enriched uranium or plutonium, natural uranium does emit enough neutrons to produce a sustained reaction if present in large amounts. But the neutrons emitted travel very fast, and simple bounce off other uranium atoms without causing them to split. The neutrons need to be slowed down to produce a sustained fission reaction. This can be done by placing the uranium in a substance that will slow the neutrons. One substance is graphite, which is pure carbon. Another is called heavy water, which is water with hydrogen atom that have twice the mass of normal hydrogen.
The United States team utilized graphite and natural uranium to construct a reactor at the University of Chicago. On December 2, 1942, they produced the world's first nuclear chain reaction. The site today is a plaza on the campus of the university commemorating the achievement, which led to the successful development of the atomic bomb. It is interesting that no special precautions were taken to mitigate the potential for high levels of radioactivity.
The German team utilized heavy water and natural uranium, suspending over 600 two inch cubes of uranium on a helical structure in a vat of heavy water. The experiment was unsuccessful, but physicists today calculate that if they had been able to obtain more uranium, it might have worked. The effort was shut down by the German government, and the cubes were seized by the Allies when they invaded Germany at the end of the war.
A 2019 article by Geoff Brumfiel of National Public Radio describes the mysterious appearance of one of the cubes in the United States in 2013. Over six hundred of the cubes are still unaccounted for. They may have even seized by the Americans and not inventoried in the hectic days at the end of the war. Or they may be somewhere else in the world.
My newest novel, The Orinoco Uranium, hypothesizes that the cubes were smuggled to South America at the end of the war by a renegade German physicist, intended as a down payment for a new life in Argentina after the German defeat.