in August 1939, when the three were called to military training. The second Uranverein began after the Heereswaffenamt (Army Ordnance Office) squeezed out the Reichsforschungsrat (Reich Research Council) of the Reichserziehungsministerium and started the formal German nuclear weapon project. The first meeting was held on 16 September 1939. A second meeting soon thereafter included Klaus Clusius, Carl Friedrich von Weizsäcker, Werner Heisenberg and Robert Döpel, his counterpart as an experimental physicist at the University of Leipzig. Here, Heisenberg was the director of the Department of Theoretical Physics until 1942. In August 1940, Döpel showed the utility of using heavy water as a moderator in a research nuclear reactor (uranmaschine) together with his wife Klara. She had studied law and worked as a lawyer until 1933, when the Nazi regime prevailed. In 1934, she married Robert Döpel and changed her area of studies to physics, and she worked with him in Leipzig without wages. They conducted experiments with a spherical geometry (hollow spheres) of uranium surrounded by heavy water. Trial L-I was done in August 1940, and L-II was conducted six months later. Results from trial L-IV, in the first half of 1942, indicated that the spherical geometry, with five metric tons of heavy water and 10 metric tons of metallic uranium, could sustain a fission reaction. So, "the Germans were the first physicists in the world, with their Leipzig pile L-IV, to achieve positive neutron production." The results were set forth in an article by Döpel, Döpel's wife, and W. Heisenberg. The article was published at first in the Kernphysikalische Forschungsberichte (Research Reports in Nuclear Physics), a classified internal reporting vehicle of the Uranverein. 1942 was the year in which supervision of the Uranverein was transferred from the Heereswaffenamt to the Reichsforschungsrat. In June 1942, Döpel's uranmaschine was destroyed by a low-speed detonation induced by hydrogen formation. This was the first in a series of accidents that destroyed nuclear energy assemblies due to wrong hydrogen handling. Already afore, a shift of the main works of Heisenberg towards the Kaiser-Wilhelm-Institut für Physik (after World War II the Max Planck Institute for Physics) in Berlin was decided. The Döpels didn't follow him despite his wishes, and they retired thereby from the uranium project. This finished the work on this topic at Leipzig. In a letter written in December 1943, Döpel recounted that air raids had destroyed 75% of Leipzig, including his institute.