From his position in the RFR, he initiated the first meeting of the Uranium Club in early 1939, the precursor to the Army Ordnance Office (HWA) German nuclear energy project, which began in September of that year.

Esau was a member of the RFR from its inception, and he was head of the physics section (Fachspatenleiter für Physik), which included mathematics, astronomy, and meteorology.

From this position in the RFR, he would play major roles in the German nuclear energy project, sometimes also referred to as the Uranverein (Uranium Club).

Additionally, for this same period, Esau was president of the Deutsche Gemeinschaft zur Erhaltung und Förderrung der Forschung (German Association for the Support and Advancement of Scientific Research), also known for short as the Deutsche Forschungs-Gemeinschaft (DFG), which had before 1937 been known as the Notgemeinschaft der Deutschen Wissenschaft (NG; Emergency Association of German Science).

Paul Harteck was director of the physical chemistry department at the University of Hamburg and an advisor to the Heereswaffenamt (HWA, Army Ordnance Office).

On 24 April 1939, along with his teaching assistant Wilhelm Groth, Harteck made contact with the Reichskriegsministerium (RKM, Reich Ministry of War) to alert them to the potential of military applications of nuclear chain reactions.

Two days earlier, on 22 April 1939, after hearing a colloquium paper by Wilhelm Hanle on the use of uranium fission in a Uranmaschine (uranium machine, i.e., nuclear reactor), Georg Joos, along with Hanle, notified Wilhelm Dames, at the Reichserziehungsministerium (REM, Reich Ministry of Education), of potential military applications of nuclear energy.

On 29 April, a group, organized by Esau, met at the REM to discuss the potential of a sustained nuclear chain reaction.

[8][9][10][11] The second Uranverein began after the Heereswaffenamt squeezed out the Reichsforschungsrat of the Reichserziehungsministerium and started the formal German nuclear energy project under military auspices.

The invitees included Walther Bothe, Siegfried Flügge, Hans Geiger, Otto Hahn, Paul Harteck, Gerhard Hoffmann, Josef Mattauch, and Georg Stetter.

A second meeting was held soon thereafter and included Klaus Clusius, Robert Döpel, Werner Heisenberg, and Carl Friedrich von Weizsäcker.

[10][11][12] When it was apparent that the nuclear energy project would not make a decisive contribution to ending the war effort in the near term, control of the KWIP was returned in January 1942 to its umbrella organization, the Kaiser-Wilhelm Gesellschaft (KWG, Kaiser Wilhelm Society, after World War II the Max-Planck Gesellschaft), and HWA control of the project was relinquished to the RFR in July 1942.

As of 1 January 1944, Esau, replacing Johannes Plendl, became the plenipotentiary of the high-frequency engineering and radar working group (A. G. Hochfrequenzphysik).

Even after Esau left his position as plenipotentiary for nuclear physics and head of the physics section at the RFR at the end of 1942, he continued to have significant authority and influence as president of the Physikalisch-Technische Reichsanstalt, as is attested to by the fact that he was able to continue research efforts for the Urainverein under the highest priority level for urgent development projects (Dringlichkeitsentwicklung, DE).

[3][20][21] After the war, the Americans turned Esau over to the Dutch to stand trial for his involvement in the plunder of research facilities of the electronics firm Philips.

[22] Thanks to support from Leo Brandt, a science policy-maker of North Rhine-Westphalia, Esau was able to establish himself back into the German scientific community.

The award was not made, due to the intervention of the physicist Max von Laue, who pointed out Esau's prominent role as a chief representative of National Socialism.