Heisenberg's main focus for the Nazi war effort was to try and create a sustainable nuclear fission, or what we might now term a nuclear reactor. The purpose of this research was to create Plutonium. Various German physicists referred to a nuclear reactor as an "engine" after the war, in Farm Hall transcripts.
Dr Fritz Houtermanns, who was trained before the war in nuclear physics by the Soviets, proposed along with Austrian physicist Prof Josef Schintlemeister, as early as 1941 that Germany develop Plutonium as a nuclear explosive.
The difficulty was that Plutonium was then just a theoretical element which could only be produced by sustained nuclear reaction, such as in a nuclear reactor. Indeed the German name for the isotopic element today called Plutonium, was eka-Rhenium.
Possibly unknown to Heisenberg The German Army was pursuing an alternate project to enrich Uranium for an atomic bomb with Uranium as the explosive under Dr Paul Harteck.
British efforts to destroy Norwegian heavy water production at Voermark had no appreciable effect on Heisenberg's efforts since heavy water was also produced at the Beck Plant in Germany. Also Heisenberg managed to create one reactor design with a graphite pile, very similar to the pile built by Enrico Fermi for the US Manhatten Project.
Heisenberg it has often been said was a brilliant theorist but poor at experiments.
The above commentry is not based upon hearsay or Wikipedia, but rather from over three years of research. Principal sources include the following publications:
The Farm Hall Transcripts, by Charles Franks;
Hitler’s Uranium Club: The secret recordings at Farm Hall, by Jeremy Bernstein;
Overheard, by David Irving;
Virus House, by David Irving;
The Rocket and the Reich, by Michael J. Neufeld.