In May 1941, Zeldovich worked with Khariton in constructing a theory, on the kinetics of nuclear reactions in the presence of the critical conditions. The work of Khariton and Zeldovich was extended into theories of ignition, combustion and detonation; these accounted for features which had not previously been correctly predicted, observed, nor explained. The modern theory of detonation accordingly is called the Zeldovich-von Neumann-Dohring, or ZND, theory, and its development involved tedious fast neutron calculations; this work had been delayed, due to the German invasion of the Soviet Union, which obstructed progress on findings that in June 1941 would be de-classified. In 1942, Zeldovich was relocated to Kazan, and tasked by the People's Commissariat of Munitions to carry out work on conventional gun powders to be supplied to the Soviet Army, while Khariton was asked to design the new types of conventional weaponry.