The 24th April 1943 saw an important milestone in the the “Manhattan Project”. “Manhattan District” had originally just been the U.S Army code name for their involvement in aspects of the project. This was the name that was gradually adopted to refer to the whole official programme for the “Development of Substitute Materials”, the atomic weapons programme, designated “Tube Alloys” by the British.
From very modest beginnings that had evolved from the the first theoretical understanding of the power of atomic energy the programme, which had only begun in 1941, had developed into a major industry. During the course of the war it would cost $2 billion – around $26 billion in 2013 values – and employed over 130,000 people in a wide variety of sites around the United States. Very few of these people knew what they were actually working on.
Only a small group of individuals at the top had a complete overview of the work they were doing. Between 15th-24th April they met in conference to discuss the outcomes of the different strands of research. They were now in a position to understand how much nuclear material they could produce and what the destructive power of a potential weapon was likely to be. The testing of a real atomic weapon device or “Gadget” still remained a long way off:
OUTLINE OF PRESENT KNOWLEDGE
Energy Release: The destructive effect of the gadget is due to radiative effects and the shock wave generated by the explosion. . The shock wave effect seems to extend over the biggest area and would be, therefore, most important. The area devastated by the shock wave is proportional to the 2/3 power of the energy release and may be simply calculated by comparing the energy release with that of TNT. If the reaction would go to completion, then 50 kg of [isotope] 25 would be equivalent to 10 tons of TNT. Actually it is very difficult to obtain a large percentage of the potential energy release.
Detonation: The second major difficulty facing us is connected with the question of detonation. . It is important that no neutron should start a premature chain reaction. . . Possible sources of neutrons are 1) Cosmic ray neutrons . . . and 2) Spontaneous fission neutrons. . .
EXPECTED DAMAGE OF THE GADGET
Comparison with TNT: The most striking difference between the gadget and a TNT charge is in the temperatures generated. The latter yields temperatures of a few thousand degrees whereas the former pushes the temperature as high as [tens of millions of degrees]. . . .
The actual damage depends much on the objective. Houses begin to be smashed under shocks of 1/10 to 1/5 of an atmosphere. For objects such as steel supported buildings and machinery, greater pressures are required and the duration of the shock is very important. If the duration of the pressure pulse is smaller than the natural vibration period of the structure, the integral of the pressure over the duration T of the impulse is significant for the damage. If the pulse lasts for several vibration periods. the peak pressure is the important quantity. . . .
Other Damage: The neutrons emitted from the gadget will diffuse through the air over a distance of 1 to 2 km, nearly independent of the energy release. Over this region, their intensity will be sufficient to kill a person,
The effect of the radioactive fission products depends entirely on the distance to which they are carried by the wind. If 1 kg of fission products is distributed uniformly over an area of about 100 square miles, the radioactivity during the first day will represent a lethal dose (=500 R units): after a few days, only about 10 R units per day are emitted, If the material is more widely distributed by the wind, the effects of the radioactivity will be relatively minor.
See The Atomic Archive for original documents.