Dr. W[illiam] C. Penney, C.S.A.R. [Chief Superintendent of Armament Research], “A Report on the Pressure Wave Caused by the Atomic Bomb Explosion in Hiroshima and Nagasaki,” 23 January 1946, Secret, excerpt, with enclosing memorandum from D.G. Sturges to J.A. Derry, 6

Date

Jan 23, 1946

Description

A leading member of the British mission to the Manhattan Project, physicist and mathematician William Penney conducted research that helped establish the optimum height of burst for the detonations above Hiroshima and Nagasaki to maximize their destructiveness. As a member of the team of scientists and medical doctors who traveled to the two cities during September-October 1945, Penney collected evidence on the physical destruction cause by the bombs, which he analyzed in this report.

Through visual inspection of flagpoles, shrines, cans, glass panels, wooden floors, and other things damaged in the blasts, and noting the distance of each from ground zero, Penney estimated the peak pressure, in pounds per square inch (p.s.i.), experienced by various objects and structures. Summarizing his results on pages 59-60, Penney estimated a peak pressure of 100 p.s.i. in Nagasaki based on damage to an earthenware drainpipe found 1200 feet from ground zero. For Hiroshima, Penney estimated a peak pressure of 35 p.s.i. at ground zero based on damage to the door of a safe. Penney’s report was probably the first systematic look at the problem of peak pressure, which would be a subject of continuing interest for war planners and civil defense specialists.

Penney’s summary of his major conclusions tacitly credited the calculations of Los Alamos scientists: Not only did “the bombs [perform] exactly according to design in their height of burst,” they were “placed in such positions that they could not have done more damage by any alternative bursting point.” Thus, the “extent of the blast damage was exactly that predicted.” In other words, the point of aerial detonation was to maximize damage, not to reduce radioactive fallout, as some later claimed.[49]

Penney explained that his report could guide any future use of atomic weapons: “the information collected would enable reasonably accurate predictions to be made of the blast damage likely to be caused in any city where an atomic explosion was conceivable.”

On destruction by fire, Penney observed that it was “tremendous” in both cities, but he did not attach any “special significance” to it, because he believed that it was a “property more of the cities themselves than of the bomb.” Writing that the explosions did not start any fires “instantly,” Penney said that “straw and many grass mats which were hanging outside to air, were ignited immediately and no doubt some fires were started in this way.” A “more likely cause” of fires, in his view, were “charcoal braziers, one of which was to be found in nearly every house.” He further added that the Japanese observed “many peculiar effects due to ‘fire winds’” that were not accurately recorded.

Commenting on radioactivity in a brief section about the “morale” effect of the bombings, Penney observed that the “danger of radioactive contamination on the ground terrified the whole population and had resulted in only spasmodic elements at clearing debris.”

Among Penney’s “Main Results” were his estimates of the explosive yields for the Hiroshima and Nagasaki detonations. Penney estimated 5 kilotons (kt) of T.N.T. for Hiroshima and 20 kt for Nagasaki. The latter estimate would turn out to be roughly accurate, but 5 kt for Hiroshima was quite low. Years later, Penney and British colleagues produced an estimate of 12 kt for Hiroshima. In 1946, when Penney prepared this report, other Los Alamos scientists and officials were trying to establish yield estimates, but this was difficult for lack of precise data, such as the Enola Gay’s “true altitude” when it dropped the weapon.[50] During the 1980s, Los Alamos published estimates of 15 kt for Hiroshima and 21 kt for Nagasaki, acknowledging that “the outside limits of uncertainties in these values are believed to be 20 percent for Hiroshima and 10 percent for Nagasaki.”

Source

National Archives, RG 77, MED Records, General Administrative Files: Reports Pertaining to the Effects of the Atomic Bomb, 1945–1946, box 91

Document published in following posting(s):

78th Anniversary of Hiroshima and Nagasaki Bombings: Revisiting the Record

Aug 7, 2023