Inter-Office Memorandum, V[ictor] Weisskopf, P[aul] Aebersold, L[ouis] H. Hempelmann, F[rederick] Reines to G[eorge] Kistiakowsky, “Calculated Biological Effects of Atomic Explosion in Hiroshima and Nagasaki,” with table attached, 1 September 1945, Secret

Date

Sep 1, 1945

Description

While declassified through several reviews during the 1970s, this memorandum for Gen. Groves on the “biological effects” of the atomic bombings in Japan document was only recently disclosed through a FOIA request.

The memo that Kistiakowsky requested but was hesitant to send to Groves -- prepared by Victor Weisskopf, Paul Aebersold, Louis H. Hempelmann, and Frederick Reines -- did not mention the news reports from Japan about burn injuries and lingering deaths and suffering (thus avoiding any risk of being seen as sympathetic to the Japanese), but the authors estimated that radiation exposure was a probable cause of some fatalities, essentially confirming those accounts.

The four Los Alamos scientists and medical doctors who prepared the memorandum explained how an atomic explosion would have deadly “biological effects,” not only from the blast but also from gamma radiation and thermal energy, with other possible effects, such as those caused by radioactive “dust particles,” not calculable. They presented their findings in a table attached to an explanatory memorandum. For many Los Alamos scientists this memorandum may have been a reminder of what they already knew.

The writers did not explain their methodology but may have been generalizing from the effects of weapons with an explosive yield in the range of 10 kilotons of T.N.T., which was Enrico Fermi’s initial estimate for the July 16, 1945, Trinity test. Whatever their methodology was, the table developed by the authors showed the impact on the ground metrically in terms of the radius and the distance from ground zero. The avoidance of references to yield estimates may have kept the classification level down.

The authors calculated the deadly impact of the blast itself (“flying debris, falling buildings, violent displacement, etc.”) and the extreme heat, which were the two primary effects contemplated by Los Alamos scientists who developed the bomb.[35] Blast effects were “lethal” within the radius of a half mile to a mile and even further, projecting overpressures of 27 pounds per square inch at 800 meters (about one-half mile) and 23 p.s.i. at 1000 meters (at 0.62 miles). The blast would not kill everyone within the range: “Freak survivals of individuals within this area would certainly occur,” according to the report, such as people who were inside hardened structures.

But those who lived through the initial blast would be exposed to “gamma radiation emitted within a few seconds after the explosion,” according to the report, which estimated that levels of 500 to 900 roentgen, found 800 to 900 meters from ground zero, would be lethal. “Freak survivors of the blast within this area would be killed by radiation,” the scientists said, although “death … might be delayed for a period of weeks.” Similarly, a 1945 U.S. Navy report found that people who were inside Hiroshima’s Bankers Club building survived the initial blast but “died later from radiation reaction, due in large part to scattered rays of secondary type.” As the Navy report explained, “This is at first difficult to understand, until one remembers that radiation entering through a window or other opening will strike floor[s] and wells and induce much secondary radiation.”

“Severe thermal burns (third degree) of the exposed skin” were expected within a mile of the blast site, although clothing or even “light shelter” would offer “considerable protection” from them. The “short intense exposure” to thermal radiation meant that the burns, later called “flash burns” were likely to be “even more serious than the most severe sunburns.” The authors used calories for measuring the levels of heat and light that people would experience at various distances. Because 8 calories/CM²was enough to cause third-degree burns, people exposed to 11 calories/CM² at 1400 meters would suffer burns that penetrated one millimeter of skin (the thickness of a credit card).

One incalculable effect was exposure to “radioactive material deposited on the ground.” While the memo said it would probably be “insignificant,” the authors noted the possibility of “unusual meteorological phenomena resulting in local precipitation or deposition of active material on falling dust particles.” Even Oppenheimer, in a statement cited by Dr. Robert Stone [See Document 13], had denied the possibility of radiation on the ground in Hiroshima or Nagasaki, but such effects would continue to be discussed in the media and in classified reports.[36]

Other “biological effects” of the bombings, such as “suffocation” or “burning to death,” could not be predicted, according to the memo. With so much emphasis placed on the blast, radiation, and thermal effects of the bombs, there is little to suggest that anyone predicted that mass fires would cause many deaths in Hiroshima and Nagasaki.

With U.S. scientists and medical doctors soon in their way to Japan to study the impact of the bombings, more information and analysis on the impact of radiation would become available, but the results of those investigations would be classified for years, just like this preliminary assessment.

Source

National Security Archive Freedom of Information Act request and appeal to National Nuclear Security Administration; document found in archives of Los Alamos National Laboratory

Document published in following posting(s):

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

Aug 7, 2023