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What was the yield of the Hiroshima bomb?

[On the moderated World War II newsgroup, the question was asked: "I have noticed in my readings that there is a very large variation in the stated yield of the Hiroshima bomb. Here are the most common that I have seen: 10.4 KT, 12 KT, 12.5 KT (often quoted), 13.5 KT, 15 KT (some online DBs), 20 KT, and More than 20 KT (some news accounts)." This was the answer that came back, from the invaluable contributer Carey Sublette. -- Dan Ford]

The most authoritative report on this is the Los Alamos National Laboratory report number LA-8819, The yields of the Hiroshima and Nagasaki nuclear explosions. Author: Malik, J. S." Here are the abstract and introduction to John Malik's report:

ABSTRACT

A deterministic estimate of the nuclear radiation fields from the Hiroshima and Nagasaki nuclear weapon explosions requires the yields of these explosions. The yield of the Nagasaki explosion is rather well established by both fireball and radiochemical data from other tests as 21 kt [one kiloton equals the explosive power of 1,000 tons of TNT]. There are no equivalent data for the Hiroshima explosion. Equating thermal radiation and blast effects observed at the two cities subsequent to the explosions gives a yield of about 15 kt [at Hiroshima]. The pressure-vs-time data, obtained by dropped, parachute-retarded canisters and reevaluated using 2-D hydrodynamic calculations, give a yield between 16 and 17 kt. Scaling the gamma-ray dose data and calculations gives a yield of about 15 kt. Sulfur neutron activation data give a yield of about 15 kt. The current best estimates for the yield of these explosions are the following:

Hiroshima 15 kt

Nagasaki 21 kt

The outside limits of uncertainties in these values are believed to be 20 percent for Hiroshima and 10 percent for Nagasaki. [In other words, the Hiroshima bomb has an outside range of 12-18 KT, and the Nagasaki bomb an outside range of 18.9-23.1 KT.]

I. INTRODUCTION

The Manhattan Project culminated in the design and fabrication of two types of nuclear weapons--Little Boy and Fat Man. The first type was exploded over Hiroshima, the second over Nagasaki. Estimates of radiation exposures depend in part on explosive yields, and much of the evaluation of radiation effects upon man depends on data from the Hiroshima and Nagasaki explosions. The yield of the Fat Man has been determined rather well, being given variously from 19-24 kt. (Present official yield is 23 kt.1) Estimates for the Little Boy have ranged from 6-23 kt. (The current official yield is 13 kt.1) The data from which estimates may be made are fragmentary, and the parameters needed for evaluation have been either missing, inconsistent, or erroneous. Part of the problem arises from President Truman's edict that the yields of both explosions were 20 000 tons of TNT; another part of the problem arises from the inadequate and faulty documentation of the combat missions by the Army Air Corps 509th Composite Group and the US Air Force historians. Many well-researched books ... are useful in resolving inconsistencies. There is also documentation by the US Strategic the Bombing Survey (USSBS), Manhattan Engineering District, and the Japanese. These sources are not particularly useful for yield evaluation; however, they contain clues.