Operation Castle and its Aftermath

Jose Garcia
March 22, 2012

Submitted as coursework for PH241, Stanford University, Winter 2012

Nuclear Weapons Testing

Nuclear weapons testing began with the Trinity project in 1945 when the United States fired a 20Kt fission blast over the New Mexico desert. [1] Shortly after, these weapons were used in combat on Hiroshima and Nagasaki ending the Second World War. After this initial effort testing in the United States progressed slowly, only 5 tests were carried out between 1946 and 1951, but in 1949 the Soviet Union conducted its own tests. Amidst the Cold War, this threat prompted the United States to increase its efforts in developing nuclear weapons. By 1963, 230 total nuclear weapons had been fired worldwide throughout 19 testing operations which required an estimated 200,000 personnel. [2]

At the height of the United States atmospheric nuclear testing program, Operation Castle, a series of high yield nuclear weapons tests was conducted in 1954. [2]

Operation Castle

Prior to this series, the United States had restricted most of its nuclear weapons testing to the Nevada Test Site. However, with the detonation of Ivy Mike, the first detonation in the Megaton range conducted by the United States, serious questions were raised as to the consequences of these detonations. The Castle series was conducted with the task of obtaining data on the properties of nuclear weapons in the Megaton range. Data on the shock generated, blast radius, and fallout produced were of particular interest since there was little information on the shock propagation, and potential damage, of these weapons in the upper atmosphere and at different water depths. [4]

Operation Castle was conducted over a span of two and a half months in Bikini Atoll and consisted of 6 detonations; Bravo, Romeo, Koon, Union, Yankee, and Nectar.

Castle Bravo

The first detonation in the series was conducted on February 28, 1954 on a coral reef on Bikini Atoll.[3] Early estimates for the blast predicted a yield of approximately 6Mt, however, it exceeded these expectations yielding a fission blast of 15Mt, now the most powerful nuclear weapons test in United States history. [5] The cloud of radioactive debris produced by this explosion reached 114,000 feet high with the base of the mushroom at approximately 60,000 feet. The explosion was so large that measurement devices in the area were destroyed and fallout persisted through the following 5 detonations. [4] This unforeseen circumstance inhibited the ability of personnel to accurately gauge the amount of fallout from each individual blast.

Castle Romeo

The second detonation in the series was the third most powerful with a total yield of 11Mt. Romeo was detonated from a barge off Bikini Atoll on March 26, 1954. [3] Despite being nearly a full month after the initial blast in the series, enough fallout from Bravo still remained in the area and made tracking of Romeo's radioactive cloud extremely difficult. The discrepancies in the progression of the radioactive fallout could not be accurately assigned to either unexpectedly high winds or as part of Bravo's cloud.

Castle Koon

Testing resumed on April 6, 1954, a mere 11 days after the second shot, with a 110Kt detonation from a barge off the atoll. [3] This was by far the weakest blast in the series and can be considered a dud since it vastly underperformed expectations; Operation Castle was to test nuclear weapons in the Megaton range. Conditions were cloudy during this test which made tracking of the radioactive cloud difficult. [3] This compounded with the remaining debris from Romeo made fallout detection for Koon impossible. In addition to these issues, the low yield from Koon produced no usable film footage and prompted the cancellation of a potential 7th shot, Echo, which used a similar design. [4]

Castle Union

Union was another barge shot, fired on April 25, 1954, which yielded a blast of 7Mt. Despite this high yield, however, Romeo's fallout continued to cloud results, making it difficult to distinguish debris from this test and the last high yield test. Much like Koon, no fallout from this detonation was accurately detected. [3]

Castle Yankee

The fifth test in the series, on May 4, 1954, was the second most powerful detonation in the operation with a yield of 13.5Mt, which exceeded even estimates adjusted after Bravo. Conditions during this test were the windiest with winds of 65 knots at an altitude of 40,000 feet. This caused a fast spread of the fallout from this detonation with reports of it reaching Mexico City 4 days after the explosion and relatively high sustained levels of fallout over the western United States for a week following the blast. [3]

Castle Nectar

Aside from Bravo, Nectar was the only detonation on land during this series. Nectar was detonated on Eniwetok Island on May 13, 1954 with a yield of just 1.7Mt. Much like Romeo, a significant amount of fallout was still present in the area, mostly from Yankee which was fired 9 days prior, leading to difficulties in measuring the fallout from this blast accurately. [3]


The fallout generated from Operation Castle exceeded previous estimates which had been based on the earlier Ivy Mike test. [5] The unforeseen amount of debris made it difficult to accurately trace fallout from each individual blast. Additionally, Bravo's large blast made the situation worse, when the detonation destroyed some of the measuring equipment in nearby Tare Island. [4] Thus, the most reliable fallout measurements from this test series are from Operation Castle as a whole.

Before the testing began, Bikini Atoll and the populations of other nearby territories were evacuated to protect the population from radioactive fallout. However, the yield by the initial blast, Bravo, was more than double the predicted amount, complicating the operation. Areas such as the Marshall Islands, which had been deemed outside of the danger area, were later found to be affected by radiation from these tests. [5] The Bikinians did not suffer any injuries from radiation but did suffer psychological damage after losing their home island. Changes in their diet also led to high rates of diabetes and their constant relocation led to them being deemed "nuclear nomads." [5]

Regardless of these complications, the operation was deemed a success. The United States recovered valuable data about Megaton bombs and also concluded that atmospheric testing could in fact pose serious health issues to populations exposed to fallout. Operation Castle was later used as a model for setting both safety and measurement standards when conducting atmospheric nuclear weapons testing.

© Jose Garcia. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] H. L. Beck and B. G. Bennett, "Historical Overview of Atmospheric Nuclear Weapons Testing and Estimates of Fallout in the Continental United States," Health Phys. 82, 591 (2002).

[2] The Five Series Study: Mortality of Military Participants in U.S. Nuclear Weapons Tests (National Academy Press, 2000), p. 13.

[3] R. J. List, "World-wide Fallout from Operation Castle," U.S. Defense Nuclear Agency, "NYO-4645 (EX)," 31 Aug 84.

[4] K. D. Coleman et al., "Operation Castle Summary Report of the Commander, Task Unit 13 - Military Effects, Programs 1-9" U.S. Nuclear Defense Agency, WT-934 (EX), 15 May 81.

[5] E. P. Cronkite, R. A. Conrad and V. P. Bond, "Historical Events Associated with Fallout from Bravo Shot - Operation Castle and 25 Y of Medical Findings," Health Phys. 73, 176 (1997).