Operation Castle - Test Execution

Test Execution

The most notable event of Operation Castle was the Castle Bravo test. The dry fuel for Bravo was 40% Li-6 and 60% Li-7. Only the Li-6 was expected to breed tritium for the deuterium-tritium fusion reaction; the Li-7 was expected to be inert. Yet J. Carson Mark, head of the Los Alamos Theoretical Design Division, had speculated that Bravo could "go big", estimating that the device could produce an explosive yield as much as 20% more than had been originally calculated. The Li-7 component turned out to be an excellent source of tritium through a previously unquantified reaction. In practice, Bravo exceeded expectations by 250%, yielding 15 Mt -- 1,000 times more powerful than the Little Boy weapon used on Hiroshima. Castle Bravo remains to this day the largest atmospheric detonation ever conducted by the United States, and the fifth largest detonation in the world.

Because Castle Bravo greatly exceeded its expected yield, JTF-7 was caught unprepared. Much of the permanent infrastructure on Bikini Atoll was heavily damaged. The intense thermal flash ignited a fire at a distance of 20 nautical miles (37 km) on the island of Eneu (base island of Bikini Atoll). The ensuing fallout contaminated all of the atoll, so much so, that it could not be approached by JTF-7 for 24 hours after the test, and even then exposure times were limited. As the fallout spread downwind to the east, more atolls were contaminated by radioactive calcium ash from the incinerated underwater coral banks. Although the atolls were evacuated soon after the test, 239 Marshallese on the Utirik, Rongelap, and Ailinginae Atolls were subjected to significant levels of radiation. 28 Americans stationed on the Rongerik Atoll were also exposed. Follow-up studies of the contaminated individuals began soon after the blast as Project 4.1, and though the short-term effects of the radiation exposure for most of the Marshallese were mild and/or hard to correlate, the long-term effects were pronounced. Additionally, 23 Japanese fishermen aboard Daigo Fukuryū Maru were also exposed to high levels of radiation. They suffered symptoms of radiation poisoning, and one crew member died in September 1954.

The heavy contamination and extensive damage from Bravo significantly delayed the rest of the series. The post-Bravo schedule was revised on 14 April 1954.

The Castle Romeo and Koon tests were complete by the time of this revision. The Echo test was canceled due to the liquid fuel design becoming obsolete with the success of dry-fueled Bravo. Yankee was similarly considered obsolete and so was conducted using a Runt II device (similar to the Union device) hastily completed at Los Alamos and flown to Bikini. With this revision, both of the wet fuel devices were removed from the test schedule.

As Operation Castle continued, the increased yields and fallout caused test locations to be re-evaluated. While the majority of the tests were planned for barges near the sand spit of Iroij, some were moved to the Bravo and Union craters. Additionally, Castle Nectar was moved from Bikini Atoll to the Ivy Mike crater at Eniwetok for expediency since Bikini was still heavily contaminated from the previous tests.

The final test in Operation Castle took place on 14 May 1954.