Mark 21 nuclear bomb
The Mark 21 nuclear bomb was a United States nuclear gravity bomb first produced in 1955. It was based on the TX-21 "Shrimp" prototype that had been detonated during the Castle Bravo test in March 1954. While most of the Operation Castle tests were intended to evaluate weapons intended for immediate stockpile, or which were already available for use as part of the Emergency Capability Program, Castle Bravo was intended to test a design which would drastically reduce the size and costs of the first generation of air-droppable atomic weapons (the Mk 14, Mk 17 & Mk 24). At long, in diameter, and weighing , the Mk-21 was half the length and one-third the weight of the Mk-17/24 weapons it replaced. Its minimum yield was specified at four megatons.
Bokak Atoll
While en route from the US to Asia in April, 1953, LST 1138, later commissioned as , dropped anchor at Bokak to search for rumored Japanese stragglers. The landing party noted the remains of the wartime outpost, but found no signs of any current occupants.
In 1954, the experience of large scale fallout from the Castle Bravo nuclear test on Bikini Atoll led to a pre-planned aerial survey of atolls adjacent to the subsequent Castle Romeo test, timed at one and four hours after the shot. The aircraft were equipped with gamma radiation detectors designed to measure ground contamination from altitudes of . An overflight of Sibylla Island measured 1.0 mrem/hr (10 μGy) an hour after the shot, dropping to 0.4 mrem/hr (4 μGy) three hours later. In 1957, Bokak was surveyed as a site for nuclear weapons testing as a part of Operation Hardtack, but due the number of improvements required to develop it, was passed over in favor of reusing the Bikini, Enewetak, and Nevada test sites. The atoll came under renewed consideration for use during Operation Dominic, but by that time the potential for political fallout from nuclear testing within a United Nations Trust Territory was deemed too great.