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Operation Castle was a series of high-energy (high-yield) nuclear tests by Joint Task Force SEVEN (JTF-7) at Bikini Atoll beginning in March 1954. Conducted as a joint venture between the Atomic Energy Commission (AEC) and the Department of Defense (DoD), the ultimate objective of the operation was to test designs for an aircraft-deliverable thermonuclear weapon. Operation Castle is (largely) considered to be a resounding success for the designs of the weapons, though there were some serious complications. One of the devices was a fizzle, and did not detonate with anything close to its predicted yield. Two other devices detonated with over twice their predicted yields' one shot in particular, Castle Bravo, resulted in extensive radiological contamination of nearby islands (including inhabitants and U.S. soldiers stationed there), as well as a nearby Japanese fishing boat, resulting in one direct fatality and continued health problems for many of those exposed. Public reaction to the tests and the awareness of the long-range effects of nuclear fallout has been attributed to being part of the foundation for the Partial Test Ban Treaty of 1963.
Background Bikini Atoll had previously hosted nuclear weapon testing in 1946 as part of Operation Crossroads where the world’s fourth and fifth atomic weapons were detonated in Bikini Lagoon. Since then, US nuclear weapons testing had moved to Eniwetok Atoll to take advantage of generally larger islands and deeper water (and therefore more real estate to stage experiments and support equipment upon). Both Atolls were part of the US Pacific Proving Grounds, however, the extremely high yields of the Castle weapons caused concern within the AEC that potential damage to the limited infrastructure already established Eniwetok would delay other operations. Additionally, the cratering from the Castle weapons was expected to be comparable to that of Ivy Mike, a 10.4 Mt weapon tested at Eniwetok in 1951 leaving a crater approximately 1 mile (1.6 km) in diameter. The Ivy Mike test had been the world’s first thermonuclear explosion that used hydrogen fusion as its primary power source. The Mike device used cryogenic hydrogen isotopes in a liquid state, commonly referred to as a "wet bomb." The complex dewar mechanisms needed to store the liquid hydrogen made the device three stories tall and 82 tons in total weight—completely impractical to be delivered by contemporary weapon systems of the cold war. With the success of Ivy Mike as a proof of concept, research began on using a solid state “dry fuel” to achieve a practical fusion weapon. The Teller-Ulam design could also use lithium deuteride as the fusion fuel, greatly reducing the size, weight, and complexity. Operation Castle was chartered to test 4 dry fuel designs, 2 wet bombs, and 1 smaller device. Approval for Operation Castle was communicated to JTF-7 by Major General Kenneth D. Nichols, General Manager of the AEC, on 21 January, 1954. Experiments Operation Castle was organized into 7 experimental detonations, all but one which were to take place at Bikini Atoll. Below is the original test schedule (as of February 1954). Operation Castle was intended to be a proof of concept for lithium deuteride fuels in fusion weapons. The so-called "dry" lithium-6 fuels were a significant advancement in weapon design over the liquid hydrogen used in the Ivy Mike device. Bravo and Union used enriched lithium in the Teller-Ulam configuration where a typical fission weapon is detonated in order to create the temperature and pressure needed for fusion in the lithium fuel capsule. Romeo and Koon used the same Teller-Ulam configuration, however, both were fueled by naturally occurring lithium deuteride (92% Li-7, 7.5% Li-6). The natural fuels, being vastly less time and resource intensive to obtain compared to enriched lithium, were important for US's ability expand the nuclear stockpile during the cold war arms race. Because the viability of dry fuels was not known, and there was significant risk of failure, development of liquid isotope weapons continued in parallel. Even though they were considered less practical due to logistical problems with transport, handling, and storage of a cryogenic device, the cold war arms race drove a technical demand for a viable weapon. Echo and Yankee were liquid fuel designs that were greatly reduced in size and weight over their Ivy Mike predecessor. The Jughead device was ultimately weaponized, despite the liquid fuel draw-backs, and saw limited fielding in the inventory until the dry fuel weapons were common. Nectar was not a fusion weapon in the same sense as the rest of the Castle series. Even though it used a dry lithium fuel for fission boosting, the principal reaction material was uranium and plutonium. Similar to the Teller-Ulam configuration, a fission device was used to create high temperatures and pressures in order to compress a second fissionable mass that would have otherwise been too large to sustain an efficient reaction if it were triggered with conventional explosives. This experiment was intended to develop an intermediate yield weapons for expanding the inventory (around 1-2 Mt vice 4-8). Test execution The most notable event of Operation Castle was the Bravo test. The dry fuel for Bravo was 40% enriched Li-6, and 60% naturally occurring Li-7, the latter of which was expected to be essentially inert. J. Carson Mark, head of the Los Alamos Theoretical Design Division, had speculated that Bravo could "go big" and estimated that it could be by as much as an additional 20%. Unexpectedly, the Li-7 was extremely reactive under the fusion conditions and produced a very high neutron and energy surplus through a previously unquantified neutron reaction. Bravo exceeded its expected yield by more than 250%, totaling 15 Mt. 1,000 times more powerful than the Little Boy weapon used on Hiroshima, Bravo is the largest detonation ever conducted by the United States, and the seventh largest weapon ever detonated. Because Bravo exceeded the designed yield by such a large margin, the JTF-7 was caught unprepared. Much of the permanent infrastructure on Bikini Atoll was heavily damaged. The intense thermal flash ignited fire at distance of 20 nautical miles on the island of Eneu (base island of Bikini Atoll). The ensuing fallout contaminated almost all of the Atoll to various levels, so much so that it could not be approached by JTF-7 for 24 hours after the test, and even then exposure time were limited. As the fallout spread downwind, more atolls were contaminated with activated calcium ash. Though people on the atolls were evacuated soon after the test, in the end 239 Marshallese on the Utirik, Rongelap, and Ailinginae Atolls were subjected to significant levels of radiation, and 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 the Lucky Dragon No. 5 were also exposed to high levels of radiation. They suffered symptoms of radiation poisoning, and one crewmember died in the fall of 1955. The heavy contamination and extensive damage from Bravo caused significant delays in the conduct the rest of the series. The post-Bravo schedule was revised on 14 April, 1954. Romeo and Koon were complete by the time of this revision. The Echo test was canceled due to the liquid fuel design being considered obsolete following the success of the dry fuel in Bravo. The Jughead device was similarly considered obsolete and Yankee 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 from the dry fuel experiments 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 move to the Bravo and Union craters. Additionally, Nectar was moved from Bikini Atoll to the Ivy Mike crater at Enewetak as a means of expediency since Bikini was still heavily contaminated from the previous tests. The final test in Operation Castle took place on 14 May, 1954. Results Operation Castle was an unqualified success for the technical implementation of dry fuels, despite the in some cases large variations from prediction. The Bravo design was quickly weaponized and is suspected to be the progenitor of the Mk-21 gravity bomb. The Mk-21 design project began on 26 March, 1954 (just three weeks after Bravo) with production of 275 weapons beginning in the fall of 1955. Romeo, relying on natural lithium, was rapidly turned into the Mk-17 bomb and was available to strategic forces as an Emergency Capability by late summer of 1954. Most of the Castle dry fuel devices eventually appeared in the inventory and ultimately grandfathered the majority of thermonuclear configurations. In contrast, the Livermore-designed Koon design was a failure. Using natural lithium and a heavily modified Teller-Ulam configuration, the test produced only 110 Kt of an expected 1.5 Mt. While engineers at the Radiation Laboratory had hoped it would lead to a promising new field of weapons, it was eventually determined that the design allowed premature heating of the lithium fuel, thereby disrupting the delicate fusion conditions. See also Bibliography | ||||||||
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