Operation Castle was a United States series of high-energy (high-yield) nuclear tests by Joint Task Force SEVEN (JTF-7) at Bikini Atoll beginning in March 1954. It followed Operation Upshot-Knothole and preceded Operation Teapot.

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 was considered by government officials to be a success as it proved the feasibility of deployable "dry fuel" designs for thermonuclear weapons. There were, however, technical difficulties with some of the tests: one device had a yield much lower than its predicted yield (a "fizzle"), while two other devices detonated with over twice their predicted yields. One test 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 (Daigo Fukuryū Maru), resulting in one direct fatality and continued health problems for many of those exposed. Public reaction to the tests and an awareness of the long-range effects of nuclear fallout has been attributed as being part of the motivation for the Partial Test Ban Treaty of 1963.

Background

Bikini Atoll had previously hosted nuclear 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. 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 at 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 device tested at Eniwetok in 1952 leaving a crater approximately 1 mile (1.6 km) in diameter.

The Ivy Mike test had been the world’s first full scale thermonuclear or fusion explosion. The Mike device used liquid deuterium, an isotope of hydrogen, making it a "wet bomb." The complex dewar mechanisms needed to store the liquid deuterium at cryogenic temperatures made the device three stories tall and 82 tons in total weight, far from being a deliverable weapon. With the success of Ivy Mike as a proof of the Teller-Ulam concept, research began on using a “dry" fuel to make a practical fusion weapon so that the United States could begin deploying thermonuclear designs in quantity. This applied the Teller-Ulam concept with lithium deuteride as the fusion fuel, greatly reducing size, weight, and complexity. Operation Castle was chartered to test four dry fuel designs, two wet bombs, and one 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 experiments, all but one of which were to take place at Bikini Atoll. Below is the original test schedule (as of February 1954).

Operation Castle Schedule

Experiment	Device	Prototype	Fuel	Date	Predicted Yield	Manufacturer	Test Location
BRAVO	Shrimp	TX-21	40% Li-6 D (dry)	1 March, 1954	6 Mt	Los Alamos Scientific Laboratory	Reef off Nam Is, Bikini
UNION	Alarm Clock	EC-14	95% Li-6 D (dry)	11 March, 1954	3-4 Mt	Los Alamos Scientific Laboratory	Barge off Iroij, Bikini
YANKEE	Jughead	TX/EC-16	Cryo H-3 (wet)	22 March, 1954	8 Mt	Los Alamos Scientific Laboratory	Barge off Iroij, Bikini
ECHO	Ramrod	N/A	Cryo H-3 (wet)	29 March, 1954	65-275 Kt	University of California Radiation Laboratory (Livermore)	Eleleron, Enewetak
NECTAR	Zombie	TX-15	Boosted fission	5 April, 1954	1.8 Mt	Los Alamos Scientific Laboratory	Barge off Iroij, Bikini
ROMEO	Runt	TX/EC-17A	7.5% Li-6 D (natural)	15 April, 1954	4 Mt	Los Alamos Scientific Laboratory	Barge off Iroij, Bikini
KOON	Morgenstern	N/A	7.6% Li-6 D (natural)	22 April, 1954	1 Mt	University of California Radiation Laboratory (Livermore)	Eneman, Bikini

Operation Castle was intended to test lithium deuteride (LiD) as a thermonuclear fusion fuel. A solid at room temperature, "dry" LiD, if it worked, would be far more practical than the cryogenic liquid deuterium (hydrogen-2) fuel in the Ivy Mike device. The same Teller-Ulam principle would be used as in the Ivy Mike "Sausage" device, but the fusion reactions were different. Mike fused deuterium with deuterium, but the LiD devices would fuse deuterium with tritium. The tritium was produced during the explosion by irradiating the lithium with fast neutrons.

Bravo and Union used lithium enriched in the Li-6 isotope while Romeo and Koon were fueled with natural lithium (92% Li-7, 7.5% Li-6). The use of natural lithium would be important to the ability of the US to rapidly expand its nuclear stockpile during the cold war arms race.

As a hedge, development of liquid deuterium weapons continued in parallel. Even though they were much less practical because of the logistical problems dealing with the transport, handling, and storage of a cryogenic device, the cold war arms race drove the demand for a viable fusion weapon. The Ramrod and Jughead devices were liquid fuel designs greatly reduced in size and weight from their Ivy Mike "Sausage" predecessor. The Jughead device was ultimately weaponized 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 in the second stage 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 intermediate yield weapons for expanding the inventory (around 1-2 Mt vs. 4-8).

It should be noted that many "fusion" or "thermonuclear" weapons still generate much or even most of their yields from fission. Although the U-238 isotope of uranium (i.e., "natural" or "depleted" uranium) will not sustain a chain reaction, it still fissions when irradiated by the intense fast neutron flux of a fusion explosion. Because U-238 is plentiful and has no critical mass, it can be added in almost unlimited quantities as a "tamper" around a fusion bomb, helping to contain the fusion reaction and contributing its own fission energy. Fission of the U-238 tamper contributed 77% -- 8 megatons -- to the yield of the 10.4 Mt Ivy Mike explosion.

Test execution

The most notable event of Operation Castle was the Bravo test. The dry fuel for Bravo was 40% Li-6 and 60% Li-7. Only Li-6 was expected to breed tritium for the deuterium-tritium fusion reaction; 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. Li-7 turned out to be an excellent source of tritium through a previously unquantified reaction. Bravo exceeded expectations by 250%, yielding 15 Mt -- 1,000 times more powerful than the Little Boy weapon used on Hiroshima. Bravo is to this day the largest detonation ever conducted by the United States, and the seventh largest ever detonated in the world.

Because 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 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 with activated calcium ash. 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 Lucky Dragon No. 5 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.

Operation Castle Schedule (Post BRAVO)

Experiment	Original Date	Revised Date	Original Yield	Revised Yield
UNION	11 March, 1954	22 April, 1954	3-4 Mt	5-10 Mt
YANKEE	22 March, 1954	27 April, 1954	8 Mt	9.5 Mt
NECTAR	5 April, 1954	20 April	1.8 Mt	1-3 Mt
ROMEO	15 April, 1954	27 March, 1954	4 Mt	8 Mt
KOON	22 April, 1954	7 April, 1954	1 Mt	1.5 Mt

The 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 the dry-fueled Bravo. Yankee was similarly considered obsolete and the Yankee test 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, Nectar was moved from Bikini Atoll to the Ivy Mike crater at Enewetak for expediency since Bikini was still heavily contaminated from the previous tests.

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

Operation Castle (Actual)

Experiment	Date	Yield	Location
BRAVO	1 March, 1954	15Mt	Reef off Nam Is, Bikini
ROMEO	27 March, 1954	11 Mt	Barge in BRAVO crater, Bikini
KOON	7 April, 1954	110 kt	Eneman, Bikini
UNION	26 April, 1954	6.9 Mt	Barge off Iroij, Bikini
YANKEE	5 May, 1954	13.5 Mt	Barge in UNION, Bikini
NECTAR	14 May, 1954	1.69 Mt	Barge Ivy-MIKE crater, Enewetak

Results

Operation Castle was an unqualified success for the implementation of dry fuel devices. 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, the US first deployable H 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

• Katsuko Saruhashi
• Pacific Proving Grounds
• Project 4.1

References

Bibliography

External links

• Downloadable/Streamable Declassified Film: Operation Castle Commanders Report, at the Internet Archive
• Downloadable/Streamable Declassified Film: Military Effects Studies Operation Castle, at the Internet Archive
• Operation Castle