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Castle Romeo was the code name given to one of the tests in the Operation Castle series of American nuclear tests. It was the first test of the TX-17 thermonuclear weapon (initially the "emergency capability" EC-17), the first deployed thermonuclear bomb.

Castle Romeo
Castle Romeo.jpg
Castle Romeo mushroom cloud.
CountryUnited States
Test seriesOperation Castle
Test siteBikini Atoll
DateMarch 27, 1954
Test typeAtmospheric
Yield11 Mt
Test chronology
Another view of the Castle Romeo mushroom cloud
The Runt device

The "Runt" TX-15 device was a weaponized "dry" fusion bomb, using lithium deuteride fuel for the fusion stage of a "staged" fusion bomb, unlike the cryogenic liquid deuterium of the first-generation Ivy Mike fusion device.

Similar to the "Shrimp" TX-21 device tested before in the Castle Bravo test, it differed from that device in using lithium deuteride derived from natural lithium (a mixture of lithium-6 and lithium-7 isotopes, with 7.5% of the former) as the source of the tritium and deuterium fusion fuels, as opposed to the relative high enrichment level of lithium (approximately 40% lithium-6) deuteride used in Bravo.

It was detonated on March 27, 1954, after several delays (which played havoc with the planned experimental measurements program) at Bikini Atoll of the Marshall Islands, on a barge moored in the middle of the crater from the Castle Bravo test. It was the first such barge-based test, a necessity that had come about because the powerful thermonuclear devices completely obliterated the small islands following detonation.

Like the Bravo test, it produced far more than its predicted yield, and for the same reason — an unexpected participation of the common lithium-7 isotope in fusion reactions. Although it had been predicted to produce a yield of 4 megatons with a range of 1.5 to 7 megatons (before the results of the Bravo test caused an upgrade in the estimates, it had originally been estimated to produce 3–5 megatons), it actually produced a yield of 11 megatons, the third-largest test ever conducted by the U.S.

Like the Ivy Mike and Castle Bravo tests, a large percentage of the yield was produced by fast fission of the natural uranium "tamper"; 7 megatons of the yield were from this source.



This became the first air-droppable thermonuclear device, the EC-17, of which only 5 were made and the first deployable staged radiation implosion Teller-Ulam thermonuclear weapon. This evolved into the Mk 17 of which 200 were made. Both of these were huge devices, weighing 39,000 and 42,000 pounds respectively. As a result, they were only capable of being carried by the B-36. They were also some of the largest yield devices deployed by Strategic Air Command—the EC-17 giving around 10 Mt and the Mk17 between 11 and 15 Mt. They were all out of service by August 1957.

Fireball in popular cultureEdit

One particular image of the Castle Romeo fireball (above, at right) has been one of the most highly reprinted images of a nuclear explosion, often serving as a stand-in for nuclear weapons in general for news stories, book covers, magazine articles, and even congressional reports (such as the Cox Report), likely because of its threatening appearance and extreme red, orange, and yellow hues. In many cases, the fact that the explosion is of a US megaton-range weapon has not prevented it to be used to represent the arsenals of other states or weapons of far lower yields, which would have a very different appearance.

One prominent usage is as the backdrop for heavy metal band Megadeth's greatest hits compilation Greatest Hits: Back to the Start. The image of Castle Romeo was also used on the cover of the New York hardcore music pioneers Cro-Mags debut album The Age of Quarrel. It is also featured on the title screen of Team17's turn-based artillery game Worms Armageddon.

The Castle Romeo photos are sometimes confused with that of Castle Bravo; the two nuclear blasts looked very similar, and they were both conducted in the same location, but much of Bravo's photographic record was destroyed because of its unexpectedly high yield.


  • Chuck Hansen, U. S. Nuclear Weapons: The Secret History (Arlington: AeroFax, 1988)

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