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Castle Romeo mushroom cloud.

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 U.S. thermonuclear bomb.

The so-called "runt" 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" device tested shortly 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 "enriched" lithium (approximately 40% Lithium-6) deuteride used in Bravo.

Another view of the Castle Romeo mushroom cloud

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 destroyed islands if they were set off on land.

Like the Bravo test, it "ran away" and 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 lb and 42,000 lb 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

• [1] List of all tested US weapons.
• 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

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