Castle Union

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The Castle Union test of the Mark 14 design.

Mark 14 nuclear bomb.

Castle Union 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-14 thermonuclear weapon (initially the "emergency capability" EC-14), one of the first deployed U.S. thermonuclear bombs.

The so-called "Alarm Clock" device (totally different from other, earlier nuclear weapon design of this name which was known as the Sloika design in the USSR) 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.

Very similar to the "Runt" device tested shortly before, in the Castle Romeo test, it differed from that device in using highly "enriched" lithium (approximately 95% Lithium-6; natural lithium was a mixture of Lithium-6 and Lithium-7 isotopes, with 7.5% of the former) in the fusion fuel.

It was detonated on April 26, 1954 at the Bikini atoll of the Marshall Islands, on a barge moored in the lagoon, off Yurochi island. The yield of 6.9 megatons was somewhat higher than the predicted 3-4 megatons. Although the barge had been moored in over 160 feet of water, the test left a crater 3,000 feet in diameter and 90 feet deep in the bottom of the lagoon.

Like the Ivy Mike, Castle Bravo, and Romeo tests, a large percentage of the yield was produced by fast fission of the natural uranium "tamper", which contributed to the extensive fallout caused by these tests.

As the artificial highly enriched lithium was both expensive as well as scarce at the time, that limited the number of these weapons that could be deployed; the "Runt" devices tested in Romeo and Castle Yankee were therefore preferred for deployment.