TheKoon shot ofOperation Castle was a test of athermonuclear device designed at theUniversity of California Radiation Laboratory (UCRL), nowLawrence Livermore National Laboratory.

The "dry" two-stagedevice was known as "Morgenstern" and had a highly innovative secondary stage. It was tested on 7 April 1954. The predictedyield was between 0.33 and 3.5 megatons, with an expected yield of 1 megaton. The actual yield was 0.11 megatons. Morgenstern was thus afizzle.

Post-shot analysis showed that the failure was caused by the premature heating of the secondary by theneutron flux of the primary. This was a simple design defect and not related to the unique geometry of the secondary. The UCRL's other shot, the "wet" (i.e., cryogenic) ramrod device, originally scheduled for theEcho shot, was cancelled because it shared the same design defect.

The name "Morgenstern" (German for morning star) was chosen because of the shape of the secondary. The secondary consisted of a central sphere from which spikes were radiating, resembling amorning star / mace. The spikes may have been^{[citation needed]} an idea from physicistEdward Teller and colleagues to use implosive jets to compress the thermonuclearcore. It was more than two decades before weapons were designed that utilized a secondary concept similar to that first tested in the Koon shot.

• Hansen, Chuck, "The Swords of Armageddon: U.S. Nuclear Weapons Development since 1945" (CD-ROM). PDF, 2,600 pages, Sunnyvale, CA, Chukelea Publications, 1995, 2007.ISBN 978-0-9791915-0-3 (2nd Ed.)

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