Operation Grapple - Grapple Y

Grapple Y

Grapple Y sought to develop a more efficient thermonuclear bomb based on the successful Grapple X design. Like Grapple X, only one detonation was conducted during Grapple Y. The bomb was detonated off Christmas Island on April 28, 1958. This bomb had an explosive yield of about 3.0 megatons, and it was the largest British nuclear weapons test ever executed anywhere.

The design of Grapple Y was notably successful because much of its yield came from its thermonuclear reaction instead of from the fast fission of a heavy uranium-238 tamper - hence making it a true H-bomb, and also because its yield had been closely predicted—indicating that its designers understood what they were doing.