Aldermaston had plenty of ideas about how to follow up Grapple X. A new design used lithium deuteride that was less enriched in lithium-6 (and therefore had more lithium-7), but more of it, thereby reducing the amount of uranium-235 in the core. Because of the possibility of an international moratorium on atmospheric testing, plans for the trial, codenamed Grapple Y, were given verbal approval by the Prime Minister, and known only to a handful of officials. The bomb was dropped off Christmas on 28 April 1958. It had an explosive yield of about , and remains the largest British nuclear weapon ever tested. The design of Grapple Y was notably successful because much of its yield came from its thermonuclear reaction instead of fission of a uranium-238 tamper, making it a true hydrogen bomb, and because its yield had been correctly predicted—indicating that its designers understood what they were doing.