Fission , primarily `` fast '' fission of the natural uranium tamper surrounding Surrounding the fusion capsule , greatly increases Cause_change_of_position_on_a_scale the yield of thermonuclear devices , and constitutes the great majority of the fallout , as nuclear fusion is a relatively clean reaction .

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