The so-called "runt" device 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. Like the Ivy Mike and Castle Bravo tests, a large percentage of the yield was produced by fast fission of the natural uranium "tamper"; 7 megatons of the yield were from this source.