Ivy King
The Ivy King bomb , designated as a Mk-18 bomb and named the "Super Oralloy Bomb" , was a modified version of the Mk-6D bomb . Instead of using an implosion system similar to the Mk-6D , it used a 92-point implosion system initially developed for the Mk-13 . Its uranium-plutonium core was replaced by 60 kg of highly enriched uranium ( HEU ) fashioned into a thin-walled sphere equivalent to approximately four critical masses . 
<triplet> highly enriched uranium (HEU) <subj> uranium <obj> subclass of