meat, nub, kernel, pith, nitty-gritty, burden, heart, essence, nucleus, inwardness, more...

periphery, exterior, surface, fringe, outer

liquid, fusion, fission, breeder, fast breeder, heavy water, light water, more...

central, very, hard, inner, common, solid, magnetic, urban, outer, essential, soft

essential part,   most important part,   central part,   central section,   heart of,   center of a ball,   part of a reactor,   pear discard,   hard-rock center,   true essence