Even without complex quantum chemistry calculations, we can deduce that the core electrons of #"U"# atom are dominated by the NINETY-ONE more protons in it compared to the ONE proton in hydrogen atom, increasing the average radius of #"U"# atom so that it is almost twice that of #"H"# atom but leaving the core electrons near the nucleus.
Due to the #1/n^2# dependence of the energy levels (ignoring angular momentum for the moment), the valence energy levels are among the densest energy levels, and the core orbital energy levels the least dense, and this easily leads to a much larger #1s-2s# gap for #"U"# atom.
If that weren't enough, the #"U"# atom has mass-velocity relativistic effects that further contract the #1s# orbital, more so than for the #2s# orbital (as the #1s# electrons are on average closer to the nucleus), further increasing the #1s-2s# gap for #"U"# atom.
