It would not just be Uranium, but Gold, Lead, and other dense elements that would be concentrated in the core. There are articles written about that, which you can find with a little googling. The fact that earth [and other inner planets] experienced heavy bombardment in its early history [and we had an interesting thread about that a few months ago] raises the question as to whether earth's crust was 'seeded' with heavy elements [such as gold and uranium]. If an outer planet [in the asteroid belt region] shattered or exploded, forming lots of debris to cause bombardments of the inner planets [and this apparently happened several different times in earth's early history, implying several distinct events of planets being shattered], some of that debris would be the cores of those planets/planetoids, with chunks of heavy elements. That's why I suggested that we might someday find a gold meteorite, and not just iron/nickel ones [which are the shattered remnants of the iron/nickel zone of such planet/planetoid]. As to moderation, distance moderation would slow down neutrons also [5 miles of Uranium is a lot of bouncing around of neutrons to slow them down], and a large bolus of uranium should have some slight nuclear fission to add to the energy release from alpha emission.