If the 500 meter diameter spherical habitat used unstacked weapons-grade uranium, enriched to 95% U-235, as a source of heat only 242 kilotonnes of Uranium Dioxide would be needed. This amount of Uranium only costs $22 billion US and would take up less than 0.02 percent of the habitat volume. But it would still provide a natural heat source for millions of years. Weapons-grade Uranium might also be useful at some point???

I might suggest that Uranium should be mined & launched from the Moon or maybe Phobos/Deimos since at $10,000/lb launching it from earth's surface would cost $5.324 trillion US. But clearly construction would be space-based, since space-based self-sufficiency must be very robust, proven with a long history of success before a Generation Starship would be attempted. I also think it would be neat if 50-100 separate Spherical Habitats were built (Borg Spheres??). Flying these City Ships in formation at a respectable distance would provide a natural state of long-distance interaction for the people aboard, including trade, political fractionation, resettlement, etc. For example a cluster of 4 ships could be created with similar religious beliefs.

bluh (News Flash --> Earth Invades Centauri Space!!!)