January 16, 2015, Jortiz3
Contrary to popular belief, the reason light-enriched-uranium reactors are used, and not thorium or breeder reactors, is due to simple economics. To run breeder reactors and thorium reactors, the neutron density and heat density must be so great that high-temperature coolants must be used throughout the core.
The systems used to manage these coolants are as exotic as the coolants are. This leads to increased costs, on the order of 20%. This 20% is enough that utilities simply choose light-enriched-uranium so that the reactor core can be cool enough that cooling with water is possible and savings can offset the cost of mining the ridiculous quantities of natural uranium required.