Thorium salt reactors require U238 to function. I pursued the details of a startup to create thorium reactors with some of my co-workers who are quite competent nuclear engineers. Frankly, the capital requirements and regulatory burdens made it unattractive to pursue.
U238 is the common, longest half-life isotope so poses little danger or prohibitive cost.
Over-regulation is probably the issue, perhaps Trump, with the help of Scott Pruitt, will change that.
I try never to underestimate the genius of the unbridled American tinkerer.
Glad to get a response - most people are unaware of the possibilty.