[beefman]

With 12 years of burnup? I've been having trouble getting burnup codes to handle thorium. What are you using? Apparently I should be using Serpent.

But anyway, 12 years is too long, even if it's feasible. Something like 4 years should be better, due to the learning curve on these. For instance, think of a reactor in the field as a ship on a long trip while technology at home continues to improve

http://en.wikipedia.org/wiki/Wait_Calculation

Edit: The lower the enrichment, and the lower the excess reactivity, the better.