There are core designs for low enriched uranium loading, mixed thorium loading, and spent fuel loadings. Have to make sure a fuel design works before it's claimed to work.
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.