Breeder reactors are for making higher grade fuel (plutonium). This was the next step up from an HTGR, which I had previously. I'm considering swapping over to ammonia to double the power output, and I have lots of low grade thorium to try in it next.
Breeder reactors generate heat while breeding breeder fuel into plutonium. In fact, the process is exactly like any other nuclear reactor, just that at the end the fuel turns into a plutonium pellet instead of just disappearing. I don't know how long either breeder fuel or plutonium lasts compared to regular uranium fuel, but considering you get those two stages PLUS four times the pellet number compared to just using straight uranium fuel, it's hard to imagine that you're getting less bang for your buck...
Course, there's also the problem of more nuclear waste to deal with, plus since it's a two-stage reaction you have to make two reactors, and the breeding phase requires using liquid sodium instead of water and sending that through a heat exchanger... but EFFICIENCY.
Also, I'm rather curious about that thorium as well, will be interesting to see your experiments... Also, how did Reika end up implementing actually getting the stuff?
EDIT:
@Reika 's post above, a three-block thick wall isn't really all that much more interesting then a one-block thick wall...