Perfect Breeder:
Water cooled it has 0 net heat gain, maintaining constant temp. It also has enough plateing to be able to reach over 9k heat without producing lava. It does evaporate water, so you need to have the source blocks far enough away from the reactor that they will not be destroyed; I have mine flowing from above. It can enrich 8 isotope cells per fuel cell if it is hot enough. You may have to move the plating around durring the heating process, as it likes to dump all its heat into adjacent cooling cells.
Heat gained: 5*10 + 1*4 = 54.
Heat loss external: 6*2+1+20=33
Heat loss internal: 20*1+10*0.1=21
Net heat: 54-54=0
Required heat to produce lava = 19250*0.85= 16262, which is after components have started to melt effectively making it 10k.
I have a piston switch to shut off the water flow to manage small losses in heat during changing out fuel or isotope cells.
With water off:
Heat gained: 5*10 + 1*4 = 54.
Heat loss external: 6*2+1+5=18
Heat loss internal: 20*1+10*0.1=21
Net heat: 54-39= +15