Okay my math was wrong, i assumed you get 1 fuel cell each.
This calculation was done for a realistic reactors mod power plant and only using liquid matter generators.

140mw x 180s x 10 = 252000 MJ (10 Fuel cells)
10 Fuel Cells= 1x U235 + 19x U238
1x U235 = 3x U238 + 36 MJ
10 Fuel Cells= 22x U238 + 36 MJ
22x U238 = 183 Uranium Ore + 184 MJ
10 Fuel Cells = 183 Uranium Ore + 220 MJ
183 Uranium Ore = 1830 Matter + 154 MJ
1830 Matter = 9150 cycles = 18300s * 10MW = 183000 MJ
10 Fuel Cells=183370 MJ

Wait what? Are you telling me the cost line up this perfectly or am I reading this wrong?

Because I did not do the math this directly. Only very roughly for a 4GW Reactor... AFAIR.

I might think about reducing uranium cost.

the profit was 70 GJ or 37%.
My first calculation was terribly off (thats why i created this discussion) but i think 37% would be kinda fair.
Now the perpetual motion machine will be very profitable, but considering the high research cost it seems reasonable

Considering that you still need Iron and Copper Production (or mod ores), I think its ok. The mod ist still pretty hardcore and lategame and I guess a well profitable uranium loop is not that big of a crime.