Start with a reactor that uses 4 fuel rods like this one.

Here you can see there is no fuel inside.

Place at least 2 yellorium blocks into the inlet.

The reactor will take 1 of the blocks and leave the other.

There is now 16000 mB instead of 9000 mB inside the reactor despite removing the other yellorium block.

Clicking "Eject Fuel" will then give you 16 yellorium ingots. If you put a full stack of 64 yellorium blocks into the inlet, you will get 1017 yellorium ingots (113 blocks worth).