I feel that a pseudo-fluid that “travels” in special pipes that are basically dummy blocks would be the easiest and probably best solution to the problem. But then again I have no idea how hard it is to code =P
EDIT: Also, since we’re talking about Cernobyl, one of the six Gen4 designs is basically the latest technology version of a LWR, the Supercritical Water Reactor: Supercritical water reactor - Wikipedia
It uses a single loop of water at >22GPa of pressure and ~400°C (aka water goes into the core and supercritical water comes out and spins a turbine). It has the highest efficiency of any design, because using s.c. water is the best way of spinning a turbine, and depending on the core design and fuel used it can either be a thermal or a fast reactor (supercritical water is not a good moderator).
Since everyone pictures a Light Water Reactor (Cernobyl style) when they think “nuclear reactor”, maybe this design could be a good high tech end-game very efficient EU generator in GT6