if you add 4 tiles of water, and block 4 other tiles, it is heat neutral.
But now to the funny part: heating it up 1: keep it offline, fill it, add one Uranium cell to the top left corner (as here) 2: coat ir in Magma 3: turn it on 4: around 9.2k hest 2 cells finish with reloading (and thus the heating nearly stops). If you want to get the other 2 cells also recharged, put them in the place of the finished cells, put cooling cells where the unfinished cells were, and replace the magma with the other stuff (as suggested above). It will heat up a bit further, but remains safe. the 2 new coolant cells will melt, if not extracted when hot, but by the time that happens, they did their job. 5: If you don't want the other cells to recharge, just wait till end of cycle, replace the surroundings, and then use it in regular service
Testet that way already, and works fine. Once heated up, the cells will recharge at aound half cycle.
But right now, i'm trying on an automated design to use with BC and red power, if there was something working well, i'll post it.