i ran into this little problem,
build a 14*14*3 big reactor ,lovely efficient design, but its build acros 4 chunks
it seems that the big reactors dump fuel on restart if you build em on more than 1 chunk
even if it's not been turned on.
so be warned and don't do it.
lost about 3 stacks of yellorium blocks and didn't get cyanite in its place so it's not all burned up