Looks like there's some kind of radiation amplification bug in the current preview branch. Up in space I've got in excess of 200 rads per tile, and in my reactor room it's in excess of 4000! It's also causing substantial slowdown in the game, I assume due to the radiation.
I've attached some screenshots, but I don't really know the steps to reproduce because I didn't catch when it first started happening.
I've included two save files, one before the bug and one after. Maybe you can extrapolate what happened between them?