Artificial radiation and shielding are quite simple, actually. The former is governed by one property of ModuleKerbalHealth: radioactivity. The formula that I use to determine this value will change in the next release, and it is quite complex. But for the purposes of TweakScale, what matters is that it is proportional to square root of maxThrust (for engines) and of EC output (for generators).
Radiation shielding is provided either by ModuleKerbalHealth directly, defined in shielding value, or by various resources present on the vessel, such as RadiationShielding (the idea is that the former is built-in shielding and the latter is the shielding you can add for better protection). Basically, it's just the amount of shielding material. It is usually proportional to crewCapacity or part's mass.
As requested here on Forum.
tech Info:
source: garwel.