In 4th edition, radiation was an attack type, resisted by armor. In 5th edition, radiation is implemented much like a poison/disease. So in my opinion, in 5th edition, radiation doesn't belong with the other damage types that you've mentioned, which are resisted by your armor. Radiation belongs with the disease and poison resistances.
Also, please check the Future Plans section of the FAQ - as noted in the Run Faster section there, the environmental section of Run Faster hasn't been finished yet.
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