Credit goes to PananaCakes for doing the hard work on this mod.

This mod makes decontamination arches only remove radiation from you if you have no more than 150 rads. Radiation is removed over time instead of instantaneously. The rate is 5 rads per second for Wasteland Workshop arches. If you have over 150 rads, you will receive a message warning that you are too irradiated to be decontaminated. Wasteland Workshop decontamination arches, the Mass Fusion decontamination chamber, and the HalluciGen decontamination chamber all use different methods to clear the player's radiation and this mod affects all three. Decontamination arches anywhere else that use the same mechanics should also be affected.

I chose these limitations because I feel Wasteland Workshop's decontamination arches are overpowered, being able to instantly cure all radiation poisoning. It's especially unbalanced in Survival difficulty as they have no side effects, unlike RadAway. But simply disabling the ability to build the arches seems wasteful to me. My intention is to keep them useful for recovering after a rad storm, or a swim across the Concord River to Sanctuary Hills. In cases such as a trek through the Glowing Sea, you'll need more than a shower to recover.

Yes, I know it doesn't make sense that you can cure any amount of internal radiation poisoning from a shower, but it's still more realistic with this mod.

Compatibility

This mod will conflict with any mod that affects the following records:

000AE175 DN102_DecontaminationHazardSpell
0014FD8B DN084_DecontaminationSpell
xx000C03 DLC02WorkshopDecontaminationArch-01Activator

That is, the spell effects used by the Mass Fusion and HalluciGen decontamination chambers, and the arch itself from Wasteland Workshop, which doesn't use a spell effect.

Credits
PananaCakes for doing almost all the scripting! And for helping to teach me.
ElminsterAU, Hlp, zilav and Sharlikran for FO4Edit