Rather than using a potion effect it might be better to create a system. This way better logic can be implemented and shielding can be used.

https://www.quora.com/How-do-substances-that-are-not-normally-radioactive-such-as-soil-metal-structures-water-vapor-become-radioactive-when-exposed-to-a-reactor-accident-or-atomic-explosion

As a note, I almost forgot about this property. Radiation can not make something radioactive, instead, something is made radioactive by having a radioactive material absorbed or placed on the object. For making the radiation system this needs to be used.

What can be done is to track radioactive material type and count per block. Then use this value to calculate the radioactive level of the area. This will also make it possible to introduce game mechanics to clear radioactive materials. While at the same time making it possible to contaminate any block in the game.