I suggest that any units that stay in fallout areas for a specified amount of time (3 hours, 6 hours, etc) will turn into radioactive units. Similar to how animals near the Hiro-Naga blasts and the Chernobyl disaster. Characteristics of radioactive units would include:
- The unit skin would turn into a different doctrine (but still belong to the same country)
- The units attack and defense values would be doubled/tripled, and their HP would be cut in half/thirds.