The Mechanics for Franz - Tier effects stack:
Rad Level 0: Clean. No penalties.
Rad Level 3 (Minor Sickness): Disadvantage to Constitution checks.
Rad Level 5 (Moderate Sickness): Disadvantage to Dexterity and Strength checks. Nausea narrative effects.
Rad Level 7 (Severe Sickness): Disadvantage to all physical stats. Max HP is effectively halved.
Rad Level 8-9 (Critical Sickness): Barely conscious. Must pass Endurance checks to take strenuous actions.
Rad Level 10 (Terminal/0 HP): Trigger 'The Ghoul Protocol'
'The Ghoul Protocol' (The Save-State): If a player hits Rad Level 10 and reaches 0 HP from radiation damage, they do not die. Instead, they pass out and awake as a Ghoul.
The Perks: Total immunity to radiation. Radiation now heals the player.
The Penalties: Disadvantage to Charisma checks with smoothskins (smoothskins are prejudiced). Stimpaks are only 50% effective.
The Catch: If a player takes massive trauma while a Ghoul, they risk going Feral (Game Over). This should be series of saves to allow players a chance at survival.