That 1.20 you see is the player "half life", basically radioactivity applies a DoT to player which halves for every cycle of the script when you move out of an irradiated zone, when both values are 0 the GUI disappears (the exposure is what causes damage)
For the rad shield it basically directs the damage that should be done to the player towards your batteries, 1kJ per radiation level is drained from batteries instead of damaging the player, if batteries get drained no matter what other shields you wear player starts taking damage regardless of shields.
EDIT: Should be noted that radiation shields do not stack with each other, wearing 1 rad shield or 10 rad shields makes no difference towards battery draining