You should use ZonePlus. You can use the .playerEntered() event to check when a player enters a specifig zone.
Something like this:
local ZoneModule = require(path.to.Zone)
local RadioactiveZone = ZoneModule.new(path.to.part)
local inZone = {}
RadioactiveZone.playerEntered:Connect(function(player)
inZone[player] = true
while inZone[player] do
--do stuff
end
end)
RadioactiveZone.playerExited:Connect(function(player)
inZone[player] = false
end)