Nebulous
Legend
I don't think that, even as a free action, you can choose to use it as an Interrupt. I don't think using it that way would be necessarily broken, but I would characterize it as 'creative rules revision' rather than rude.
I've had this same question before and i tend to treat it as an interrupt, even if that's not quite by the book. And i would have thought "radiant" was elemental damage, never would have occured to me otherwise.