Here's TLDR version: (1) the steam released has radioactivity lasting on the order of seconds, (2) if/when meltdown occurs, cesium/iodine radionuclides can exist in the steam but are apparently blown out to sea away from Japan, and (3) with the exception of controlled steam releases, a steel containment surrounding the core & related components will absolutely contain 100% of worst-case scenarios, meltdowns, etc.

At least that's how I understood it. Not an expert so I have no idea how much of it is true or not.