Can anyone verify there is, in fact, a 3rd containment at the reactor in question?
First, the GE BWR-1 reactor in question has NO 3rd containment. If the "pressure cooker" is breached, the fuel is exposed to the environment.
Secondly, Cesium-137 has a 30yr half-life. If you were to get a big breath full of that steam, you would likely have health problems, if not serious health problems. That's not to say it was concentrated or didn't adequately disperse at sea.