Most of the dispersed radiation is from gases that have short half lives: once the leaks are contained it will dissipate quickly.
Naaah, I dont consider Cesium 137 as a "short lived nuclide". And its volatile besides. Ive seen reports of Cs in the atmosphere and that tells us that at least some of the Zr cladding has been compromised
Your post was addressed to someone who knows exactly what it means, but there are many posters here who actually worry about potential radioactive fallout (cesium, iodine, etc) beyond Fukushima Daiichi, first, and, second, don't know cesium from zirconium, so here's a quick update for them:
1. Quit worrying! This low-level radioactivity may keep seeping out of that plant's reactors and spent fuel pools for weeks to come but
it poses no danger to anyone beyond the current safe perimeter. Helicopter pilots dropping water on the plant - obviously passes last less than 1 hr since water tanks must be refilled with 7+ tons of seawater, pilots wear protective suits, helicopter provides shielding - receive radiation equivalent of 10 chest X-rays/hr.
2. Source for above is US-supplied drone and U-2 plane which have been taking detailed radiation readings over the plant at various altitudes.
3. Anybody in North America or Eurasia still worried - please ignore the clueless idiot we got as "US surgeon general" who advised residents to swallow potassium iodide tablets. If you now reside near sea level, and you move to a house located 1,000 ft uphill, you'll get more radiation than can ever be generated by Daiichi - even if this ongoing mess continues for weeks to come, as seems likely. The plant has to be cleaned up before getting buried in boron-loaded concrete, btw, because otherwise radioactivity will poison Japanese ground waters and unknowable volumes of Pacific ocean waters.
4. Anybody in the southern hemisphere, keep in mind the jet stream cannot cross the equator: here is today's jet stream map - centered on North pole:
5. Cesium exists in the atmosphere as a leftover from nuclear tests in both north and south hemispheres. Everyone born after 1949 already has some.