Although visually spectacular, these explosions are not necessarily dangerous in terms of releasing radioactivity. The buildings are an external shell, with the task of sealing radioactive materials falling to a metal containment vessel constructed inside the concrete shell.

"The explosion... wasn't a terribly important event," according to Malcolm Grimston from the Energy Policy and Management Group at Imperial College, London.

"The building was designed to fall outwards" - preventing damage to the thick steel containment vessel inside.
http://www.bbc.co.uk/news/science-environment-12726628

A little reassuring I thought. That is assuming that he/they really know what they are talking about.