The news reports still don't know what part of the plant is blowing up here. It could be the turbine building. The main generator uses hydrogen gas for cooling, but that explosion is pretty big...
If the reactor itself did explode, this could be as bad as Chernobyl, depending on the condition of the reactor. Chernobyl was particularly catastrophic because it was actually burning, and spewing particulate fuel material out with the smoke over a wide area. If the reactor here simply exploded due to the pressure but otherwise remained more or less in tact, the radiation release would hopefully not be as bad. Furthermore apparently the prevailing winds are going out to sea, so there would be minimal exposure as well. In any event, extremely bad situation here.