Are we prepared?

Listening to the news of a possible meltdown of the Nuclear Power Plant in Japan was more than unsettling. They said the "worse case scenario", would be if the reactor indeed melted down, it would fall into it's enclosure...if the enclosure didn't hold, it would leak out into the air, ground and water. My question is: if we are brilliant enough to build and harness Nuclear Activity, why are we not capable of anticipating a worst case scenario and build a structure THAT WOULD contain such a tragedy?  I wonder if we in the US and the countries that have Nuclear Plants are planning on fortifying these structures. A fail safe plan on something so dangerous should have been in place long before we write it off as... Hindside is 20/20.