Since the author seems so certain that the third containment shell would hold the meltdown safely, and explosions keep happening: Can someone answer me why won'

Since the author seems so certain that the third containment shell would hold the meltdown safely, and explosions keep happening: Can someone answer me why won't they just allow it to melt down, so they can clean up the rest and get the whole nuclear plant back online? Is it just an economical reason? Or is there any other risk?