I do not think “cracked” is a word you want to ever see in this context:

Japan’s nuclear regulator said one reactor core at the quake-damaged Fukushima Dai-Ichi power plant may be cracked and leaking radiation.

Not to be too alarmist, because that never happens on blogs…but I do believe a cracked reactor core could lead to one of the worst-case scenarios.

But one of the disadvantages is that the containment structure is a lightbulb-shaped steel shell that’s only about 30 or 40 feet [nine to 12 meters] across—thick steel, but relatively small compared to large, dry containments like TMI [Three Mile Island]. And it doesn’t provide as much of an extra layer of defense from reactor accidents as containments like TMI [do]. So there is a great deal of concern that if the core does melt, the containment will not be able to survive. And if the containment doesn’t survive, we have a worst-case situation.”

And the Japanese nuclear safety agency adds to the concern:

The agency said while the reactor appears to have partially retained its function to contain radiation leaks, there’s a strong possibility that some part of the reactor is now damaged and the containment function is weakening.