Here is a NEJM review which explains things nicely and puts the history of plant accidents in perspective:
In the partial meltdown at Three Mile Island, the plant's containment structure fulfilled its purpose, and a minimal amount of radiation was released.2 However, there was no such containment structure in place at the Chernobyl reactor — the explosions and the subsequent fire sent a giant plume of radioactive material into the atmosphere. Although the Three Mile Island accident has not yet led to identifiable health effects,3-5 the Chernobyl accident resulted in 28 deaths related to radiation exposure in the year after the accident.6,7 The long-term effects of the Chernobyl accident are still being characterized, as we discuss in more detail below. The situation at Fukushima, though still in daily flux, will probably end up ranking between these two historical accidents in terms of radiation releases and health consequences.
More from the NEJM blog.