It is generally accepted that nuclear plants cannot explode through anything except planned detonation (i.e., an attack of some kind), and then it is not a nuclear explosion. A meltdown can release significant radiation, such as at Chernobyl, but probably only if the plant is as badly maintained as that one was. Most likely there would be no measurable release of radiation, which was the case at TMI. The "100% increase of radiation" reported by some media sources refers to radiation in the plant control room, which is normally a shirt-sleeves environment.