No, a nuclear explosion cannot occur at commercial nuclear plants. Fuel for nuclear plant uranium is mined from the earth and then goes through the process of "enrichment." From that process, comes uranium-235 (which makes up approximately 4% of nuclear fuel used at a commercial facility) and uranium-238 (which makes up the other 96% of the fuel). In order to have an explosion, unranuim-235 must make up nearly 100% of the fuel. Scientifically speaking, an explosion at a nuclear facility in the U.S. would counter the laws of physics.