You've been reading too much science fiction and/or listening to the anti-nook kooks. The physics of LWR technology does not allow a nuclear explsoion to take place. The fissile material is too dispersed and there is no credible mechanism to assemble it into a supercritical mass which would produce even a low-yield nuclear explosion.
And no, NOT like Chornobil. That was a power transient that produced fuel damage and moderator burnup. The explsoive reaction was from steam and hydrogen. In an unconatined system, like the RBMK design, disperion can occur. LWR system like we have in this country have containment, so widespread dispersion is mitigated.