It's unlikely that hackers could manage even a Chernobyl-scale problem on a modern nuclear plant. Chernobyl had several horrendous flaws. For one, it had a positive feedback: as the fuel got hotter, the reaction sped up. With modern plants the opposite occurs.
Also Chernobyl had no containment dome.
Even the old GenII designs in the U.S. have much better inherent safety than Chernobyl had. There's no way to hack away physical barriers. Even TMI, our worst accident ever with a full meltdown, did not breach the containment barriers.
And of course with any commercial plant, there's absolutely zero chance of an actual nuclear detonation, much less a thermonuclear one as mentioned in the comment above. The fuel just isn't enriched enough to work as a bomb.
