A nuclear reactor exploding like a bomb is an element of many stories ("Pacific Rim", etc). From some of these stories it looks like at least some reactors can be blasted just by sending a command through the user friendly interface. How much of this is true?
A nuclear reactor is a complex device, not designed to be a bomb, and a bomb is also a complex device built for the different purpose. The known reactor accidents (Chernobyl, Fukushima, SL-1) resulted in dangerous contamination but all explosions were just "steam explosions" or at least much weaker than could be achieved with the dedicated nuclear bomb.
Is there a fundamental reason why nuclear reactors do not explode as true bombs or is this simply because the design is focused to make them safer and not to behave so? Can we build a useful nuclear reactor capable of the true nuclear explosion if required?