It's really simple, everyone. Any time there's a large amount of energy contained in a small amount of space, and that energy is released in an uncontrolled event, it's essentially an explosion.
A nuclear detonation in a nuclear power station is almost certainly "impossible" unless something is maliciously done to the system in order to turn it in to a bomb. But, by design, the system contains huge amounts of energy, potential energy, and if that energy is not appropriately controlled, contained, and released, there will be spectacular consequences. There are many places in any nuclear power station where that energy is stored or released, and all of those places have carefully designed control systems that, to hopefully excruciating standards, manage that energy. There are high-pressure systems, systems with nuclear potential energy, chemical potential energy, immense kinetic energy, systems with large amounts of heat, and systems that combine all those potential energy components. Each system has a set of failure modes that the engineers have hopefully accounted for in their designs, but for each failure mode, there's a risk that the failure exceeds the ability of the system to contain it, or even that the probability of that failure is deemed too low to even implement a failure-containment system. Even worse, if one (even small) failure triggers, or exacerbates another failure, then a cascade of failures can easily overwhelm whole system.
An exploding power plant is certainly possible - it has happened in a number of instances.
An exploding Nuclear power plant is also possible - it has happened in a number of instances, but the differences is that the explosion is not likely going to come from a system that compromises the containment of radioactive consequences (although even that happens in real life too, and not just to old/badly-maintained power stations).
In real life terms, there's a reason why all power stations (nuclear and non-nuclear) have comprehensive safety plans, failure management systems, and containment systems. Things break. In systems with more energy, the standards of both the primary and safety systems have to be more demanding. In nuclear power stations they are more demanding.
In factorio, though, we, the player, are responsible for ensuring that these standards are met, and that our power generation systems can contain/control their energy. Frankly, there should be changes made to our existing power systems (solar, steam) such that sometimes they explode, causing damage, for example, if you drive in to a steam engine, it should explode, damaging things around it, not just the engine itself. A Biter damaging an accumulator should take out all the other accumulators around it since they have batteries in them that can "vent with flame".
In factorio, we can throw a cluster-grenade too close to ourselves, and kill ourselves. If we behave badly with a nuclear station can we not kill ourselves too?