What's The Difference Between A Nuclear Reactor And Nuclear Weapon?

Let's get the obvious difference out of the way first: A nuclear reactor generates electricity that can power the world around us. A nuclear weapon, well, we know plenty enough about the damage one of these devices can cause. However, this leads to an interesting question: What prevents a power station from becoming a nuclear weapon (and vice versa)? The most common type of nuclear reactor uses two water loops; one to cool the reactor and another to generate power. In other words, the defining difference is: Nuclear reactors rely on controlled fission, whereas nuclear weapons require an uncontrolled reaction.