AI is not going to take over the world. AI isn't even that smart. I'm sick to death of hearing these ridiculous stories from science celebrities that people take seriously because they figured out something, if that logic holds true then we should all use alchemy because Isaac Newton was a big fan of this.
To understand why this isn't the case you need to understand the AI, the deep learning that is used so often in these scare stories works by using a set of input neurons, a hidden layer of a few up to millions of neurons, and an output layer.
An input is given and it propagates though the structure, and gets an output, if the output doesn't match the input then the structure is adjusted slightly and this is repeated until it does, this is called the learning phase. The important word here is structure the thing about these structures is it takes ages to train them, but when you do they can pick out patterns from examples they previously haven't seen before. If you then give it a completely different set of input data to learn from the structure changes to fit that data, which means the previously learned stuff might be lost or altered in such a way that the network no longer becomes good at finding these patterns and thus need to be re-trained.
This means that these learning structures are only good for specialised things such as reading an individual's hand writing. The larger the hidden layer the more things it can do without risk of interfering with another structure. The thing that makes neural nets so good though is they are short and don't take much time in order to do specialised things, if you add in millions of more neurons it will take longer to process the information and become less useful.
Neural nets are a tool, a specialized tool that are only good at doing a few things at a time, killing humans isn't that easy, not to mention we has humans would have to set up the scenario perfectly for this to happen, and I don't see scientists getting authority to hook neural networks up to nuclear systems any time soon, or ever.