This is the last post in our series of posts on nuclear energy. Here, I shall describe the basic principles behind the design and continued operation of a nuclear reactor. In our last post, we looked at the techniques responsible for making fissile matter release energy as quickly as possible (in a nuclear explosion). Today, we will instead look at techniques to control nuclear fission reactions and usefully harness the resultant energy. Since there are a variety of various nuclear reactor designs and fuels, I shall stick to talking about a reasonably common (although a bit aged) design known as the pressurized-water reactor using U-235 fuel.