Lots of great answers here, but only a couple really get down to brass tacks. TGFlynn answers the question of why we can't use fusion bombs: "Thermonuclear bom

Lots of great answers here, but only a couple really get down to brass tacks. TGFlynn answers the question of why we can't use fusion bombs: "Thermonuclear bombs require a fission explosion to trigger them (in fact I believe most produce more energy from fission than from fusion)." So in short, even setting off "fusion" bombs produces lots of bad byproducts. The other part of your question is, "why not a big piston and cylinder?" Car engines are heat engines as well. They convert heat in to kinetic energy (motion). The problem is that they're relatively inefficient when compared to the typical means of heat/motion conversion used in a power plant. They're also far more complex and require more maintenance than other, simpler designs. The design of a car engine has advantages that suit it well to use in an automobile. Namely that it's power output is easily variable. You don't need peak horsepower when sitting at a stoplight, so you throttle the engine back and generate far less power. This requirement is almost nonexistent in power plants. There is some variability, but you're dealing with average power consumption from thousands of households. The rate of increase/decrease is much, much slower than you find in an automobile. Because of this, power plants use heat engines that might require a lot of energy to get started, but are more efficient once operating. Steam generators coupled with turbines are most common. The heat is used to boil water, which creates steam, which is used to drive some type of turbine. Turbines are wildly efficient if they're designed properly. The primary inefficiency in a steam/turbine power plant is startup and shutdown, but plant operators try to minimize this impact by employing operation strategies that distribute load in a way that can scale up and down minimally. So, why not use a large piston in cylinder design? Because it's less efficient, more complex, requires more maintenance, and requires more materials to construct.