I have focused on nuclear reactors that utilize nuclear fission to generate power because they have existed and functioned for decades. I have briefly covered some esoteric reactor types such as sodium cooled fast breeder reactors, thorium reactors and small modular reactors although they not proven technologies even after a great deal of research and development. Today I am going to begin a series of posts on the possibility of utilizing nuclear fusion reactors for power generation. My only mention of fusion in the past has been in the context of hydrogen bombs where a fission bomb is used to ignite a fusion reaction.
Nuclear fission occurs when a heavy unstable nucleus breaks apart, releasing energy and leaving behind nuclei of lighter elements which may or may not be radioactive. Nuclear fusion occurs when the nuclei of lighter elements fuse to form heavier elements, releasing energy in the process.
Our sun is powered by nuclear fusion. Hydrogen nuclei fuse to form helium nuclei. This process can continue up the periodic table until it hits iron. There is a mathematical model of fusion called the nuclear packing faction curve. It shows how much energy can be derived from a particular fusion reaction. Hydrogen to helium produces the most energy. with less and less energy as you get to heavier and heavier elements. Beyond the formation of iron, the heavier nuclei actually require energy to fuse instead of producing it. Stars form concentric spheres where heavier and heavier elements form in the core. When iron dominates, new processes take over that sometimes lead to collapse and violent novas. It is in such novas that elements heavier than iron are formed.
When scientists came to understand the nuclear fusion process going on in the sun, they realized that if they could harness fusion for power, it would be able to provide huge amounts of energy for human civilization. Initially, it was thought that the sun was just a glowing ball of gas compressed by gravity to the point where the fusion reaction was initiated. According to existing theories at the time, there would be no internal structure to the gas ball. Unfortunately for the effort to develop fusion for energy production, it turns out that the sun has a great deal of internal structure with a number of different processes happening simultaneously in different zones within sphere. When scientists tried to reproduce the fusion reaction by compressing hydrogen gas, instead of a uniform ball of gas, various sorts of transitory structures emerged which interfered with the fusion process and impeded their progress.
There is an old joke that fusion power is just forty years away but, like a rainbow, as time passes, it remains forty years away. A great deal has been learned about stellar dynamics and the process of nuclear fusion over the past few decades and progress has been made. Recently, for the first time, researchers were able to get more energy out of a fusion reaction than they had put in for very brief time. There is still far to go but a milestone has been passed. In coming posts, I will deal with the physics of, the history of and the prospects for safe and economical generation of electricity via nuclear fusion.
Internal structure of our sun:
