How Does a Nuclear Fission Power Plant Work?

The Reaction

At the heart of nuclear fission power plants is the reactor core. It is here that reactions are initiated, sustained,and regulated. The substance at the core of a nuclear reactor can be either plutonium-239, plutonium-241 or uranium-235. While the physics are quite complex (nuclear physicists can run circles around brain surgeons and rocket scientists), the basic concept involves the firing of a neutron into a single atom. If we were to take an atom of uranium-235 and fire a neutron at it, the resulting reaction would break the uranium-235 into lighter elements and potentially scatter one or more neutrons. The uranium-235 is said to be fissile if these scattered neutrons are sufficient enough to break down other uranium-235 atoms around it in a chain reaction. The goal of a reactor core is to gather enough of the radioactive isotope in a dense enough space to allow for the chain reaction to continue indefinitely.

No One Wants an Explosion

Chain react indefinitely, and the reaction inevitably becomes more and more violent. Since we're building a power plant and not a bomb, the next step in operating a nuclear plant is containing the exponential growth of neutron reaction. The two main options are neutron poisoning and neutron moderation. The former involves a series of long rods composed of neutron-soaking elements such as xenon. The deeper the rods are inserted into the core, the more the rods can absorb neutrons and slow the reaction. In contrast, neutron moderators change the proportion of neutrons that will continue to fission, controlling the number of neutrons that are zinging around. The most commonly used moderator is water, with far fewer plants using graphite or heavy water (contains deuterium). In addition to neutron moderation, water is often used to cool the entire operation, with miles of pipeline drawing heat away from the reactor.

Making Power

With all the aspects of control surrounding the carefully guided nuclear reaction, it can be hard to discern from where the power is coming. Nuclear reactors actually produce usable heat energy through several methods. When nuclei collide into nearby atoms, kinetic energy is converted into heat energy. Gamma rays emitted by the fissile material heat the metal in the reactor. Even the radioactive decay of fissile material inside the reactor produces a great deal of heat. The cooling system already installed, the one that draws cool water over the reactor, actually doubles as the means of power generation. As the water is heated by the reactor, it is turned to steam, which is carried away and used to power a turbine, producing the kind of power that can light up an entire grid.

Resources

• List of Facilities
• World Nuclear Association