- Last Updated: 28 March 2016 28 March 2016
Nuclear power plants produce power by creating large amounts of heat. In this respect, they're very similar to coal- and oil-fired power plants. They key difference is in how the heat is generated. Rather than burning fossil fuels, nuclear plants use the heat of a self-sustaining nuclear fission reaction.
A form, or isotope, of the element uranium (U) is the most common fuel for commercial nuclear power. U-235 is very rare. In natural uranium, 99.3% of the atoms are U-238, which is stable and not useful as fuel. U-235, as it is called, makes up only 0.711% of what we find in nature, and it's necessary to use very complicated means to enrich natural uranium to at least 3-5% U-235 before it can be used as reactor fuel. This is known as LEU (Low-Enriched Uranium), as opposed to HEU (Highly Enriched Uranium) which is the weapons-grade stuff ... 20-90% U-235.)
U-235 is useful because it has an atomic structure that isn't stable; atoms of U-235 are constantly decaying, or falling apart, releasing tiny, subatomic particles called neutrons. Neutrons are like bullets, moving very, very fast. When one of them strikes another atom of U-235, the struck atom splits and releases neutrons of its own. Meanwhile, the pieces of the former U-235 atom (usually isotopes of Iodine (I) and Strontium (Sr)) are sent spinning away at high velocities, and as they slow down by colliding with other atoms, their kinetic energy is converted to heat in the fuel. Sometimes the fission products are unstable and decay also, releasing even more neutrons. This process of splitting atoms is called nuclear fission.
The intensity of this reaction is proportional to fuel density. The closer together we pack our U-235 atoms, the greater the chance that a neutron from one will actually split another, rather than flying uselessly off into space. If we achieve a state where exactly one neutron from each and every fission causes another fission, we call the reaction self-sustaining. Except when starting up and shutting down, this is the region where we want the reactor to operate, because it will produce constant thermal power (heat).
In a nuclear reactor, then, our goal is to bring a great deal of fuel into close proximity, so that enough atoms will be split by flying neutrons to create a self-sustaining chain reaction. The reactor core at TMI contained 36,000 long, slim rods known as fuel pins. Each pin consisted of a thin zirconium alloy tube filled with uranium oxide fuel pellets, as shown in the drawing. The pins were bound together into a number of bundles or assemblies, and were held vertically in racks also made from a zirconium alloy. These racks hold the fuel pins spaced closely enough to sustain the reaction, but far enough apart that cooling water can pass among them to carry away heat.
With such awesome power at our disposal, we need a means of control. To reduce heat, we must reduce the number of atoms split, and therefore we must reduce the number of neutrons that are flying around. So, interspersed among the fuel pins are several rods made of hafnium, cadmium, or boron, elements which have the property of absorbing neutrons. These control rods are used like throttles for the reactor. As they're pulled out of the core, heat output increases. As they're pushed in, heat output decreases. In an emergency, or in the event of a loss of power, magnetic drive mechanisms on the rods release them, allowing them to fall quickly to their fully inserted positions in the core. This is called a reactor trip or SCRAM, and is a very fast way of shutting down the reactor.
(The word "SCRAM" originated in the first nuclear reactor, in which the main safety control rod was suspended above the core on a rope. To quickly shut down the reactor in an emergency, the rope was cut with an axe. The person responsible for doing this was called the Safety Control Rod Axe Man.)
The entire reactor core is encased in a large, thick-walled casing known as the pressure vessel. Cooling water is forced through this vessel, via side-mounted nozzles, to carry heat from the reactor core to the rest of the plant. The control rods enter the reactor vessel from the top; in-core instrument wiring enters the vessel from below. The diagram you see at left is an actual layout diagram for the core at TMI, looking down from the top, showing locations of control rods, inlet and outlet nozzles, and instrumentation.
The interior of the reactor pressure vessel is at once one of the most inhospitable places on earth, and also one of the most heavily instrumented. Thermocouples and flow instruments operate here in an environment where hard radiation can make ordinary metals turn brittle and crumble. The water here is hundreds of degrees above its normal boiling point and under a pressure of more than 2,000 PSI.
In order to ensure that power is produced most efficiently in all parts of the core, and also to ensure that the fuel "burns" evenly, the control rods are arranged in groups which are independently adjustable. The plant's design documents specify which rods are to be raised, and in what order, during each operating regime from cold shutdown to full power. Rod positions can also be altered as the core becomes depleted so that the fuel "burns" evenly among the fuel bundles.
Note that depicted on the diagram are several neutron detectors, which are used to measure reactor power. The amount of neutron activity in the core is directly and reliably proportional to the thermal power the reactor is producing, so by monitoring neutron flux we can keep tabs on the reactor's heat output. The instruments are in three ranges because no one instrument could be accurate over the wide range of power levels the reactor covers during startup and shutdown. The source range instrument is used at extremely low power levels, primarily when the reactor is shut down or is being started up from a cold shutdown. The power range instrument covers levels found when the reactor is operating at high power. The intermediate range instrument covers levels in between the other two instruments.
The top of the reactor pressure vessel is a heavy plate known as the head. The head is removed during maintenance and refueling operations when the reactor is completely cold. It's secured to the top of the pressure vessel by dozens of bolts which are all carefully torqued. Since the pressure within the vessel is in excess of one ton per square inch, one can scarcely imagine the force that acts on the head when the reactor is at full pressure. These are strong bolts!