Friday, March 18, 2011

An Introduction to Nuclear Power

As I've discussed recently, a lot of information on nuclear power has been long on hype and short on accurate information. My own experience with friends and family has shown me that many otherwise intelligent people have little to no understanding of nuclear power. I recommend that people refer to Wikipedia for detailed explanations of how nuclear fission works or how a nuclear reactor uses fission to make power, since those subjects deserve a more in-depth treatment than I can given them in a blog post. However, the news coming out of Japan may make more sense if we briefly review what nuclear fuel is and how the fuel works.

A uranium fuel pellet
Most commercial reactors use enriched uranium in the form of pellets for their fuel. Uranium consists mostly of the isotope uranium-238 along with a small amount of other isotopes. When uranium is enriched to be used as nuclear fuel, the isotope uranium-235 is increased above the 0.7% found in natural uranium (most power plants use fuel that's 3% to 5% uranium-235). As shown in the photo, these pellets are relatively safe to handle before undergoing nuclear fission. The pellets are sealed into tubes (often called "cladding"), which are usually composed of a zirconium alloy. Zirconium is the material of choice due to certain favorable properties, not the least of which is its near-transparency to neutrons (neutrons will have to be able to pass from one tube to the other in order to sustain a fission reaction). These tubes are then bundled together into fuel assemblies. In a boiling water reactor (BWR), like those used at Fukushima, the assemblies are encased in an additional thin-walled tube. A removable control rod is inserted into the fuel assembly to absorb neutrons and prevent a premature fission reaction. In a pressurized water reactor (PWR) (the most common reactor type in the United States), the control rods enter the fuel assembly from above; in a BWR the control rods enter from below. Notice that all elements of the fuel, the cladding, and the control rods are made of solid materials. I am always surprised to find that many people believe that nuclear fuel is some sort of glowing liquid.

Nuclear fuel rod assemblies
The assemblies are inserted in a reactor vessel and the vessel is sealed. To start up the reactor, the control rods are withdrawn. Neutrons produced by the spontaneous fission of a relatively small number of uranium atoms strike other uranium atoms nearby, which causes still more fissions. A nuclear chain reaction (or "criticality") occurs when, on average, at least one neutron produced by each fission goes on to produce an additional fission. The control rods are partially inserted or removed as necessary to keep the reaction at the desired rate. The control rods contain materials like cadmium or boron that absorb neutrons and prevent an excessive rate of fission. Water within the reactor cools the fuel assemblies, slows the emitted neutrons (slower neutrons are more effective at producing a fission reaction in uranium), and is used in conjunction with steam turbines to produce electricity. In a BWR, the fission reaction boils the water within the reactor vessel. In a PWR, the water in the vessel is maintained at a pressure that prevents it from boiling; the heat from the water in the pressurized "primary loop" is transferred to water in the "secondary loop", which is allowed to boil.

Fuel assemblies in a nuclear reactor opened for servicing

The typical commercial reactor will operate for about 12 to 24 months before the fuel must be replaced. By this time, much of the uranium-235 within the assemblies has been changed into fission products (some of the most common being iodine-131, cesium-137, and strontium-90). Isotopes of plutonium will have also been produced when neutrons were absorbed by atoms of uranium-238. The reactor is shut down by fully inserting the control rods. At this point the reactor is "subcritical", meaning that, on average, each fission produces less than one additional fission. The fuel assemblies are then removed from the reactor and are placed in a water-filled pit or pool. Although the nuclear chain reaction has been stopped, the water is required to cool the assemblies since radioactive decay of the fission products produces a significant amount of heat. Eventually the rate of decay will fall sufficiently that the fuel assemblies can be removed from the pool and be placed in a dry cask.

Spent fuel assemblies in a cooling pool

I'll be explaining what the above means for Fukushima in a later post.

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