Science / Medicine : The Fundamentals of Using Uranium--and Controlling It
All commercial nuclear reactors are fueled by essentially the same thing: rods containing a uranium compound, typically uranium dioxide. When struck by a neutron, uranium atoms in this compound can fission, or split into two smaller atoms (of lighter atomic weight). This fission reaction produces more neutrons, which in turn cause fission in other uranium atoms, setting up a self-sustaining chain reaction.
Enormous quantities of heat are released by this reaction. Commercial nuclear reactors use this heat to generate steam, which drives a turbine to generate electricity. The heart of the reactor is a pressure vessel that houses the fuel rods in an assembly called the core. Operators control the reaction within the core by inserting rods made of materials that absorb neutrons.
In most commercial reactors, water is also important. Besides keeping the fuel rods in the core from overheating dangerously, water slows down the neutrons emitted during fission reactions, increasing the probability that they will cause fission in uranium (fast neutrons have much lower probabilities of causing a fission reaction).
Water has worked reasonably well as both coolant and moderator, but other materials, notably graphite, also have been used in experimental and demonstration plants built since the mid-1960s in the United States, and in operational plants in Europe. Although commercial reactors of this type are for the most part no longer being built, industry observers think that advanced versions of these plants will be major power sources of the future.
A reactor that uses helium gas for cooling and graphite for moderation has been developed by GA Technologies in San Diego. Fuel, in the form of minuscule uranium-carbide pellets, is embedded in graphite blocks. Helium gas flows through holes in the blocks, keeping them cool--if several thousand degrees Fahrenheit can be called cool. The advantage is that graphite remains solid at temperatures up to nearly 6,000F, minimizing the chance of radioactive materials from the fuel spreading beyond the reactor core in an accident. Another advantage is that helium is an inert gas that would not corrode or erode plant structures, even at high temperatures. Such corrosion is a chronic problem in water-cooled designs.
Lack of corrosion is one of many advantages of a second type of advanced reactor under development in the United States. Cooled by liquid sodium metal, it has no moderator at all; the nuclear reaction is sustained by using fuel containing higher densities of fissionable (more reactive) nuclear material. Although extremely corrosive to some substances, liquid sodium has almost no effect on stainless steel, and it is also a much better heat conductor than either helium or water.
Though very hot water and steam tarnish steel fixtures in a matter of hours, such fixtures are largely unaffected by liquid sodium, according to researchers at Argonne National Laboratory’s Idaho campus, where an experimental liquid-sodium-cooled reactor has been in operation for more than 20 years.
