The common nuclear fuel uranium oxide also known as UOX is usually compressed and cooked into cylindrical ceramic pellets. These pellets manufactured to exacting standards and are machined to precise dimensions. The pellets are then sealed into long metal tubes called fuel rods
Different reactor designs use different metals for the rods. This layer of metal is called cladding and prevents the nuclear fuel from contaminating the coolant. Originally, stainless steel was used because of its strength and resistance to corrosion. Now a zirconium alloy is used in most reactors because it is even more corrosion resistant than stainless steel and has low neutron absorption. Fuel rods are combined into assemblies which are used to create the core of a reactor.
In a pressurized water reactor (PWR), the one centimeter diameter tubes are filled with helium to improve heat conductions. They are assembled into square bundles that are from fourteen rods by fourteen rods to seventeen rods by seventeen rods. Many rods are placed end to end to from the bundles with from one hundred eighty to two hundred sixty rods per bundle. One hundred twenty to one hundred ninety bundles are loaded into the reactor core. There is a hollow center in the bundles which allows control rods to be inserted.
Boiling water reactors have rod and bundle configurations that are similar to the pressurized water reactors but, in addition, each fuel rod bundle is surrounded by a thin tube. This helps prevent local variations in neutron flux and heat exchange with the circulating coolant. There are about ninety fuel rods in each bundles and up to 800 bundles in the core.
Canadian Deuterium Uranium or CANDU reactors have fuel bundles that are ten centimeters in diameter and one half meter long. They contain ceramic uranium oxide pellets in zirconium alloy tubes that are welded to zirconium alloy plates at each end. Each bundle has about fourty fuel rods arranged in concentric rings. They weight about twenty kilograms and cores have from fourty five hundred to sixty five hundred bundles.
There are many other configuration of nuclear fuels, rods, bundles and cores but these are used mostly for research and military applications.
In the high radiation environment of a reactor core, materials can swell, crack, change porosity. As different isotopes are produced by the fission process, there can be pitting, formation of bubbles, buildup of such isotopes, out gassing and other processes that change chemical properties and material properties. One important property is thermal conductivity. As all these changes take place, the thermal conductivity of uranium oxide changes and overheating of the center of the fuel pellets can occur.
Every configuration of nuclear fuel elements has its benefits and drawbacks. There is ongoing research into what chemical composition and physical configuration is best for the nuclear fuel needed to create a economical, reliable and safe nuclear reactor.