Fresh Fuel Assembly
In PWRs, the reactor core consists of assemblies of fuel rods featuring a zirconium alloy cladding, holding uranium oxide pellets (with uranium enriched to ~ 4% U-235) or MOX pellets (mixed uranium-plutonium oxides [(U, Pu)O2], with a Pu content of 5–10%). Fuel fabrication is the final step of the front end of the nuclear fuel cycle. In this step, a complete fuel assembly is fabricated. Since a fuel assembly consists of several structural parts, this step may be processed at different locations, and these parts may also be prefabricated.
The fuel assembly constitutes the base element of the nuclear reactor core. The reactor core (PWR type) contains about 157 fuel assemblies (depending on the reactor type). Western PWRs use a square lattice arrangement, and assemblies are characterized by the number of rods they contain, typically 17×17 in current designs. The enrichment of fuel rods is never uniform, and the enrichment is differentiated in the radial and axial directions. This arrangement improves power distribution and improves fuel economy.
Russian VVER-type reactors use a fuel characterized by their hexagonal arrangement but is otherwise of similar length and structure to other PWR fuel assemblies.
A PWR fuel assemblies stand between four and five meters high, are about 20 cm across, and weigh about 800 kg (of which about 500kg is uranium). The assemblies have vacant rod positions for control rods or in-core instrumentation. Control rods, in-core instrumentation, neutron source, or a test segment can be vertically inserted into a vacant tube called the guide thimble.
Special Reference: CEA, Nuclear Energy Division. Nuclear Fuels, ISBN 978-2-281-11345-7
Spent Nuclear Assembly
Spent nuclear fuel is a nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant or an experimental reactor), and a fresh fuel must replace that due to its insufficient reactivity. Spent fuel is characterized by fuel burnup, a measure of how much energy is extracted from nuclear fuel, and a measure of fuel depletion. Due to fuel depletion and fission fragments buildup, spent nuclear fuel is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and must be replaced by fresh fuel. It may have considerably different isotopic constituents depending on its point along the nuclear fuel cycle.
It must be noted that irradiated fuel is due to the presence of a high amount of radioactive fission fragments and transuranic elements that are very hot and very radioactive. Reactor operators must manage the heat and radioactivity that remains in the “spent fuel” after it’s taken out of the reactor. In nuclear power plants, spent nuclear fuel is stored underwater in the spent fuel pool on the plant, and plant personnel moves the spent fuel underwater from the reactor to the pool. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away. After several years (> 5 years), decay heat decreases under specified limits so that spent fuel may be reprocessed or interim storage.
At first glance, it isn’t easy to recognize fresh fuel from used fuel. From a mechanical point of view, the used fuel (irradiated) is identical to the fresh fuel. In most PWRs, used fuel assemblies stand between four and five meters high, are about 20 cm across, and weigh about half a tonne. A PWR fuel assembly comprises a bottom nozzle into which rods are fixed through the lattice, and it is ended by a top nozzle to finish the whole assembly. There are spacing grids between these nozzles. These grids ensure an exact guiding of the fuel rods. The bottom and top nozzles are heavily constructed as they provide much mechanical support for the fuel assembly structure. Western PWRs use a square lattice arrangement, and assemblies are characterized by the number of rods they contain, typically 17×17 in current designs. In contrast to the fresh fuel, which is simply shiny, the oxide layer forming on the surface of used fuel assemblies during the four-year fuel cycle makes them dark. Moreover, Cherenkov radiation is typical only for spent nuclear fuel. The glow is also visible after the chain reaction stops (in the reactor). The Cherenkov radiation can characterize the remaining radioactivity of spent nuclear fuel. Therefore it can be used for measuring fuel burnup.