The key feature of LWRs fuel cycles is that many fuel assemblies in the core and these assemblies have different multiplying properties because they may have different enrichment and different burnup. Generally, a common fuel assembly contains energy for approximately 4 years of operation at full power. Once loaded, the fuel stays in the core for 4 years, depending on the design of the operating cycle. During these 4 years, the reactor core has to be refueled. During refueling, every 12 to 18 months, some of the fuel – usually one-third or one-quarter of the core – is removed from the spent fuel pool. At the same time, the remainder is rearranged to a location in the core better suited to its remaining level of enrichment. The refueling machine usually does this. The removed fuel (one-third or one-quarter of the core, i.e., 40 assemblies) has to be replaced by fresh fuel assemblies. 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.
The refueling operation is divided into six major phases:
- preparation,
- reactor disassembly,
- fuel handling,
- reactor reassembly,
- preoperational checks, tests,
- reactor startup.
At first glance, it isn’t easy to recognize fresh fuel from used fuel. From the 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 visible also 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.
Refueling and Samarium Removal
An important difference between Xe-135 and Sm-149 is that samarium 149 is a stable isotope and remains in the core after shutdown. There are only two ways of samarium removal, and one of these processes is the samarium burning up when the reactor is in power operation. The samarium burnup rate depends upon the neutron flux and the samarium 149 concentration (i.e., the reaction rate).
The second process is associated with refueling. During refueling, one-third or one-quarter of the core is usually removed from the spent fuel pool and replaced by fresh fuel assemblies. At the same time, the remainder is rearranged to a location in the core better suited to its remaining level of enrichment. Therefore, refueling naturally decreases the overall samarium content in the core.
