2.15.3.2.1 PWR UO2 fuel assembly

Figure 432 shows an example of a PWR fuel assem­bly. PWRs have 197-230mm square, ductless assem­blies that traverse the full 2635-4550 mm height of the core. They comprise a basic support structure of unfueled zirconium alloy guide tubes attached to the top- and bottom-end fittings, an array of 14 x 14 to 18 x 18 fuel elements (minus the number of guide tubes), and several axially spaced grids that hold the array together. About half of the assemblies have rod control clusters attached at their upper end; these consist of 18-24 slender stainless-steel-clad absorber rods of AgInCd alloy or B4C, individually located in the guide tubes. The absorber rods are withdrawn for startup and are repositioned after

Control rod guide thimble

Instrumentation guide thimble

refueling; the reactor is controlled at power by altering the concentration of an absorber (boric acid) in the coolant. The bottom-end fitting is located on the core grid plate and the assembly is spring loaded against a hold-down system to compensate for differential expansion or growth during irradiation.

Fine control is obtained by incorporating a burn­able poison like Gd2O3 in some of the elements, in which it is admixed with UO2 in the core region, and with the upper and lower sections of natural UO2. By minimizing power changes in this manner, the inci­dence of pellet-clad interaction (PCI) failures can be kept to very low, acceptable values. Various improve­ments in fuel assembly design have been adopted. To improve reliability, for instance, debris filtering was adopted in the structural design of the bottom part of the fuel assembly, the grid structure design was mod­ified against fretting corrosion, and an intermediate flow mixer grid was added to enhance the margin to depart from nucleate boiling (DNB). Zirconium alloy grids for better neutronics, optimized distribu­tion of fissile and fertile materials, and a burnable poison to improve fuel cycle economy and to extend reactor cycle length were all introduced for econ­omy in the current assembly designs, as also the removable top nozzle to reduce operation and main­tenance costs.