Neutronic Characteristics of Using Zirconium Diboride and Gadolinium in a Westinghouse 17 X 17 Fuel Assembly
There has been a trend by the utilities to extend the operating cycle of their power reactors in the production of electricity. In order to achieve this, variations in fuel composition, fuel pin arrangement, and burnable absorber distribution have been proposed. Burnable absorbers are materials that absorb the neutrons that cause a nuclear fission reaction and are consumed (burned) in the process. With a higher initial 235U fuel load, the burnable absorbers make it possible to extend the operating cycle while maintaining uniform power throughout the core. There are various schemes in the use of burnable absorbers. For example, Westinghouse uses an integral fuel burnable absorber (IFBA) rod in their fuel assemblies where the enriched UO2 fuel pellet is coated with a thin layer of zirconium diboride (ZrB2). Other fuel manufacturers such as Combustion Engineering, Siemens, General Electric, and Framatome incorporate gadolinium directly into the fuel matrix which yields the gadolinium-uranium (Gd2O3 - UO2) integral burnable absorber (IBA) rods. A drawback of burnable absorbers is that it produces, on a global level, an excess reactivity increase in the middle of the operating cycle due to the consumption of the poisons. Compensation is made by increasing the soluble boron concentration in the moderator. Operating experience has found that a more refined control over reactivity within the fuel assembly can be accomplished by using burnable absorbers which lead to better fuel utilization. There is, however, little evidence that multiple burnable absorbers are used simultaneously in any given fuel assembly in power reactors. It is believed that a proper combination of burnable absorbers, both IFBA and IBA's, would be able to minimize or level out the reactivity increase. Therefore, the focus of this study is to determine the operational characteristics of two different types of fuel assemblies, both having a 17 X 17 matrix design that are commonly used in today's pressurized water reactors (PWR). The first is manufactured by Westinghouse and the second by Babcock & Wilcox. Westinghouse has 9 different IFBA rod configurations ranging from 16 to 200 IFBA rods per assembly. The Babcock & Wilcox design has 4 different IBA configurations ranging from 8, 12, 16, and 20 IBA rods per fuel assembly with gadolinium concentrations ranging from 2, 4, 6, to 8 w/o gadolinium. An analysis of these assemblies will be processed and evaluated by using the Scale5.1 and PARCS software packages. This data will be used to create a hybrid fuel assembly which will employ the desired characteristics from both the Westinghouse and Babcock & Wilcox assemblies.
Networked Digital Library of Theses and Dissertations