Improved reflector modeling for LWR analysis - 347

Special treatment of the reflector in reactor analysis is required due to the drastic differences of neutronics properties between the core and reflector regions. The strong spectrum change observed at core/reflector boundaries combined with geometry complexity, material and structural heterogeneity of radial and axial reflectors make such treatment a challenging task. The correct modeling of the reflector response is important for accurate predictions of core power distribution especially in regions next to the reflector. For this reason special care is taken in generation of reflector homogenized cross-sections and discontinuity factors (DFs). Historically, one-dimensional (1-D) color set problems are used for the reflector, which is different from the unit fuel assembly cross-section generation models. The investigations presented in this paper are further extension of studies performed elsewhere to achieve a more correct modeling by introducing improved color set models for reflector cross-section and DF generation and parameterization. These color set models more accurately capture the 2-D effects that occur on reentrant surfaces. From the transport solution of the color set model, discontinuity factors are calculated, which in turn preserve the transport solution in the nodal code calculations. Two sensitivity studies have been performed. The first study evaluates the effect of the size of the color-set model on reflector constants as compared to full 1/8 core sector of symmetry. The second sensitivity study is conducted with the aim of determining a parameterization for the discontinuity factors as function of the core conditions such as boron concentration, moderator temperature and density. In addition, the effects of the loading pattern next to the reflector region on the discontinuity factors are examined through the use of color sets that include Mixed Oxide (MOX) fuel and the traditional UO2 fuel. The prediction improvements that are achieved in both the global eigenvalue and power distribution from the selected optimal 2-D color sets as compared to the 1-D models currently in use are discussed in this paper for two nuclear power plants. The newly calculated discontinuity factors show an improvement in predicting the global eigenvalue and power distribution and correcting the power tilt that was previously observed. (authors)