Dimensionally adaptive neutron kinetics for multidimensional reactor safety transients - II: Dimensionally adaptive switching algorithms

A dimensionally adaptive, automatic switching algorithm is presented that h as been developed for the RELAP5/PANBOX coupled thermal-hydraulics and neut ron kinetics code system to switch between three-dimensional (3-D), one-dim ensional (1-D), and point neutron kinetics models during a transient calcul ation. The su itching criteria from higher- to lower-dimensional models are based oil the time evolution of the flux shape, while the switching criter ia from lower-dimensional models to the 3-D model are based on error estima tes and reactivity criteria. Calculations of main-steam-line-break, control -rod-ejection, and boron-dilution transients have been used to validate the dimensionally adaptive automatic switching algorithm. These validation cal culations show that the results produced by the automatic switching algorit hm retain the accuracy of the 3-D reference calculations. Notably, they are considerably faster, typically requiring only 30 to 70% of the CPU time ne eded by the 3-D reference calculations. Furthermore, our calculations confi rm that a 3-D neutron kinetics model is indeed required for these reactor s afety transients by showing that the point-kinetics and 1-D models are by t hemselves very inaccurate.