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.