Dimensionally Adaptive Neutron Kinetics for Multidimensional Reactor Safety Transients - I: New Features of RELAP5/PANBOX

A dimensionally adaptive, automatic switching algorithm has been developed for the RELAP5/PANBOX coupled thermal-hydraulics and neutron kinetics system to switch between three-dimensional (3-D), one-dimensional (1-D), and point neutron kinetics models during a transient calculation. The 3-D, 1-D, and point neutron kinetics models are developed and analyzed. The basis of this development is the consistent and stable nodal expansion method. The 1-D and point neutron kinetics models are derived in a unified manner from the 3-D model using the adiabatic approximation. The operator formulation of perturbation/sensitivity theory is consistently used to determine the reactivity for the point-kinetics model. Furthermore, the new features of the coupled RELAP5/PANBOX code are described. This provides the basis underlying the dimensionally adaptive algorithm.