Generation of nodal neutronic parameters’ library based upon high-fidelity core-wide simulations - Part II: Partially inserted absorber rods

Partial absorber rod insertions require special treatment in nodal core simulators to accurately predict a reactor core's attributes. Several methods have been employed to address the so called rod cusp effect on reactivity. This occurs due to improper flux-volume weighting of nodal parameters. The NESTLE nodal code has been employed to examine four alternative approaches for addressing not only the effect of partially rodded nodes on reactivity, but also on pin power with a focus on predicting powers in the unrodded and rodded spans of partially rodded nodes. Approach A uses the 1-D axial transverse integrated equations solution spanning the original axial mesh. Approaches B and C solve the nodal balance and 1-D axial transverse integrated equations on an axial sub-mesh, with Approach B doing so for just the partially rodded nodes and Approach C doing so for all the nodes in the radial planes containing an axial sub-mesh. Approach D uses a weighted average of partially rodded and axially adjacent nodes fluxes. Differences between NESTLE and the higher fidelity code VERA in steady state reactivity, transient power level, and nodal and pin-span powers have been determined for the four approaches. Approach A has been determined to be inadequate. Approaches B and C are both adequate and nearly equivalent, with Approach C demonstrating superior performance in predicting the power distribution within nodes adjacent to the partially rodded nodes. Approach D performed well regarding predicting core reactivity but was inadequate in predicting the power distribution within partially rodded nodes.