Open upper plenum of LOF thermal hydraulics and inherent control rod insertion

In liquid-metal reactor (LMR) hypothetical transients for which normal scram is postulated not to occur, the thermal expansion of the control rod drivelines (CRDs) as they are washed by the hotter core effluent tends to insert the control assemblies (CAs) further into the core, thereby providing negative reactivity. A number of concepts to enhance the heatup-induced elongation of drivelines is being proposed involving both design features of the drivelines as well as flow control features of the drivelines and the upper internals structure (UIS). Reported here are the results of an analysis in which the COMMIX-1A computer code was used to investigate the three-dimensional thermal-hydraulic behavior in the upper plenum of a 425-MW(t) pool-type LMR during a loss-of-flow (LOF) transient and its influence on the driveline heatup and expansion. The calculations consider an open plenum geometry, which does not incorporate a UIS or CRD shroud tubes such that the drivelines are directly exposed to the multidimensional plenum flow. The objective of the present work is to define reference cases for inherent CRD insertion in which thermal-hydraulic features that might enhance the driveline heatup but, on the other hand, whose effects may be quantitatively sensitive to design details are completely absent