Development of Effective Algorithm for Coupled Thermal-Hydraulic-Neutron-Kinetics Analysis of Reactivity Transient

Analyses of nuclear reactor safety have increasingly required the coupling of full three-dimensional neutron-kinetics (NK) core models with system transient thermal-hydraulic (TH) codes. To produce results within a reasonable computing time, the coupled codes use different spatial descriptions of the reactor core. The TH code uses few, typically 5 to 20, TH channels that represent the core. The NK code uses the explicit node for each fuel assembly. Therefore, a spatial mapping of coarse grid TH and fine grid NK domain is necessary. However, improper mappings may result in the loss of valuable information, thus causing inaccurate prediction of safety parameters.

The purpose of this investigation is to study the sensitivity of spatial coupling (channel refinement and spatial mapping) and develop recommendations for NK-TH mapping in the simulation of safety transients - control rod drop, turbine trip, and feedwater transient - combined with stability performance (minimum pump speed of recirculation pumps).

The research methodology consists of a spatial coupling convergence study, as an increasing number of TH channels and different mapping schemes approach the reference case. The reference case consists of one TH channel per one fuel assembly. The comparison of results has been done under steady-state and transient conditions. The obtained results and conclusions are presented in this paper.