Fluent and Trac-Pei Analyses of Boron Dilution

In this paper a chronological overview is presented of the work done at KEMA on boron dilution in a Pressurized Water Reactor (PWR). This work has been focused on calculating boron dilution when an un-borated water slug enters the reactor vessel at a pump start-up. Two different thermal-hydraulic codes have been used for this purpose: FLUENT and TRAC-PF1. FLUENT is a computational fluid dynamics code, while TRAC-PF1 is specifically written to analyse transients in a pressurized water reactor. This paper gives an idea of the advantages and the disadvantages of these two codes and shows some comparisons between the results. Both codes appear to provide reasonable results for scoping analyses purposes and show that prompt criticality would require a substantial volume of un-borated water feed. A phenomenon which appeared to be very important is numerical diffusion. For plant specific quantitative analyses it is recommended to improve TRAC-PF1 to better model the boron dilution (reduce numerical diffusion, model turbulent diffusion). A detailed comparison between TRAC-PF1 and a code as FLUENT for a relatively simple case would enable a quantitative verification of the improved TRAC-PF1 model, and would provide confidence to use TRAC-PF1 for more complicated transients