BORON DILUTION IN PRESSURIZER WATER REACTORS

The research deals with boron dilution that might occur during some transient conditions in PWR, VVER-1000. A boron diluted coolant plug may cause a neutron flux excursion. The phenomenon is analyzed by available techniques: 1- System Thermalhydraulics (TH) to calculate boron dilution process and transport; 2-Computational Fluid Dynamics (CFD) to study mixing in boron diluted and non-diluted, 3-Three-dimensional Neutron Kinetics (3D NK) to study effects of boron diluted plugs in the physics of neutrons. The key findings were to set-up a suitable computational system to address relevant phenomena involved. Regarding TH system codes, it was demonstrated that TH system codes are suitable to investigate formation and transport of significant non-borated plugs in loop seals of PWRs after a SB LOCA. Regarding 3D NK and CFD codes, information, working knowledge and expertise were acquired. Ways of coupling between TH and 3D NK and TH and CFD codes and efforts underway to promote validation of coupled tools were discussed. The procedure developed by the University of Pisa to produce judgment about acceptability of code analysis results was used.