Boiling water reactors (BWRs) are equipped with a standby liquid control system (SLCS). The SLCS is used to inject boron to shutdown the reactor from full power condition in the event that the control rods fail to insert. In order for the SLCS system to shut down the reactor, adequate mixing of the borated solution with the reactor coolant is necessary. In BWRs prior to BWR 5, the boron injection points are located in the lower plenum. The objective of this project is to evaluate the impact of the operating conditions on the boron injection based on the understanding of the behavior of multi-species flow in typical pre-BWR 5 reactors by using computational fluid dynamics (CFD). The project is divided into two phases. At the first phase, a CFD model based on the test configurations of GE 1/6 scale test program was established. The results were validated against measurements conducted by GE during the 1/6 scale test program performed in 1981. The validation shows that the CFD can give accurate predictions of the boron mixing. The technical approach employed in the CFD model was adequate to capture the boron mixing process in the BWR lower plenum. The second phase of the project is the sensitivity study based on the same technical approach developed in the first phase. However, a simplified BWR lower plenum model was used due to the time constrain. In the sensitivity study, the baseline case and four additional cases with different operating conditions were investigated using the same CFD approach.

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