CFD was used to simulate boron mixing in the ESBWR core during a limiting ATWS event. During the ESBWR design process, questions were raised regarding the Standby Liquid Control System (SLCS), which adds a sodium penta-borate solution to the core during ATWS events for the purpose of halting reactivity and core shutdown. The questions involved the design prediction of boron transport and mixing in the core bypass spaces, and how that influenced the core shutdown time prediction associated with the SLCS. In response, a CFD simulation was performed to study the mixing of the sodium penta-borate solution in bypass spaces of the ESBWR core. The CFD solution was a transient, Reynolds-Averaged Navier-Stokes (RANS) solution using two-equation turbulence modeling and 25 million computational nodes. Compared with the design predictions, the CFD results showed a much faster transport of boron toward the center of the core and a much greater quantity of boron is ingested into the fuel bundles. These findings demonstrate an adequate level of conservativism in the core shutdown time predictions, and confirm the effectiveness of the ESBWR SLCS system.

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