This report summarizes the progress of the MIT/TEPCO project entitled “Thermal Striping in LWR Piping Systems” during the second year. Progress during the first year was summarized in the MIT CANES report MITR-NSP-TR-007. Thermal striping has raised safety concerns due to incidents at some nuclear power plants. The current study investigates thermal striping at tee junctions of light water coolant systems through numerical simulations. A series of benchmark and sensitivity study was performed through the first and second year of this project.
Further improved results were obtained from FLUENT simulations during the second year due to several refinements. These include: use of a smaller time step (~ 1 ms vs. 10-50 ms), an improved sub-grid scale model, a refined tee junction geometry, and the use of a central –differencing spatial discretization scheme. Sensitivity studies of these refinements and the flow velocity ratios are described in this report.
A new version of FLUENT, FLUENT6.0, was released in January 2002. All calculations that were performed since March 2002 used FLUENT6.0. Although modifications in FLUENT6.0 do not involve enhancements of LES subgrid models, one feature related to LES in FLUENT6.0 is the incorporation of the second-order accurate central-differencing scheme. The central-differencing scheme is available for the momentum equations when using the LES turbulence model. This scheme provides improved accuracy for LES calculations. A sensitivity study was performed to compare the calculated results using the second order upwind and the central differencing schemes. Comparisons of the normalized temperatures show that the central differencing scheme produces significantly higher temperature fluctuations than the secondary upwind scheme at all three azimuthal positions. It is also noted to have a significant effect in turbulent mixing which in turn promotes a more uniform temperature distribution in the main coolant flow down stream the tee junction. However, one problem associated with use of the central-differencing scheme is that it can produce unbounded solutions and can lead to stability problems. Overshoot and undershoot of the calculated coolant temperatures were both found when the central-differencing scheme was chosen.
Nuclear Systems Enhanced Performance