PNNL

Abstract

The Pacific Northwest National Laboratory researchers used a computational fluid dynamics (CFD) computer code to evaluate the mixing at a sampling system location of a research and development facility. The facility requires continuous sampling for radioactive air emissions. Researchers sought to determine whether the location would meet the criteria for uniform air velocity and contaminant concentration as prescribed in the American National Standard Institute (ANSI) standard, Sampling and Monitoring Releases of Airborne Radioactive Substances from the Stacks and Ducts of Nuclear Facilities. Standard ANSI/HPS N13.1-1999 requires that the sampling location be well-mixed and stipulates specific tests (e.g., velocity, gas, and aerosol uniformity and cyclonic flow angle) to verify the extent of mixing.. The exhaust system for the Radiochemical Processing Laboratory was modeled with a CFD code to better understand the flow and contaminant mixing and to predict mixing test results. The CFD results were compared to actual measurements made at a scale-model stack and to the limited data set for the full-scale facility stack. Results indicated that the CFD code provides reasonably conservative predictions for velocity, gas, and aerosol uniformity. Cyclonic flow predicted by the code is less than that measured by the required methods. In expanding from small to full scale, the CFD predictions for full-scale measurements show similar trends as in the scale model and no unusual effects. This work indicates that a CFD code can be a cost-effective aid in design or retrofit of a facility’s stack sampling location that will be required to meet Standard ANSI/HPS N13.1-1999.