Assessment of the 296-S-21 Stack Sampling Probe Location

Tests were performed to assess the suitability of the location of the air sampling probe on the 296-S-21 stack according to the criteria of ANSI N13.1-1999, Sampling and Monitoring Releases of Airborne Radioactive Substances from the Stacks and Ducts of Nuclear Facilities. Pacific Northwest National Laboratory conducted most tests on a 3.67:1 scale model of the stack. CH2MHill also performed some limited confirmatory tests on the actual stack. The tests assessed the capability of the air-monitoring probe to extract a sample representative of the effluent stream. The tests were conducted for the practical combinations of operating fans and addressed: (1) Angular Flow--The purpose is to determine whether the velocity vector is aligned with the sampling nozzle. The average yaw angle relative to the nozzle axis should not be more than 20. The measured values ranged from 5 to 11 degrees on the scale model and 10 to 12 degrees on the actual stack. (2) Uniform Air Velocity--The gas momentum across the stack cross section where the sample is extracted should be well mixed or uniform. The uniformity is expressed as the variability of the measurements about the mean, the coefficient of variance (COV). The lower the COV value, the moremore » uniform the velocity. The acceptance criterion is that the COV of the air velocity must be ?20% across the center two-thirds of the area of the stack. At the location simulating the sampling probe, the measured values ranged form 4 to 11%, which are within the criterion. To confirm the validity of the scale model results, air velocity uniformity measurements were made both on the actual stack and on the scale model at the test ports 1.5 stack diameters upstream of the sampling probe. The results ranged from 6 to 8% COV on the actual stack and 10 to 13% COV on the scale model. The average difference for the eight runs was 4.8% COV, which is within the validation criterion. The fact that the scale model results were slightly higher than the actual stack suggests that the other test results on the scale model are conservative relative to the actual stack. (3) Uniform Concentration of Tracer Gases--A uniform contaminant concentration in the sampling plane enables the extraction of samples that represent the true concentration. This was first tested using a tracer gas to represent gaseous effluents. The fan is a good mixer, so injecting the tracer downstream of the fans provides worst-case results. The acceptance criteria are that (1) the COV of the measured tracer gas concentration is ?20% across the center two-thirds of the sampling plane and (2) at no point in the sampling plane does the concentration vary from the mean by >30%. The results on the scale model at the point simulating the sampling probe ranged from 0.3 to 6 %COV, and the maximum single point deviation from the mean was -10%. (4) Uniform Concentration of Tracer Particles--Uniformity in contaminant concentration at the sampling probe was further demonstrated using tracer particles large enough to exhibit inertial effects. Particles of 10-?m aerodynamic diameter were used. The acceptance criterion is that the COV of particle concentration is ?20% across the center two-thirds of the sampling plane. The scale model results ranged form 2 to 9%. Based on these tests, the location of the air sampling probe on the 296-S-21 stack meets the requirements of the ANSI/HPS N13.1-1999 standard.« less