PNNL

Abstract

Proposed construction of or changes to a radiological effluent stack must meet the mixing criteria at the sampled stack location required by the standard ANSI/HPS N13.1-2011 and ISO 2889. Meeting the mixing criteria may be a requirement for operations under a permit. For a well-mixed sample location, the standard requires less than 20° flow angle variation and less than 20% variation of the air velocity, gas tracers, and particle tracers measured at the center two-thirds of the sampling point cross-section. An additional criteria for gases is that at no point in the sampling plane does the concentration vary from the mean by greater than 30%. For more than a decade, three-dimensional computational fluid dynamics (CFD) models of such stacks have been used to assist in characterizing the sample location suitability. Results of modeling have shown stacks designs where mixing is both adequate and inadequate. For situations in which inadequate mixing in the stack at the sampling location has been identified, additional modeling with design modifications (e.g., adding an air blender) was used to asses if mixing criteria improved. Testing results have confirmed the CFD model blind predictions with similar values (i.e., ± 5% coefficient of variance) for the mixing criteria in both scale model and full-scale stack tests. CFD simulations of flow and mixing within a stack scheduled for modifications can indicate if the retrofitted stack passes mixing criteria for baseline operating conditions, can evaluate stack mixing performance at the limits of operation, and can recommend modifications if needed. This work demonstrates that CFD modeling is an effective tool for designing a new or retrofitting a facility stack and should be considered for use to help qualify a stack sampling location to meet standard requirements.