SME Associates LLC Providing Innovative Solutions RMS- Optimization of Sampling Systems James P. Menge PE, CHP

Background Every day various quantities of particulates, vapors, gases and radionuclides are discharged to the environment from various power generation plants and production facilities. In general, the airborne effluent from nuclear power plants is discharged continuously through one or more vents or stacks of various heights.  

Background (cont’d) The released airborne radioactive pollutants can be summarized into the following categories: Gaseous Vapor Particulate The radionuclides are produced by a result of two processes: The first process is the formation of fission products, which are produced when uranium atoms in the fuel are split. The second process is activation, which occurs when atoms of other material within the system are made radioactive by neutron bombardment.

Aerodynamic Equivalent Diameter (AED) Key Terms A term used as a single dimension to describe a particle size in a fluid medium. Aerodynamic Equivalent Diameter (AED) The particle size distribution curve relating to one size particle Monodisperse Dimension less number that relates variables describing flow, velocity, density and viscosity. Reynolds Number The percent of particulate that is transported from the nozzle to the collection media. Transport Efficiency The region of Reynolds numbers between laminar (<2500) and turbulent flow (>4000) which relates variables describing flow, velocity, density and viscosity. Transition Region

System Description The system draws a continuous air sample from the sampling probe located high in the effluent stack. The air sample is drawn through the sample lines to a series of filters consisting of a particulate filter, an iodine filter, a noble gas chamber and a pump then discharged to the stack. In the noble gas chamber, a sodium iodide scintillation detector is used to obtain the gross gamma count rate of the noble gases. The count rate is converted to a release rate (μCi/sec) by manually multiplying it by a stack flow conversion factor.

Sampling Systems FAQ’s Is there one “Universal” monitoring system configuration? No. Various monitoring system configurations are used by the nuclear power plant operators or utilities to comply with NUREG 0737 and Reg Guide 1.97. Why do the sampling systems have problems? These sampling systems can have inherent problems associated with representative sampling because of the complexity and length of the sampling line. How effective are the sampling systems? One utility initiated a program to measure the effectiveness of the sampling systems and found several mechanical problems (fans, baffles, probes, etc.) that affected their effluent monitoring program. What did reviews of the stacks show? Reviews performed for stacks found that the stack flows and the flow profiles were significantly different than the design flows used in their emission calculations.

Sampling System FAQ’s (cont’d) What is the purpose of a sample system model? A sample system model provides an insight of which factors can influence the sample transport efficiency of a sampling system. The sample flow velocity (v) is an important factor in the transport efficiency of particles through a sampling system. What are the various design parameters, and what do they do? The sample flow rate, tubing diameter and Reynolds Number are design parameters that directly determine the flow velocity through the sample system

Particle Transmission Efficiency – Fx of Sample Flow & Reynolds Number

Sampling System Maintenance Perform a maintenance check on the sampling system actual pump flow variance Check each probe and sample line for cleanliness. Check system installation for the number of bends and bend curvature radius. Higher transport efficiencies can be achieved when sampling line installations minimize flow direction changes. Check Moisture content residual in sampling lines Pressure test for Air-Leakage within Sample lines A periodic check of the flow profile should be performed to check the effluent stream characteristics for sampling location.

Sampling Point Challenges The sampling point available for use may not provide a valid representation sample of the effluent. A flow profile should be generated for each sample point location. This ensures that under varying plant conditions, the sample point is representative of the system stream. The adequacy of the proposed location requires a flow profile to determine the flow velocity at several points. The flow measurements need to be taken during different operating process flow conditions.

RMS: A Dynamic System Flow rate and tubing diameter will affect transport efficiency based on particle sizing, plate out and impaction. The transport efficiency of particles through a sample line may vary by 50% or more due to sample flow rate, and tubing diameter. The RMS accuracy increases by reducing the sampling error and the extent of the scaling factors used to determine the particulate effluent release. The effluent particulate monitoring repeatability increases due to the transport efficiency being optimized. A Dynamic System will effect performance 2)The use of scaling factors has the tendency to overestimate the effluent release 3) This characteristic is important in predicting the performance of the sampling system to allow for standardization of the particle transport model for correlation to other radiation monitoring systems.

Transport Efficiency The transport efficiency is a function of particle size: as the particle size increases, the transport efficiency decreases. The median particle aerodynamic size should be determined for the distribution range of the mixture. The limitation of the sample transport efficiency of the radiation monitoring sampling system is that it is based on a particular particle size and not the mixture. Therefore, the recommendation is to provide a set of efficiencies for various particle sizes or a sample transport efficiency range based on particle size.

Optimization of Design Parameters The transmission of a 5μm AED particle with a density of 1.5 g/cm3 drawn through the sample system model at 56.630 l/min yields only a 26% transmission efficiency. The transmission efficiency of a particle is a function of the sample tube diameter. The optimum internal diameter for this sample system model with the design flow of 56.630 l/min is 22.926 mm. The sample line transmission efficiency for this sample system model can be increased to 27% for a 5μm AED particle with a density of 1.5 g/cm3 by increasing the flow to approximately 70 l/min. The transmission efficiency of all particles less than 10μm AED is increased by raising the sample flow rate to 70 l/min. AED -

Particle Transport Efficiency

Optimization of Design Parameters The transmission efficiency of particles is a function that is inversely proportionate to the particle density. As the particle density increases, the transmission efficiency decreases. At the optimum sample flow rate of 70 l/min, the transmission efficiency for AED particle with a density range of 1.00 to 2.00 g/cm3 varies by only 0.1%. The sampling system Reynolds number is in the transition region to achieve optimum transport efficiency.

Recommendations: Sampling System Perform an analysis of the sample stream to determine the range of particle sizes and density. Develop a model for the sampling system to determine the transmission efficiency for the system based on particle size and density. It is not a monodisperse particle. Determine the optimum sample flow rate for the sampling system. Do not rely on what the vendor or manufacturer has stated the sample flow rate should be for their radiation monitoring system skid.

Filter Efficiency vs Particle Size Review the filter collection media specifications to ensure the filter is sized appropriately for the flow velocity.

Conclusions The RMS Sampling System is key to proper system operation and dose measurement RMS Sampling systems need periodic maintenance It’s important to understand what the design parameters are for the sampling system Performing detailed reviews will ensure system is not operationally challenged

References Menge J, (1997); “Optimization of Airborne Effluent Sampling Systems for Nuclear Power Plants” Schmidt, A (1988); “ A Three-Part Strategy for Continuous Extractive Monitoring for Hazardous Particulate Emissions from Exhaust Stacks” Schwendiman, Stegen, Glissmeyer (1975); “Methods and Aids for Assessing Particle Losses in Sampling Lines” ANSI N13.1-1995; American National Standard Institute “Guide to Monitoring Releases of Airborne Radioactive Substances from Ducts and Stacks of Nuclear Facilities”. NUREG 0737 (1983); “Clarification of TMI Action Plan Requirements” US NRC Regulatory Guide 1.97 Rev 3 (1983); “Instrumentation for Light-Cooled Nuclear Power Plants to Assess Plant and Environs Conditions During and Following an Accident”

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