harvey Atmospheric Dispersion Factors What by xiaoyounan

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									    Atmospheric Dispersion Factors:
What Are They and Why Do We Use Them?
            R. Brad Harvey, C.C.M.
             Theodore A Messier
                Marlboro, MA
                   978-568-2727
             rbharvey@framatech.com
               Atmospheric Dispersion Factors

Outline
> Components of an Atmospheric Dispersion Model
> Gaussian Plume Model
> Applications in the Nuclear Power Industry
> Atmospheric Dispersion Factors (CHI/Q Values)
  as a Function of Averaging Time and Probability
  Level

> References

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              Atmospheric Dispersion Factors

Components of an Atmospheric Dispersion Model
> Emissions
   Source location
   Source effective release height
      Stack height plus plume rise
   Source Emission Rate

> Meteorology
   Wind speed
   Wind direction
   Delta-temperature (stability class)

> Receptor
   Receptor location
   Receptor height
   Receptor pathways

                                             3
            Atmospheric Dispersion Factors

Gaussian Plume Modeling Assumptions
> Conservation of Mass
   Dry Deposition
   Decay in transit

> Crosswind and Vertical Gaussian (Normal)
 Concentration Distributions
> Hourly Dispersion Factors
   Steady-State Meteorological Conditions
   Continuous Emission Rate

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                   Atmospheric Dispersion Factors

Coordinate System




                                 Receptor coordinates:
                                        (x,y,z)

 Source coordinates:
       (0,0,H)


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              Atmospheric Dispersion Factors

Emission and Downwind Factors

 Low Wind Speed            High Wind Speed




    Emissions factor  Q   Downwind factor  1
                                                 u


                                                        6
            Atmospheric Dispersion Factors

Crosswind Factor




                                              
                                                    2
                                                        
                           1          1  y          
       Crosswind factor         exp
                          2  y      2 y
                                          
                                                
                                                       
                                                       

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                 Atmospheric Dispersion Factors

 Vertical Factor




                   1      1  H  z 2      1  H  z 2       
                                                                     
Vertical factor                      exp  2    
                         exp                                     
                  2  z   2  z           z  
                                                                 
                                                                   

                                                                8
                      Atmospheric Dispersion Factors

Resulting Basic Equation
              Concentration = emissions factor x
 downwind factor x crosswind factor x vertical factor


> Simplifying assumptions:
      Receptors at ground level (z=0)
      Receptor beneath plume centerline (y=0)


                              Q(Ci / sec)                 1  H ( m)  2 
 (Ci / m 3 )                                       exp    ( m)  
                                                                       
                     u (m / sec)  Y (m)    Z ( m)      2 Z        
                                                         



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            Atmospheric Dispersion Factors

Applications in the Nuclear Power Industry

> Routine Releases (ODCM)

> Design Basis Accident

> Emergency Response




                                                10
                      Atmospheric Dispersion Factors

Applications in the Nuclear Power Industry


Application        Receptors             Time Averaging   Probability Level
 Routine       Site Boundary         Variable (batch     (Actual
 Release                                 releases)            Meteorology)
               Specific Receptors
                                                             15% (default)

                                      Annual Average
  Design       EAB                     2h                5%
  Basis
 Accident      LPZ                     8h
               CR                      24hr
                                        96hr
                                        720hr
Emergency      Site Boundary         ¼hr                 (Actual
Response
               Specific Receptors                            Meteorology)




                                                                             11
                                    Atmospheric Dispersion Factors

CHI/Q Values as a Function of Averaging Time and Probability Level


                         Maximum Site Boundary CHI/Q Values

                      1.0E-03
    CHI/Q (sec/m^3)




                      1.0E-04                                         5%
                                                                      15%
                      1.0E-05                                         50%

                      1.0E-06
                                1      10     100    1,000   10,000
                                      Averaging Time (hrs)



                                                                          12
              Atmospheric Dispersion Factors

Potential Causes for Predicting Year-to-Year
 Variations in CHI/Q Values

> Changes in Local Topography
    Ground Cover, Nearby Buildings

> Changes in Met Monitoring Program
    Sensors, Calibration Procedures, Data Processing

> Calibration Accuracy
    WS ± 0.5 mph; WD ± 5 deg; Delta-T ± 0.1 deg C

> Climatic Variability

                                                           13
               Atmospheric Dispersion Factors

How Often Should Meteorological Data be Updated?
> Whenever there are changes to the met monitoring program
  or its nearby topography

How Many Years of Met Data Should be Used in
  Dispersion Calculations?
> RG 1.70 (1978): Two consecutive cycles, and preferably
  three or more whole years

> ANSI/ANS-3.11 (2000): At least three to five years
> DG-1111 (2001): Five years (minimum: one complete year)


                                                          14
                Atmospheric Dispersion Factors

References
> Routine Releases (ODCM)
    RG 1.111
    NUREG/CR-2919 (XOQDOQ)

> Design Basis Accident
    RG 1.145
    NUREG/CR-2858 (PAVAN)

> Emergency Response
    NUREG/CR-5247 (RASCAL 2.0)
    RASCAL 3.0 (to be published)




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