Regulatory Guide 1.145

Atmospheric Dispersion Models for Potential Accident Consequence Assessments at Nuclear Power Plants

ML12216A014
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Issue date:	08/31/1979
From:	Office of Nuclear Regulatory Research, NRC/OSD
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RG-1.145
Download: ML12216A014 (14)

U.S. NUCLEAR REGULATORY COMMISSION August 1979

)REGULATORY GUIDE

COFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 1.145 ATMOSPHERIC DISPERSION MODELS FOR POTENTIAL ACCIDENT

CONSEQUENCE ASSESSMENTS AT NUCLEAR POWER PLANTS

A. INTRODUCTION

Regulatory Guide 1.3, "Assumptions Used for Evaluating the Potential adiological Con- Section 100.10 of 10 CFR Part 100, "Reactor sequences of a Loss of 'tlant Accident for Site Criteria," states that meteorological condi- Boiling Water Reactors,"'. gulatory Guide tions at the site and surrounding area should 1.4, "Assumptions Use fo aluating the be considered in determining the acceptability Potential Radiological seque es of a Loss of a site for a power reactor. Section 50.34 of of Coolant Accident Pressurized Water

10 CFR Part 50, "Domestic Licensing of Reactors." A nn ther regulatory Production and Utilization Facilities," requires guides also inclu e endations for or that each applicant for a construction permit or references to r olo analyses of potential operating license provide an analysis and accidents. The lp abi]* of the specific cri- evaluation of the design and performance of teria discusse inAo these other analyses structures, systems, and components of the will be conide a case- by- case basis.

facility with the objective of assessing the risk Until suc pe generic guidelines are to public health and safety resulting from the developed h analyses, the methodology operation of the facility. Section 50.34 of 10 provid in .s ide is acceptable to the NRC

CFR Part 50 also states that special attention staff.

should be directed to the site evaluation factors identified in 10 CFR Part 100 in the - "

B. DISCUSSION

assessment of the site.

spheric diffusion' models described The regulatory positions presented in this t__gde reflect review of recent experi- guide represent a substantial change from pro- ata on diffusion from releases at cedures previously used to determine relative n level without buildings present and concentrations for assessing the poten ro releases at various locations on reactor offsite radiological consequences for a range cility buildings during stable atmospheric postulated k.accidental releases of radioacti ditions with low windspeeds (Refs. 1 material to the atmosphere. These procedure rough 6). These tests verify the existence of now include consideration of plume me r, I ffluent plume "meander" under light wind- directional dependence of rs-ion speed conditions and neutral (D) and stable conditions, and wind frequencies for rious (E, F, and G) atmospheric stability conditions locations around actual exclusion area o (as defined by the AT criteria in Regulatory population zone (LPZ) boundaries. Guide 1.23, "Onsite Meteorological Programs").

Effluent concentrations measured over a period The direction-dependent approach was devel- of 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> under such conditions have been oped to provide an improved basis for the Part shown to be substantially lower than would be

100-related review of propose ctor and site predicted using the traditional curves (Ref. 7)

considerations. Accordingl de i pro- vides an acceptable meetho Lo deter- mining site-specific relativ ~ncentrations tIn discussions throughout this regulatory guide, atmos- pheric dispersion w/il be considered as consisting of two compo- nents: atmospheric transport due to organized or mean airflow (x/Q) and should be t de abk tining x/Q within the atmosphere and atmospheric diffusion due to values for the eov atiohte discussed in disorganized or random air motions.

USR U 'ORY GUIDES Comments should be sent to the Secreta" of toheCommilsio U.S. Nudes Regulatory Commission. Washington. D.C. 2M Attention: Docketing and Ratb" Ginesa we rs ai d makowsilota to thoe pcbli Service Branch.

mohotids foceeoivdwithoINa "mort1 part oe the commisuion's higiuido, the ned the son in oark- The guides am issued m the following ton broed divisions:

of F"It' l" or or so Provide guidanc in

1.Power Reactors 6. Products pinlmg ih diorn is ImrowuirodMLod& and soluions d"Word from 2. RPsemch end Teat Reactors 7. Transporttion so out in to guids willb acceptbli N mw wdo a boo for O --- go 3. =e mid Materias Faclties &HOccupetiol'Health

.

4. end Sti 9 . Antitrust and Financial Review ConUT6ior 5. Materii nd Pn Prootection 10. General commet and sugndons for irpovrwo es on

• gulds we - Requests for singto copies of issued guides 1Iwiuich mey be rrocdior for at am,M and gulden will be revisd, as approrio, t ooo Corn- Planant soan en autoutfeic distribution list for @in&Uopese Of future gudea inee and tor now vrowsidenor S '0 Howeve. omfwf an In l:e 11icdivsionasahould be indef In vuviting to U.S. NWoolt Regulatory Mb guide, N MOO" dut too 01 abo gme*ft IMIM Y. 130 Commi ,ion.

WNington, D.C. 2056, Attenaion: Director, Division of Psitiolef unlul Inowb~n ft need for an a* rolso Tesdts" kormetlon

• nd Document Control.

of lateral and vertical plume spread, which are position in Regulatory Guide 1.23, calms should functions of atmospheric stability and down- be assigned a windspeed equal to the vane or wind distance. anemometer starting speed, whichever is 0

higher. Otherwise, consideration of a con- The procedures in this guide also recognize servative evaluation of calms, as indicated by that atmospheric dispersion conditions and the system, will be necessary. Wind directions wind frequencies are usually directionally during calm conditions should be assigned in dependent; that is, certain airflow directions proportion to the directional distribution of can exhibit substantially more or less favorable noncalm winds with speeds less than 1.5 meters diffusion conditions than others, and the wind per second. 2 can transport effluents in certain directions more frequently than in others. The pro- cedures also allow evaluations of atmospheric 1.2 Determination of Distances for x/Q Calculations dispersion for directionally variable distances such as a noncircular exclusion area boundary. For each wind direction sector, x/Q values for each significant release point should be

C. REGULATORY POSITION

calculated at an appropriate exclusion area boundary distance and outer low population This section identifies acceptable methods for zone (LPZ) boundary distanc

e. The following

(1) calculating atmospheric relative concentra- procedure should be used to determine these tion (x/Q) values, (2) determining x/Q values distances. The procedure takes into considera- on a directional basis, (3) determining x/Q tion the possibility of curved airflow tra- values on an overall site basis, and (4) Jectories, plume segmentation (particularly in choosing X/Q values to be used in evaluations light wind, stable conditions), and the poten- of the types of events described in Regulatory tial for windspeed and direction frequency Guides 1.3 and 1.4. shifts from year to year.

Selection of conservative, less detailed site For each of the 16 sectors, the distance for parameters for the evaluation may be sufficient exclusion area boundary or outer LPZ bound- to establish compliance with ,regulatory ary x/Q calculation should be the minimum guidelines. distance from the stack or, in the case of releases through vents or building penetra- I. CALCULATION OF ATMOSPHERIC RELATIVE tions, the nearest point on the building to the CONCENTRATION (x/Q) VALUES exclusion area boundary or outer LPZ

boundary within a 45-degree sector centered Equations and parameters presented in this on the compass direction of interest.

section should be used unless unusual siting, meteorological, or terrain conditions dictate the For stack releases,, the maximum ground- use of other models or considerations. High- level concentration in a sector may occur quality site-specific atmospheric diffusion tests beyond the exclusion area boundary distance may be used as a basis for modifying the equa- or outer LPZ boundary distance. Therefore, tions and parameters. for stack releases, x/Q calculations should be made in each sector at each boundary distance

1.1 Meteorological Data Input and at various distances beyond the exclusion area boundary distance to determine the The meteorological data needed for x/Q cal- maximum relative concentration for considera- culations include windspeed, wind direction, tion in subsequent calculations.

and atmospheric stability. These data should represent hourly averages as defined in regu- 1.3 Calculation of X/Q Values at Exclusion Area Boundary latory position 6. a of Regulatory Guide 1. 23. Distances Wind direction should be classed into 16 com- Relative concentrations that can be pass directions (22.5-degree sectors, centered assumed to apply at the exclusion area on true north, north-northeast, etc. ). boundary for 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> immediately following an accident shouid be determined. 3 Calculations Atmospheric stability should be determined based on meteorological data averaged over a by vertical temperature difference (AT) 1-hour period should be assumed to apply for between the release height and the 10-meter the entire 2-hour period. This assumption is level or by other well-documented parameters reasonably conservative considering the small that have been substantiated by %diffusiondata. variation of x/Q values- with averaging time Acceptable stability classes are given in Table (Ref. 8). If releases associated with a postu-

2 of Regulatory Guide 1.23. lated event are estimated to occur in a period Calms should be defined as hourly average

2 windspeeds below the vane or anemometer Staff experience has shown that noncalm windspeeds below starting speed, whichever is higher (to reflect 1.5 meters per second provide a reasonable range for defining the distribution of wind direction during light winds.

limitations in instrumentation). If the instru- mentation program conforms to the regulatory 3See 100.II of 10 CIR Part 100.

1.145-2

of less than 20 minutes, the applicability of the A is the smallest vertical-plane cross- models should be evaluated on a case-by-case sectional area of the reactor build- basis. ing, in m 2 . (Other structures and/

or : directional consideration may Procedures for calculating "2- hour" x/Q be justified when appropriate. )

values depend on the mode of release. The procedures are described below. x/Q values should be calculated using Equations 1, 2, and 3. The values from Equa- tions I and 2 should be compared and the

1.3.1 Releases Through Venzts fn Othee Ruilding P-enetrations higher value selected. This value should be compared with the value from Equation 3, and Ihis class of release modes includes all the lower value of these two should be selected release points or areas that are effectively as the appropriate xiQ value. Examples and a lower than two and one-half times the height of detailed explanation of the rationale for deter- adjacent solid structures (Ref. 9). Within this mining the controlling conditions are given in class, two sets of meteorological conditions are Appendix A to. this guide.

treated differently, as follows:

b. During all other meteorological condi- a. During neutral (D) or stable (E, F, tions [unstable (A, B, or C) atmospheric or G) atmospheric stability conditions when the stability and/or 10-meter level windspeeds of 6 windspeed at the 10-mete.r level is less than 6 meters per second or more], plume meander meters per second, horizontal plume meander should not be considered. The appropriate x/Q

can be considered. X/Q values may be deter- value is the higher value calculated from mined through selective use of the following set Equation 1 or 2.

of equations for ground-level relative concen- trations at the plume centerline: 1.3.2 Stack Releases

1 (1) This class of release modes includes all x/Q =

UIo(1OyOz + A/2) release points at levels that are two and one- half times the height of adjacent solid struc- tures or higher (Ref. 9). Nonfumigation and X/Q - 1 (2) fumigation conditions are treated separately.

3 Uio( u ya Z)

a. For nonfumigation conditions, the equation for ground-level relative concentration X/Q - I (3) at the plume centerline for stack releases is:

Uloltly az x/Q 1 r-h 1 where (4)

nyz x/Q is relative concentration, in sec/

where ms, n is 3.14159, Uh is windspeed representing conditions at the release height, in m/sec, U1 0 is windspeed at 10 meters above plant grade, 4 in m/sec, he is~effective stack height, in m:

a is lateral plume spread, in m, a hhe = ht, Y function of atmospheric stability and distance (see Fig. 1),

o is vertical plume spread, in m, a h is the initial height of the plume z function of atmospheric stability (usually the stack height) above plant grade, in m, and and distance (see Fig. 2),

Y is lateral plume spreaswith meander Y and building wake effects, in m, a ht is the maximum terrain height above function of atmospheric stability, plant grade between the release windspeed U 1 0 , and distance [for point and the point for which the distances of 800 meters or less, calculation is made, in m; ht cannot I = Mo , where M is determined exceed hs.

frvom Fil. 3; for distances greater than 800 meters, y = (M - 1)

ay800m + y]I, and b. For fumigation conditions, a "fumiga- tion x/Q" should be calculated for each sector as follows. The equation for ground-level rela-

4 tive concentration at the plume centerline for the 10-meter level is representatve of the depth through which the plume is mixed with building wake effects. stack releases during fumigation conditions is:

1.145-3

determine sector X/Q values at outer LPZ

x/Q = 1 ayhe

, h > 0 (5) boundary5 distances for various longer time

(2701/2Uh periods.

ey

2. DETERMINATION OF MAXIMUM SECTOR x/Q

where VALUES

Eh is windspeed representative of the e layer of depth he , in m/sec; in lieu The x/Q values calculated in regulatory posi- of information to the contrary, the tion 1 are used to determine "sector x/Q

NRC staff considers a value of 2 values" and "maximum sector x/Q values" for meters per second as a reasonably the exclusion area boundary and the outer LPZ

conservative assumption for h of boundary.

about 100 meters, and e

2.1 Exclusion Area Boundary o is the lateral plume spread, in m, y that is representative of the layer at 2.1.1 General Method a given distance; a moderately stable (F) atmospheric stability condition is Using the x/Q values calculated for each usually assumed. hour of data according to regulatory posi- tion 1.3, a cumulative probability distribution Equation 5 cannot be applied indiscrimi- of x/Q values should be constructed for each of nately because the x/Q values calculated, using the 16 sectors. Each distribution should be this equation, become unrealistically large as described in terms of probabilities of given x/Q

h becomes small (on the order of 10 meters). values being exceeded in that sector during Tie x/Q values calculated using Equation 5 the total time. A plot of x/Q versus probability must therefore be limited by certain physical of being exceeded should be made for each restrictions. The highest ground-level x/Q sector, and a curve should be drawn to form values from elevated releases are expected to an upper bound of the data points. From each occur during stable conditions with low wind- of the 16 curves, the x/Q value that is speeds when the effluent plume impacts on a exceeded 0.5% of the total time should be terrain obstruction (i.e., h = 0). However, selected (Ref. 10).. These are the sector x/Q

elevated plumes diffuse upv$ard through the values. The highest of the 16 sector values is stable layer aloft as well as downward through defined as the maximum sector x/Q value.

the fumigation layer. Thus ground-level relative concentrations for elevated releases 2.1.2 Fumigation Conditionsfor Stack Releases under fumigation conditions cannot be higher than those produced by nonfumigation, stable Regulatory position 1.3.2 gave proce- atmospheric conditions with h = 0.. For the dures for calculating a fumigation x/Q for each fumigation case that assumes F stability and a sector. These sector fumigation values, along windspeed of 2 meters per second, Equation 4 with the general (nonfumigation) sector values should be used instead of Equation 5 at obtained in regulatory position 2.1.1, are used distances greater than the distance at which to determine appropriate sector x/Qs for fumi-.

the x/Q values, determined using Equation 4 gation conditions, based on conservative with he = 0, and Equation 5 are equal. assumptions concerning the duration of fumiga- tion. These assumptions differ for inland and coastal sites, and certain modifications may be

1.4 Calculation of x/Q Values at Outer LPZ Boundary appropriate for specific sites.

Distances Two- hour x/Q values should also be cal- a. Inland Sites: For stack releases at culated at outer LPZ boundary distances. The sites located 3200 meters or more from large procedures described above for exclusion area bodies of water (e.g., oceans or Great Lakes),

boundary distances (see regulatory posi- a fumigation condition should be assumed to tion 1.3) should be used. exist at the time of the accident and continue for 1/2 hour (Ref. 11). For each sector, if the An annual average (8760-hour) x/Q should sector fumigation x/Q exceeds the sector non- be calculated for each sector at the outer LPZ fumigation x/Q, use the fumigation value for boundary distance for that sector, using the the 0 to 1/2-hour time period and the nonfumi- method described in regulatory position 1.c of gation value for the 1/2-hour to 2-hour time Regulatory Guide 1.111, "Methods for Estimat- period. Otherwise, use the nonfumigation ing Atmospheric Transport and Dispersion of sector value for the entire 0 to 2-hour time Gaseous Effluents in Routine Releases from period. The 16 (sets of) values thus deter- Light-Water-Cooled Reactors." (For stack re- mined will be used in dose assessments requir- leases, h should be determined as described ing time-integrated concentration considera- in regulaeory position 1.3.2.)

These calculated 2-hour and annual average values are used in regulatory position 2.2 to tions.

'58M 5100.11 of 10 CFR Part 100.

0

1.145-4

b. Coastal Sites: For stack releases at x/Q for a given sector is determined as sites located less than 3200 meters from large described in regulatory position 1.4.

bodies of water, a fumigation condition should be. assumed to exist at the exclusion area The logarithmic interpolation procedure boundary at the time of the accident and produces results that are consistent with continue for the entire 2-hour period. For each studies of variations of average concentrations sector, if the sector fumigation x/Q exceeds with time periods up to 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> (Ref. 8).

the sector nonfumigation x/Q, use the fumiga- Alternative methods should also be consistent tion value for the 2-hour period. Otherwise, with these studies.

use the nonfumigation value for the 2-hour period. Of the 16 sector values thus deter- For each time period, the highest of the mined, the highest is the maximum sector x/Q 16 sector x/Q values should be identified. In value. most cases, these highest values will occur in the same sector for all time periods. These are c. Modifications: These conservative as- then the maximum sector x/Q values. However, sumptions do not consider frequency and dura- if the highest sector x/Qs do not all occur in tion of fumigation conditions as a function of the same sector, the 16 (sets of) values will be airflow direction. If information can be pre- used in dose assessments requiring time- sented to substantiate the likely directional integrated concentration considerations. The occurrence and duration of fumigation condi- x/Q values for the various time periods within tions at a site, the assumptions of fumigation in that sector should be considered the maximum all appropriate directions and of duration of sector x/Q values.

1/2 hour and 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> for the exclusion area boundary may be modified. Then fumigation 2.2.2 Fumigation Conditionsfor Stack Releases need only be considered for airflow directions in which fumigation has been determined to Determination of sector x/Q values for occur and of a duration determined from the fumigation conditions at the outer LPZ

study of site conditions. 6 boundary involves the following assumptions concerning the duration of fumigation for in-

2.2 Outer LPZ Boundary land and coastal sites:

2.2.1 GeneralMethod a. Inland Sites: For stack releases at sites located 3200 meters or more from large Sector x/Q values for the outer LPZ bodies of water, a fumigation condition should boundary should be determined for various be assumed to exist at the outer LPZ boundary time periods throughout the course of the at the time of the accident and continue for 1/2 postulated accident. " The time periods should hour. Sector x/Q values for fumigation should represent appropriate meteorological regimes, be determined as for the exclusion area bound- e.g., 8 and 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> and 3 and 26 days as ary in regulatory position 2.1.2.

presented in Section 2.3.4 of Regulatory Guide 1.70, "Standard Format and Content of b. Coastal Sites: For stack releases at Safety Analysis Reports for Nuclear Power sites located less than 3200 meters from large Plants--LWR Edition," or other time periods bodies of water, a fumigation condition should appropriate to. release durations. be assumed to exist at the outer LPZ boundary following the arrival of the plume and continue For a given sector, the average x/Q for a 4-hour period. Sector X/Q values for values for the various time periods should be fumigation should be determined as for the approximated by a logarithmic interpolation exclusion area boundary in regulatory posi- between the 2-hours sector x/Q and the annual tion 2.1.2.

average (8760-hour) x/Q for the same sector.

The 2-hour sector x/Q for the outer LPZ c. The modifications discussed in regula- boundary is determined using the general tory position 2.1.2 may also be considered for method given for the exclusion area boundary the outer LPZ boundary.

in regulatory position 2.

1. The annual average

3. DETERMINATION OF 5% OVERALL SITE x/Q

6For example, examination of site-specific information at a lo- VALUE

cation in a pronounced river valley may indicate that fumigation conditions occur only during the downvalley "drainage flow" The x/Q values that are exceeded no more regime and persist for durations of about 1/2 hour. Therefore, in this case airflow directions other than the downvalley direc- than 5%. of the total time around the exclusion tions can be excluded from consideration of fumigation condi- area boundary and around the outer LPZ

tions. and the duration of fumigation would still be considered boundary should be determined as follows as 1/2 hour. On the other hand, data from sites in open terrain (noncoastal) may indicate no directional preference for fumiga- (Ref. 10):

tion conditions but may indicate durations much less than 1/2 hour. In this case, fumigation should be considered for all Using the x/Q values calculated according directions, but with durations of less than 1/2 hour.

to regulatory position 1, an overall cumulative

?See §100.11 of 10 CFR Part 100. probability distribution for all directions com-

• The X/Qs are based on 1-hour averaged data but are as- bined should be constructed. A plot of x/Q

sumed to apply for 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />. versus probability of being exceeded should be

1. 145-5

made, and an upper bound curve should be used in the evaluation of applications tendered drawn. The 2-hour x/Q value that is exceeded on or after the implementation date to be

5% of the time should be selected from this specified in the active guide (in no case will curve as the dispersion condition indicative of this date be earlier than November 1, 1979) as the type of release being considered. In follows:

addition, for the outer LPZ boundary the maximum of the 16 annual average x/Q values 1. For early site review applications.

should be used along with the 5% 2-hour x/Q

value to determine - X/Q values for the 2. For construction permit applications (in- appropriate time periods by logarithmic cluding those incorporating or refer- interpolation. encing a duplicate plant design and those submitted under the replicate plant

4. SELECTION OF x/Q VALUES TO BE USED IN option of the Commission's standardiza- EVALUATIONS tion program).

The x/Q value for exclusion area boundary For the following cases, either the proposed or outer LPZ boundary evaluations should be guide or the procedures described in Standard the maximum sector x/Q (regulatory position 2) Review Plan Section 2.3.4 (1975) may be used:

or the 5% overall site x/Q (regulatory posi- tion 3), whichever is higher. All direction- 1. Construction permit applications tend- dependent sector values should be presented ered before the implementation date.

for consideration of the appropriateness of the exclusion area and outer LPZ boundaries and 2. Operating license applications whose con- the efficacy of evacuation routes and emer- struction permits precede the implemen- gency plans. Where the basic meteorological tation date.

data necessary for the analyses described herein substantially deviate from the regula- 3. Operating reactors.

tory position stated in Regulatory Guide 1.23, consideration should be given to the resulting This proposed guide does not apply to the uncertainties in dispersion estimates. following options specified in the Commission's standardization policy under the reference

D. IMPLEMENTATION

system concept:

This proposed guide has been released to 1. Preliminary design approval applications.

encourage public participation in its develop- ment and is not intended to foreclose other op- 2. Final design approval, Type 1, appli- tions in safety evaluations. Except in those cations.

cases in which an applicant proposes an acceptable alternative method for complying 3. Final design approval, Type 2, appli- with specified portions of the Commission's cations.

regulations, the method to be described in the active guide reflecting public comments will be 4. Manufacturing license applications.

1.145-6

- -. -. -- ~-----~ -.- 4--~-

I IjIj Ii

103

-T- I -4----- -- 4- i ill

-4--4-4-4-4-4-4---~--, 4

/ic,

• 1 iI

-4~'4- 4 f 41

5 ,D~L

E' I

2

10

z

0

05 A- EXTREMELY UNSTABLE

MODERATELY UNSTABLE.

C - SLIGHTLY UNSTABLE

T - NEUTRAL

E- SLIGHTLY STABLE

F MODERATELY STABLE-

I01

2 l

=_, 0 [

4.10 102 2 5 105

5 103 2 5 104 2 DISTANCE FROM SOURCE (W

Figure 1. Lateral diffusion without meander and building wake effects, oa, vs. down- wind distance from source for Pasquill's turbulence types (atmospheric stability) (Ref. 7).

For purposes of estimating u during extremely stable (G) atmospheric stability conditions, without pl~ne meander or other lateral enhancement, the following approximation is appropriate:

Oy(G) = 3-y(F)

1.145-7

3. 03

2-

,.2 z

0

10

S0I

b"

2

0

10lo

101 2 5 105

2 5 103 2 5 DISTANCE FROM SOURCE (m)

Figure 2. Vertical diffusion without meander and building wake effects, z, vs. downwind distance from source for Pasquill's turbulence types (atmospheric stability) (Ref. 7).

For purposes of estimating oz during extremely stable (G) atmospheric stability conditions, the following approximation is appropriate:

az(G) = Vz(F)

1.145-8

Stabi I ity Class

6G

a-

3-

0E

1 2 3 4 5 6 10

WINDSPEED (m/sec)

Figure 3. Corect!on factors for Pasquill-Gifford a values by atmospheric stability class (see Appendix A to this guide)

1.145-9

APPENDIX A

ATMOSPHERIC DIFFUSION MODEL FOR RELEASES THROUGH VENTS

AND BUILDING PENETRATIONS

Rationale The conditional use of Equations 1, 2, and 3 is considered appropriate because (1) horizon- The effects of building wake mixing and am- tal plume meander tends to dominate dispersion bient plume meander on atmospheric dispersion during light wind and stable or neutral condi- are expressed in this guide in terms of condi- tions and (2) building wake mixing becomes tional use of Equations 1, 2, and 3. more effective in dispersing effluents than meander effects as the windspeed increases and Equations 1 and 2 are formulations that have the atmosphere becomes less stable.

been acceptable for evaluating nuclear power plant sites over a period of many years (Ref. 7 Examples of Conditional Use of Diffusion Equations and Regulatory Guides 1.3 and 1.4) but have recently been found to provide estimates of Figures A-l, A-2, and A-3 show plots of ground-level concentrations that are consist- xUo/Q (x/Q multiplied by the windspeed Ulo)

ently too high during light wind and stable or versus downwind distance based on the condi- neutral atmospheric conditions for 1-hour re- tional use (as described in regulatory posi- lease durations (Refs. 1 through 6). tion 1.3.1) of Equations 1, 2, and 3 during atmospheric stability class G. The variable M

Equation 3 is an empirical formulation based for Equation 3 equals 6, 3, and 2 respectively on NRC staff analysis of atmospheric diffusion in Figures A-l, A-2, and A-3 (M is as defined experiment results (Ref. 2). The NRC staff in regulatory position 1.3.1). The windspeed examined values of lateral plume spread with conditions are those appropriate for G stability meander and building wake effects (I ) by and M =6, 3, and 2.

atmospheric stability class (based on ATY, cal- culated from measured ground-level concentra- In Figure A-l, the XU1 o/Q from Equation 3 tions from the experimental results. Plots of (M = 6) is less than the higher value from the computed Y values by atmospheric stabil- Equation I or 2 at all distances. Therefore, for ity class and downwind distance were analyzed M = 6, Equation 3 is used for all distances.

conservatively but within the scatter of the data points by virtually enveloping most test In Figure A-2, the xUo/Q from Equation 3 data. The resultant analysis is the basis for (M = 3) is less than the higher value from the correction factors applied to the Pasquill- Equation 1 or 2 beyond 0.8 kln. Therefore, for Gifford a values (see Fig. 3 of this guide). M = 3, Equation 3 is used beyond 0.8 km. For Thus, Eq~aation 3 identifies conservatively the distances less than 0.8 kin, the value from combined effects of increased plume meander Equation 3 equals that from Equation 2.

and building wake on diffusion in the Equation 2 is therefore used for distances less horizontal crosswind direction under light wind than 0.8 km.

and stable or neutral atmospheric conditions, as quantified in Figure 3. These experiments In Figure A-3, the x-uo/Q from Equation 3 also indicate that vertical building wake mixing (M = 2) is never less than the higher value is not as complete during light wind, stable from Equation 1 or 2. Therefore, for M = 2, conditions as during moderate wind, unstable Equation 3 is not used at all. Instead, Equa- conditions although the results could not be tion 2 is used up to 0.8 km, and Equation 1 is quantified in a generic manner. used beyond 0.8 km.

1.145-10

CY

0.1 1.0 10

PLUME TRAVEL DISTANCE (km)

Figure A-1. xU 10 /Q as a function of plume travel distance for G stability condition using Equations 1, 2. and 3 (M = 6).

1.145-11

o t*

0.1 1.0 10

PLUME TRAVEL DISTANCE (km)

Figure A-2. x910/0 as a function of plume trvel distance for G stability using Equations 1, 2, and 3 (M - 3).

1.145-12

10-2 I k Eq. 3 (M=2) I II

-- H

E

q. I

10-

__ _i I _

o

10-

-4

___ ___ I '!ii q. 3 j(M=2)

__

____ __ -

.q. Eq. 2

10-s

0 .1 1.0 10

PLUME TRAVEL DISTANCE (km)

Figure A-3. xUj10 /Q as a function of plume travel distance for G stability condition using Equations 1, 2, and 3 (M = 2).

1.145-13

REFERENCES

1. Van der Hoven, I., "A Survey of Field Nuclear Power Station," Preliminary Safety Measurements of. Atmospheric Diffusion Analysis Report, Amendment 24, Docket Under Low-Wind Speed Inversion Condi- Numbers 50-458 and 50-459, 1974.

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2. Start, G. E., et al., "Rancho Seco Build- at Three Mile Island Nuclear Station Under ing Wake Effects On Atmospheric Diffu- Low Wind Speed Inversion Conditions,"

sion," NOAA Technical Memorandum ERL Final Safety Analysis Report, Amend- ARL-69, Air Resources Laboratory, Idaho ment 24,' Docket Number 50-289, 1972.

Falls, Idaho, November 1977, available from Publication Services, Environmental 7. Gifford, F. A., Jr., "An Outline of Theories Research Laboratories, National Oceanic of Diffusion in the Lower Layers of the At- and Atmospheric Administration, Boulder, mosphere," Chapter 3 in Meteorology and Colorado-80302.- Atomic Energy--1968 (D. H. Slade, Ed.),

available as TID-24190 from the National

3. Wilson, R. B., et al., "Diffusion Under Technical Information Service, Springfield, Low Windspeed Conditions Near Oak Ridge, Virginia 22151.

Tennessee," NOAA Technical Memorandum ERL ARL-61, Air Resources Laboratory, 8. Gifford, F., "Atmospheric Dispersion Models Idaho Falls, Idaho, 1976, available from for Environmental Pollution Applications,"

Publication Services, Environmental Re- Lectures on Air Pollution and Environmental search Laboratories, National Oceanic and Impact Analyses, American Meteorological Atmospheric Administration, Boulder, Society, pp. 35-38, 1975.

Colorado 80302.

9. Snyder, W. H., and R. E. Lawson, Jr.,

4. Sagendorf, J. F., and C. R. Dickson, "Determination of a Necessary Height for a Stack Close to a Building - A Wind Tunnel

"Diffusion Under Low Windspeed, Inversion Conditions," NOAA Technical Memorandum Study," Atmospheric Environment, Vol. 10,

ERL ARL-52, Air Resources Laboratory, pp. 683-691, Pergamon Press, 1976.

Idaho Falls, Idaho, 1974, available from

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Publication Services, Environmental Re- Denton, dated July 25, 1978, Subject:

search Laboratories, National Oceanic and

"Meteorological Model for Part 100 Evalua- Atmospheric Administration, Boulder, tions," and August 2, 1978 reply.

Colorado 80302.

11. Van der Hoven, I., "Atmospheric Transport

5. Gulf States Utilities Company, "Dispersion and Diffusion at Coastal Sites," Nuclear of Tracer Gas at the Proposed River Bend Safety, Vol. 8, pp. 490-499, 1967.

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