Atmospheric Dispersion in Atmospheric Dispersion in Nuclear Power Plant SitingNuclear Power Plant Siting

IAEA Safety SeriesIAEA Safety Series

50-SG-S350-SG-S3

IntroductionIntroductionThe atmosphere is an important The atmosphere is an important

pathway for the transport of radioactive pathway for the transport of radioactive releases from a nuclear power plant to the releases from a nuclear power plant to the environment and there by to man. environment and there by to man. Meteorological phenomenon and mechanism Meteorological phenomenon and mechanism is involved in the dispersion of the released is involved in the dispersion of the released effluent in the atmosphere. To calculate the effluent in the atmosphere. To calculate the concentration and deposition in a region, concentration and deposition in a region, different models may be used. Specific different models may be used. Specific inputs data is required to for such type of inputs data is required to for such type of calculation. Meteorological data is one of the calculation. Meteorological data is one of the inputs. This guide emphasis Selection of inputs. This guide emphasis Selection of models and input data to fit a specific models and input data to fit a specific situation for accurate measurement of situation for accurate measurement of dispersed radionuclide. dispersed radionuclide.

Meteorological investigation at Meteorological investigation at different stagesdifferent stages

Site SurveySite SurveyMeteorological characteristics along with Meteorological characteristics along with

population density and land use in a region population density and land use in a region are important factors while determining the are important factors while determining the suitability of a site for power plant siting. At suitability of a site for power plant siting. At the stage of site survey climatic information the stage of site survey climatic information may be gathered from nearby synoptic may be gathered from nearby synoptic weather station. Adverse meteorological weather station. Adverse meteorological characteristics i.e. channeled flow, unfavorable characteristics i.e. channeled flow, unfavorable wind prevailing wind direction or stability wind prevailing wind direction or stability conditions should be studied in combination conditions should be studied in combination with population distribution. IAEA safety series with population distribution. IAEA safety series No. 50-SG-S4 is also helpful in site selection No. 50-SG-S4 is also helpful in site selection and evaluation for Nuclear Power Plant.and evaluation for Nuclear Power Plant.

Site EvaluationSite Evaluation

During the design and licensing of a During the design and licensing of a nuclear power pant, evaluations shall be nuclear power pant, evaluations shall be made of the concentrations due to made of the concentrations due to effluent which may release from the plant effluent which may release from the plant in operational states and accidental in operational states and accidental conditions. Therefore significant conditions. Therefore significant meteorological variables are determined meteorological variables are determined for input into dispersion calculations for for input into dispersion calculations for statistical analysis, which might be statistical analysis, which might be helpful to obtain probability distribution helpful to obtain probability distribution of concentration of radioactive material of concentration of radioactive material released from the plant.released from the plant.

Need of Meteorological DataNeed of Meteorological DataMeteorologicaMeteorological l characteristic of the site characteristic of the site

should be well understood preceding the licensing should be well understood preceding the licensing stages. However need of meteorological data stages. However need of meteorological data during the routine operation is not much during the routine operation is not much extensive These information will be helpful to extensive These information will be helpful to determine: determine: The short and long-term routine radioactive The short and long-term routine radioactive releases to the environment. releases to the environment. To take countermeasures after an accidental To take countermeasures after an accidental release (contingency Planning).release (contingency Planning). A programme for meteorological measurement A programme for meteorological measurement along with forecast is essential for implementing along with forecast is essential for implementing of emergency measuresof emergency measures

Limitation of Meteorological Limitation of Meteorological MeasurementsMeasurements

The actual dispersion is ofThe actual dispersion is of Lagrangian Lagrangian characteristics in which each parcel of fluid characteristics in which each parcel of fluid during its travel may be subjected to the during its travel may be subjected to the following conditions very different from the following conditions very different from the source. source. Physical and chemical state of effluentsPhysical and chemical state of effluents Terrain and height of the sourceTerrain and height of the source Meteorological conditionMeteorological condition Variation in plume w.r.t distance and timeVariation in plume w.r.t distance and timeConsideration of the above conditions can Consideration of the above conditions can permit a proper evaluation of the dispersion permit a proper evaluation of the dispersion using dispersion models.using dispersion models.

Terrain ConsiderationTerrain Consideration

Meteorological measurement are oftenly Meteorological measurement are oftenly determined by the terrain due to the determined by the terrain due to the reasons.reasons. Hill side meteorological measurements Hill side meteorological measurements provide data that is applicable only to its provide data that is applicable only to its immediate surrounding.immediate surrounding. Plant structure in particular cooling Plant structure in particular cooling towers may effect atmospheric dispersion.towers may effect atmospheric dispersion.While collecting meteorological data, such While collecting meteorological data, such local effects should be considered such local effects should be considered such that these may not affect the values of the that these may not affect the values of the variables to be measured.variables to be measured.

Type of Meteorological data Type of Meteorological data requiredrequired

The precision, nature and scope of The precision, nature and scope of meteorological data to be collected should be meteorological data to be collected should be compatible with the methods and models in compatible with the methods and models in which they may be used.which they may be used.The bases of such programme are:The bases of such programme are:

General Meteorological Data:General Meteorological Data: Air TemperatureAir Temperature Air flow ( wind direction and speed with Air flow ( wind direction and speed with duration)duration) Thickness if mixing layerThickness if mixing layer PrecipitationPrecipitation HumidityHumidity

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Indicators for Turbulence:Indicators for Turbulence: Air temperature and temperature laps Air temperature and temperature laps rate.rate. Sky cover during day ,night or net Sky cover during day ,night or net radiation radiation Wind direction fluctuationsWind direction fluctuations Wind speed at different heightsWind speed at different heightsIt is useful to have good knowledge of the It is useful to have good knowledge of the space and time distribution of such space and time distribution of such variables to understand and determine variables to understand and determine effluent trajectory. The inaccurate effluent trajectory. The inaccurate information collected about turbulence information collected about turbulence indicators may lead to wrong estimates of indicators may lead to wrong estimates of the concentrations.the concentrations.

Instrumentation for data collectionInstrumentation for data collection

The meteorological instrumentation The meteorological instrumentation system should have performance within system should have performance within the limit described in the manual. The the limit described in the manual. The instruments systems should be protected, instruments systems should be protected, maintained, serviced and calibrated. maintained, serviced and calibrated. Latest instruments available in the Latest instruments available in the market for the collection of this type of market for the collection of this type of data should be preferred.data should be preferred.

Terrain ComplexityTerrain ComplexityWind and air temperatures are Wind and air temperatures are

affected by terrain irregularities and the affected by terrain irregularities and the presence of large water bodies. There are presence of large water bodies. There are two main types of complex terrains.two main types of complex terrains. Uneven terrain e.g. ridges and valleysUneven terrain e.g. ridges and valleys Coastal areas, or an area near large Coastal areas, or an area near large water water bodiesbodiesUneven TerrainUneven TerrainIn uneven terrainIn uneven terrain the mean atmospheric the mean atmospheric flow comprises offlow comprises of Lower regime where wind are locally Lower regime where wind are locally effected by structures, hills and valleys.effected by structures, hills and valleys. Upper regime where winds are not Upper regime where winds are not affected.affected.

Coastal SitesCoastal SitesSites near large water bodies are subject Sites near large water bodies are subject to distinct following local wind system.to distinct following local wind system. In fair weather wind direction depends In fair weather wind direction depends on the position of the sun i.e. towards on the position of the sun i.e. towards land during day and toward water during land during day and toward water during night.night.The on-shore and off-shore winds The on-shore and off-shore winds experience a change of surface roughness experience a change of surface roughness and temperature gradient patterns which and temperature gradient patterns which under appropriate condition may result in under appropriate condition may result in recirculation of fumigation.recirculation of fumigation.

Atmospheric DispersionAtmospheric Dispersion A radioactive gas once become A radioactive gas once become airborne travels and disperses in a airborne travels and disperses in a manner that is influenced by its own manner that is influenced by its own physical properties and those of ambient physical properties and those of ambient atmosphere into which it is released. atmosphere into which it is released. The effluent once released into the The effluent once released into the atmosphere with certain velocity and atmosphere with certain velocity and temperature are generally different that temperature are generally different that of the ambient atmosphere. The effluent of the ambient atmosphere. The effluent motion has vertical component due to the motion has vertical component due to the effects of vertical velocity and difference effects of vertical velocity and difference of temperature until they are dissipated.of temperature until they are dissipated. The vertical rise of the effluent is called The vertical rise of the effluent is called the plume rise and it changes the the plume rise and it changes the effective height of the release point. effective height of the release point.

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The effluent movement is also The effluent movement is also affected by wind during and after rise affected by wind during and after rise which is called Guide transport. which is called Guide transport. The turbulent motion of the The turbulent motion of the atmosphere causes random movement atmosphere causes random movement of the effluent resulting in a of the effluent resulting in a progressive lateral and vertical progressive lateral and vertical spreading by mixing with air called spreading by mixing with air called Guide atmospheric dispersion. Guide atmospheric dispersion. The combination of transport and The combination of transport and diffusion is called atmospheric diffusion is called atmospheric dispersion .dispersion .

Types of Dispersion ModelsTypes of Dispersion Models

Selection of models and input data to Selection of models and input data to fit a specific situation depends upon the fit a specific situation depends upon the characteristics of the site and its region characteristics of the site and its region and conditions of release. Some typical and conditions of release. Some typical uses of atmospheric dispersion models are:uses of atmospheric dispersion models are: To derive shot-term concentration and To derive shot-term concentration and deposition values.deposition values. To determine long-term time integrated To determine long-term time integrated concentration and depositionconcentration and deposition

The model we will discuss are applicable to The model we will discuss are applicable to the situation where the meteorological the situation where the meteorological parameters are reasonably uniform in time parameters are reasonably uniform in time and space.and space.

The Effluent BehaviorThe Effluent BehaviorThe effluent undergoing plume rise The effluent undergoing plume rise

may subject to processes such as:may subject to processes such as: Radioactive decay and buildup of Radioactive decay and buildup of

daughter products.daughter products. Wet deposition Wet deposition (Rainout/snow out by water (Rainout/snow out by water

droplets as precipitation or Washout by falling droplets as precipitation or Washout by falling precipitation)precipitation)

Dry deposition (Gravitational settling)Dry deposition (Gravitational settling) Formation of aerosols and coalescence Formation of aerosols and coalescence

of aerosols.of aerosols. Re-suspension of material deposited on Re-suspension of material deposited on

surfacesurface

Indicators of TurbulenceIndicators of Turbulence

While using dispersion models it is While using dispersion models it is necessary to specify atmospheric stability as necessary to specify atmospheric stability as function of meteorological parameters such function of meteorological parameters such as:as:

1.Temperature laps rate:1.Temperature laps rate: The rate of decrease The rate of decrease of temperature with height i.e. -dT/dZ. The of temperature with height i.e. -dT/dZ. The temperature distribution in the atmosphere temperature distribution in the atmosphere frequently differs from normal adiabatic frequently differs from normal adiabatic behavior and may have any of following behavior and may have any of following conditions:conditions:

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Adiabatic condition:Adiabatic condition: If the rate of change in If the rate of change in temperature is per unit altitude then it is temperature is per unit altitude then it is called the adiabatic lapse rate. This lapse called the adiabatic lapse rate. This lapse rate is about 1rate is about 1ooC/100 m as shown in C/100 m as shown in fig.fig.

Super adiabatic condition:Super adiabatic condition: Over certain Over certain ranges in altitude the rate of temperature ranges in altitude the rate of temperature decrease may be more rapid than the decrease may be more rapid than the adiabatic rate, and in this case, the adiabatic rate, and in this case, the stratum of air is said to exhibit super stratum of air is said to exhibit super adiabatic behavior.adiabatic behavior.

Stable condition:Stable condition: In stable condition at the In stable condition at the altitude the rate of temperature decrease altitude the rate of temperature decrease is less rapid as compared to adiabatic rate is less rapid as compared to adiabatic rate and the stratum.and the stratum.

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Inversions:Inversions: When the temperature When the temperature increases over some range in altitude and increases over some range in altitude and some where not then the condition exist some where not then the condition exist called an inversion.called an inversion.

Isothermal condition:Isothermal condition: When the When the temperature remains constant across a temperature remains constant across a stratum, it is then known as isothermal.stratum, it is then known as isothermal.

22. Wind direction Fluctuation:. Wind direction Fluctuation: The wind The wind direction fluctuation is also used to direction fluctuation is also used to characterize the atmospheric stability. The characterize the atmospheric stability. The standard deviation of the wind direction is standard deviation of the wind direction is obtained by electronic devices or by wind obtained by electronic devices or by wind direction records. The wind direction direction records. The wind direction record shows different width of trace for record shows different width of trace for different stability conditionsdifferent stability conditions

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3. Insolation, cloudiness and wind speed:3. Insolation, cloudiness and wind speed:Insolation, cloudiness and wind speed are Insolation, cloudiness and wind speed are

also used to determine the stability classes. also used to determine the stability classes. Thermal turbulence is related to heat flux. The Thermal turbulence is related to heat flux. The cloud cover reduces the heat losses or heat gain cloud cover reduces the heat losses or heat gain tending to make the stability neutral. tending to make the stability neutral. Strong winds increase mixing and lead to Strong winds increase mixing and lead to neutral stability. neutral stability. Clear skies at night with low winds make for Clear skies at night with low winds make for stable atmosphere. Whereas, the similar stable atmosphere. Whereas, the similar conditions in day time make for an unstable conditions in day time make for an unstable atmosphere.atmosphere. During the day solar radiation measurement or During the day solar radiation measurement or estimates are useful in determining the stability estimates are useful in determining the stability classes .classes .4. Richardson and Bulk Richardson Number:4. Richardson and Bulk Richardson Number:

These reflect the imbalance between These reflect the imbalance between thermal or convective turbulence.thermal or convective turbulence.

Stability ClassesStability Classes

Following are the six stability classes Following are the six stability classes using solar radiation and net night radiation using solar radiation and net night radiation with wind speed.with wind speed.

AA Extremely stable Extremely stable

BB Moderately stable Moderately stable

CC Slightly unstable Slightly unstable

DD Neutral Neutral

EE Slightly stable Slightly stable

FF Moderately stable Moderately stable

The incoming solar radiation may be The incoming solar radiation may be measured by pyrheliometer. At night net measured by pyrheliometer. At night net radiation flux is measured by net radiation flux is measured by net radiometer.radiometer.

Wind speedU(m/s)At 10 m

Stability class, day,with solar radiation RD

(langleys/h)

Stability class, day,with solar radiation RD

(langleys/h)RD≥50 50> RD ≥ 25>RD≥12.5 12.5> RD

RN> -1.8 -1.8≥ RN ≥-3.6 -3.6>RN

U<22≤U<33≤U<44≤U<55≤U

A A-B B DA-B B C DB B-C C DC C-D D DC D D D

D - -D E FD D E D D DD D D

Note: 1 langley= 1 cal.cm-2=4.187 J.cm-2

STABILITY CLASSIFICATION TABLE, USING SOLAR RADIATION AND NIGHTNET RADIATION, WITH WIND SPEED (Pasquill-Gifford Table)

Atmospheric Dispersion Calculations Atmospheric Dispersion Calculations ApproachesApproaches

Theoretically following approaches Theoretically following approaches may be adopted to treat atmospheric may be adopted to treat atmospheric dispersion with their own limitations:dispersion with their own limitations:

The gradient transfer or K-theory approachThe gradient transfer or K-theory approach The statistical theory approachThe statistical theory approach The dimensional analysis approachThe dimensional analysis approach

The K-theory approach is mainly used The K-theory approach is mainly used for such type of calculations which can take for such type of calculations which can take space-time variations of wind and stability space-time variations of wind and stability conditions and terrain complexity into conditions and terrain complexity into account.account.

Nature of SourcesNature of SourcesIn atmospheric dispersion calculations In atmospheric dispersion calculations

different kinds of sources are usually different kinds of sources are usually classified in terms of spatial configuration classified in terms of spatial configuration and duration of release which are:and duration of release which are: Point sourcesPoint sources Line and area sourcesLine and area sources Volume sourcesVolume sources

Actual sources of radioactive effluent Actual sources of radioactive effluent are not point sources but may be considered are not point sources but may be considered for mathematical simplicity. Line and area for mathematical simplicity. Line and area sources may be treated by subdivision into a sources may be treated by subdivision into a number of effective point sources. For large number of effective point sources. For large volume sources a virtual point source may be volume sources a virtual point source may be considered.considered.