Waste Treatment Plant Project
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Waste Treatment Plant ProjectAdapting Dispersion Software to DOE Standard 3009
Jorge SchulzThomas R. McDonnellBechtel National, Inc.
2012 EFCOG Safety Analysis WorkshopMay 8, 2012
22OverviewThis paper describes work performed for the WTP project to enable use of MACCS2 version 2.5 to generate 95th percentile overall site atmospheric dispersion factors (/Q) in accordance with the statistical treatment required by DOE-STD-3009-94The maximum sector 99.5th percentile /Q was also determined by post-processingAppendix A, section A.3.3, of STD-3009 states that the 95th percentile /Q should:Account for variations in distance to the site boundary as a function of directionBe consistent with the statistical treatment of /Q values described in regulatory position 3 of NRC Regulatory Guide 1.145 for the evaluation of consequences along the exclusion area boundaryDetermine the distance to the site boundary in accordance with the procedure outlined in position 1.2 of Regulatory Guide 1.145MACCS2 determines directionally independent /Qs at a specific distance; thus, there is no way of obtaining the direction-dependent 95th percentile /Q without post-processing223Site Boundary Distance Regulatory Guide 1.145, position 1.2, states:For each of the 16 sectors, the distance for . . .X/Q calculation should be . . . the nearest point on the building to the exclusion area boundary within a 45-degree sector centered on the compass direction of interest.WTP has several large processing buildings that are clustered together on the siteThe site boundary distances were determined from the building cluster perimeter to the closest points on the established public boundary at each compass sectorThe site boundary distances range from 9.1 to 13.9 km3
4Combined Effects of Site Boundary Distance and Prevailing Wind Direction
Although the SSW, SW boundary distance is the closest, prevailing winds are from the WSWMACCS2 does not take actual site distances into accountMACCS2 boundary445Meteorological Data Collection and ProcessingPrevious meteorological data were1-hour averages rounded to the nearest mph (~0.5 m/s), resulting in discrete steps when plotted as a cumulative distribution (red curve)Subsequently, ten years of 15-minute average met data were obtained and processedThe CCDF curve is significantly smoother (blue curve)The processed 15-minute average data can be used for events involving short term accident releaseThis processed data was also used to generate hourly averages, which were then converted into the format used by MACCS25
66Software ValidationOriginal V&V of MACCS2 was performed before the WTP project had fully implemented DOE-STD-3009-94Subsequently, it was determined that the 95th percentile /Qs produced by MACCS2 did not meet STD-3009 requirementsRequired post-processing of MACCS2 expanded output to reflect actual site boundary distances and to be directionalNecessitated also V&Ving the calculation of individual hourly sequence /Qs that MACCS2 generates in the process of determining its 95th percentile values A spreadsheet-based approach (using Microsoft Excel) was developed and validated for calculating atmospheric dispersion factors based on the methodology given in NRC Regulatory Guide 1.145Basis for confirming that the 95th percentile /Q results produced by MACCS2 ATMOS module meet the requirements for V&V in accordance with DOE Quality Assurance Order DOE O 414.1C and Safety Software Guide DOE G 414.1-466Spreadsheet routines are Visual Basic for Applications (VBA) function procedures:Function Sigz(BldHgt, sigmaz)' This function calculates the building wake correctionFunction PlumeRise(Uz, Distance, Stability, MixH, SubCall)' This function determines the plume rise for elevated releasesFunction RG1145(Wind, Distance, Stability)' This function determines the hourly accident X/Q using RG1.145 methodologyFunction Sigy(Distance, Stability, Wind, sigmay, s800table)' This function calculates the lateral plume spread with meander using the Mcor functionFunction Mcor(Stab, WindSpeed)' This function calculates the meander correction factorFunction Gz(H, heff, z, sigmaz)' Calculates the G(z) termFunction Vdep(Stability, Wind)' This function calculates the dry deposition velocityFunction Qprime(Distance, Stability, Vdep, Wind, XQType)' This function determines the depletion of the source using the methodology described in the GENII Version 2 Software Design DocumentFunction sigmay(Distance, Stability, Table)' This function calculates the lateral dispersion coefficientsFunction sigmaz(Distance, Stability, Table)' This function calculates the vertical dispersion coefficientsFunction MixHeight(Stability, z0, M_O_Data, MetHeight, alpha, Wind, k)' Calculates the mixing heightFunction MixDistance(MixH, Stability, Table)' This function determines the distance at which the vertical dispersion factor equals the mixing height' This function assumes that the distance will be greater than 1000 metersFunction RelWind(Stability, z0, M_O_Data, MetHeight, alpha, Wind, k, heff)' This function determines the wind speed at the release height for elevated releasesFunction psi(alpha, Linv, x, Stability, z)' This function calculates the psi term for the effects of stability' on the wind profile.Function InvMol(Stab, z0, Table)' InvMol computes an estimate of the inverse of the Monin-Obukhov length (1/L) based on the relationship between stability class, z0, and 1/L as implemented in the Arcon96 code. ' The argument to this function named Table is implemented in a worksheet range that duplicates the data values of the INVMOL2 subroutine in ARCON96.
Independent calculation being revised to address closer distances and plume rise. ## new test cases.7MACCS2 RunsMACCS2 expanded outputs were obtained using 10 years of site meteorological data and each spatial intervalExpanded output consists of individual /Q sequences for each hourEach /Q sequence tracks the plume as it travels downwind from the release location Spatial intervals cover intermediate distances and each unique site boundary distanceUndepleted and depleted individual sequence /Qs were determined for both ground-level and elevated releasesDispersion CoefficientsThe MACCS2 Gaussian model uses spatially-dependent dispersion parameters, y and zFor WTP, y and z values are input to MACCS2 as lookup tables using the Briggs Open Country parameterizationsFor ground-level releases, /Qs are calculated both with and without plume meander78Post-Processing of MACCS2 Expanded OutputThe MACCS2 expanded outputs described in the previous slide were post-processed in an Excel spreadsheetMACCS2 /Qs were imported into Excel worksheets and sorted by clockwise meteorological sectorFor each sector, the site boundary distance was entered and the 99.5th percentile /Qs were determinedThe overall 95th percentile /Q was determined by using the Excel function PERCENTILE on an array of hourly /Qs for each sector899Comparison of MACCS2 Results to Post-Processed /Q Results The post-processed, 95th percentile directionally-dependent, undepleted and depleted overall site boundary /Qs are compared below to those determined using the MACCS2 directionally-independent methodologyThe post-processed /Qs ranged from 13 to 21% lower than those obtained by the MACCS2 method
99Largest difference was 32%.1010Identified MACCS2 Software ErrorsIn the course of this effort, two software errors were discovered in MACCS2, which are described in the following slidesThe first error was in the treatment of dispersion coefficients from a lookup table (freezing of y values)The second involved the method of accounting for plume meander using the Regulatory Guide 1.145 model implementationAn evaluation of these errors determined that they either produce conservative results or have no significant impact on existing WTP accident analyses11y Freezing Error BackgroundAn intermediate step in the MACCS2 computation process is determination of lateral and vertical dispersion coefficients (y and z)MACCS2 calculates the dispersion coefficients for each user-defined distance interval for all meteorological sequencesMACCS2 tracks the plume travel downwind over time; thus, depending on the wind speed, meteorological conditions can change at some downwind distance intervalIn those cases, MACCS2 calculates a virtual distance that would provide the same dispersion coefficients at the transition point for the current stability class11In this example, the plume expands in the first hour based on the stability class and wind speed of that hour. In the next hour, the stability class changes to a more stable condition, and the wind speed decreases to one half of the initial wind speed; therefore, the plume expands more slowly and takes longer to travel a similar distance.
12y Freezing Error DescriptionIf the virtual distance exceeds the dispersion coefficient look-up table (e.g., for stable atmospheric conditions), the last z is used until the meteorology changes againTypically, the z limit is reached, while the y is noti.e., the plume does not expand in the vertical direction, but can continue to expand in the horizontal directionHowever, once the value of z reaches its limit, two distance intervals later MACCS2 v. 2.5 also freezes the value of yThis should not occur because the limit of the y look-up table is not reached/Q values are also frozen12
MACCS2 Expanded Output Extract13Effect of y Freezing ErrorFreezing y rather than allowing it to grow with distance, results in a sequence /Q that is higher than it should be, which is conservativeFor WTP meteorology, the error typically occurs in about 4% to 8% of the sequencesThe reduction in a sequence /Q after correction would be no more than about 10-20%Therefore, for a sequence to have an impact on the 95th percentile /Q, the uncorrected /Q value would h