The ANSI/ANS 2.15 Standard for Modeling Routine Radiological Releases

from Nuclear Facilities

John Ciolek

AlphaTRAC, Inc.

Introduction

…setting the stage

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Modeling Standards Plan

• Idea: Produce voluntary consensus standards for atmospheric modeling– ANS 2.15: Modeling routine releases of radiological

material from nuclear facilities

– ANS 2.16: Design basis accident scenario modeling

– ANS 3.8.10: Real-time emergency response modeling for accidents

• Working group decided to create the standards sequentially– Build off previous work

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Process for Creating a Standards Documents

• Form working group• Create Project Initiation Notification (PIN)

– Defines scope and intention

• Create standards document• Obtain subcommittee technical content approval• Obtain consensus committee and public review• Review for compliance with ANSI• Obtain approval as American National Standard

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The 2.15 Standard

…what is considered

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Subjects Considered in 2.15…

• Models– Model types– Reference frames

• Time scales• Release modes

– Sources– Ground-level and elevated releases– Mixed-mode releases– Plume rise– Aerodynamic effects of buildings

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…Subjects Considered in 2.15

• Removal mechanisms– Radioactive decay, wet and dry deposition

• Geospatial data• Meteorological data• Meteorological networks• Quality assurance

Note: Standard applies only to offsite areas

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Special Subjects

• Recirculation and complex flow• The modeling process• Requirements• Quality assurance

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Recirculation

…and other complexities

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Recirculation Group

• Subgroup formed to investigate influence of complex flow which includes:– Recirculation– Stagnation– Flow reversal– Wind shear

• Subgroup charged with determining:– Where complex flow occurs, does it significantly

contribute to total dose?– If so, what should be recommended?

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Recirculation Evaluation Process

• Determine current state of knowledge– Literature search for complex flow atmospheric

dispersion studies (journals, conference papers, books, etc.)

– Interview authors of XOQDOQ– Review recent communications within NRC

• Analyze influence of complex flow on total dose• Develop recommendations to working group• Write white paper to document analysis and findings

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Recirculation Group Findings

• Only two studies specifically examine this issue!– Response to Pilgrim Watch (O’Kula and Hanna, 2011)– Southern Great Plains (LLNL - NUREG/CR-6853, 2004)

• Problems:– Model grids too coarse

• 4 km grids will only see features > 8km in extent

– Low resolution of meteorological observations– Did not look at multiple time scales found to be important

in literature• Hours, diurnal, one to three days

– Meteorological averaging times too long• Used hourly averaged meteorological data

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Problem Using Hourly Observations

• Canyon release from Los Alamos, NM

• One hour plume travel

• Instantaneous puff shown

• Plume returns to release after one hour

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Diurnal Recirculation

• Tetroon Study• Rocky Flats, CO• Feb. 9, 1991• Tetroon returned

to release point about ~ 12 hours after release

Release

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Findings

• Can’t base recommendations on Pilgrim Watch or Plains studies

• Found several studies that document complex flow– Hourly, diurnal, and daily signatures

• Impacts can be 2 to 5 times greater– With pooling and flow reversal, impacts can be even

greater• Stability influences recirculation• Climatology influences recirculation frequency• Complex flow can occur 15% to 50% of time

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Unanimous Recommendations

• Eliminate recirculation factor• Explicitly treat complex flow

– If that might have significant effect on results

• We need a conclusive study– Determine if complex flow significantly affects routine release

modeling

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DOE Model Comparison Study

• Rapid study (not published)• Straight-line (EPIcode®) vs. variable trajectory 3-D

model (CAPARS® system)• Random sample of start times from one year

– 215 out of 35,040 15-minute data sets

• Four-hour simulations• Maximum concentration binned at select distances• No deposition or resuspension

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Complex Domain

Release

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Comparison Results

EPIcodeCAPARS95th Percentile50th PercentileInterquartile

EPIcodeCAPARS95th Percentile50th PercentileInterquartile

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Analysis of Modeling

• Results differ based on how material was released• Results match at some distances (1 - 5 km)• Differences can be 1 - 2 orders of magnitude• Suggests that straight-line models:

– Are not necessarily the most conservative in complex environments

– May greatly over-estimate consequences > 20 km from the release

• Considering complex terrain and flow probably will produce significantly different results

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The Modeling Process

…an engineering perspective

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Modeling Process

• Added “Modeling Process” as first section of 2.15 document

• Explicitly treats:– Requirements– Quality assurance– Complex modeling processes

• Requires variable-trajectory modeling under some circumstances

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Requirements…

• Must explicitly develop requirements• Must be achievable• Must be checked at end of project for completion

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…Requirements

• Should include:– Regulatory agencies governing project– Applicable regulatory limits– Model selection – Time scales– Release modes– Removal mechanisms– Input data– Modeling domain definitions– Quality assurance

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Quality Assurance

• Must first define QA process to be used– Can be company-based or national/international-based standard

• IEEE, NRC, DOE, etc.

• Must define what components will be applicable to project

• Must follow process agreed upon

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Variable-trajectory Models Shall Be Used IF…

• You have a requirement to use them

OR

• Straight-line Gaussian modeling results are > 10% of regulatory limits AND

• You have the potential for complex flow within your domain AND

• Complex flow occurs > 15% of the year

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Potential for Complex Flow

Determined by:

• Documented flow features– Requires qualified meteorologist

OR • Known topographical features:

– Large bodies of water (> 500 sq km)• Smallest area with documented recirculation

– Mountain(s)• Can ignore mountains < 150 m tall unless within 2 km of release

– Valleys more than 50 m deep

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Frequency of Complex Flow

• Can be difficult to determine• Must quantify how often complex flow features

happen– Left to modelers– Allwine & Whiteman (1994) method is acceptable

• Uses one profiler– Assumes uniform wind field

• Currently investigating practical implementation of this process

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Status of the 2.15 Document

…the present state

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ANS 2.15 Document

• Final (Rev. 1) finished– Sent to ANS-24 February 16, 2011

• ANS-24 review completed – Comments returned April 15, 2011

• Next steps:– Incorporate comments and return to ANS-24– Once approved, send to consensus committee (Nuclear

Facilities Standards Committee)

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The Next Standards Document

…what to expect

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ANS 2.16: Design Basis Accident Modeling

• First working group meeting held April 14, 2011– Additional DOE design basis modeling expert added to working

group

• Jeremy Rishel (PNNL) added as co-chair• Working meeting to be held at June 2011 NUMUG

meeting• Still issue of complex modeling role in design basis

accident modeling

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Questions?

jciolek@alphatrac.com

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