dresden nuclear power station pre-application meeting

Dresden Nuclear Power Station Pre-Application Meeting

Providing Natural Gas as a Primary Fuel to Station Auxiliary Heating Boilers

January 14, 2015

Agenda• Introductions• Dresden Nuclear Power Station (DNPS) Heating Boiler ( ) g

Background• Applicable Regulatory Requirements• Heating Boiler Fuel Hazard Analysis• Description of FLACS Capabilities

Brief Description of DNPS FLACS Analysis• Brief Description of DNPS FLACS Analysis• Validation of FLACS Analysis Results• SummarySummary• Discussion/Regulatory Challenges/Feedback

Dresden Nuclear Power Station Natural Gas Fuel for Station Heating Boilers Pre-Application Meeting2

Introductions - Exelon Personnel in Attendance

• Paul Wojtkiewicz – DNPS Senior Manager Plant Engineering• Filippo Gavelli – GexCon US• Jayram Patel DNPS Design Engineering• Jayram Patel – DNPS Design Engineering• Tom Griffith – DNPS Regulatory Assurance• Mitch Mathews – Exelon Corporate Licensing


DNPS - Heating Boiler Background

• The recently installed Auxiliary Heating Boilers:− Provide plant heating to DNPS buildings and heatingProvide plant heating to DNPS buildings and heating

steam to various vaporizers. No Safety Related functions associated with these boilers

− Dual fuel design (i.e., can operate with natural gas or fuel oil)

− Currently operated using fuel oilCurrently operated using fuel oil


DNPS - Heating Boiler Background (Cont) - Site Layout


Applicable Regulatory Requirements

• NRC Regulatory Guide (RG) 1.78, Rev. 1, “Evaluating the Habitability of A Nuclear Power Plant Control Room During A Postulated Hazardous Chemical Release,” is used to assess the habitability of theHazardous Chemical Release, is used to assess the habitability of the control room during and after a postulated external release of hazardous chemicals from mobile or stationary sources, offsite or onsite.

• RG 1.91, Rev. 2, " Evaluations of Explosions Postulated to Occur at Nearby Facilities and on Transportation Routes Near Nuclear Power Plants " can be used to evaluate onsite hazards as discussed in NRCPlants, can be used to evaluate onsite hazards as discussed in NRC Standard Review Plan (SRP) Section 2.2.1-2.2.2, “Identification of Potential Hazards in Site Vicinity.”


DNPS - Heating Boiler Fuel Hazard Analysis

• Hazard analysis utilized the same rationale as described in the Staff Regulatory Guidance of RG 1.91, Rev. 2. The following hazards were considered sufficiently low:considered sufficiently low:1. Those that are greater than 491 feet of vital structures, systems,

and components (Lower effective yields may be justified by analyses accounting for reaction kinetics)analyses accounting for reaction kinetics)

2. Those that have a frequency that is less than 10-6 occurrences per year, when based on conservative assumptions, or 10-7 per year when based on realistic assumptionswhen based on realistic assumptions

3. Those that do not impair the ability of safety-related structures to perform as-designed (Effective yields based on analyses accounting for reaction kinetics)accounting for reaction kinetics)

• Previous analyses were based on the method of evaluation presented in the NRC Safety Evaluation Report for the Hartsville Nuclear Plant, dated March 1976


dated March 1976.

DNPS Hazard Analysis (Cont.) –Current Analysis

• Utilizing currently approved methods (i.e., Hartsville method), a number of plant structures would experience an unacceptable pressure wave.− Boiler House

T bi B ildi W ll− Turbine Building Wall− Main Power Transformer


DNPS Hazard Analysis (Cont.) - Concerns• Analysis determined Control Room habitability not a concern, however:• Since the heating boilers are in close proximity to vital SSCs the hazard

is within the 491 foot radius described in RG 1.91.is within the 491 foot radius described in RG 1.91.• While safety systems are designed to detect and automatically isolate

natural gas leakage in the Heating Boiler Building, Exelon has not yet concluded that the hazard frequency is less than 10-7 per year.concluded that the hazard frequency is less than 10 per year.

• The use of TNT equivalency results in effective yields that are unacceptably high; therefore, a new method of evaluation is required to determine realistic yields based on an analysis technique that accountsdetermine realistic yields based on an analysis technique that accounts for reaction kinetics as discussed in RG 1.91, Rev. 2.

• Exelon chose GexCon US’s FLame ACceleration Simulator (FLACS):− Accounts for vapor cloud explosion (VCE) interaction with site SSCs− Accounts for vapor cloud explosion (VCE) interaction with site SSCs− Provides realistic blast yields based on experimental data− Widely recognized as accurate by government agencies such as

U S Department of Transportation Pipeline and Hazardous


U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration (PHMSA).

DNPS Hazard Analysis (Cont.) - Comparison of MethodsTNT Methodology:• Point source detonation• Sharp rise in overpressure and has a very short durationp p y• Over predicts VCE near field pressure and under predicts in the far field• Efficiency factor can only be estimated• Results in plant structures experiencing an unacceptable pressure wave.p p g p p

− Turbine Building Wall− Main Power Transformer− Dry Cask StorageDry Cask Storage

FLACS computer code (CFD 3-D modeling):• Covers large area and begins as a deflagration• Produces a more gradual rise in pressure over longer period of timeProduces a more gradual rise in pressure over longer period of time• Evaluates the congestion and confinement in areas within the vapor cloud• Conservatively assumes the boiler house was filled with a stoichiometric mixture

of natural gas and air for dispersion analysesg p y


Description of FLACS Capabilities

FLACS is a process-safety specific consequence modeling software tool which includes:

I t t d l i fi d di i d li biliti• Integrated explosion, fire and dispersion modeling capabilities• Geometry import from several 3D Computer Aided Drafting programs• Gas mixtures• Transient (time dependent) modeling• Mitigation and preventive measures

− Vent panels water deluge inertingVent panels, water deluge, inerting• Extensive validation


Description of FLACS Capabilities (continued)

• Structured Cartesian grid• Distributed porosity concept (sub-grid modelling)• RANS standard k-epsilon model• Modified SIMPLE solver• Compressible and incompressible solversp p• OpenMP parallel processing• Explosion Combustion model

− Artifically thickened flame zoneArtifically thickened flame zone− Tuned parameters based on 35 years of validation− Semi-empirical model for turbulent flame speed

• Eddy dissipation concept (EDC) combustion model for fire modelingEddy dissipation concept (EDC) combustion model for fire modeling• Atmospheric boundary layer profiles• Methodology for calculating equivalent stoichiometric gas clouds for

deterministic or probabilistic explosion consequence modelingdeterministic or probabilistic explosion consequence modeling


Brief Description of DNPS FLACS Analysis

• The FLACS computer code used for internal and external vapor cloud explosion (VCE) deflagration analyses when variables affecting the explosion (e.g. unusual geometry or highly variable congestion)

• Computational fluid dynamics (CFD) modeling of the• Computational fluid dynamics (CFD) modeling of the natural gas explosion scenario in the DNPS Heating Boiler Building using FLACS

• CFD modeling to predict the explosion energy, explosion severity, as well as blast pressure and impulse at various distances/locations of interestdistances/locations of interest


DNPS FLACS Analysis (Cont) – Key Analysis Assumptions

• A natural gas leak occurs • All gas detection and fail safe systems fail g y• The entire open air volume of Heating Boiler Building is

filled with natural gas and air (Conservative)• Ignition occurs in the center of the building• Wall panels fail and relieves internal pressure

Heating Boiler Building windows allow venting of the• Heating Boiler Building windows allow venting of the explosion


FLACS Analysis (Cont) - Overpressure Blast Contours for Accidental NG Explosion in Heating Boiler Building

BOILER HOUSEBOILER HOUSE

Turbine Building


FLACS Analysis (Cont) - Overpressure Blast Impulse for Accidental NG Explosion in Heating Boiler Building

BOILER HOUSE

Turbine Building


FLACS Analysis (Cont) - Results

• Turbine Building (TB) walls will not be superficially damaged - not expected to be perforated nor will it experience scabbingTh iti l MCC i t ithi th TB ld i ti l• The critical MCC equipment within the TB would remain operational

• A natural gas explosion in the Heating Boiler Building does not present a credible event threat to Dry Cask Storage Area based on distance

• SSCs required for plant safe shutdown remain available• Assume the Unit 3 Main Power Transformer would be lost due to the

VCE and resulting pressure wave• There will be no offsite release or offsite dose associated with a

postulated VCE in the DNPS Heating Boiler Building


Validation of FLACS Analysis Results

• FLACS widely used; however, no precedent for its use in any licensing actions

• Numerous experiments have been conducted to validate FLACS analysesFLACS l i t di h b t d b• FLACS explosion studies have been accepted by government agencies worldwide (DOT/PHMSA in the US, but also in Australia and several countries in Europe)

• Results of experiments that compare analyses to empirical data is readily available for NRC audit and or review


Summary• Through utilization of FLACS, EGC has determined that no

SSCs required for safe shutdown will sustain damage from a VCE in the DNPS Heating Boiler Building that will render them incapable of performing their designated safety functionfunction.

• Explosion will not result in exposures that result in radiological dose in excess of the 10 CFR 50.34(a)(1) as it relates to the requirements of 10 CFR 50.67

• NRC review is required to allow implementation for the use of FLACS as a departure from a method of evaluationof FLACS as a departure from a method of evaluation described in the Updated Final Safety Analysis Report used in establishing the design bases or in the safety analyses.


Discussion/Regulatory Challenges/Feedback


dresden nuclear power station pre-application meeting

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