ML20083C124

Effects of Internal Containment Stratification & Mixing on AP600 Passive Containment Cooling Sys Design Basis Analysis Evaluation Models

ML20083C124
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Issue date:	05/08/1995
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ML20083C113	List: ML20083C124 NRC-95-4459, Forwards Effects of Internal Containment Stratification & Mixing on AP600 Passive Containment Cooling Sys Design Basis Analysis Evaluation Models
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Effects of Internal Containment Stratification and Mixing on AP600 Passive Containment Coolics System Design Basis Analysis Evaluation Models 5

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Effects of Internal Containment Stratification and Mixing on AP600 Passive Containment Cooling System Design Basis Analysis Evaluation Model Table of Contents Executive Summary 1

1.0 Introduction 1-1 2.0 Containment Design Basis Analysis Requirements 2-1 3.0 Mixing Effects on AP600 Containment Performance 3-1 3.1 AP600 Design Characteristics with Respect to Mixing 3-1 3.2 Mixing Effects on Mass Transfer 3-1 j

3.3 Mixing in the AP600-Loss-of-Coolant Accident (LOCA) 3-3 3.4 Mixing in the AP600-Main Steam Line Break (MSLB) 3-5 4.0 Evaluation Model Ability to Predict AP600 Performance 4-1 4.1 EGOTHIC Momentum Formulation and Noding Effects on Mixing 4-1 4.2 Heat Transfer Surfaces in Design Basis Analysis (DBA)

Models 4-2 4.3 Distributed Parameter Model for Peak Pressure Calculations 4-2 4.4 Lumped Parameter Model for LOCA-Long-Term 4-4 4.5 Model Assessment for LOCA Blowdown 4-5 4.6 Evaluation Model Assessment Matrix 4-6 5.0 EGOTHIC Verification and Validation with LST 5-1 5.1 LST Scale Atypicalities 51 j

5.2 Range of Noncondensibles above Operating Deck 5-1 5.3 Tests Selected for EGOTHIC Validation 5-3 6.0 Framework for Passive Containment Cooling System Evaluation Models 6-1 7.0 Conclusions 7-1 i

8.0 References 8-1 i

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EXECUTIVE

SUMMARY

This report provides a summary of the strategy for developing an evaluation model for passive containment cooling system design basis analysis and a framework for assessing the success of the methodology.

The physics of passive cooling in the AP600 plant are described for both loss-of-coolant-accident (LOCA) and main steam line break, and a sound, straightforward approach to developing and jus;ifying the evaluation models relative to stratification and mixing is discussed.

Westinghouse has explored and is developing a detailed model to calculate the pressure transient during the early, peak-pressure stage of postulated design basis analysis LOCA, and a practical, more coarsely noded model for examining the 24-hour criterion for LOCA is being prepared in parallel. A comparison of the coarser model to results from the detailed model, in addition to the scheduled large-scale test (LST) validation, will provide a basis for the acceptability of the coarser calculation through 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. An evaluation model approach for steam line break is currently under development. The characteristics ut :hese evaluation models are described in this report.

A matrix of accident phases versus important phenomena identified by the phenomena identification and ranking table (PIRT) is provided. 'Ihe well-understood characteristics of the evaluation models allow the use of these matrices to assess the acceptability of passive containment cooling system design basis analysis methodology.

An understanding of the evaluation model approach and its bases enhances a focused review and audit in the most significant areas with regard to containment pressure analyses.

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1.0 INTRODi;CTION During the AP509 design pognm in early 1994, it became apparent that there was a need to establish a phenomenologica! rnpri en !!w dfects of stratilication and mixing on heat removal inside containmeN durin3 mg:, Sasis aedyces (DBA), lhe intent was to identify nondimensional groups d

and scaling considerations relati<r to s:. ratification and mixing. That objective has been met with the preliminuy and 6-21 pa.ssive cor Linm:nt cooling system (PCS) scaling reports."" The purpose of this report is to extend tr.e scaling results in order to examine the AP600 PCS evaluation models with respect to mixing and stratPication.

In the preliminary scaling report, the phenomeiia of jet entrainment, wall boundary layer entrainment, and mixing were discussed in some detail. In 'Ae final scaling report it was shown that mass transfer is the governing phenomenon. It can therefe.e be concluded that analysis methods should be assessed relative to parameters tha: are important to mass transfer. Mixing and stratification inside containment affect mass transfer to the internal containment surfaces; both the PCS and internal heat sinks play an important role. This repon provides the following overview relative to mixing and stratification:

Summary of phenomena related to PCS DBA Outline of the evaluation model strategy l

Applicability of the large scale tests (LSTs) for code validation Framework for assessing DBA methods An understanding of the evaluation model approach and its bases enhances a focused review and audit calculation effort in the most important area', relative to containment heat removal.

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2.0 CONTAINMENT DESIGN HASIS ANALYSIS REQUIREMENTS i

An evaluation model, of the combination of the WGOTHIC computer code and the input, is defined

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during code validation for the purpose of calculating the containment response to the PCS design basis accidents - LOCA and main steam line break. Examples of the evaluation model definition are shown in Table 2-1.

De containment DBA criteria establish the goal for analyses. De AP600 PCS evaluation model is i

being used to assess the following criteria:

P s P.,,[45 psig (60 psia)]

g Pm,5 50% P.,,

i T(t)ou 5 T(t)%

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it is necessary to show that the above criteria are met with sufficient margins while accounting for the eirects of mixing and stratification.

l A re-analysis of the limiting PCS design basis transients is scheduled for May 1995 (Preliminary SSAR Markups). The balance report will provide an overview of the evaluation model strategy that will be followed for the $ u abmittals. De pressure transient from the June 30,1994, PCS analysis

• is shown in Figure 2-1, and will be used for discussions in this report for reference.

Two evaluation models are being developed for the PCS--ore for short-term pressure peaks and one for long term pressure reduction. A distributed parameter WGOTHIC model will be used for peak pressure calculations for LOCA. De LOCA calculation will be carried beyond the second peak through approximately 1000 seconds, during which time the pressure most closely approaches the Pg criterion. A relatively coarsely noded lumped parameter WGOTHIC model will be used to calculate the entire transient through 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Subsequent portions of this report define the bases of the choice of these PCS evaluation models.

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Table 21 Passive Containment Cooling System Design Basis Analysis Evaluation Model Evaluation Aspect Examples Model Yart (in baseline GUTHIC unless noted)

Code Models/ Correlations Momentum equatior, types Pressure drop correlations Flow paths (junctions)

In EGOTHIC upgrade:

Heat and mass transfer correlations Liquid film governing equations Wall-to-wall radiation Noding Definitions and Junctions Lumped parameter node Distributed parameter node Flow junctions Boundary conditions Governing Equations / Solution Matrix solver Techniques Time step control Stability criteria l

Convergence criteria Input Design Data Geometry Flow areas Volumes Protection system configuration Noding Selection Type (lumped /distnbuted parameter)

Size Number Locations l

Connections Accident Boundary Condidons Mass and energy releases Equipment assumptions Initial Conditions Pressure Temperature Humidity Ambient conditions Model/ Correlation Selection and Uchida condensation correlation input Friction factors I

in EGOTHIC upgrade:

Channel correlations for extemal beat and mass transfer Flat plate correlations for internal shell heat and mass transfer mA1951w.*T.lbo42595 2-2 4

s 1

l M PCS Water Conservatively Delayed to 660 sec.

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0.2 m

wm PCS Long-term Cooling Blowdown Transition (Free volume) l (Internal Heat Sinks) l, (PCS heat transfer)

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0 I

10 100 1.000 10,000 100,000 i

Time l

(sec) a Figure 2-1 AP600 PCS LOCA Design Basis Analysis Pressure Transient (from Reference 3) mA1951w wpf:lbe42595 2-3

3.0 MIXING EFFECTS ON AP600 CONTAINMENT PERFORMANCE The final scaling report

• concluded that mass transfer is the dominant phenomenon governing heat transfer through the containment shell. The degree of mixing within the AP600 containment affects mass transfer. The dominant phenomena affecting mixing are different during each accident / phase, for example LOCA blowdown, LOCA long-term cooling, and steam line break. De amount of steam and noncondensible mixing predicted by,WGOTHIC affects mass transfer rates calculated by the evaluation model due to the strong effect noncondensibles have on condensation mass transfer. De following discussion of the effects of mixing in AP600 and the influence of the evaluation model on mixing predictions provides a basis for assessing the evaluation model.

3.1 AP600 Design Characteristics with Respect to Mixing The AP600 design is conducive to mixing. Table 3-1 provides a comparison of parameters related to mixing between the regions above and below the operating deck; the comparison is made with a current 4 loop Westinghouse plant for which design data is readily available; therefore, relative values are used in this comparison. The AP600 relative flow area through the deck, or deck porosity, will be similar to that in a standard plant, so the resistance to mixing between regions below and above deck is similar. The AP600 containment will have more area through the deck relative to the volume to be mixed, so that similar driving forces through the operating deck would have even more propensity to mix the entire containment volume than in current operating plants.

De AP600 design also has compartments below deck with relatively open interconnections. Table 3-2 summarizes these AP600 design features. As can be seen, the AP600 has even greater propensity for mixing than standard Westinghouse operating plants. Rese characteristics are considered in the input to the PCS evaluation model related to flow paths.

3.2 Mixing Effects on Mass Transfer Plumes and jets entering a containment atmosphere and entrainment into wall boundary layers provide sufficient driving forces to move steam to the containment wall, so that mass transfer is limited by the ability of steam to diffuse through the boundary layer to the containment wall at a given elevation.

Mass transfer is affected primarily by the bulk-to-film steam partial pressure difference near the condensing surface. It is also affected to soma extent by velocity near the condensing surface, as is j

the case with high kinetic energy such as that which occurs with the high velocity steam jet released in i

steam line breaks.

Mass transfer surfaces can be considered in two categories--the PCS (containment shell) and the internal heat sinks (primarily below the operating deck). De heat removal by internal heat sinks is dominant during steam line break and the early phase of a LOCA, and heat removal by the PCS becomes dominant in the long-term cooling after a LOCA.* For a main steam line break (MSLB),

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I the PCS heat sink is not dominant during the early limiting portions of the transient. Steam line releases, typically less than 500 seconds in duration, are limited by steam line and feedwater isolation and steam generator dryout. Longer term c(xiling and depressurization of the containment is provided by the PCS; however, since there are no long-term steam line releases, long-term containment response l

is bounded by the long-term LOCA. The following discussion shows how these physical processes can be related in a matrix for assessing the evaluation model.

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Table 31 AP600 Design Characteristics Relative to Mixing Comparison of AP600 to Standard Large Dry Containment Current Plant Parameter (4 loop)

AP600 Containment Free Volume (ft')

3.1 x 10 1.7 x 10' 6

Containment Plan View Cross Sectional Area at Deck 15,400 13,300 2

Elevadon (ft )

Approximate Flow Area Between Lower Compartments

-2,800

~1,9(X) and Above Deck Volume (ft')

Flow Area Reladve to:

Deck area

-18%

~ 14%

Flow Area Relative to:

Free volume (ft /ft')

~0.9 x 10-3

~1.1 x 10-3 2

Table 3 2

" Porosity" within and from Lower Compartments in AP600 AP600 Design Features Relative to Mixing Location Operating Plants Expected Mixing Effect Large, open, well connected lower Valve and CMT rooms have Withm lower Compamnents stairwells with large openings compartments are conducive to instead of closed doors mixing Lower compartments not sectioned off into small rooms; compartments are larger and more open Flow paths from lower Accumulator room is small open From Lower

+

Compamnents to volume with stairwell open to compartments to open volume Open Volume volume above deck above deck are conducive to mixing CMT room is 30% of containment

+

volume, with operating deck grating and stairwells, so compartment is very open to flow communication SG (loop) compartments are open

+

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A stratified fluid can be defined as a volume of fluid with negligible horizontal density, temperature, or concentration gradients. A stratified volume may have vertical gradients or may be vertically well-mixed. The physics of a buoyant plume entering a large volume lead to a stratified fluid and any resulting atial gradient will have a higher concentration of the lighter fluid at the top (for example, richer steam concentrations at the top). Since there are negligible horizontal gradients in a stratified fluid, the distribution of steam and noncondensibles in containment can be represented by the axial steam density gradient */w. This definition is convenient for discussing the effects of mixing on heat removal by the two categories of heat transfer surfaces: the internal heat sinks located below the operating deck, and the PCS above the operating deck.

For low Froude numbers, there is negligible momentum introduced by the break flow, velocities are low, and mass transfer is dominated by free convection. For high Froude numbers developed during steam line breaks, the momentum leads to mixing throughout containment and to higher velocities along the walls, which enhances mass transfer due to mixed (free and forced) convection. Thus, for high Froude number jets, the effects of velocity must also be considered.

Therefore, the assessment of an evaluation model for inside containment can be reduced to examining models relative to how they affect */, and velocities near the containment shell and how these 3

parameters affect mass transfer rates to solid surfaces as a function of time during a transient.

3.3 Mixing in the AP600--Loss-of-Coolant Accident (LOCA)

The postulated LOCA is a double-ended guillotine break of a primary system reactor coolant pipe,

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which releases significant quantities of high temperature, high-pressure steam and water inside the f

ste.im generator companment. As shown in Figure 3-1 the steam that pressurizes containment l

circulates and condenses on the internal containment walls. Heat is ultimately removed from containment by evaporation of PCS liquid film to air flowing through the external PCS flow path.

The focus of mixing discussions is on how the steam circulates and mixes with noncondensibles within containment.

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Mixing inside the AP600 during a LOCA is dominated during blowdown by pressure-driven Dows, and during long-term cooling by large-scale natural circulation driven by density head differences in adjacent compartments. During long-term cooling, additional mixing within the open volume above the operating deck occurs because of entrainment into the steam-rich plume rising from the steam generator compartment.

During blowdown, the steam generator companment pressurizes by about 2 psi relative to adjacent compartments, forcing flow out of all openings from that companment. His can be seen in Figure 3-2 where the pressure difference between the steam generator and adjacent compartments is shown as a function of time through blowdown. The evaluation model should be assessed relative to its ability to predict containment pressure under blowdown conditions of pressure-driven flow.

Because of the pressure-driven flow during blowdown, lower compartments become filled with relatively high steam concentrations. His leads to mixing during the transition to long term as the hotter, lighter steam rises and is replaced by cooler, drier gases from the boundary layers on condensing surfaces. During blowdown, the containment pressure is governed by volume pressurization. During the transition to long-term cooling, internal heat sinks, which are primarily below the operating deck, begin to absorb energy and reduce pressure.

As the transient progresses to long term, cooler, drier gases fall down along the walls and fill the bottom of containment up to a level at which they can be entrained into the break room. A quasi-steady flow field is reached relatively quickly, and is shown qualitatively in Figure 3-3.

Evaluations of larger scale containment test data (NUPEC M-4-3,'" HDR**) have shown qualitatively that mixing within containment is strongly affected by the elevation of the steam injection. When steam is introduced at a low elevation, mixing occurs due to large-scale circulation driven by the density head in companments adjacent to the break room. Dere is also a degree of mixing within the volume above the operating deck where the gases exit the steam generator companment, since the rising plume entrains gases above the operating deck. While the NUPEC and HDR tests are in many ways dissimilar to AP600, these general mixing phenomena are expected to be qualitatively similar for AP600. Since the Froude number for a LOCA is very low, there is effectively no mixing due to momentum in the long term. De evaluation model should be assessed by its ability to model the longer term LOCA containment mixing phenomena of density head circulation and plume entrainment.

3,4 Mixing in the AP600-Main Steam Line Break (MSLH)

De limiting portion of the MSLB scenario is short (less than 600 seconds) since the accident is terminated by the main steam isolation valve and feed water isolation. Since the PCS mA1951w.wpf;1bc4 505 3-6

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l PCS Water Storage Tank Air Air inlet inlet Concrete teel Shield ontainment essel Building Air Flow Bame Large-Scale Circulation iP600 3

Through Lower Cwamnents Jitimate ieat Sink

'9000.C410 Figure 3-3 AP600 PCS Containment Vessel - Steam /Noncondensible Mixing Mechanisms during Long-Term LOCA mA1951w.opf:lbe42595 3-8

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i external water is not assumed to be available until 660 seconds, the PCS has no influence on MSLB performance. The high Froude numbers associated with MSLB indicate that the break results in very high kinetic energy into containment. In addition, the limiting steam line breaks occur at the elevation of the main steam line at the top of the steam generator, resulting in very high momentum flow introduced into the containment, tending to drive the containment to a well-mixed condition.

The LST tests with 3-inch steam delivery pipe achieve Froude numbers representative of an MSLB.

The data show mixing throughout the test vessel. Thus, for the MSLB, the AP600 is expected to be well mixed throughout containment, both above and below deck. Test data evaluations based on the LST are being performed to confirm the expected mixing. These will be factored into the devek>pment of an evaluation model for MSLB which can be assessed according to the framework provided herein.

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4.0 EVALUATION MODEL ABILITY TO PREDICT AP600 PERFORMANCE ne following section summarizes the modeling capabilities of the WGOTHIC transient momentum equation formulations and the effects of the formulation and noding on the ability of the evaluation model to predict AP600 performance. A discussion of the two momentum formulations as they will be applied to AP600 DB A is given, followed by the effects of relative heat removal by the internal heat sinks versus the PCS as the transient progresses. Model validation for blowdown calculations is also discussed.

4.1

,WGOTHIC Momentum Formulation and Noding Effects on Mixing The traditional single-node containment code and WGOTHIC formulations are compared and contrasted in Figure 4-1. The lumped parameter formulation in WGOTHIC differs from traditional single-node codes. In single-node codes, the entire open volume is represented as one node and there can be no resolution of velocities or noncondensible distributions within containment.

WGOTHIC provides analysis capabilities beyond those of containment codes used for operating plants.

De following are definitions of key terms used in WGOTHIC discussions. For PCS DBA evaluations, compartments below deck are modeled in WGOTHIC as lumped parameter volumes in a node-network solution, which is referred to as the lumped parameterformulation. In this formulation, a transient momentum equation is solved * * "'""""Sh "* through the junctions joining nodes. For pressure and density head-driven flows that exist below deck, node-network solutions, such as the WGOTHIC lumped parameter formulation, are acceptable. The transient momentum equation for flow junctions linking the volumes provides a coarse representation of transient fluid velocities, and the discretization of the containment allows coarse representation of steam / air concentrations throughout containment.

Based on LST valida: ion, an accurate representation of entrainment into a buoyant plume rising into an open volume requires a more detailed model than can be obtained with lumped parameter volumes.

, _ GOTHIC includes a finite difference solution to the transient momentum equation within an open W

volume *"" "'h' "*h "* whid wh de wM ddW in m& sim is Mmd m a h distributed parameterformulation, he distributed parameter formulation is a user option to define a more detailed matrix of nodes within an open volume. Such a subdivided volume allows a better resolution of flow fields such as those arising from plume entrainment. Subdivided volumes can be i

connected to lumped parameter volumes below deck using junctions, as described below.

The WGOTHIC evaluation model predictions for AP600 have well-understood characteristics. He distributed parameter formulation of the momentum equation in WGOTHIC, in combination with sufficient nodes in critical locations, has been shown to provide a reasonably detailed resolution of vekvity and noncondensible distributions within the LST.* Additional validation for MSLB is currently underway.

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T.

Traditional Plant Containment Analysis (Single Node Lumped Pararneter)

Sorays

-wy T

TV (WecMixed ConcF00n)

WGOTHICLumpedParameter

• WGOTHICDistributedParameter *

(Node-Network)

(Finite Difference, Large Mesh) f 11 r

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=

=-

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Figure 41 Comparison of Traditional Lumped Parameter Containment Codes to WGOTHIC Momentum Formulations mA1951w.wpf:1bo42595 4-2

He distributed parameter model will be used to evaluate short term peak pressures of the LOCA. De distributed parameter model requires long compute times which make its use for evaluating the 24-hour pressure criterion impractical. Derefore, a less detailed lumped parameter model will be used to evaluate the 24-hour pressure criterion when containment pressure is well below the design pressure. Comparison of the results of distributed and lumped parameter models over the first 1(XK) seconds of a LOCA is expected to show that the lumped parameter model is a reasonable basis for evaluation of the AP6(X)long-term cooling. The following sections provide some background considerations which set the stage for an evaluation model assessment.

4.2 Heat Transfer Surfaces in Design Basis Analysis (DBA) Models De axial steam density gradient can be examined relative to its effect on mass transfer to surfaces, the dominant process for pressure reduction. Surfaces on which mass transfer takes place can be divided into two categories, the heat sinks that are primarily located below the operating deck (" heat sinks"),

and the interior PCS vessel surface that is above the operating deck ("PCS surface"). The dominant surfaces for heat removal are different depending on the postulated accident and the time in the transient.

In the postulated DBA LOCA, the final scaling report showed that pressure mitigation is dominated by volume pressurization and heat sinks below deck during the early parts of the transient. During the LOCA blowdown phase the PCS vessel surface can be considered simply as an externally adiabatic hea; sink above deck, representing only a fraction of the total heat transfer surface area available.

l Re PCS becomes the dominant henz removal surface during the LOCA long-term heat removal phase.

During the same period, heat sinks below deck become saturated and eventually become heat sources.

Postulated steam line breaks are over in about 600 seconds, so that the PCS is not the dominant heat removal surface during the limiting portion of that transient, although the containment shell metal heat capacity does contribute to total heat removal.

l 4.3 Distributed Parameter Model for Peak Pressure Calculations ne distributed parameter evaluation model provides increased resolution to more accurately represent entrainment into a rising plume above the operating deck. Compartments below deck are modeled with lumped parameter nodes-one per compartment, and the Uchida condensation mass transfer coefficient is applied to ad internal heat sinks.

For-long term heat removal, the LST has provided a database from which to establish a valid distributed parameter model.* Re distributed parameter model has shown good agreement with LST measured noncondensible distributions, total pressure, and available velocity measurements as well as mal 951=gf a t@42595 4-3

with other local test data from the extensive LST instrumentation. Noding studies have led to a final distributed parameter LST model. A corresponding AP600 distributed parameter model is being built.

4.4 Lumped Parameter Model for LOCA-Long-Term For LOCA long-term containment cooling, the lumped parameter model of the LST has been shown to slightly over-predict the containment vessel pressure.( This results from two competing effects:

over-mixing of noncondensibles in the vessel, and over-predicting the velocity. Both effects are caused by the tendency of lumped parameter models to our-entrain.

De impact of these competing effects on pressure in a lumped parameter model are as follows. In the long term, heat removal is dominated by the FCS. Over-mixing carries noncondensibles above the operating deck, and increased noncondensibles above the operating deck degrade mass transfer, thereby penalizing PCS heat removal. Herefore, over-mixing tends to increase the predicted pressure for long-term cooling.

Over-predicting velocity tends to over-predict heat and mass transfer using mixed free and forced convection correlations, and therefore tends to decrease the predicted pressure. De balance between mixing and velocity yields a slight net over-prediction of p*cssure for the LST.(8)

He AP600 internal mass transfer is expected to be dominated by free convection during a LOCA based on the relatively low Froude number. Forced convection effects will be neglected in the lumped parameter evaluation model, that is, the mixed convection correlation will be disabled by setting the forced convection component to zero, effectively eliminating the calculated velocities from consideration. Therefore, assessing the lumped parameter model will reduce to consideration of its ability to predict mixing.

The LST does not have a flow path into the simulated steam generator compartment (see Section 5.1),

so that the tests show a rather steep axial steam density gradient that is not well represented by the LST lumped parameter model. He AP600 has sufficient flow area into the steam generator compartments to allow large-scale circulation, so that the plant is expected to be well mixed (see Section 3.3). Since the AP600 is expected to be well mixed and dominated by free convection, the use of the lumped parameter evaluation model, with free convection only, will provide a good representation of the AP600 conditions.

Validation of the use of the lumped parameter evaluation model will be based on comparisons to LST covering a range of conditions expected in the AP600. Comparisons between the lumped and distributed parameter results over the early limiting portion of the containment response transient will provide additional support for the use of the lumped parameter for long term depressurization.

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4.5 Model Assessment for LOCA lilowdown ne lumped parameter model is being validated by comparison to CVTR tests, and by comparison to standard review plan methodology. During the AP600 blowdown (the first 30 seconds of the transient) containment pressure is governed by volume pressurization, with die second order effect of heat removal by internal heat sinks, including the containment shell heat capacity, similar to current operating plants. Since the AP600 design is at least as open to mixing as currently operating plants, the CVTR tests are equally applicable for AP600 blowdown methods validation.

For currently operating plants, the standard review plan allows, a single-node containment code using.

the Uchida correlation, based on CVTR test comparisons. A comparison will be provided between die WGOTlilC code and CVTR data using a single WGOTHIC node with Uchida specified for the total heat transfer coefficient. The WGOTHIC single-node results will be compared to results of a similar GOTHIC comparison to CVTR data.""'8"" *) He single-node /Uchida WGOTHIC model can therefore serve as a basis for comparison to validate the evaluation models during blowdown, while the external containment surface can be considered to be adiabatic.

To provide additional comparison to current plant methodology, the single-node /Uchida.W_ GOTHIC will be run with AP600 blowdown mass and energy releases and compared to the evaluation models.

De blowdown pressurization predicted by the evaluation model is expected to be similar to that of the single node /Uchida case; therefore, the evaluation model is expected to be equivalent to models in the standard review plan for blowdown calculations. He AP600 is expected to perform equivalently to standard Westinghouse operating plants using approved methodology during blowdown.

4.6 Evaluation Model Assessment Matrix A matrix of accident phases and important phenomena are shown in Table 4-1, along with an indication of the dominant heat sink surface for the accident phase and whether or not velocity plays a significant role. De framework of Table 4-1 can be used to systematically assess the evaluation model. In later sections, the matrix is applied specifically to the LOCA phases of interest.

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Table 4-1 Matrix for Systematic Assessment of PCS Evaluation Model Consideration for Accident / Phase Parameter Influencing LOCA LOCA MSLB Mass Transfer (01500 seconds)

(>1500 seconds)

(0-600 seconds) dp/3z

+ Blowdown steam

+ For the low elevation

+ High-momentum jet distnbutions lead to break, large scale leads to well mixed initially well mixed circulation leads to containment containment well mixed containment Heat sinks below deck

+ PCS is dominant Heat sinks below are dominant surface surface

+

during transition deck are dominant surface Significant effect Negligible effect (free Velocity

+ Negligible effect (free

+

+

convection dominated) convection dominated)

(forced convection after b!owdown dominated)

+ Volume pressuri-zation is dominant mechanism 1

1 I

mW" Iw wptitO42595 46

I 5.0 EGOTilIC VERIFICATION AND VALIDATION WITH LST The LST is a credible database for validation of WGOTHIC. His has been shown through a detailed scaling analysis.* The following provides a brief summary of the most important considerations for code validation-atypicalities identified in scaling analyses and the ranges of noncondensible concentrations in the above deck region near the PCS.

5.1 LST Scale Atypicalities The scaling analysis identified two atypicalities in the LST facility relative to the AP600; a small yet higher fraction of cooling in the test due to sensible heating of the external liquid film; and the lack of a flow path into the simulated steam generator compartment in the LST. Since sensible heating of the liquid film is a relatively small fraction of the total heat removal in both AP600 (5 percent) and LST (5 to 20 percent), and a mechanistic (and therefore, scalable) model of the sensible film heating is included in WGOTHIC, this is not a significant atypicality for internal mass transfer. The effect of higher cooling rates in the LST database is simply to increase the range of condensation rates over which WGOTHIC is validated.

The lack of a flow path into the simulated LST steam generator compartment, shown in Figure 5-1, has two effects relative to code validation. The first is that the LST has a more emphasized axial gradient since there is no large scale circulation through the operating deck as shown in Figure 3-3 for the AP600. Mixing in the LST at low Froude numbers is therefore driven only by entrainment into the plume rising out of the simulated steam generator compartment, and not by a density head driven large-scale circulation through the below-deck regions. Since entrainment into a plume is one of the more difficult phenomena to model with a containment code, the LST provides a rather severe test for

_ _ GOTHIC validation.

W At high Froude numbers, mixing is also driven by momentum introduced by the high velocity jet. The lack of a flow path into the simulated steam generator compartment provides additional resistance to mixing, and therefore causes the LST to conservatively under-represent mixing in the AP600 due to momentum. Even so, the LST still showed near perfect mixing at Froude numbers as low as the minimum that occurs during the limiting portions of an AP600 MSLB.

Based on the above discussions, the LST atypicalities relative to AP600 identified by the scaling analysis can readily be factored into the WGOTHIC code validation.

5.2 Range of Noncondensibles above Operating Deck The LST database covers a wide range of internal conditions. The range of conditions includes a wide range of noncondensible concentrations above the operating deck, which compares favorably to the range of noncondensible concentrations expected in the AP600.""

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5.3 Tests Selected for WGOTillC Validation The phenomena identification anfi ranking table (PIRT) developed in the scaling evaluation identifies the most important phenomena for predicting containment pressurization. Based on the PIRT and the considerations discussed above, LST runs have been selected that address code validation for the parameters with the largest effect on vessel pressure. He bases for selection of LST cases for code validation have been discussed with the NRC.'"

Data from the entire LST database is also being used to examine such topics as:

= The validation of heat and mass transfer correlations in an integral setting

• The degree of mixing as a function of Froude number

= De effects of break clevation and orientation Therefore, there is a sufficient database being utilized for WGOTHIC validation, as well as methodology and phenomena validation.

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6.0 FRAMEWORK FOR PASSIVE CONTAINMENT COOLING SYSTEM EVALUATION MODELS A detailed model of the LST has been constructed consisting of a relatively large number of nodes interior to containment, a distributed parameter momentum formulation, and the best available thermal-hydraulic correlations. He model has been developed through noding sensitivities and by incorporating mechanistic models for the dominant phenomena. The mechanistic models include boundary layer heat and mass transfer correlations with noding sufficient to define properties for use in the correlations. This model is referred to as the distributed parameter evaluation model.

The AP600 distributed parameter evaluation model will be used to calculate the LOCA peak containment pressure which occurs prior to approximately 1000 seconds. The phenomena in this model are well represented. A coarser noded, lumped parameter model will be used to calculate containment pressure through 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, when the pressure is well below containment design. Thus, there will be two PCS DB A evaluation models as shown in Table 6-1..

Table 6-1 PCS Evaluation Models for AP600 DBA Accident / Phase LOCA LOCA (1000 seconds -24 MSLB (0--1000 seconds) hours)

(0 600 seconds)

PCS DBA Evaluation Distnbuted Parameter Lumped Parameter (Currently under 4

Model investigation)

Support for the acceptability of the evaluation model will be drawn from the areas of scaling, code validation and test comparisons, and uncertainty and margin assessments. The primary source of conservatism is in boundary and initial conditions as shown in Figure 6-1. The evaluation models have well-understood characteristics that can be assessed according to the matrix in Tables 5-1 and 6-1. A code uncertainty will also be appropriately considered. Tables 6-2 through 6-4 provide a more detailed breakdown of the considerations of dominant phenomena during a LOCA according to the approach outlined in Tables 5-1 and 6-1. A similar framework for evaluating MSLB is under development.

An assessment of the margins due to these code inputs has been provided previously.*

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Table 6-2 Assessment ef LOCA Blowdown (0-30 seconds)

Evaluation Model j

Topic Characteristic Relative to Topic Effect of Characteristic Basis for Characteristic 3p/dz During blowdown, break Blowdown pressurization For the relatively high compartment pressurizes will drive mixing throughout pressurization of the break containment.

compartment, a lumped parameter (node-network) formulation is applicable below deck.

The evaluation models will Node-network solution will provide a reasonable initial Both distributed parameter show steam is driven into condition for transition and and lumped parameter lower compartments during long term cooling, models use node-network blowdown below the operating deck The Evaluation Model is expected to give results similar to SRP 6.2 methods (single node, Uchida).

AP600 design is more conducive to mixing than standard plants.

Velocity Distributed Parameter Use Heat transfer to surfaces is Distributed Parameter mixed convection for PCS not dominant during Low velocities effectively and Uchida for internal heat blowdown.

give free convection sinks Lumped Parameter Predicted velocities are not Lumped Parameter utilized in beat / mass Use free convection for PCS transfer correlations and Uchida for internal heat sinks i

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1 Table 6 3 LOCA Transition (30-1500 seconds)

Evaluation Model Characteristics Relathe to Topic Topic Effect of Characteristics Basis for Characteristic dp/dz Distributed Parameter Distributed Parameter will Distributed Parameter model accurately represents be used to calculate pressure methodology has been

• /u through the second peak qualified for entrainment when the containment into buoyant plumes with design pressure may be LST comparisons challenged.

Lumped Parameter Lumped Parameter Lumped Paran;eter model Mixing noncondensibles LST validation shows will overmix from below deck penalizes lumped parameter model PCS heat transfer will overmix Velocity Distributed Parameter Distributed Parameter Distributed Parameter Use mixed convection as Results in free convection Low Fr in AP600 leads to currently implemented effecdvely due to low expectadon of free predicted velocides.

convection during transition period.

Lumped Parameter Lumped Parameter Lumped Parameter Same as distributed Use free convection Neglects effects of high parameter

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a Table 6-4 LOCA Long Term PCS Cooling ( > -1500 seconds)

Evaluation Model Characteristic Relative Topic to Topic Effect of Characteristic liasis for Characteristic dp/dz Distributed Parameter Distributed Parameter Distributed Parameter Accurately represents Accurate representation of Model has been qualified with LST expected */

expected AP600 gradients wherein buoyant plumes drive the 3

internal flow field.

Lumped Parameter Lumped Parameter Coarse noding increases May drive somewhat more Lumped Parameter predicted mixing mixing than expected for AP600 is expected to be well AP600 mixed by large scale circulation

>l500 seconds, PCS is dominant For PCS, it is conservative heat removal surface, and mixing to mix noncondensibles noncondensibles from below deck from below deck.

suppresses mass transfer Velocity Distributed Parameter Distributed Parameter Distributed Parameter Accurately represents Effectively is free Low Fr in AP600 leads to velocities, so use mixed convection in code expectation of low velocity convection as currently correlations, due to low implemented velocity predicted for AP600 Lumped Parameter Will neglect forced convection in the model Lumped Parameter Conservatively neglects Lumped Parameter velocity effects Velocities are over-predicted by model, and neglecting forced convection is conservative based on LST results m:\\l951w.wpf:lbe42595 6-4

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Assessment of Code Validation Code Uncertainty Engineering Tests for Boundary Cond6ons and Geometry

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7.0 CONCLUSION

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'Ihe physics and modelling characteristics for the Passive Containment Cooling System Design Basis Analysis evaluation models have been described.

A sound, straightforward approach to developing and justifying the evaluation models relative to stratification and mixing has been discussed. Westinghouse has explored and is developing a reladvely detailed model, the distributed parameter evaluation model, to calculate the pressure transient during the early stages of LOCA when the containment design pressure may be challenged. A practical lumped parameter evaluation model for examining the 24-hour criterion is also being i

prepared. Comparison of the lumped and distributed parameter model results will provide additional basis for the acceptability of the calculation at 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.

t A matrix of accident phases versus important phenomena has been provided. The well-understood.

characteristics of the evaluation models allows the use of these matrices to assess the acceptability of passive containment cooling system design basis analysis methodology. An appropriate strategy for use of Wf, GOTHIC for steam line break is under development.

An understanding of the evaluation model approach and its basis will allow focused review and audit efforts in areas of most significance to containment pressure analyses.

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8.0 REFERENCES

1.

AP600 Passive Containment Cooling System Preliminary Scaling Report, July 28,1994.

2.

WCAP-14190 (Proprietary), Scaling Analysis for AP600 Passive Containment Cooling System, October 1994.

3.

AP600 PCS Design Basis Analysis Models and Margin Assessment Report, June 30,1994.

4.

ENISil244, Circular litter to CSNI PWG4 Participants in ISP 35 NUPEC Hydrogen Distribution Tests, inne 30,1994.

S.

Detailed Assessment of the HDR-Hydrogen Mixing Experiments Ell, L. Wolf, H. Holzbauer, T. Cron, International Conference on %w Trends in Nuclear System Thermohydraulics.

Proceedinns. Volume II. pp.91-103, May 1994. Pisa, Italy.

6.

GOTHIC Verification on Behalf of HDR-Hydrogen Mixing Experiments, H. Holibauer, L.

Wolf, op. cit., pp 331-340.

7.

Comparisons Between Multi-Dimensional and Lumped-Parameter GOTHIC-Containment Analyses with Data, L. Wolf, H. Holzbauer, M. Schall, op. cit., pp. 321-330.

8.

Westinghouse /NRC Meetings on PCS Analysis Program (11/1517/95), November 21,1994.

1 9.

NAl 8907-CC, GOTHIC Containment Analysis Package Technical Manual.

10.

NAl 8907-09, GOTHIC Containment Analysis Package Qualfication Reportfor GOTHIC _S.

I1 Enperimental Basisfor the AP600 Containment Vessel Heat and Mass Transfer Correlations, WCAP-14326 (Proprietary), WCAP-14327 (Nonproprietary), April 1995.

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