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LEVEL-SWELL PREDICTION WITH RETRAN-3D AND ITS APPLICATIONTO A BWR STEAM-LINE-BREAK ANALYSIS

Y. Aounallah, K. Hofer

Level-swell experiments have often been simulated using system codes, such as TRAC and RELAP, butonly cursory assessments have been performed with the operational-transient code RETRAN-3D, themain system code used within the STARS project. The present study, initiated in the framework of a BWRSteam-Line-Break (SLB) accident scenario, addresses this lacuna by performing RETRAN simulations ofthe General Electric Level-Swell experiments, and by investigating their implications on power plantaccident analyses. Parameters to which the predicted level swell is sensitive have been identified, andrecommendations on code options are made. The SLB analysis objective was to determine the amount ofsteam and liquid discharged through the break under specified boundary conditions, and to gauge theresults against reference values. The impact of the nodalization of the upper part of the reactor pressurevessel was investigated and found to play an important role, whereas the level swell induced from flashingwas found not to be the predominant factor for these simulations.

1 INTRODUCTIONThe prediction of the two-phase mixture-levelbehaviour during blowdown conditions is of interest toboth the chemical and power industries. It isparticularly salient for Light Water Reactors, forexample for the prediction under accidental situationsof core uncovery and steam generator dryout. Also,an a priori conservative assumption may not alwayslead to a conservative result. For example, aconservative increase of reactor power may lead tooverprediction of the level swell of an uncovered core,or a steam generator, resulting in an overprediction ofthe cooling; hence the need for complementary best-estimate calculations with accurate mixture-levelpredictions.The capability of the operational-transient codeRETRAN-3D [1], the main system code in the STARS(Safety Research related to Transient Analysis of theReactors in Switzerland) project, to accurately predictthe mixture level behaviour, has been the object ofonly cursory effort, based on reported studies [2], incontrast to other system codes, such as TRAC andRELAP [3,4,5]. The present simulation of the GeneralElectric blowdown experiments [6], which was initiatedin the framework of a steam line break analysis, aimsat addressing this shortcoming.One objective was to assess the code models as"stand-alone" components of more complex systems,such as the one for the present postulated mainsteam line break in a BWR plant; the break isassumed to occur inside the turbine building. Themain interest of the study is to investigate the basis forcalculations of the radioactive isotopes contained inthe primary coolant, and therefore entrained in thesteam and liquid mass discharged through the break.The break outflow is interrupted by a complete closureof the main steam isolation valves (MSIVs). A ruptureof a main steam line in the turbine building is mostcritical from the viewpoint of release of activity to theenvironment, since this building is not included in thebarrier formed by the containment. The present study

consists of calculating the mass of steam and liquiddischarged from the break under different conditions:e.g. break size, MSIV closure time, and variations ininitial conditions. Three cases were identified for theanalysis, two of them with a guillotine break and onewith a small break size equivalent to 10% of the cross-sectional area of one main steam line. The analysespresented here were all performed at "hot-standby"conditions.

2 GENERAL ELECTRIC LEVEL-SWELLEXPERIMENTS

2.1 Description of experiments and modelsThe General Electric (GE) Level-Swell experimentswere designed to investigate separate-effectphenomena, such as critical flow, mixture level swell,and axial void distributions, during blowdownconditions. The experiments consisted ofdepressurizing a quiescent pool of water, initially inthermodynamic equilibrium with its vapour phase. Thefast rate of depressurization (up to 10 bar/s at thebeginning of the transient) resulted in a sudden rise,or swell, of the liquid free surface due to flashing. Theinitial level swell was followed by a level shrink as thesteam production in the mixture decreased as aconsequence of the reduction in depressurization rate.A series of experimental tests were performed withSmall Blowdown and Large Blowdown vessels, both4.27!m long, vertically-oriented cylinders, withrespective internal diameters of 0.305 m and 1.219 m.The depressurization rate was controlled by mountingdifferent size venturi nozzles on the discharge line.Experimental uncertainty was reported to be typically+/-_0.12 m for the mixture level, and up to +/-_0.05 forthe void fraction. No information was available in theopen literature regarding the uncertainty of the systempressure measurements.Two modeling schemes were used, both aimed atmaximizing consistency between the componentassessment model and (i) the BWR plant model, and

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(ii) the experimental discretization scheme for themeasured variable. This was reflected in thenodalization schemes in both cases, as well as in useroptions. Thus, while a two-node model was used inthe framework of the plant study, a seven-node modelwas used for the detailed simulation of the GEexperiments. The impact of the radial nodalization inthe phase separation region was investigated for theBWR system model, as described below (Section3.5).2.2 Base-case calculations and sensitivity testsThe nodalization for the base-case calculations (GETest 5801-15) and sensitivity tests is depicted inFig.!1.

Fig.!1: Nodalization for the base-case model.

The RETRAN 3-D bubble-rise model (BRM) was usedfor the Control Volume 2 (Fig. 1) and, in the context ofthis model, the salient parameters identified for thesetests were the bubble-rise velocity (Vb), the bubble-gradient parameter (Co), and the break-valvecontraction coefficient (Cn), as listed in Table 1 below.Unless otherwise stated, these base-caseparameters, used in the preliminary calculations forsteam line break analysis (see Section 3), wereimplemented in the calculations.Table 1: Base-case parameters.

Parameters Values Remarks

Bubble-rise velocity 1.3 [m/s]

Bubble-gradient parameter 0.0 [-] Homogeneous

Break valve contraction coeff. 1.0 [-] No correction

The result shown in Fig. 2 for the Large Vessel tests,and subsequently confirmed within the range of thesensitivity tests performed, is that, for the base case(i.e. with Vb=1.3 m/s), the rate of level swell and levelshrink is lower than the experimental values. Also, thepeak elevation of the two-phase mixture level is

underpredicted, and the timing of this peak is delayed.Sensitivity calculations, stemming from the base-casesimulation, were performed to assess the impact ofvarying the bubble-rise velocity, the bubble-gradientparameter, and the break valve contraction coefficient.Figure 2 shows the effect that the bubble-rise velocity(Vb) has on the mixture level, particularly when thereseems to be no supportive evidence in the openliterature for selecting a particular value within therange considered (0.3 m/s to 1.5 m/s). Very low orvery large values of Vb can be selected for limitingcases, such as the determination of the maximumlevel swell or the collapsed liquid level, respectively.

Fig.!2: Effect of the bubble-rise velocity (Vb) on themixture level (with Co=0.0 [-]).

The effect of varying the bubble-gradient parameter isshown in Fig. 3, where the value Co=0 corresponds toa uniform void profile, and Co=1.0 corresponds to themaximum void gradient. The base-case value(Co=0.0) maximizes the level swell, since the voidfraction at the free interface is then at a minimum, andthe vapour flow rate leaving the mixture is then thesmallest. Conversely, the maximum void gradientleads to the lowest mixture level, since it maximizesthe vapour flow rate leaving the mixture free interface.

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Fig.!3: Effect of the bubble-gradient parameter (Co)on the mixture level (with Vb=1.3 m/s).

Figure 4 shows the combined effect of variations in Vband Co, and thus indicates the domain swept by themixture level velocity during depressurization, andidentifies the envelope cases. It can be seen thatdrastically different results can be obtained for arelatively narrow range of Vb. The bubble-gradientparameter becomes increasingly important as rates oflevel swell decrease (Co would be inconsequential forVb~0 m/s since no vapor would then cross the mixturefree interface).

Fig.!4: Effect of combined bubble-rise velocity (Vb)and bubble-gradient (Co) on the mixture level:envelop cases.

With the base-case parameters (Vb=1.3 m/s, Co=0.0),the system pressure is also underpredicted duringvessel depressurization, as illustrated in Fig. 5. Theinitial dip in the measured system pressure, followedby a recovery, is caused by the time delay in flashing(not included in the code) as steam is vented from thesteam dome, with no significant void formation takesplace in the superheated liquid. After this brief period,typically less than one second, significant vaporbubble formation takes place, causing the two-phasemixture level to swell and compress the steam, asmanifested in the recorded pressure recovery.Significant correction of the break-valve contractioncoefficient (Cn=0.8) was required to obtain the correctdepressurization rate, as can be seen in Fig. 5. Theimpact of the bubble-rise model parameters,particularly Vb, on the depressurization rate is alsoshown in Fig. 6. The changes in slope of the pressure-decay curves correspond to conditions where themixture level reaches the break elevation (as can beseen in Fig. 2). The initial single-phase steam flowvented out of the pressure vessel is then replaced bya two-phase mixture which reduces the vapour massvented, and thereby slows down the pressure decay.Since the process of interest is the loss of liquidinventory in the pressure vessel, the main outcome

from these calculations is that the base-caseparameters would give conservative results(overestimation of liquid losses) only if the mixturewater level reaches the break-line elevation. This isbecause the experimentally observed level-shrinkrate, which follows the initial level swell, isunderpredicted, and therefore the predicted mixturelevel remains high for a longer period of time.

Fig.!5: Effect of the valve contraction coefficient (Cn)on the mixture level.

Fig.!6: Effect of the bubble-rise velocity (Vb) on thesystem pressure.

2.3 Prediction of the axial void profileA model was used to assess the code’s capability topredict the axial void profile during blowdownsituations. The experimental Test 1004-3 has beenthe most frequently investigated one, including by theRETRAN code developers [3], and was therefore alsoselected here.A seven-node model (Fig. 7) was used to match theaxial distribution of the void fraction measured

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experimentally. In other words, the node lengthcorresponds to the axial distance between thesuccessive pressure tappings. Thus, there isconsistency in the control volumes used in theexperiment and those in the code simulations.The void fractions are very well predicted by the code,as can be seen in Fig. 8, as long as neither theexperimental nor the calculated mixture level reachesthe BRM lower boundary (Node 6 in Fig. 7). Thisactually occurs at 100_s for Test 1004-03, but waspredicted to occur at ~150_s, as shown in Fig. 9. As aconsequence of this delay, the void fraction increasein Node 5 is also correspondingly delayed.

Fig.!7: Nodalization for GE Test 1004-3.

Among the RETRAN-3D void model options, thealgebraic slip model ( isflag=3) and the dynamic slipmodel ( isflag=5), both based on the Chexal-Lellouchedrift-flux model, gave very similar results.

Fig.!8: Comparisons of axial void profiles (GE Test1004-3).

2.4 Comparison against the GE Level-Swelldatabase: use of the Wilson BRM

In addition to the base-case and sensitivitycalculations, the eleven experimental tests

constituting the GE Level-Swell database were alsoused to identify possible inconsistencies in theidentified trends. The Large-Vessel experimentsconsisted of four tests with different break sizes, theinitial water level being the same for all tests. TheSmall-Vessel experiments included cases with fourbreak sizes (some of which included a vesselrestriction plate), and two initial water levels. All testswere initiated at a system pressure of about 70 bar.

Fig.!9: Effect of the bubble-rise velocity (Vb) on themixture level (Test 1004-3).

For comparison of the code predictions with theexperimental database, the nodalization shown inFig.!2 was used, together with the BRM base-caseparameters and the pressure-dependent, Wilsonbubble-rise velocity model [1]. Good predictions of thedepressurization rates were obtained by adjusting thevalve contraction coefficients, found to be Cn~0.9 andCn~0.7 for the Large-Vessel and Small-Vessel tests,respectively. These adjustments resulted in loweringof the maximum elevation reached by the two-phasemixture, and a reduction in the discrepancy for thetiming of this maximum.

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Fig.!10: Wilson bubble-rise velocity for differentexperimental tests: Vb=1.3 m/s being thebase-case value and (dP/dt)i the initialdepressurization rate.

The main finding is that the base-case parameterslead to an underestimation of the mixture level peakelevation for cases where the steam line was notreached, and an overestimation otherwise. TheWilson model (Fig. 10), on the other hand, isconsidered to have performed well. It must be statedthat this option was used together with thehomogeneous model option (Co=0) for consistencywith the Wilson’s methodology (which is based on anaxially-averaged value of the bubble-rise velocity). InFig. 10 is shown the bubble-rise velocity for four tests(1004-3, 1004-2, 5801-15, 5702-16) as calculated byRETRAN-3D. Each of these tests is characterized byits initial depressurization rate (dP/dt)i. The Figureshows how the bubble-rise velocity increases over thefirst 5 to 20 seconds of the transient, from a low valueto a quasi steady-state value, and how both the rate ofincrease and final level depend upon thedepressurization rate. The base-case value of Vb=1.3m/s is also shown.Among the Small-Vessel tests, two tests (8-28-1 and1004-2) clearly indicate venting of a two-phasemixture, and from the analysis of these tests comesthe observation that the use of the base-caseparameters delays significantly the calculated level-shrink rate, as illustrated in Fig. 11, while the use ofthe Wilson model provides much closer agreement.

Fig.!11: Comparison of the mixture levels obtainedwith the base-case and the Wilson bubble-rise velocities (GE Test 1004-2).

It must also be added that the conclusions reachedhere are not sufficient to fully characterize thebehaviour of the mixture level in more complexsituation such as the mixture level in a BWR pressurevessel during blowdown conditions, since, in additionto flashing, the net mass and enthalpy balance over

the nodes under consideration can play an importantrole as well, as will be further shown in Section 3.5.

3 APPLICATION TO BWR STEAM-LINE-BREAK SCENARIO

3.1 Problem descriptionThe objective of the main steam line break analysisapplied to a BWR plant consists of calculating therelease of steam and liquid transported to the turbinebuilding following a break in a main steam line. Thetwo most relevant components for mitigating thisevent are the flow limiters and the main steamisolation valves (MSIVs). The design of the integratedventuri-type flow limiters defines the maximum steamblowdown rate, while the release of primary coolant islimited by the closure of the MSIVs (Fig. 12).Therefore, one objective of the steam line breakanalysis is to assess the impact of an early reactorisolation as a consequence of a reduced MSIVclosure time. The following cases were investigated:

• 200% break, 10 second MSIV closure time,• 200% break, 5 second MSIV closure time,• 10% break, 5 second MSIV closure time.

The first case is used for benchmarking the RETRAN-3D analysis against the results of a BWR safetyanalysis report (SAR).

3.2 Sequence of eventsThe first response to a break in one of the main steamlines inside the turbine building is a rapid increase insteam flow as the reactor depressurizes. A steam linehigh-flow signal detected in the two flow limitersinvolved initiates MSIV closure (for all valves) shortlyafter the break, which is followed by a reactor scramas soon as the MSIVs reach the 90% open position.The flow limiters restrict the steam flow rate out of thereactor pressure vessel (RPV) to the maximumallowed under critical flow conditions. Referring to oneflow limiter, this value is approximately twice as largeas the rated flow for 100% power operation.As a consequence of the reactor blowdown, and thesubsequent decrease in reactor pressure, the reactorwater level will swell, due to flash evaporation, withthe result that the two-phase level will rise up to thesteam nozzles and liquid will flow out into the steamlines. Finally, this event is terminated by the completeclosure of the MSIVs.Table 2:Values used for initial conditions at hot-

standby in the RETRAN-3D BWR steamline-break analysis.

Parameter Value Used in Analysis

Core power 5% of nominalReactor water level 100% of nominalCore mass flowrate

35% of nominal

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System pressure 93% of nominalPressure in turbinebuilding

1 bar

MSIVs MSIVs related to turbogroup A open;MSIVs related to turbogroup B closed

Position of turbinebypass valves

Closed

Position of turbinestop valves

Open

Fig.!12: Main steam piping system with break in turbine building.

3.3 Initial ConditionsBecause the water inventory for a given reactor waterlevel is larger at hot-standby than at full power, as aconsequence of the lower void fraction in the core, theupper plenum and the separators, the amount ofprimary coolant discharged by the steam line breakwill be larger than under normal operating conditions.The RETRAN-3D analyses were therefore performedfor hot-standby conditions, for which the mainparameters are summarized in Table 2.3.4 RETRAN-3D model and analyses3.4.1 NodalizationThe main steam line break analysis with RETRAN-3Dwas carried out based on a comprehensive systemmodel including the reactor pressure vessel and itsinternals, the reactor core, the two recirculation loopsand pumps, the complete main steam piping system,the turbines, the turbine bypass lines, the condensers,and the blowdown lines. A total of about 200 nodeswas utilized. A general overview of the main steampiping system and the postulated break in one of thetwo steam lines in the turbine building are given in Fig.12. Four main steam lines (denoted by A, B, C and D)are connected to the RPV. Each of these steam linesconsists of the piping system, an integrated, venturi-type, steam-flow restrictor (here referred to as flowlimiter), and two MSIVs. In addition, a dual-functionsafety relief valve, and either a safety valve or amotor-operated pressure relief valve, are flanged toeach main steam line upstream of the flow limiters.Inboard and outboard MSIVs are located on each sideof the primary containment, which provides theseparation of the drywell and pressure suppressionpool from the reactor building.

The 16-inch diameter main steam lines coming fromthe RPV join together in pairs, and the resulting two20-inch diameter lines (denoted by A and B in Fig. 12)penetrate the reactor building, and furtherdownstream enter the turbine building. Both linesdivide again into two 16-inch diameter lines that arefinally connected to the turbines valves.

For the RETRAN-3D analysis, the postulated break ismodelled for the 20-inch diameter main steam line Aat the entrance to the turbine building. The break islocated at about 5_m downstream of the locationcalled "Junction" in Fig. 12, and about 9_m from thecorresponding outboard MSIV location. The details ofthe nodalization for the ruptured steam line in thevicinity of the turbine building are given in Fig. 13.Node 700 is a time-dependent volume — a specialRETRAN control volume for which the constantthermodynamic conditions of the turbine buildingatmosphere are imposed. The activation of valvesV18, V19, and V20 actually simulates the break(Section 3.4.3).

Fig.!13: RETRAN-3D model of the main steam linebreak in the turbine building.

During the model development stage, it was observedthat the calculated coolant mass discharged in theturbine building is sensitive to the nodalization of theRPV components that involve phase separation

563 564 565

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Section of Figure 13(see below)

Drywell ReactorBuilding

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Section of Figure 13(see below)

DrywellDrywell ReactorBuildingReactorBuilding

TurbineBuildingTurbineBuilding

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during the depressurization phase. This mainlyconcerns the nodalization of the steam separators, thesteam dryer, and the upper downcomer. Severalnodalization schemes were examined, andreasonable results were obtained with the one shownin Fig. 14.The behaviour of the reactor water level is determinedin RETRAN-3D by the bubble-rise model (BRM). Theparameters that can be specified in this model, asalready described in Section 2, are: (i) the bubble-risevelocity Vb, which can be either a user-definedconstant (or a function defined within the code"control" system) or a code-calculated value based onthe Wilson correlation; (ii) the initial two-phase mixturelevel; and (iii) the bubble-gradient Co. It should benoted that the input values of the BRM, in particularthe bubble-rise velocity, can be adjusted by the codeduring steady-state initialization, if this option isemployed.The obvious reason for using a bubble-rise model isthat liquid flows out of a phase-separated volume onlyif the mixture level reaches the outlet elevation. This isdifferent for standard RETRAN-3D control volumes inwhich the liquid and steam phases arehomogeneously mixed. The RETRAN bubble-risemodel is therefore the most appropriate option fortracking the reactor water level, and the associatedprimary coolant discharge out of the RPV.Special attention was given to the development of themodel so that it conforms to the flow behaviourexpected to occur in the RPV upper region of thenuclear power plant during a steam line break event.In contrast to the GE Level-Swell facility, there arethree flow paths that need to be considered (Fig. 14a):Path 1: steam separators Æ steam dryer Æ steam

dome Æ main steam lines;

Path 2: upper downcomer inside of the steam dryerassembly seal skirt Æ steam dryer Æ steamdome Æ main steam lines;

Path!3: annulus of the upper downcomer between theRPV and the steam dryer assembly seal skirtÆ main steam lines.

During the blowdown period of the main steam linebreak, the reactor water level swells for all threepaths. When the level in Path 3 reaches the bottom ofthe main steam line nozzles, liquid is entrained. Thetwo-phase mixture level in Path 1 can rise up to thetop of the steam separator, and from there merge withthat from Path 2, and cause swelling in the steamdryer. When the steam dryer is completely full, liquidcan also flow into the main steam lines.It is expected that when fluid enters the steam lines,most of the liquid would flow through Path 3, and mostof the steam through Paths 1 and 2.The expected flow behaviour as described above wascorrectly simulated with the nodalization schemeillustrated in Fig. 14. The radial nodalization of theRETRAN-3D plant model in the RPV upper regionincludes three separated volumes: one for the steamseparators (here denoted as Separator Interior), andtwo for the upper downcomer (here denoted asSeparator Exterior and Dryer Skirt). The separatorexterior node comprises the volumes delineated bythe steam dryer assembly seal skirt and the steamseparators. The dryer skirt node represents thevolume between the steam dryer assembly seal skirtand the RPV, and extends vertically up to bottom ofthe steam lines. In order to obtain the correct flowbehaviour, the height of the steam separator interiorand steam separator exterior were extended toinclude the steam dryer volume.

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Steam Dryer

Steam Dome

Steam Dryer Assembly Seal Skirt

SteamSeparators

Main SteamLine Nozzle

(a) Schematic of upper RPV (b) RETRAN model of upper RPV

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Exterior

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Downcomer FeedwaterDowncomerDowncomer FeedwaterFeedwater

Dome 2 Steam LinesDome 2Dome 2 Steam LinesSteam LinesSteam Lines

Fig.!14: RETRAN-3D nodalization for the upper RPV.

3.4.2 Code options selectedThe major code options used for the main steam linebreak analysis are summarized in Table 3. Theanalysis was carried out without coupling betweenthermo-hydraulic and neutronics. Instead, the corepower was kept constant at 5% of full powerthroughout the transient.The rise of the reactor water level during thedepressurization period depends on the separation ofliquid and steam, which is mainly expressed by thebubble-rise velocity. It is obvious that smaller valuesresult in a more rapid and larger rise of the level. Inthe context of a main steam line break analysis, theliquid phase would then reach the main steam linenozzles earlier, and more liquid would be entrained inthe steam lines, and finally through the break into theturbine building. An analysis with the bubble-risevelocity close to zero is therefore considered as aconservative approach. Based on the results obtainedfrom the separate-effect study (Section 2), the Wilsoncorrelation for the bubble-rise velocity was selectedfor this analysis. The correlation is used together withthe bubble-gradient parameter Co=0.85 for theseparator interior, and Co=0 for the separator exteriorand dryer skirt nodes.Table!3: Major code options of the RETRAN-3D main

steam line break analysis.

Code Option Details

Code version RETRAN-3D MOD003

Numerical scheme Four-equation option, thermodynamicequilibrium

Slip Algebraic slip equation based on driftflux model of Chexal-Lellouche(isflag=3)

Power generation Power table,no reactor kinetics model used

Critical flow model Isoenthalpic expansion modelWilson bubble risevelocity correlation

The correlation is used for all thebubble-rise volumes

3.4.3 Modelling of the sequence of eventsThe core power and recirculation pump speed remainunchanged after the initiation of the postulated break.After the break, the feedwater mass flow rate is keptconstant at the value equivalent to 5% of steamproduction at full power. The undamaged 20-inchmain steam line (i.e. line B in Fig. 12), is isolated priorto the break by closing its outboard MSIVs. Themaximum initial steam vessel blowdown rate is set byadjusting the flow areas of the junctions associatedwith the flow limiters.The main steam line break is simulated by the threevalves V18, V19 and V20 shown in Fig. 13. Thevalves V19 and V20 control the flow from the steamline to the turbine building, and valve V18 is used tosimulate the piping misalignment in the case of a200% break. For the break sizes investigated here,the valve operation sequences are as follows:

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• 200% Break: V19 and V20 open completely andV18 closes at break activation timewithin 0.1 ms.

• 10% Break: V19 opens by 10% at break activationtime within 0.1 ms, V18 remains fullyopen, and V20 is not used (i.e.remains closed).

In the RETRAN-3D model, the turbines are simulatedwith time-dependent volumes, to provide the thermal-hydraulic boundary conditions. In order to limit thesteam flow from the turbine out of the break, TurbineA is isolated shortly after the break by the turbine stopvalves. The closure of the outboard MSIVs connectedto the ruptured main steam line is not triggered by asteam line high-flow signal. In accordance with theresults of the BWR SAR analysis, they are closed at0.5 s after the activation of the break for both the200% and the 10% break analysis. Finally, the MSIVclosure characteristic is based on a linear, time-dependent table for the valve flow area.3.5 ResultsThe break mass flow rates calculated by RETRAN-3Dfor the three cases analyzed are shown in Figs. 15a to15c. The results are normalized to the nominal mainsteam mass flow rate. The origin of the time scale isrelated to the break activation time. In addition, itshould be noted that the break mass flow rate isdefined here as that discharged at the steam linebreak; that is, referring to the model of Fig. 13, thesum of the flow rates from the steam line Nodes 561and 562 to the turbine building Node 700.In Fig. 15a, the results obtained for the 200% breakwith 10_s MSIV closure time are compared againstthe data of the BWR SAR analysis. One of the mostimportant differences is the time delay for the liquid tostart to flow out of the break. It takes 2.4_s in theBWR SAR analysis, and 3.4_s according to theRETRAN-3D simulation. Furthermore, the slope ismuch steeper for the BWR SAR analysis. The timedelay and slope are mainly determined by the speedof the downcomer level swell, which itself is influencedby the timing of the void formation, by the bubble-risemodel parameters, and by the initial reactor waterlevel. The large peak (up to 390% of nominal mainsteam mass flow rate) at the beginning of the transientfor the RETRAN-3D plot in Figs. 15a and 15b is theresult of the rapid steam discharge out of both sides ofthe guillotine break in the steam line. Such a peak isnot evident from the BWR SAR analysis.As expected, the RETRAN-3D analyses for the twocases with MSIV closure time of 5_s result inconsiderably smaller values of the total mass releasedcompared with that for a closure time of 10_s.Moreover, there is almost no liquid mass release inthe former case.The influence of the small break size of 10% is shownin Fig. 15c. After the closure of the MSIVs at 5.5_s,

the ruptured main steam line is isolated at both ends,and it takes another 15_s for completedepressurization to the ambient pressure of theturbine building.

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Time [s]

Fig.!15b: RETRAN-3D result for 200% main steamline break and 5_s MSIV closure time.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

0

5

10

15

20

25

30

35

RETRAN: Total Flow RETRAN: Liquid Flow RETRAN: Steam Flow

Rel

ativ

e B

reak

Mas

s F

low

Rat

e [

%]

Time [s]

Fig.!15c: RETRAN-3D result for 10% main steamline break and 5 s MSIV closure time.

38

The numerical values of the integrated break massflow rates are summarized in Table 4. The numbersare normalized to the total mass release of the BWRSAR analysis.Table 4: Relative integrated break mass flow rates for

different steam line break scenarios.

Analysis Total Mass[%]

Liquid Mass[%]

Steam Mass[%]

BWR SAR_ 200% break,

10 s MSIVclosure time

100.0 75.5 24.5

RETRAN-3D_ 200% break,

10_s MSIVclosure time

_ 200% break,5_s MSIVclosure time

_ 10% break,5_s MSIVclosure time

90.8

37.3

16.5

37.5

3.0

0.3

53.3

34.3

16.2

As shown in Fig. 2 for the GE experiments, theswelling of the reactor water level duringdepressurization of the RPV depends on the bubble-rise model. A sensitivity study was carried out with theRETRAN-3D BWR plant model for the bubble-risevelocity of the separator exterior and dryer skirt nodes(Fig. 14). The goal was to find out by how much thetime delay td, for the onset of the liquid mass flow ratedetermined at the RPV main steam line nozzles, canbe modified in order to eventually match the results ofthe BWR SAR analysis (Fig. 16). The time delay isdefined as the time needed by the downcomer two-phase mixture level to reach the RPV main steam linenozzles. The investigation was performed using thesum of the relative mass flow rates of the two 16-inchdiameter main steam lines C and D at the exit of theRPV (Fig. 12). It should be noted that the RETRAN-3D results shown in Fig. 16 and Fig. 15a are different,since they were calculated at different locations: i.e.the RPV exit and the break location, respectively.

0 1 2 3 4 5 6 7 8 9 10 11 12

0

50

100

150

200

250

300 BWR SAR: Liquid Flow BWR SAR: Steam Flow RETRAN: Total Flow RETRAN: Liquid Flow RETRAN: Steam Flow

Sum

of R

elat

ive

Mas

s F

low

Rat

es a

t RP

V S

team

Lin

e N

ozzl

es [

%]

Time [s]

td

Fig.!16: Time delay td for the onset of liquid massflow for a 200% main steam line break and10 s MSIV closure time.

The bubble-gradient parameter was set at Co=0(uniform void profile) for the separator exterior anddryer skirt nodes, and at Co=0.85 for the separatorinterior node. For the entire study, the valuedetermined by RETRAN-3D for the bubble-risevelocity of separator interior was Vb_=_4.41_m/s. Asmentioned above, this value is calculated at thebeginning of the transient. Results are summarized inTable 5.

Table 5: Effect of the bubble-rise velocity Vb on thetime delay td for the onset of liquid flow for a200% main steam line break and 10 s MSIVclosure time.

Analysis Time delay td [s]BWR SAR 2.4RETRAN-3D

Vb based on Wilson correlation aVb = 1.29 m/s b

Vb = 0.61 m/s b

Vb = 0.15 m/s b

Vb = 0.03 m/s b

a for separator interior, separatorexterior and dryer skirt

b for separator exterior and dryer skirt

3.43.73.53.33.3

The results of this study show a weak dependency ofthe time delay td on the bubble-rise velocity. In thiscontext, an important difference is that the RETRAN-3D model used for this sensitivity analysis is acomplete plant model. Therefore, the results are alsofunctions of the response of the nodes representingthe RPV lower region, especially the lowerdowncomer. During an RPV depressurization as aresult of a main steam line break, void is formed in thelower downcomer region. This effect is enhanced bythe low subcooling for the downcomer for an RPV athot-standby conditions. As a consequence of this voidformation, a two-phase mixture flows into the upperdowncomer, in particular in the dryer skirt, andtherefore the downcomer mixture level is displacedupwards. It was found from this analysis that thecontribution of mixture level rise due to inflow from thelower downcomer is the predominant factor comparedto flashing that occurs in the upper downcomer. Thisis due to the relatively low depressurization rate (~1.5bar/s over the initial 3.5 seconds of the transient)when compared against the analysis of GeneralElectric experiments (e.g. ~4 bar/s for the base case),in Section 2 above. The results from the Vb studytherefore show that the RETRAN-3D results cannotyield a better match with the BWR SAR analysis datasolely by selecting a conservative value of the bubble-rise velocity.

4 CONCLUDING REMARKSSimulations of the General Electric Level-Swellexperiments with the operational-transient codeRETRAN-3D have identified important modelparameters for accurate predictions of the level swell

39

during depressurization under blowdown conditions:namely, the bubble-rise velocity, the bubble-gradientparameter, and the valve contraction (or discharge)coefficient. While a fixed value for bubble-rise velocityis useful for calculating bounding limits, it remains oflimited applicability when accurate prediction isrequired; for example, when the mixture level is atriggering signal in the course of the simulation. TheWilson correlation for the bubble-rise velocity wasfound to yield consistently good results. Significantadjustments needed to be made to the break-valvecontraction coefficient in order to obtain the correctvessel pressure decay rate.In the case of more complex modelling, such as areactor pressure vessel for the present BWR steamline break analysis, the axial and radial nodalization ofthe upper region was also found to play an importantrole in predicting the reactor water level duringdepressurization, and therefore particular attentionwas given to the nodalization scheme. The accidentscenarios included different boundary conditions forthe break sizes and MSIV closure times. The bubble-rise model parameters, which are relevant to thecalculated reactor water level swelling duringdepressurization, were selected based on simulationsof the General Electric Level-Swell experiments.Agreement between the RETRAN-3D results andthose of the reference analysis of the BWR SARsteam line break analysis was reasonable. Thedifferences concerned primarily the delay for themixture level to reach the steam-line nozzle, and alsothe quality of the two-phase mixture vented throughthe nozzle. The delay could not be explained with thebubble-rise velocity alone since flashing in the upperdowncomer was found not to be the predominantfactor in determining the level rise, due to therelatively low depressurization rate (compared to theGeneral Electric experiments). A conservative, asopposed to the present best-estimate, methodologycould explain the observed discrepancy, along withother factors, such as the initial water level.

ACKNOWLEDGMENTSThis work was partly funded by the Swiss FederalNuclear Safety Inspectorate (Hauptabteilung für dieSicherheit der Kernanlagen, HSK), and the SwissFederal Office of Energy (Bundesamt für Energie,BFE).

REFERENCES[1] M.P.!Paulsen, J.H.!McFaden, G.C.!Gose,

C.E.!Peterson, P.J.!Jensen, J.G.!Shatford,J.A.!McClure, J.L.!Westacott, "RETRAN-3D – AProgram for Transient Thermal-HydraulicAnalysis of Complex Fluid Flow System; Volume1: Theory and Numerics", EPRI NP-7450,Revision 4, July 1999.

40

[2] M.P.!Paulsen, J.H.!McFaden, P.J.!Jensen,D.L.!Johnson, C.E.!Peterson, G.C.!Goose,J.G.!Shatford, and J.L.!Westacott, "RETRAN-3D– A Program for Transient Thermal-HydraulicAnalysis of Complex Fluid Flow System; Volume4 Assessment Manual", EPRI NP-7450, Revision5, July 1999.

[3] P.!Saha, J.H.!Jo, L.! Neymotin, U.S.!Rohatgi,G.C.!Slovik, C.!Yuelys-Miksis, "IndependentAssessment of TRAC-PF1 (Version 7.0),RELAP5/MOD1 (Cycle 14), and TRAC-BD1(Version 12.0) Codes using Separate-EffectsExperiments", NUREG/CR-4359, BNL-NUREG-51919, August 1985.

[4] Y.A.!Hassan, "Transient Two-Phase BlowdownPredictions of an Initially Stagnant SaturatedLiquid Steam in a Vessel Using TRAC-PF1",Nuclear Technology, 69, 388 (1985).

[5] C.E.!Hendrix, "Analysis of the General ElectricCompany Swell Tests with RELAP4/MOD7”,Transactions of the American Nuclear Society,Atlanta, Georgia, USA, June 3-7, 1979.

[6] J.A.!Findlay, G.L.!Sozzi, "BWR Refill-RefloodProgram – Model Qualification Task Plan",GEAP-24898, NUREG/CR-1899, October 1981.