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Nuclear Engineering and Design 263 (2013) 187– 192

Contents lists available at SciVerse ScienceDirect

Nuclear Engineering and Design

j ourna l h om epa ge: www.elsev ier .com/ locate /nucengdes

ontainment fan cooler heat transfer calculation during main steam line breakor Maanshan PWR plant

ng-Ruey Yuann ∗, Lain-Su Kaonstitute of Nuclear Energy Research Atomic Energy Council, R.O.C., 1000, Wenhua Road, Jiaan Village, Longtan Township, Taoyuan County 32546, Taiwan

i g h l i g h t s

Evaluate component cooling water (CCW) thermal response during MSLB for Maanshan.Using GOTHIC to calculate CCW temperature and determine time required to boil CCW.Both convective and condensation heat transfer from the air side are considered.Boiling will not occur since TB is sufficiently longer than CCW pump restart time.

r t i c l e i n f o

rticle history:eceived 17 October 2012eceived in revised form 5 April 2013ccepted 10 April 2013

a b s t r a c t

A thermal analysis has been performed for the Containment Fan Cooler Unit (FCU) during Main SteamLine Break (MSLB) accident, concurrent with loss of offsite power, for Maanshan PWR plant. The analy-sis is performed in order to address the waterhammer and two-phase flow issues discussed in USNRC’sGeneric Letter 96-06 (GL 96-06). Maanshan plant is a twin-unit Westinghouse 3-loop PWR currentlyoperated at rated core thermal power of 2822 MWt for each unit. The design basis for containment tem-perature is Main Steam Line Break (MSLB) accident at power of 2830.5 MWt, which results in peak vaportemperature of 387.6 ◦F. The design is such that when MSLB occurs concurrent with loss of offsite power(MSLB/LOOP), both the coolant pump on the secondary side and the fan on the air side of the FCU loosepower and coast down. The pump has little inertia and coasts down in 2–3 s, while the FCU fan coastsdown over much longer period. Before the pump is restored through emergency diesel generator, thereis potential for boiling the coolant in the cooling coils by the high-temperature air/steam mixture enter-ing the FCU. The time to boiling depends on the operating pressure of the coolant before the pump isrestored. The prediction of the time to boiling is important because it determines whether there is poten-tial for waterhammer or two-phase flow to occur before the pump is restored. If boiling occurs then thereexists steam region in the pipe, which may cause the so called condensation induced waterhammer orcolumn closure waterhammer. In either case, a great amount of effort has to be spent to evaluate thewaterhammer pressure and even the structural integrity. With respect to this, GOTHIC computer code isused to simulate the thermal response of FCU during MSLB/LOOP. The transient response of containmentvapor temperature and vapor saturation temperature presented in FSAR are used as boundary conditions

of GOTHIC to drive the heat transfer across the cooling coils given a predetermined condensation heattransfer coefficient and convective heat transfer coefficient. The coolant temperature response has beencalculated and compared with the saturation temperature corresponding to the coolant operating pres-sure to determine the time to boiling. The information is then used as the basis to determine whether thecurrent system design has a sufficient margin to prevent boiling from occurring or whether a betterment

requ

engineering and design is

∗ Corresponding author. Tel.: +886 3 4711400x6083; fax: +886 3 4711404.E-mail addresses: ryyuann@iner.gov.tw (Y.-R. Yuann), lskao@iner.gov.tw

L.-S. Kao).

029-5493/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.nucengdes.2013.04.007

ired to increase the operating pressure at the coolant pump suction.© 2013 Elsevier B.V. All rights reserved.

1. Introduction

For the current BWR and PWR design, it was identified inUSNRC’s Generic Letter 96-06 (GL 96-06) [NCR, 1996] hat the cool-

ing water system used to cool the containment air/steam mixturethrough the operation of containment air cooler may be exposed tothe hydrodynamic effects of waterhammer during either a loss-of-coolant accident concurrent with loss of offsite power (designated

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88 Y.-R. Yuann, L.-S. Kao / Nuclear Engi

s LOCA/LOOP) or a main steamline break concurrent with loss offfsite power (designated as MSLB/LOOP). During LOCA/LOOP orSLB/LOOP, the coolant flow in the air cooler cooling coils tube is

topped and the high-temperature containment air/steam mixtureay boil the coolant and form a substantial void region in the tube.

ubsequent start of the coolant pump will pressurize the voidedegion and condense the bubble abruptly to induce the waterham-er effect. It was also identified that cooling water system serving

he containment air coolers may experience two-phase flow con-itions during postulated LOCA/LOOP and MSLB/LOOP scenarios.ecause the cooling water systems were not designed to withstandhe hydrodynamic effects of waterhammer and the heat removalssumptions used in design-basis accident analysis did not accountor the two-phase flow conditions it was stimulated in GL 96-06 thatorrective actions may be needed to satisfy system and operabilityequirements. A study has been conducted to determine whetherhe current design and operation of domestic operating plants,ncluding Chinshan BWR-4, Kuosheng BWR-6 and Maanshan PWR,re vulnerable to the conditions discussed in GL 96-06. This paperresents how the Maanshan plant addresses the waterhammer andwo-phase flow issues. The design of Containment Fan Cooler UnitFCU) is reviewed and the static pressure of the fluid in the coolingoil tube after stoppage of the coolant pump is determined based onhe operating pressure at the pump suction and the elevation dif-erence between the top of cooling coils and pump suction. Higherontainment air temperature will heat up CCW more severely. TheSLB is the limiting case for CCW heatup and is chosen for the

tudy because the analysis in Maanshan FSAR (Final Safety Anal-sis Report) shows that the containment temperature responsend peak value resulting from MSLB are more severe than thoseesulting from LOCA. The GOTHIC [NAI, 2006] computer model iseveloped to perform the thermal analysis through the FCU. Theapor temperature and saturated vapor temperature presented inSAR MSLB analysis [Maanshan FSAR, 2005] are used as boundaryonditions to drive the heat transfer across the cooling coils. The cal-ulated coolant temperature response is used to determine whenhe temperature will reach the saturated point. The time to boilings compared with the time the coolant pump is restored and usedo determine whether the current system design has a sufficient

argin to prevent boiling from occurring or whether a bettermentngineering and design is required to increase the operating pres-ure at the coolant pump suction. It is justified that if boiling is notccurring in the coolant then the waterhammer discussed in GL 96-6 is not an issue to Maanshan plant. It is also justified that if theime to boiling is well beyond the time the coolant pump is restoredhen there is no concern for the continuous applicability of theriginal single-phase analysis. This is because the original analysisssumes that FCU secondary side starts to operate and remove con-ainment heat at the time when coolant pump is restored and theresent analysis shows that there will be no two-phase situationeyond that time.

There are four sections in the paper. Section 1 is an introduction.ection 2 is the description of containment FCUs in Maanshan plant.ection 3 describes the GOTHIC computer model used to simu-ate the thermal response during MSLB accident. Section 4 presentshe analysis results in terms of the calculated coolant temperatureesponse and the time to boiling. Section 5 is a conclusion.

. Description of Maanshan containment fan cooler system

Maanshan plant is a twin-unit Westinghouse 3-loop PWR cur-

ently operated at rated core thermal power of 2822 MWt (theriginal licensed core thermal power is 2775 MWt and has beenncreased to 2822 MWt through Measurement Uncertainty Recap-ure, or so called MUR, power uprating) for each unit [Maanshan

g and Design 263 (2013) 187– 192

FSAR, 2005]. The design basis for containment temperature is MSLBaccident at power of 2830.5 MWt, which results in peak vapor tem-perature of 387.6 ◦F. The flow diagram for the containment aircooling of component cooling water (CCW) system is shown inFig. 1. CCW system consists of two redundant independent safetytrains. Each train is comprised of one surge tank, two pumps, oneheat exchanger and associated pipings. The surge tank is main-tained at pressure of 18–20 psig during normal operation. The CCWpump suction pressure during normal operation is maintained atapproximately 57.3 psia. The containment fan cooler system con-sists of four separate FCUs inside the containment. Each train ofCCW system supply cooling water to two FCUs. Each FCU consistsof a two-speed vane axial fan, a fan motor, cooling coils with cop-per fins, and painted housing. Each FCU contains two sets of coolingcoils; one receives CCW during normal and post-accident operationwhile the other receives central chilled water during normal oper-ation only and is isolated upon receipt of a safety injection signal(SIS). The coil receiving central chilled water is not considered inthe present analysis since it is not safety related. During normaloperation, three of the four FCUs operate continuously to removeheat from the containment atmosphere to maintain the averagetemperature below 120 ◦F and the relative humidity below 70%. IfSIS (containment high pressure of 3.1 psig is one of SIS.) occurs, thefans are designed to start automatically at low speed if not previ-ously running or if power is interrupted. If running in high speed,the fans automatically shift to low speed to prevent fan motor fromoverloading because the air-steam mixture to be cooled becomesdenser than air at atmospheric pressure as a result of the increase ofcontainment pressure. The FCU will reach the required low speed of580 rpm within 15 s upon receiving SIS. The 15 s consists of 10 s tostart diesel generator and 5 s to load and come up to full speed. TheCCW will be restored within 33 s of SIS. This consists of 10 s to startdiesel generator, 20 s to load the CCW pump, and 3 s to come up torated speed. The CCW flow through the cooling coil for each FCU isa constant of 1050 gpm. The set of the CCW cooling coils consistsof 6 turbex finned coils (designated as coil # 7, 8, 9, 10, 11 and 12)and the CCW flow distribution in it is shown as Fig. 2. The inlet flowQin and the outlet flow Qout are both equal to 1050 gpm. Among the6 coils coil, # 9 has the highest flow rate of 184 gpm and hence isselected for the conservative determination of forced heat transfercoefficient between tube inner surface and coolant in this study.There are 44 tubes for coil # 9 and the outside diameter and tubewall thickness are 0.65′′ and 0.049′′, respectively. The fluid velocityfor coil # 9 is calculated as 5.6 ft/s.

3. GOTHIC model

GOTHIC is a general purpose thermal-hydraulics computer codewhich can be used to analyze a simple separate heat transferproblem to a complicated containment configurations involvedin traditional or advanced light water nuclear power plants. Toaddress the waterhammer or two-phase flow issue described inGL 96-06, one needs to predict the transient coolant temperatureinside the cooing coil tube during MSLB concurrent with LOOP.The idea is to use containment vapor temperature and saturatedvapor temperature response presented in FSAR design basis as aboundary condition to perform the thermal analysis. There aremany ways to solve this problem. In the present study we employGOTHIC code for the calculation because GOTHIC provides variousoptions which can be easily applied to perform this kind of heattransfer calculation. The approach we adopt is to use containment

vapor temperature and saturated vapor temperature response ofFSAR, in conjunction with an appropriate heat transfer coefficientfrom the vapor side to the tube wall, as a boundary conditionto drive the heat transfer through the tube. The heat transfer

Y.-R. Yuann, L.-S. Kao / Nuclear Engineering and Design 263 (2013) 187– 192 189

Fig. 1. Containment air cooling of CCW system.

CCW Flow in Qin

Q1

Q2

Q3

Q11

Q12Q10

Q9Q2

Q1

Q4

Q4

Q7

Q8

CCW Flow Return Qout

COILS 7 and 8 COILS 9 and 10 COILS 11 and 12

8"4"

2 1/2"

5/8"

Fig. 2. CCW flow distribution in each coil of CCW cooling coil set.

190 Y.-R. Yuann, L.-S. Kao / Nuclear Engineering and Design 263 (2013) 187– 192

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ig. 3. FSAR MSLB containment vapor temperature and saturated vapor tempera-ure response.

rom the containment environment to the tube wall is governedy Eq. (3-1):

′′ = hcond(TV,sat − Twall) + hconv(TV − Twall) (3-1)

here q′′ is the heat flux (Btu/h-ft2), TV,sat the saturated vapor tem-erature (◦F) inside containment, TWall the coils tube outer surfaceemperature (◦F), Tv the vapor temperature (◦F) inside containment,cond the condensation heat transfer coefficient (Btu/h-ft2-◦F),conv = convection heat transfer coefficient (Btu/h-ft2-◦F).

Since the option we use is to calculate heat transfer rate usinghe given containment environmental temperature and given heatransfer coefficient, therefore, we need to combine hcond and hconv

s one single effective heat transfer coefficient, i.e. Eq. (3-1) will isecast as

′′ = (TV − Twall)[

hconv + TV,sat − Twall

TV − Twallhcond

]= heffective(TV − Twall)

(3-2)

here

effective = hconv + TV,sat − Twall

TV − Twallhcond (3-3)

n Eq. (3-3), TV and TV,sat are known from FSAR (as shown inig. 3), while hcond and hconv are pre-determined. As mentionedreviously, the GOTHIC option we use is to specify containmentnvironmental temperature and wall heat transfer coefficient.nd in this case the wall heat transfer coefficient is heffective,hich is dependent on the unknown tube outer surface tem-erature, TWall. Therefore, the heffective in Eq. (3-3) needs to beetermined in an iterative way. To facilitate the iterative calcu-

ation we have defined a control variable CVh. CVh is calculateds CVh = hconv + ((TV,sat − Twall)/(TV − Twall))hcond, with TWall fromOTHIC calculation at each time step. The calculated CVh is thensed to represent the effective heat transfer coefficient heffective atach time step.

With regard to the determination of hconv, we have accountedor the effect of fan coast down. As MSLB/LOOP occurs, containmentan will begin to coast down since power is lost. For Maanshanesign, CCW pump will be restored to full speed operation with3 s of the accident, we therefore conservatively assume that hconv

oasts down from initial 14.0 Btu/h-ft2-◦F to 2.0 Btu/h-ft2-◦F in

3 s, where 14.0 Btu/h-ft2-◦F and 2.0 Btu/h-ft2-◦F correspond toorced convection and natural convection heat transfer coefficient,espectively. With regard to hcond, we have conservatively used

constant value of 1112.0 Btu/h-ft2-◦F throughout the transient,

Fig. 4. GOTHIC model of FCU thermal analysis.

where 1112.0 Btu/h-ft2-◦F corresponds to 4 times of the UCHIDAcondensation correlation upper limit of 278.0 Btu/h-ft2-◦F.

The forced convection between tube inner surface and coolant iscalculated using Dittus–Boelter heat transfer coefficient correlationas follows:

Assume P = 150 psig, T = 98 ◦F, then � = 62.05 lbm/ft3, � =1.0029 × 10−4 Btu/s-ft-◦F, � = 4.6793 × 10−4 lbm/s-ft, cp =0.9973 Btu/lbm-◦F. The inside diameter and coolantvelocity are 0.552′′ and 5.6 ft/s, respectively, as indi-cated in Section 2. Therefore, we have Re = �vD/� =(62.05 × 5.6 × 0.552/12)/(4.6793 × 10−4) = 3.4159 × 104 andPr = �cp/� = (4.6793 × 10−4 × 0.9973)/(1.0029 × 10−4) = 4.653

Using Dittus–Boelter correlation, we haveNu = 0.023Re0.8Pr0.4 = 180.15, and

h = �(Nu)/D = 1.0029 × 10−4 × 180.150.552/12

× 3600 = 1413.96 Btu/h-ft2-◦F

The CCW pump will coast down very fast with typical coast downtime of 2–3 s. In this study it is conservatively assumed that thepump cost down time is 6 s and the convection heat transfer coef-ficient from inside tube wall to the coolant decreases from 1413.96to 20.0 Btu/h-ft2-◦F in 6 s.

The GOTHIC model is shown in Fig. 4 and all analysisassumptions stated above have been included in the model-ing of GOTHIC. In this figure volume 1 represents the coolingcoil tube fluid. There are 44 tubes and each tube has insidediameter Di of 0.552′′, outside diameter Do of 0.65′′ andlength L of 93′′, therefore, the volume is V = (�/4)D2

iL(44) =

(�/4)(0.552/12)2(93/12)(44) = 0.567 ft2, and the correspond-ing outer surface area is 44�DoL = 44�(0.65/12)(93/12) = 58.03 ft2.However, the effective heat transfer area shown in the design cal-culation for coil #9 is 1856 ft2 with fin effect considered. GOTHIConly looks the tube outer surface area which is much smaller thanthe effective area. In order to account for this additional area, amultiplier of 1856/58.03 = 31.983 has been applied on heffective.

4. Analysis results

Based on the modeling described in Section 3, the calculatedshort term and long term calculated coolant temperature insidecooling coil tube are shown in Figs. 5 and 6, respectively.

From the design data it is known that the normal operatingpressure at the CCW pump suction is 57.3 psia. As shown in Fig. 1,the elevation difference between the highest point of FCU and theCCW pump suction is 172′11′′ − 102′ 91/2′′ = 70.13 ft. Therefore,

the static pressure at the highest point of FCU after pump trip isconservatively estimated to be 57.3 − (61.73 × 70.13/144) = 27.24psia, assuming the fluid density is �(150 psia, 120 ◦F) = 61.73lbm/ft3. If the measurement uncertainty associated with the pump

Y.-R. Yuann, L.-S. Kao / Nuclear Engineering and Design 263 (2013) 187– 192 191

Table 1Time to boiling vs. CCW surge tank operating pressure.

CCW surge tank pressure Ps (psia) Static pressure at top of FCU Pc (psia) Saturation temperature corresponding to Pc (◦F) Time to boiling (s)

20 9.14 188.99 23.5122 11.14 198.36 34.0824 13.14 206.41 46.1726 15.14 213.50 57.6828 17.14 219.86 68.8430 19.14 225.63 79.2432 21.14 230.92 89.2434 23.14 235.82 98.8436 25.14 240.38 107.8538 27.14 244.65 118.4040 29.14 248.67 124.8042 31.14 252.48 133.0544 33.14 256.09 141.5446 35.14 259.52 148.2948 37.14 262.80 168.0749.75 38.86 265.50 282.05>49.75 >38.86 No boiling

100

150

200

250

300

350

400

0 10 20 30 40 50 60 70 80 90 100

Time (second)

Tem

per

ature

(F

)

Containment Vapor Temperature (FSAR)

Containment Saturated Vapor Temperature (FSAR)

Coolant Temperature Inside Cooling Coil Tube (Predicted)

FM

otsi2

FM

0

50

100

150

200

250

300

20 25 30 35 40 45 50

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ig. 5. Short term coolant temperature response inside the cooling coil tube duringSLB/LOOP.

perating pressure is conservatively assumed to be 5.0 psi, thenhe static pressure at the highest point of FCU is 22.24 psia. The

aturation temperature corresponding to 22.24 psia is 233.66 ◦F. Its known from Fig. 5 that it takes 94.6 s for the temperature to reach33.66 ◦F, which is much longer than the time of 33 s at which the

ig. 6. Long term coolant temperature response inside the cooling coil tube duringSLB/LOOP.

Surge Tank Pressure (psia)

Fig. 7. Time to boiling vs. CCW surge tank operating pressure.

CCW pump is restored to full speed. Therefore, it is determinedthat boiling will not occur within the time frame that CCW andFCU are restored to operation and sufficient margin to saturationpoint exists. Because it is determined that boiling will not occurbefore CCW pump is restored to full speed, it is concluded thatwaterhammer is not an issue to Maanshan CCW’s containmentcooling. It is also assumed in FSAR analysis that containment FCUstarts to perform its heat removal function when CCW pump isrestored. However, the present calculation shows that boilingwill not occur and two-phase flow will not occur after pump isrestored. Therefore, it is concluded that the existing single-phaseheat removal analysis in FSAR remains applicable.

It can also be deduced from Figs. 1 and 6 the relationshipbetween time to boiling and the operating pressure of CCW surgetank. Referring to Fig. 1, the elevation difference between top ofFCU and normal surge tank water level is 172′11′′ − 159′3′′ = 13.67ft, therefore the pressure at top of FCU is

Ptank − 5 − 13.67 × 61.73144

= Ptank − 10.86 psia (4-1)

Using Eq. (4-1) and Fig. 6, we can get the relationship between timeto boiling and the operating pressure of CCW surge tank, as shownin Table 1. Table 1 is also plotted, as shown in Fig. 7.

The design pressure of CCW system is 164.7 psia, and thenormal operating pressure at surge tank, pump suction and pumpdischarge are 32.7–34.7 psia, 57.3 psia and 142.5 psia, respectively.This implies that the maximum surge tank pressure allowed to

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92 Y.-R. Yuann, L.-S. Kao / Nuclear Engi

perate, with 10% margin to the design pressure for the result-ng maximum system pressure, is 164.7 − 142.5 + 34.7 − 16.47

40.43 psia. From Table 1, the time to boiling for this operatingressure is approximately 126.2 s. Therefore, additional 31.6 s126.2 − 94.6 = 31.6 s) has been gained for the calculated time tooiling if the current maximum CCWS system operating pressure

s increased to within 10% of design pressure.

. Conclusion

To address the waterhammer and two-phase flow issues dis-ussed in GL 96-06, a comprehensive GOTHIC modeling andalculation of containment fan cooler thermal response during

ain steam line break concurrent with loss of offsite power has

een performed. The analysis results show that the coolant in theooling coil will be heated by the high-temperature containmentir/stem mixture and it takes 94.6 s for the coolant temperature to

g and Design 263 (2013) 187– 192

reach the saturation temperature of 233.66 ◦F corresponding to thecoolant pressure of 22.24 psia at top of FCU, which is much longerthan the time of 33 s at which the CCW pump and FCU are restoredto operation. Because many conservative assumptions have beenmade in the calculation and the results show that there is sufficientmargin to the point of boiling, it is determined that the water-hammer and two-phase flow will not be an operability issue inMaanshan plant.

References

1996. NRC MM Generic Letter 96-06, “Assurance of Equipment Operability andContainment Integrity During Design-Basis Accident Conditions”, September

2006. NAI 8907-06 Rev 16, “GOTHIC Containment Analysis Package Technical Man-ual, Version 7.2a(QA)”, January.

2005. Final Safety Analysis Report for Maanshan Nuclear Power Station Units 1 and2, Amendment 39.