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Available online at www.sciencedirect.com

Nuclear Engineering and Design 238 (2008) 453–459

Analysis of thermal stratification in the primary circuitof a VVER-440 reactor with the CFX code

Ildiko Boros ∗, Attila AszodiBudapest University of Technology and Economics, Institute of Nuclear Techniques, Budapest, Hungary

Received 17 October 2006; received in revised form 23 January 2007; accepted 9 February 2007

bstract

The thermal stratification can lead an important role in the aging of the NPP piping because of the stresses caused by the temperature differencesnd the cyclic temperature changes. These stresses can limit the lifetime of the piping, or lead to penetrating cracks. For the stress analyses, the

etermination of the thermal hydraulic parameters of the stratified flow is necessary, which can be simulated by computational fluid dynamicsCFD) codes. The results of the simulation show the time development and the breaking up of the stratification and the temperature distributionf the stratified flow. The main difficulty of these CFD simulations is the uncertainty of the boundary conditions because of the unknown flowircumstances. In this paper, some results of CFX simulations are presented concerning the pressurizer surge line, and the injection pipe of thePIS for VVER-440 type reactors.

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2007 Elsevier B.V. All rights reserved.

. Introduction

If two medium with different densities (i.e. with differentemperatures) flow inside a pipe, thermal stratification can occur.he development and the stability of the stratified flow depend on

he temperature difference and on the relative velocity betweenhe fluids (Cortesi, 1996). The stratified flows induce thermaltresses in the piping through different manners. There is aglobal” stress caused by the temperature differences of thepper and lower layer of the coolant, which can induce a newrack in the pipe wall, or the growing of an existing crack. Theaving of the middle mixed coolant layer (the so-called thermal

triping) and the so-called “turbulent penetration” cause stressesuctuating with high frequency. The maximum value of theseuctuating stresses is on the inner pipe wall, and it decreasesoing outside of the pipe, because of the thermal inertia of theipe wall. Turbulent penetration occurs when coolant of higherass flow penetrates into a pipe with stagnating or nearly stag-

ating coolant. A typical occurrence of turbulent penetration ist the connection of the pressurizer surge line and the main loop.

∗ Corresponding author.E-mail address: boris@reak.bme.hu (I. Boros).

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029-5493/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2007.02.036

According to the international operating experiences, theaterial fatigue of the NPP piping caused by thermal stratifiedows may limit the lifetime of the pipes (NRC, 1988a,b; NEA,005), therefore the consideration of thermal stratification is cru-ial in the aging management and for the lifetime-extension ofuclear power plants. In PWRs, the most affected pipes are usu-lly the feed water lines of the steam generator, the pressurizerurge line and the injection pipes of the emergency core coolingystems. The stratification is caused mainly by leaking valves.n the US, more than 12 penetrating cracks have been identifiedince 1979, as a result of thermal stratification. Cracks due tohe stratification were found in France, Belgium and Japan, asell. The Loviisa NPP in Finland reported a penetrating crack

t the pressurizer surge line in 1994 as a common result of ther-al stratification and material defects. The settled temperatureonitoring system confirmed the existence of a stratified flowith temperature differences of 220–270 ◦C.The particular importance of the problem of the thermal

tratification in the Paks NPP is caused by the fact, that theignificance of the phenomenon was not known at the construc-ion of the plant (as in other plants of similar age), therefore it is

ot included in the design base of the plant. An extensive inves-igation performed for Paks NPP (VEIKI, 1998) resulted in theist of the possibly affected primary pipes. These are the follow-ng: the primary loops, the pressurizer surge line, the pressurizer

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pray system, the surge lines of the HPIS and LPIS, the surgeine of the hydro accumulators, the feed water pipe of the steamenerator and the auxiliary emergency feed water connectionnto the steam generator.

For the determination of mechanical effects of the strat-fied flows, stress analyses have to be done, for which thehermal–hydraulic parameters of the flow (e.g. the maximalemperature differences along the given pipe section and therequency of the temperature changes) are required as an inputarameter. The thermal–hydraulic behavior of the flow cane simulated by computational fluid dynamics (CFD) codes.ecause of the quite simple geometry, the modeling may beerformed with reasonable low computational efforts. The mainifficulties of the simulations are the uncertainties of the bound-ry conditions, because the necessary parameters are usually noteasured at the boundaries of the flow region.In this paper, some CFD calculations are presented concern-

ng the development of thermal stratification in VVER-440 typeeactors. With the CFX code (ANSYS, 2005), stratified flow inhe pressurizer surge line, and in the injection pipe of the HPISf the VVER-440 has been investigated.

. Thermal stratification in the pressurizer surge line

The surge line of the VVER-440 connects the lower nozzlef the pressurizer to the hot leg of the loop 6 (in Units 1 and 3)r to the loop 1 (in Units 2 and 4). Under the pressurizer, aftershort vertical section, the surge line is divided into two legs

igned as YP10 and YP20 legs that follow a shifted path to theain loop (see Fig. 1). The inner diameter of the surge line is

07 mm, the length of one leg is about 14 m. The surge line ishermally insulated.

During the operation of the pressurizer heaters, the high tem-

erature medium of the pressurizer dilates and flows into theurge line. This warmer coolant can be stratified on the loweremperature coolant of the surge line and flows stratified abovehe cooler layer toward the main loop.

Fig. 1. Arrangement of the pressurizer surge line.

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g and Design 238 (2008) 453–459

In normal operation, the pressurizer heaters operate periodi-ally. The pressurizer temperature is then about 325 ◦C, while theoolant of the hot leg of the main loop has a temperature of about97 ◦C. In the surge line, there is an alternating flow with veryow coolant velocities in the surge line in both directions. Theeating of the primary circuit during the start-up of the reactor isanaged in part with the pressurizer heaters, which means that

hey operate almost continuously. Therefore, the temperature ofhe pressurizer is always higher than the main loop temperature,o there is a slow permanent flow downward from the pressurizeruring the heat-up period. The maximal temperature differencesan reach even 140–150 ◦C.

.1. Temperature monitoring system in the pressurizerurge line in the Paks NPP

In the Paks NPP, an extensive temperature measurement pro-ram was implemented in 2000 in order to find out the possiblehermal stratification in the pressurizer surge line (Paks, 2001).ltogether, 70 Pt resistance thermometer were installed on theP20 leg of the surge line of the Unit 1. The temperature of

he pipe wall was measured at 10 positions between the pressur-zer and the main loop (see Fig. 2). In one measuring position,here were seven thermocouples operating, arranged verticallyquidistant (see Fig. 3).

The monitoring system collected temperature data minutelyor about 3 months. The operating period included the start-upf the unit, and normal operation as well. The measured temper-tures – about 100,000 data per thermometer – were recordedy the unit computer.

The measurements show that in normal operation there isperiodic stratification in the surge line with a cycle period

f about 45 min which corresponds to the operation of theressurizer heaters. However, the maximum of the temperatureifferences is about 30 ◦C, therefore no critical fatigue occurs

n normal state. On the other hand, the monitoring system con-rmed the existence of a stable stratification during the heat-uperiod (see Fig. 4). The maximum of the temperature differenceseaches 130–140 ◦C. The stratification is very stable, particularly

ig. 2. Planned measuring positions on the surge line (realized only on the YP20eg) (Paks, 2001).

I. Boros, A. Aszodi / Nuclear Engineering and Design 238 (2008) 453–459 455

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Fig. 5. CFX model of the surge line and the arrangement of the monitor lines.

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ig. 3. Installation of the thermocouples at the measuring positions (Paks, 2001).

n the first horizontal section of the pipe. (The monitoring sys-em was installed only on the YP20 leg of the surge line, so theossibly asymmetric flow fields could not be proven.)

.2. CFX simulation of thermal stratification in the surgeine

The development of the thermal stratification during the start-p of the reactor has been investigated with the CFX code. For theimulation of the stratification, the CFX model of the pressurizerurge line has been built, which contained the T-junction at theressurizer, the two legs of the surge line and a short (1 m) sectionf the main loop (see Fig. 5). For the simulation, different vol-me meshes were created, and checked for mesh-independency.

he production mesh – due to the simple geometry – contained78,000 hexahedral volume elements (Fig. 6).

With the model of the surge line, transient CFX calculationas performed concerning the development of the stratified flow.

Fig. 6. Hexahedral mesh of the model.

ig. 4. Measured temperatures at the cross-sections 0 (near the vertical inlet nozzle, at the left side) and 4 (lower horizontal section, at the right side) for the same4-h period, on the top and the lowest thermometers, during the heat-up of the Unit 1.

456 I. Boros, A. Aszodi / Nuclear Engineering and Design 238 (2008) 453–459

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Fig. 7. Coolant temperature in the surge line at t = 900 s.

he boundary conditions were estimated based on the unit oper-tional indicators, performing an extensive sensitivity study, andere set as follows: the coolant with higher temperature (240 ◦C)

nters from the pressurizer into the surge line with a velocityf 0.03 m/s. In the main loop, coolant with lower temperature140 ◦C) flows with a mass flow of 1500 kg/s. As outlet boundaryondition, 0 Pa relative pressure was set at the main loop. Theimulation run 2200 s, with a timestep of 1 s, assuming laminarow.

The simulation confirms the development of a stratified flown the surge line. The stratification is very stable in the firstorizontal section of the surge line, the maximal temperatureifference between the upper and lower layer is about 100 ◦Ct the beginning of the transient. However, the results show a

ery asymmetric flow in the surge line that eventuates in ansymmetric stratification in the lower sections of the surge linesee Fig. 7).

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Fig. 9. Measured and calculated coolant temperatu

Fig. 8. Coolant velocity field in the surge line at t = 1200 s.

The cause of this special flow field is that – according to thealculation – there is a permanent recirculation of colder coolantn the lower layer caused by the asymmetric arrangement of theurge line legs and the asymmetric connection of the two legsnto the main loop (see Fig. 8). The recirculating coolant blockshe flow of the warmer coolant coming from the pressurizer in theower section of one of the legs (in the YP20 leg), therefore it canow only in the other (YP10) leg. This result is not supportedy the measurements in the Paks NPP, where only one of theegs has been monitored, and no definite sign of flow blockingas noticed. The differences between the measured and calcu-

ated development of the stratification can be originated possibly

rom the uncertainties of the boundary conditions (mainly thenlet velocity, which is calculated from the pressurizer level).o bypass this problem an extensive parameter study should be

re in the surge line at monitor lines 0 and 4.

I. Boros, A. Aszodi / Nuclear Engineering and Design 238 (2008) 453–459 457

Fig. 10. Path of the HPIS surge line between the main loop and the hermetic compartment wall.

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tinsulated along the section A, but after the check valve, it is notinsulated.

In the first (“A”) section of the surge line, the coolant temper-ature corresponds to the temperature of the primary circuit and

Fig. 11. CFX model and meshing o

ade, which is quite time-demanding. However, some measure-ents in the Bohunice NPP confirm the existence of this type

f recirculation.Fig. 9 shows the comparison of the calculated and measured

oolant temperatures during the transient at two vertical monitor-ines shown in Fig. 5. The similarity of the measurement andimulation is obvious; however, the CFD simulation predictsetter mixing (i.e. thicker mixed layer) between the cold and hotayers.

. Thermal stratification in the surge line of the highressure injection system (HPIS)

According to the international operating experiences (NEA,005; VEIKI, 1998; PaB, 1998), the surge lines of the emer-ency cooling systems can be affected concerning the thermaltratification. The surge line of the high pressure injection sys-em is connected to the cold leg of the loops 2, 3 and 5. The surgeine runs from the HPIS pump to the box wall on one path and

hen it is divided into two legs that are closed with pneumaticuick-stop valves. Inside the box wall, there are inner quick-stopalves and after it, the pipe legs join together again (see section Cn Fig. 10) and connects after a check valve (see “B” in Fig. 10)

rst section of the HPIS surge line.

o the main loop (see section A). The surge line is thermally

Fig. 12. Coolant temperature in the surge line at t = 180 s.

458 I. Boros, A. Aszodi / Nuclear Engineering and Design 238 (2008) 453–459

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Fig. 13. CFX model of the second section of the HPIS surge line.

hermal stratification may occur due to heat losses through theinsulated) pipe wall. Between the check valve and the quick-top valves, the temperature corresponds to the so-called boxhermetic compartment) temperature, i.e. about 50 ◦C. In thisection, the leakage of the check valve – that could be caused,.g. by thermal expansion – may lead to thermal stratification.ome demonstrative calculations were performed with the CFXode for the sections “A” and “C” of the surge line to simulatehe development of the thermal stratification.

In section “A”, the effect of heat losses through the pipeall was investigated with assuming that there was no leak-

ge through the check valve. Heat flux through the pipeall was determined by analytical calculation and through

he check valve by additional CFX simulation. The modelf the section “A” contained the surge line up to the checkalve (without exact modeling of the valve itself) and amall part of the main loop. The volume mesh of theodel contained 157,000 tetrahedral volume elements (seeig. 11).

The transient simulation performed with the model run80 s with a time step of 1 s, with assuming laminar flow

Re = 5000, . . ., 10,000) and with the following boundary con-itions: 1500 kg/s mass flow in the main loop (according to theormal operation) with a temperature of 267 ◦C. A 1200 W/m2

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Fig. 15. Thermal stratification

Fig. 14. Thermal striping in the surge line.

eat flux was specified at the wall of the surge line (accordingo the analytical calculations, assuming no insulation conser-atively), 5000 W/m2 heat flux at the closed end of the surgeine (according to the CFX simulation of the check valve).ccording to the calculation, no stratified flow evolved in

he first section of the HPIS surge line because the maximalemperature differences are only 4–5 ◦C (see Fig. 12). The

or the formation of stratification. The colder coolant flowsn large eddies and mixes with the warmer water in the surgeine.

in the HPIS surge line.

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For the simulation of the second section of the HPIS surgeine (see section “C” in Fig. 10), the CFX model of the pipeection has been built, assuming the leakage of the check valve.he model contained the surge line between the check valvend the quick-stop valves. The valves are modeled as a simplelosed end of the pipe. The volume mesh of the model contained74,000 hexahedral volume elements (see Fig. 13).

The development of the stratification was investigated with aransient simulation, which ran for 420 s, with a time step of 1 s,ssuming laminar flow in the pipe. As inlet boundary condition,t/h mass flow from the primary circuit with a temperature of67 ◦C, as outlet boundary condition, 0 Pa relative pressure athe quick-valves were assumed. The heat losses through the pipeall were neglected in order to keep the numerical stability of

he simulation.The results show the development of a thermal stratification

n the surge line but the stratified flow is not stable, at t = 300 she layers break up in the first section of the investigated pipe.he maximal temperature difference reaches 200 ◦C in the firstection of the pipe for a short period (see Fig. 15). The thermaltriping (the waving of the mixed layer) can be observed as wellsee Fig. 14).

. Summary

In the paper, some results of CFX simulations concerninghe thermal stratification in primary piping of VVER-440 wereresented. The calculations – in accordance with the availableeasured data – confirm the development of stratified flow in

he surge line of the pressurizer during the heat-up of the reactor.owever, the CFX simulation shows a stable asymmetric flow

n the surge line, which is not directly confirmed by the mea-urements. For the exact assessment of the phenomenon, furthereasurements would be necessary, installed on both legs of the

urge line.

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g and Design 238 (2008) 453–459 459

In the second part of the paper, simulations of thermaltratification in the HPIS injection line were presented. Thenvestigation of the surge line of the HPIS shows similar strati-cation, in case the check valve is assumed to be leaking. Theseimulations cannot be validated by Hungarian measurements.or the precise description of the boundaries (as the leakageass flow rate on the check valve), the extensive study of the

nternational operating experience is necessary.The results showed that the CFD codes provide an effec-

ive method for the demonstration of the thermal stratificationhen sufficient operational data are available for the concernediping. Otherwise, measurements or additional simulations areequired, and additional sensitivity studies should be performedor the accurate analysis. Qualitative CFD simulations can alsolay an important role in the ageing management of NPP pip-ng: they can help deciding, whether the possibility of thermaltratification exists in given pipe sections.

eferences

NSYS, 2005. ANSYS CFX, Release 10.0. 2005 ANSYS Europe Ltd.ortesi, A.B., 1996. Direct Numerical Simulation of a Temporally-Developing,

Stability-Stratified Mixing Layer for a Wide Range of Richardson andPrandtl Numbers. Swiss Federal Institute of Technology, Zurich.

EA, 2005. NEA CSNI, CSNI Integrity and Ageing Working Group. Ther-mal Cycling in LWR Components in OECD-NEA Member Countries,NEA/CSNI/R(2005)8.

RC, 1988a. NRC Bulletin No. 88-08 (Supplement 1, 2, 3): Thermal Stressesin Piping Connected to Reactor Coolant Systems, http://www.nrc.gov.

RC, 1988b. NRC Bulletin No. 88-11: Pressurizer Surge Line Thermal Strati-fication, http://www.nrc.gov.

aB, 1998. Analysis of thermal stresses due to thermal stratification in pipelines.

aks, 2001. Assessment of the Results of the Temperature Monitoring System,Installed on the Pressurizer Surge Line. Paks NPP, in Hungarian.

EIKI, 1998, Ageing management of nuclear power plant equipment—piping,background material for regulatory guidelines. Report in Hungarian.