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Nuclear Engineering and Design 229 (2004) 2546

Analysis of natural circulation phenomena in VVER-1000

Seyed Khalil Mousavian a,b,, Francesco DAuria b, Mahmoud A. Salehi a

a Department of Mechanical Engineering, Sharif University of Technology, NPP of AEOI, P.O. Box 11365-9567 Tehran, Iranb Dipartimento di Ingegneria Meccanica Nucleare e Della Produzione, Universit di Pisa, Via Diotisalvi 2, I-56126 Pisa, Italy

Received 1 July 2003; received in revised form 13 October 2003; accepted 18 November 2003

Abstract

In all light water reactors (LWR), natural circulation is an important passive heat removal mechanism. In the present pa-

per, the natural circulation phenomena are studied with reference to step-wise coolant inventory reduction and a small break

loss-of-coolant-accident (SBLOCA) in the cold leg of VVER-1000. The natural circulation flow map (NCFM) approach is con-

sidered to evaluate the natural circulation performance of the VVER-1000 NPP also comparing VVER-1000 and PWR systems.

Three different elevations between heat source (core) and heat sink (steam generators) zones have been considered in order to

characterize the buoyancy force in a VVER-1000. The influence of power and the cold legs loop seal upon the natural circulation

performance is also evaluated. In the second part, a series of SBLOCA simulations with break area ranging from 0.5 to 11.7% of

the cold leg cross sectional area are performed starting with the VVER-1000 system in nominal conditions. The effect of Emer-

gency Core Cooling System (ECCS) including passive and active parts of ECCS are evaluated. The simulations were performed

by the help of the system code RELAP5. Within the framework of the qualification of the adopted computational tools, the

results are compared with experimental data from Kozloduy NPP unit 6 test and PSB-VVER integral test facility available fromthe literature. Namely, the qualification of the adopted nodalisation in steady state conditions is achieved by using experimental

data. The accuracy of selected results have been estimated in quantitative terms by applying the fast Fourier transform based

method (FFTBM). Finally, the relevance and the potential for the occurrence of the reflux condensation mode, i.e., one of the

Natural Circulation regimes, for cooling of reactor core in VVER-1000 are discussed.

2003 Elsevier B.V. All rights reserved.

1. Introduction

Natural circulation phenomena play an important

role in energy extraction from hot zones and rejectionto the cold zones without using a mechanical pump

in several engineering systems, such as solar water

heaters, geothermal heat extraction, electrical machine

motor cooling and reactor core cooling (Zvirin, 1981).

The density difference between fluid leaving the heat

Corresponding author. Tel.: +39-050-836675;

fax: +39-050-836665.

E-mail address: kh [email protected]

(S.K. Mousavian).

source at one elevation and the heat sink(s) in higher

elevation induces flow and provides a passive means

of core cooling.

Naturally driven systems have been used in theexisting nuclear reactors both to remove the core

heat under normal operation conditions including

full power operation (like the Russian boiling wa-

ter reactor VK-50 or the Dodewaard reactor in the

Netherlands) and to fulfil some safety functions (Hart

et al., 2001; Warnke et al., 2001). Many experiments

have been performed to study of the natural circula-

tion phenomena in Western pressurized water reactor

(PWR) test facilities (DAuria and Galassi, 1992;

Loomis and Soda, 1982; Tasaka et al., 1988; Ferng

0029-5493/$ see front matter 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.nucengdes.2003.11.012

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26 S.K. Mousavian et al. / Nuclear Engineering and Design 229 (2004) 2546

Nomenclature

A flow area

AA average amplitudeACCU accumulators

AS wall cross sectional area

ECCS emergency core cooling system

F function

f frequency in FFT analysis

FFT fast Fourier transform

FW feed waterG mass fluxg gravity acceleration

HPIS high pressure injection system

ISB integral test facilityL length

l axial length

LPIS low pressure injection system

MCP main coolant pump

MSH main steam headerN number of points in FFT analysis

NCFM natural circulation flow map

NPP nuclear power plant

NC natural circulation

PACTEL integral test facility

PWR pressurized water reactor

PMK integral test facilityPSB integral test facility

RELAP reactor excursion and leak analysis

program

REWET integral test facility

RPV reactor pressure vessel

RM residual mass

SBLOCA small break loss-f-coolant-

accident

SG steam generator

St. St. steady state

t timeVVER water-cooled water-moderated

reactor

w weighting factor

WF weighted frequency

Z elevation

Greek letters

difference symbolsummation symbol

F difference between experiment

and calculation function in

frequency domain

P pressure loss

Z average elevation difference

friction coefficient

Subscripts

accl acceleration term

B buoyancy

cal calculation

CL cold leg

CL-HL between CL and HL

exp experimental

f frictional termHL hot leg

j index of summation in FFT analysis

k index of summation through loop

m power of 2 in FFT analysis

mo model

mix mixture in two-phase flown index of summation in FFT analysis

norm normalization

pipelines pipe lines through loop

pr proto-type

pump-off pump is offRPV reactor pressure vessel

R ratio

saf safety

SGs steam generators

var index of number of parameters in

FFT analysis

and Lee, 1994). Other studies focused on evaluation

and modeling of natural circulation behavior dur-

ing small break loss-of-coolant-accident (SBLOCA)

and transients in Western-type reactors (Duffey andSursock, 1987; Hsu et al., 1990; Mousavian and

Salehi, 2001).

The natural circulation experimental database in

VVER geometry, especially in VVER-1000, is lim-

ited if compared with the database available for

Western PWRs. In VVER-440 geometry experiments

were performed in REWET-III, PMK and PACTEL

integral test facilities (Kervinen and Hongisto, 1986;

Toth, 1988; Lomperski and Kouhia, 1994; Puustinen,

2002). In the case of VVER-1000 there are two inte-

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S.K. Mousavian et al. / Nuclear Engineering and Design 229 (2004) 2546 27

gral test facilities: ISB (small scale, with a two-loop

model of primary circuit) and PSB (large scale, with a

four-loop model of primary circuit), both of those in-

stalled at Elektrogorsk in Russia. Natural circulationexperiments relevant to SBLOCA and the stepwise

reduction of mass inventory are reported for ISB

(Krepper, 1997, 1999). So far, some data related to

natural circulation in PSB-VVER integral test facility

were published (Blinkov et al., 2003). In addition, the

VVER-1000 natural circulation database is enlarged

by the results from the Kozloduy NPP unit 6 program

(Pavlova et al., 2000).

In Western type PWRs by decreasing mass inven-

tory three or four main modes of natural circulation

are identified: single phase flow, stable two phase

flow, unstable two phase flow with siphon effect andboiler-condenser or reflux condensation. The primary

circuit mass flow rate is a strong function of buoy-

ancy pressure head, pressure losses, mass inventory

or void fraction, and a weak function of power.

Two main objectives of this paper can be distin-

guished:

Natural circulation performance study in VVER-

1000; In depth study of natural circulation phenomena in

complex systems.

Related to the second objective, reflux condensa-

tion cooling is significant at small mass inventories

of primary system (e.g., Kawanishi et al., 1991). Dur-

ing SBLOCA, reflux condensation does not contribute

much to heat removal when the break is larger than

a defined value, around 1% of cold leg size in West-

ern PWR, as reported by Jeong (2002). The loop seal

effect on heat removal capability and qualification of

natural circulation conditions is noticeable (DAuria

and Frogheri, 2002; Krepper, 1999).

In order to achieve both the defined objectives, qual-ification of the adopted tools and of the analysis pro-

cedure is needed.

2. Nodalization and qualification

The Relap5/Mod3 code is used for the present anal-

yses to simulate natural circulation phenomena in a

VVER-1000. This code is based on one-dimensional

six-equation models and was developed for best-

estimate simulation of thermalhydraulic behavior of

LWRs during accidents and transients. The adopted

nodalization of one loop of primary and secondary

sides is shown in Fig. 1.The nodalisation qualification is achieved by a com-

plex process as discussed by DAuria and Galassi

(1998), where the steady-state and the on-transient

qualifications levels are distinguished. A sample result

derived from the steady-state nodalisation qualifica-

tion process is given in Fig. 2. Selected steps and re-

sults from the on-transient nodalisation qualification

are discussed below (Sections 2.1 and 2.2).

2.1. Comparison and quantitative assessment with

Kozloduy NPP unit 6

Natural circulation test data from Kozloduy NPP

unit 6 were measured by Bulgarian and Russian ex-

perts during the plant commissioning phase (Pavlova

et al., 2000). Initial and boundary conditions related to

one of the selected test conditions are given in Table 1.

Fig. 3 shows the comparison between results from

the VVER-1000 code-nodalisation and those from Ko-

zloduy NPP unit 6 test. Two code calculations have

been performed, owing to inadequate knowledge of

all boundary conditions of the NPP test: in case one,

main feed waters (FW) flow rates is 20% of nominalvalue and initial pressurizer level is 7.2 m; in case two,

mass flow rates of main FW is 10% of nominal value

and initial pressurizer level is 5.2 m.

From a qualitative point of view, the calculated re-

sults are judged to be sufficiently close to the measured

data, also considering the unknown uncertainty that

characterizes the measurements. Therefore, it was de-

cided to apply the fast Fourier transform based method

(FFTBM; Ambrosini et al., 1990; DAuria et al., 2000;

Prosek et al., 2002) in order to quantify the accuracy,

even though the FFTBM tool is normally applied whenan experimental database wider that the present one is

available.

The accuracy quantification of a code calculation

considers the amplitude, in the frequency domain, of

the experimental signal Fexp(t) and the error function:

F Fcal(t) Fexp(t) (1)

In particular, the method defines two values which

are characteristic of every calculation: average ampli-

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Fig. 1. RELAP5/mod3.2 nodalization for NC study in a VVER-1000.

tude (AA) and weighted frequency (WF);

AA =

N/2=2mn=0 |F(fn)|N/2=2m

n=0 |Fexp(fn)|and

WF =

N/2=2mn=0 |F(fn)|fnN/2=2m

n=0 |F(fn)|(2)

The overall accuracy is obtained by defining average

performance indexes, total average amplitude:

AAtot =Nvarj=1

(AA)j (wf)j, (WF)tot

=

Nvarj=1

(WF)j (wf)j, with

Nvarj=1

(wf)j = 1

(3)

(wf)j =(wexp)j (wsaf)j (wnorm)jNvar

j=1 (wexp)j (wsaf)j (wnorm)j(4)

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Table 1

Initial and boundary conditions and sequence of events at Kozloduy unit 6 test

Initial and boundary conditions Sequence of events

Parameters Unit Values Time (s) Values/conditions

Reactor power MW 151 0 All MCPs are switched off

Primary side pressure MPa 15.70

Pressure in MSH MPa 6.13 60 Auxiliary FW pumps start

Pump heads MPa 0.62

Pressurizer water level m 5.20 90 Control rod group #8 starts withdrawal

Hot/cold legs temperatures C 283.5/281

PRZ steam temperature C 344.2 220 Auxiliary FW flow rate reaches to 21/42 kg/s

SG water level m 2.45

Temperature of main FW C 161.7 385 Make-up flow rate reaches to 9 kg/s

Make up/let-down flow rates kg/s 8.4/8.4

Emergency FW flow rates kg/s 44.5/36.5 590 Let-down flow rate reaches to 5.5 kg/s

SG pressure MPa 6.13

Core exit temperature C 282.1 630 End of experiment

where Nvar is the number of the parameters analyzed,

and (AA)j, (WF)j and (wf)j are average amplitude,

weighted frequency and weighting factors for i-th ana-

lyzed parameter, respectively. Each (wf)j accounts for

experimental accuracy, safety relevance of particular

parameters and its relevance with respect to pressure

(or primary pressure normalization). Weighting factor

components of Eq. (4) are shown in Table 2 (Bovalini

et al., 1992).

For FFTBM calculations the primary side pressure,hot and cold leg temperatures, pressurizer water level,

and steam generator down-comer level are selected.

Table 3 shows the results of calculations for the two

-20

0

20

40

60

80

100

120

140

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Height (m)

Volume(m^3)

RELAP5

EXPERIMENT

Fig. 2. Qualification of nodalization at steady state level for sec-

ondary side of SG.

cases as mentioned. We assumed N/2 = 212, also cut

frequencies (fcat) 1, 3 and 5 Hz are selected in FFTBM

calculations.

With reference to the accuracy of a given calcula-

tion, the following acceptability criterion is defined:

AAtot K (5)

where K is acceptability factor valid for whole the

transient. Based on several calculations K = 0.4 hasbeen fixed (DAuria et al., 1996). It can be noted:

1. AAtot 0.3 characterize very good predictions,

2. 0.3 AAtot 0.5 characterize good code predic-

tions,

3. 0.5 AAtot 0.7 characterize poor code predic-

tions,

4. AAtot 0.7 characterize very poor code predic-

tions.

Table 2

Weighting factor components for the analyzed quantities (Bovaliniet al., 1992)

Quantity wexp wsaf wnorm

Pressure drops 0.7 0.7 0.5

Mass inventories 0.8 0.9 0.9

Flow rates 0.5 0.8 0.5

Primary pressure 1.0 1.0 1.0

Secondary pressure 1.0 0.6 1.1

Fluid temperatures 0.8 0.8 2.4

Clad temperatures 0.9 1.0 1.2

Collapsed levels 0.8 0.9 0.6

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15

15.5

16

16.5

17

17.5

18

18.5

19

0 100 200 300 400 500 600Time (s)

Primarypressure(MPa)

case1

case2

Kozloduy

2.32

2.34

2.36

2.38

2.4

2.42

2.44

2.46

2.48

2.5

2.52

0 100 200 300 400 500 600

Time (s)

SGlevel(m)

case1

case2

Kozloduy

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

0 100 200 300 400 500 600

Time (s)

Pressurizerlevel(m)

case1

case2

Kozloduy

260

270

280

290

300

310

320

0 100 200 300 400 500 600

Time (s)

Coldandhotlegtemperatures(C)

case1 hot -leg case1 cold- leg

case2 hot -leg case2 cold- leg

Kozloduy hot-leg Kozloduy cold-leg

(a)

(c) (d)

(b)

Fig. 3. Comparison of case 1 and 2 of the present study with Kozloduy NPP unit 6.

In addition, the acceptability thresholdK =

0.1 hasbeen fixed for the primary pressure, because of its

importance in characterizing any transient scenario.

The most interesting information is given obviously

by AAtot, which represents the relative magnitude of

these discrepancies; WF adds a further information

allowing to better identify the character of accuracy.

As it can be seen from Table 3, the primary pres-

sure criterion (i.e., K = 0.1) was not fulfilled but

the total accuracy is well below 0.3 indicating very

good predictions. The reason for discrepancies be-

tween measured and calculated trends is attributed toinadequate knowledge of the boundary conditions, as

already mentioned. As a conclusion, based on this ap-

plication of the FFTBM, the VVER-1000 nodalisation

was considered qualified at the on-transient level.

2.2. Comparison with PSB-VVER integral test facility

The PSB integral test facility is a four loop, full pres-

sure scaled down model of the primary system of the

NPP with VVER-1000 in Russia. The volume-power

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Table 3

Results of quantitative analysis for two cases at different cut

frequenciesa

Variables Case 1 Case 2

AAj WFj AAj WFj

Primary pressure 0.20 0.09 0.23 0.10

0.22 0.26 0.25 0.30

0.22 0.31 0.25 0.36

HL temperature 0.04 0.13 0.04 0.13

0.04 0.38 0.04 0.37

0.04 0.44 0.04 0.43

CL temperature 0.04 0.11 0.05 0.11

0.05 0.34 0.06 0.33

0.05 0.39 0.05 0.39

PRZ collapsed level 0.17 0.05 0.24 0.040.18 0.16 0.24 0.13

0.18 0.19 0.24 0.15

SG collapsed level 0.05 0.07 0.04 0.07

0.05 0.22 0.04 0.21

0.05 0.26 0.04 0.24

Total (AAtot, WFtot) 0.08 0.11 0.09 0.10

0.09 0.31 0.10 0.31

0.09 0.36 0.10 0.36

a Upper, middle and lower numbers are related to 1, 3 and 5 Hz

cut frequencies, respectively.

scale is 1/300 while the elevation scale is 1/1. Fourloops provide an adequate simulating of the emer-

gency and transient conditions leading to asymmetri-

cal thermalhydraulic behavior of the loops. A natural

circulation experiment in PSB has been performed

(Blinkov et al., 2003) and the related main initial and

boundary conditions values are reported in Table 4.

In the PSB-VVER some geometrical similar-

ity criteria including axial length, flow area, wall

Table 4

Initial and boundary conditions in PSB-VVER (Blinkov et al., 2003)

Parameters Unit Initial conditions

(forced conditions)

Natural circulation

conditions

Core power MW 0.500 0.500

Core outlet pressure MPa 15.5 15.7

Core volumetric flow rate m3/s 0.0055

Fluid velocity in the core m/s 0.39

SG pressure MPa 6.5 6.5

Core mass flow rate kg/s 54.3

Core inlet/outlet temperatures C 281.5/283.6 277.0/305.2

SG inlet/outlet temperatures C 283.2/282.1 303.2/281.8

Pump inlet/outlet temperatures C 282.0/281.7 281.4/277.3

cross-sectional area (with the exception of pipelines)

and friction coefficient are considered to be the same

for the model and the proto-type:

lR

lmo

lpr

= 1, AR

Amo

Apr

= 1,

ASR

ASmo

ASpr

= 1, R

mo

pr

= 1 (6)

In reality, the model friction factors exceed the sim-

ilar values in the prototype in each section of the

loop. Thus, the overall pressure drop in the prototype

(VVER-1000) at nominal conditions equals 0.61 MPa,

while in the test facility (PSB-VVER) this value is

0.87 MPa.

Also the scaling criteria which have been developedby Ishii and Kataoka such as Richardson, modified

Stanton, Biot, time ratio and heat source numbers are

satisfied (Ishii and Kataoka, 1983).

The on-transient nodalisation qualification of the

VVER-1000 nodalisation has been performed fol-

lowing the requirements foreseen by the so-called

Kv-scaled calculation (DAuria and Galassi, 1998).

Following this procedure, boundary and initial condi-

tions of the NPP nodalisation are fixed utilizing source

data from the Integral Test Facility, ITF (PSB in the

current case) and proper scaling criteria to scale-upfacility related values. At the end, experimental data

from the ITF are compared with calculation results

from the NPP nodalisation (or input deck).

The comparison between the calculated results from

the VVER-1000 nodalisation and the measured data

in the PSB integral test facility for fluid temperatures

distribution through the loop (given at the time of start

of the experiment as a function of the function of a

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260

270

280

290

300

310

320

1 2 3 4 5 6 7 8

Mesh points through loop : 1-core outlet, 2-UP, 3-SG inlet,4-SG outlet, 5-MCP inlet, 6-MCP outlet, 7-dow n comer, 8-core inlet

Temperature(C

)

present study PSB-VVER

265

270

275

280

285

290

295

300

305

310

315

0 100 200 300 400 500 600

Time (s)

FluidtemperaturesatRPVinletandoutlet

(C)

PSB-VVER-CL

PSB-VVER-HL

present-study-CL

present-study-HL

0 50.0 100.0 150.0 200.0 250.0 300.0 350.0 400.0

Time (s)

0

1.00

2.00

3.00

4.00

5.00

6.00

Liquidvelocitythroughcore(m/s)

XXX PSB-VVER

X

X

X

X

X

X

XX

XX X X X X X X X X

YYY present-study

Y

Y

YY

Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y

(a) (b)

(c)

Fig. 4. Comparison of present work with PSB-VVER experimental data.

geometrical abscissa along the loop) including core

inlet and outlet that are given as a function of time

and liquid velocity through the core can be found in

Fig. 4. There is a good agreement between the consid-

ered curves (NPP calculated results and ITF measured

data). No quantitative evaluation of accuracy was done

in this case by the application of the FFTBM.

3. Natural circulation scenario

It is assumed that at time 0 all MCPs trip and re-

actor power scram occurs, so transient conditions for

establishing the natural circulation will initiate. After

about ten minutes (at t = 700 s), mass inventory re-

duction starts, caused by fluid draining of primary side

coolant based upon the steps characterized by Fig. 5.

0.0E+00

2.0E+04

4.0E+04

6.0E+04

8.0E+04

1.0E+05

1.2E+05

1.4E+05

1.6E+05

1.8E+05

-1000 1000 3000 5000 7000 9000 11000 13000 15000

Time (s)

Integralofdrainingmas

sflow

rate(kg)

66% of total mass inventory, at nominal conditions

Fig. 5. Integral of mass flow rate draining.

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Table 5

Initial and boundary conditions in NC study

Initial conditions Boundary conditions

Parameters Unit Values Parameters/systems Values/conditions

Reactor power MW 3000.0 ECCS (LPIS, HPIS, ACCU) Not activated

Primary/secondary sides pressures MPa 15.7/6.44 Auxiliary/emergency FW Not activated

Total mass of RPV/primary-side kg 83 103/241 103 Mass flow rate of FWs at: (kg/s)

Steady-state conditions; 163.0

From St.St to 700 s; 16.3

From 700 s to end; 4.9

Fluid temperatures at RPV inlet/outlet C 289/318 All MCPs trip at time, s 0

Fluid temperatures at SG inlet/outlet C 318/288 Starting time of draining, s 700

Pressurizer temperature C 345 Reactor scram at time, s 0

Maximum fuel/cladding temperatures C 340/2100 Mass flow rate of draining: (kg/s)

At time 0 to 800 s 0

From 800 to 3100 s According to Fig. 5

From 3100 s to end 6.0Collapsed level of pressurizer m 8.4

Safety relief valves set points:

Collapsed level of core m 3.25 Opening (MPa) P(SG) 8.24

Closing (MPa) P (SG) 6.86

Collapsed level of SG m 1.8

The 700 s period is needed in order to ensure that MCP

coast down effects are negligible and that the loop

mass flow rates reach a stable values corresponding to

single phase stable natural circulation. The primary

coolant draining design outlined in Fig. 5 is used in

the calculations documented in this chapter and de-rives from an experiment performed in the LOBI fa-

cility available at the European Community research

center of Ispra in Italy.

Table 5 reports the initial and boundary conditions

of the VVER-1000 natural circulation scenario for our

analysis.

By stepwise decrease of the mass inventory in the

primary side single phase, two phase, reflux con-

densation, and dry-out phenomena or events occur

(Mousavian et al., 2003a, 2003b). The chronological

major events are described in Section 3.1. In the sub-sequent sections the effects of buoyancy force, power

and loop seal on natural circulation are discussed.

3.1. Effect of buoyancy on NC in a VVER-1000

The schematic of a simplified VVER-1000 loop in-

cluding vessel, hot leg, steam generator, cold leg loop

seal and cold leg horizontal part is shown in Fig. 6.

By integrating of one-dimensional momentum equa-

tion in natural circulation conditions (Ppumps = 0)

around in the loop of Fig. 6 we find (Todreas and

Kazimi, 1990):

k

Lk(Gm)k

t= PB + Paccl Pf (7)

Fig. 6. Schematic diagram of a VVER-1000 loop.

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where

PB [(m)CL (m)HL]g(Z)CL-HL,

Paccl

k

Gm2

m

k

, and

Pf PRPV + PSGs

+Ppipelines + Ppump-off+ (8)

In Eq. (8) the parameters PB, Paccl and Pf refer

to buoyancy, acceleration and frictional pressure head

terms, respectively. The Eqs. (7) and (8) show the

importance of the term Z. Otherwise, the effect of

acceleration pressure head with respect to other terms

is negligible. Owing to this, the attention is focused

toward the difference in elevation between the heatsink (steam generator in the present case) and the heat

source (core in the present case).

Therefore, natural circulation phenomena are eval-

uated by the help of the RELAP5 code, considering

three different distances (or three elevation differ-

ences) between hot and cold zones in next three

sub-sections (the words high-level and low-level

are used as synonymous and abbreviation for high-

elevation-difference and small-elevation-difference

between heat source and heat sink, respectively).

3.1.1. NC in nominal case

In this sub-section, the natural circulation phenom-

ena are studied in nominal conditions (ZCL-HL =

9.28 m). Variations of fluid and saturation tempera-

tures through the whole loop in natural circulation

conditions (at t = 2000 and t = 10,000 s) can be

found in Fig. 7. The average sub-cooling of the loop

at t = 2000 s can be deduced from Fig. 7a. On the

contrary, the Fig. 7b shows that saturation conditions

occur throughout the loop at t= 10,000 s and differ-

ences shown in the figure give an idea of numericalerrors and of errors (typical) in interpreting the code

results by the code user. Single phase natural circula-

tion occurs between 100 and 82% of primary side mass

inventory. By decreasing mass inventory, at 75% of

nominal conditions, two phase mass flow rate reaches

its maximum value. At low primary coolant mass in-

ventory, i.e., about 40%, the flow rate stagnates.

Fig. 8 shows the variations of pressure in primary

and secondary side reported as a function of total in-

ventory of the primary side. The primary side depres-

surizes slowly and finally reaches the secondary side

pressure and saturation conditions are basically estab-

lished in both primary and secondary sides.

3.1.2. NC in high-level case

By increasing the elevation of steam generators

(heat sinks) with respect to the reactor core (heat

source) the effect of buoyancy force is studied

(ZCL-HL = 15.7 m). The natural circulation mass

flow rate in the loop for both single and two phase

flow regimes is higher (for about 20%) than in the

nominal case. In addition, smaller amplitudes for the

oscillations are calculated, Fig. 9. The conclusion,

as expected is that the cooling capability by natural

circulation is enhanced and is more stable when in-

creasing the elevation difference between heat sink

and heat source.

At very low mass inventories, i.e., coolant mass25% of the nominal value, the dry-out phenomenon

occurs. Fig. 10 shows the consequences of dry-out

upon the clad surface temperature.

3.1.3. NC in low-level case

The height difference between hot and cold zones is

decreased in this case by about two meters with respect

to the nominal case (ZCL-HL = 7.48 m). Core inlet

mass flow rate in single and two phase flow conditionsis calculated to decrease for about 7%. In this case,

two phase flow instability is not observed between

100 and 43% of mass inventory (Fig. 11), with main

reference to the time window around 7000 s.

Considering all the three code runs (nominal,

high-level and low-level), it may be noted that when

low mass inventories (starting from 40% of nominal

value) occur the core inlet mass flow rate is quite sim-

ilar, thus independent upon the elevation difference

between heat source and heat sink.

Table 6 summarizes the chronological events duringnatural circulation in the three analyzed cases. It shall

be noted, as expected, that the event under consider-

ation for the nominal case occurs within the ranges

and/or the occurring times calculated for the high-level

and the low-level cases.

3.2. Effect of loop seal on NC in a VVER-1000

Loop seals are the U-shaped bends in the cold

legs piping connecting steam generators outlet to

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At time=2000 s

250

260

270

280

290

300

310

320330

340

350

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

mesh points through first loopFluidandsaturatedtempera

ture(c)

Lower plenum Core region Upper plenum HL SG CL & down

comer

Fluid temperature

Saturated temperature

279.4

279.5

279.6

279.7

279.8

279.9

280

280.1

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

Mesh points through first loop

Fluidandsaturationtemperature(C)

At time=10000 s

(a)

(b)

Fig. 7. Fluid and saturation temperatures through the loop at NC conditions.

MCPs suction. The influence of eliminating the loop

seals upon the natural circulation scenario is dis-

cussed hereafter. This has been done starting from the

VVER-1000 nodalisation representing the real NPP.

The capability of heat removal by natural circulation

in Western PWRs, by using of L-shaped config-

uration instead of U-shaped loop seal, has been

already estimated (DAuria and Frogheri, 2002): such

a capability was calculate to increase for about 20%.

Fig. 12 shows the distribution of liquid velocity

and void fraction through vertical section of cold

leg between steam generator and pump suction that

result from the present VVER-1000 related cal-

culation.

In the case when loop seals are eliminated from the

system configuration, the velocity of liquid after re-

flux condensation is higher than in the original case

(with loop seals). Also during the reflux condensation

mode of natural circulation, the vertical section be-

tween steam generator and pump suction is cleared at

an earlier time in the case when the loop seal is present

compared with the case without loop seal. Moreover,

by eliminating of the loop seals, the instability re-

gion of two phase flow is decreased. Limited or no

effect of the loop seal is detected in other modes of

the natural circulation, thus confirming that removal

of loop seal is beneficial from the point of view of the

thermalhydraulic design of a VVER-1000.

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4.00

5.40

6.80

8.20

9.60

11.00

12.40

13.80

15.20

30.040.050.060.070.080.090.0100.0

Inventory of primary side (%)

Primaryandsecondarypress

ure(MPa)

primary

secondary

Fig. 8. Primary and secondary side pressures in nominal case ofNC conditions.

3.3. Effect of core power on NC in a VVER-1000

The core power level is one of the important pa-

rameters affecting the natural circulation scenario in

VVER-1000. Natural circulation experiments from

ISB-VVER (with vertical steam generators) integral

test facility showed that by increasing power from

100 to 200 kW the mass flow rate through the loops

increased by 25% of single phase nominal natural

circulation value (Krepper, 1999).

Therefore, by keeping the reactor power at 2.5, 5

and 10% of nominal power, the cold leg mass flow

-100

0

100

200

300

400

500

600

700

-1000 1000 3000 5000 7000 9000 11000 13000 15000

Time (s)

Massflowt

hrou

ghtheloop(kg/s) Nominal

High-level

Fig. 9. Mass flow rates through the loop in nominal and high level

cases.

20 0

40 0

60 0

80 0

1000

1200

1400

1600

1800

0 5000 10000 15000 20000 25000 30000

Time (s)

Maximumc

laddingtempera

ture(C)

Starting of dry-out

Fig. 10. Maximum cladding temperature in high level case of NC

study (dry-out effect).

rates are obtained (Fig. 13) by using the VVER-1000

nodalisation.

When the power level is decreased from 5 to 2.5%

the core inlet mass flow rate was found to decrease

for about 28% of its nominal value and the two phase

flow instability occurs at a later time (Fig. 13). At

2.5% of nominal power, about 50 minutes after the

natural circulation transient initiation, two phase flow

starts and the void fraction increases up to about 0.3

without dry-out occurrence (till the time when 30% oftotal mass inventory).

When the power level is kept at 5% of the nominal

value, the void fraction is increased and after 180 min

-100

0

100

200

300

400

500

600

700

0 2500 5000 7500 10000 12500 15000

Time (s)

Massflow

rateth

roughtheloop(kg/s)

Fig. 11. Mass flow rate through the loop at low level case of NC

study.

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Table 6

Chronological of events for different cases of natural circulation phenomena

Time (s) Residual mass (%) Events

Nominal conditions (1) High-level conditions (2)

0 t 2800 (1) 82 RM 100 Single-phase flow

(2) 86 RM 100 Single-phase flow

(3) 85 RM 100

t= 3100 (1) RM= 75 Two-phase stable (maximum flow rate)

(2) RM= 77 Two-phase stable (maximum flow rate)

(3) RM= 74

(1) 4000 t 6000 53 RM 58 Two-phase & unstable

(2) 4000 t 4650 57 RM 59 Two-phase and very low unstable

(2) 4000 t 7500 48 RM 57

(1) t= 7850 Reversed flow starts (RPV outlet)

(2) t= 9000 RM= 48

(3) t= 7600 Reversed flow starts (RPV outlet)

(1) 10200 t 11700 40 RM 43 Two-phase with oscillations

(2) 12800 t 14550 38 RM 40 Two-phase with oscillations

(3) 9750 t 10800 41 RM 43

(1) 1170 t 15000 35 RM 40 Reflux condensation (without dry-out)

(2) 14550 t 23000 27 RM 38 Reflux condensation (without dry-out)

(3) 10800 t 15000 34 RM 41

(2) t= 23000 RM= 27 Dry-out starts

(2) 23000 t 25000 24 RM 27 Dry-out and core inlet mass flow rate

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-1

0

1

2

3

4

5

6

-500 2500 5500 8500 11500 14500

Time (s)

Liquidvelocity(m/s

)

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Voidfraction

with loop sealwithout loop sealwithout loop sealwith loop seal

Fig. 12. Loop seal effect on liquid velocity and void fraction.

(or at a value of 38% for total mass inventory) dry-out

occurs.

When the power level is increased from 5 to 10%

of nominal power, about 50 min after the natural cir-

culation transient initiation, two phase flow occurs but

only for 12 min (or 38% of total mass inventory) and

after that the dry-out would be initiated (Fig. 14).

Definitely the increase of power causes:

Larger value for single and stable two-phase natural

circulation, Early appearance of instabilities,

-400

100

600

1100

1600

2100

2600

3100

0 2000 4000 6000 8000 10000 12000 14000 16000

Time (s)

Coreinletm

assflow

rate(kg/s)

(1) power: 300 MW

(2) power: 150 MW(3) power: 75 MW

Single-phase

Two-phase & dry-out (1)

Two-phase (2)Dry-out (2)Two-phase (3)

Two-phase unstable (3)

Two-phase stable (3)

Dry-out (3)

(1), (2) and (3)

Fig. 13. Core inlet mass flow rates at three different power levels.

0

0.2

0.4

0.6

0.8

1

-500 1000 2500 4000 5500 7000 8500 10000 11500

Time (s)

Voidfraction

power 2.5%

power 5%

power 10%

Fig. 14. Void fraction through reactor core at three different powers.

Anticipation of main events with main reference to

the time when dry-out is calculated.

4. Natural circulation performance using

NCFM

One of the methods for evaluating the natural circu-

lation phenomena is the natural circulation flow map

(NCFM), as already mentioned. Core inlet mass flow

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-5

0

5

10

15

20

25

30

35

40

200 300 400 500 600 700

Residual mass per volume (kg/m^3)

Massflowrateperpower(Kg/s-M

Wth)

NominalLow-levelHigh-level

Fig. 15. NCFM at different heights (buoyancy effect) of a

VVER-1000.

rate, reactor core power, total mass inventory and vol-

ume of primary side are considered within the NCFM

approach to evaluate the natural circulation perfor-

mance of systems (e.g., including code nodalisation

representative of natural circulation systems) not used

to construct the NCFM itself. A NCFM has been ob-

tained by the envelope of experimental data (lower

and upper limits) from several commercial NPPs andintegral test facilities such as Bethsy, Lobi, Semis-

cale, Lstf, PKL and SPES (DAuria and Frogheri,

2002).

The natural circulation performance of VVER-1000

at three different heights can be derived from Fig. 15

where the NCFM is not reported. It clearly ap-

pears that the natural circulation performance of

the high-level design is better than the two other

designs.

The NCFM is used in Fig. 16 for comparing the

natural circulation performance of VVER-1000 withthe performance of Western PWRs integral test facili-

ties.

The maximum values of mass flow rate (per unit

volume of primary system) for VVER-1000 and PWR

NPPs are similar. However, in VVER-1000, related to

Western PWRs, the mass flow rates through loop re-

main stable and above the zero value (characteristic of

the reflux condensation in the case of PWR) at low val-

ues of primary system inventory. Thus, VVER-1000

shows a better NCFM than PWR.

-5

0

5

10

15

20

25

30

35

40

200 300 400 500 600 700 800 900 1000

Residual mass per volume (kg/m^3)

Massflowrateperpower(kg/s-MWth)

lower-limit (PWR)

upper-limit (PWR)

nominal (VVER-1000)

high-level (VVER-1000)

Fig. 16. Comparison of NCFM for VVER-1000 in nominal and

high level conditions with lower and upper limits in Western PWR.

5. Natural circulation relevant to SBLOCA

A series of SBLOCA analyses with break area rang-

ing from 0.5% (5 mm diameter) to 11.7% (100 mm

diameter) of the VVER-1000 cold leg diameter are

performed starting from NPP nominal conditions. The

initial and boundary conditions are summarized in

Table 7. The break is placed in the cold leg of theloop with pressurizer between the pump outlet and the

down-comer.

Generally during SBLOCA in Western PWRs there

are five different phases (Prosek and Mavko, 1999) in-

cluding sub-cooled blow-down, two-phase natural cir-

culation, reflux condensation, loop seal clearing, core

re-flooding, and long term cool-down.

In the SBLOCA experiments, in PACTEL facility

(medium scale facility of VVER-440), flow stagna-

tion and system re-pressurization were observed when

the water level in the upper plenum fell below the en-trances to the hot legs at a time when coolant flow into

the hot legs changed from single to two-phase flow

(Lomperski and Kouhia, 1994). The core inlet mass

flow rate for different break sizes is depicted in Fig. 17.

In the SBLOCA with 11.7 and 5.8% break sizes,

primary side pressure decreases and reaches secondary

side pressure at 75 and 425 s, respectively (Fig. 18).

However, in different SBLOCA scenarios, primary

side pressure was predicted to remain higher than sec-

ondary pressure.

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Table 7

Initial and boundary conditions in NC relevant to SBLOCA

Initial conditions Boundary conditions

Parameters Unit Values Parameters/systems Values/conditions

Reactor power MW 3100 Time of trip and reactor scram t= 0 s

Primary/secondary sides pressures MPa 15.7/6.2

Mass inventory of RPV/primary-side kg 81.7 103/232.6 103 ECCS (LPIS, HPIS, ACCU) Activated

Fluid temperature at RPV inlet/outlet C 287.0/322.7 Auxiliary/emergency FW Not activated

Fluid temperature at SG inlet/outlet C 322.7/287.0 Main FWs Activated

PRZ temperature C 345.8 MSIV valves Closed (after transient)

Fuel/cladding maximum temperatures C 2200/351.5 PORV valves Not activated

Collapsed level of PRZ m 8.5

Collapsed level of core m 3.25 Break valve opening time t= 0 s

Collapsed level of SG m 1.9

FWs temperatures C 220.3 Break area (%) 0.5, 1.1, 3, 5.8 and 11.7

0 .2 .4 .6 .8 1 1.2 1.4 1.6

X 104Time (s)

-200

0

200

400

600

800

1000

1200

1400

1600

Core

inletmassflow

rate(kg/s)

XXX 11.7%-with-ECCSX

X

X

X X X

YYY 5.8%-with-ECCS

Y

Y

Y

YY Y

ZZZ 5.8%-without-ECCS

Z

Z

Z

Z

Z Z

0

200

400

600

800

1000

1200

1400

1600

1800

0.0E+00 2.0E+03 4.0E+03 6.0E+03 8.0E+03 1.0E+04 1.2E+04 1.4E+04 1.6E+04

Time (s)

Coreinletmassflowrate(kg/s)

0.5%-with-ECCS

1.1%-with-ECCS

1.1%-without-ECCS

3.0%-with-ECCS

3.0%-without-ECCS

(a)

(b)

Fig. 17. Core inlet mass flow rate in SBLOCA at different breaks

and conditions.

SBLOCA experiments by using ECCS on the

ISB-VVER integral test facility showed that the core

collapsed level and the primary pressure decrease

slower than in the case without using emergency

core cooling system. Also, the maximum cladding

temperatures do not undergo to dangerous values

(Krepper, 1997). In the SBLOCA (5.8% break) with-

out ECCS (including HPIS, LPIS and ACCU), at

less than 26% of total mass inventory, dry-out oc-

curs. Fig. 19 shows the gas void fraction through

reactor core when different breaks are assumed tooccur.

In the study of SBLOCA (break area less than 3%

of the pipeline area) with actuation of ECCS, void

fraction through core is zero. But in the case of without

ECCS at 3% and also1.1% void appear (Fig. 19), as

expected.

The comparison between SBLOCA (5.8% break

area with and without ECCS) and Natural circula-

tion scenarios (with draining method) is shown in

Fig. 20 in the time domain even though the repre-

sentation in the phase-space (i.e., with coolant inven-tory on the horizontal axis and pressure in the vertical

axis) could be more representative. The main results

of comparison showed that, during the SBLOCA with

ECCS:

Instability of two-phase flow is not observed; The maximum value of mass flow rate is decreased

for about 30%.

Both the two differences between natural circulation

and SBLOCA scenarios are connected with the exis-

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Fig. 18. Pressure distribution of primary and secondary side in different SBLOCAs.

tence of the driving-force represented by the pres-

sure difference across the break that is not present in

the case of the natural circulation.The SBLOCA study confirmed that some safety

margins, e.g., time of dry-out occurrence, Figs. 18c

and 19, increases more than eight times (see Fig. 19)

in the cases when ECCS are used, related to cases

when ECCS are not part of the system design.

Definitely, the performed SBLOCA analyses

showed that natural circulation is hugely affected

by the selected sequence of events where break area

and actuation modes of emergency system play the

greatest role.

5.1. Reflux condensation cooling in VVER-1000

As the mass inventory of the primary side is de-creased, the natural circulation is terminated and the

steam begins to be condensed in steam generators

tubes (starting point is the hot packages of upper pipes)

and propagates through hot legs, see Fig. 21, and the

reactor pressure vessel (Mousavian et al., 2003a). The

propagation of the discontinuity in the liquid velocity

(Fig. 21) is actually representative of the propagation

of void fraction into the cold leg.

Table 8 reports some information related to the start-

ing of flow reversal through hot leg at different posi-

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Fig. 19. Gas void fraction through core in different SBLOCAs.

tions along the hot leg itself (these are characterized

by a node number pertaining to the nodalisation) based

on mass inventory. When the collapsed liquid level in

the reactor pressure vessel goes below the level of the

Table 8

Starting point of reflux condensation between RPV outlet and ascending side of SG

Residual mass (%) Component (relevant to Fig. 1) Nominal case High-level case Low-level case

38.0 201-01 Reversed flow






Fig. 20. Comparison between SBLOCA (5.8%) and NC-with-

draining in a VVER-1000.

hot leg (entrance) there is the potential for flow rever-

sal (Figs. 21 and 22). Looking at Fig. 22, this occurs

at a time bounded by 3000 s and 12,000 s in the con-

sidered SBLOCA scenario.

When the reactor cools down by reflux condensation

the counter current flow limitation (CCFL) or flood-

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-3

-2

-1

0

1

2

3

4

5

6

-500 1500 3500 5500 7500 9500 11500 13500 15500

Time (s)

Liquidvelocitythroughhotlegfrom

RPV

toSG

(m/s)

comp. 200

comp. 205

comp. 210

Fig. 21. Liquid velocity through hot leg from RPV to SG.

ing could occur. The supply of cooling liquid (i.e.,

from the ascending side of steam generators, namely

of PWR type NPP) into the reactor core is limited by

the occurrence of CCFL at the steam generators inlet

plenum (namely, in PWR types NPP) or in the connec-

tion between hot legs and reactor pressure vessel (this

could be of some relevance in both PWR and VVER).

The potential effect of reactor power upon reflux

condensation can be envisaged from observing thetime trends in Fig. 23. By increasing the reactor

power from 75 to 150 MWth the time when negative

liquid flow-rate is calculated for the horizontal tubes

of steam generators occurs earlier with value of mass

7.0

8.0

9.0

10.0

11.0

12.0

13.0

0.0E+00 3.0E+03 6.0E+03 9.0E+03 1.2E+04 1.5E+04

Time (sec.)

Collapsedlevelofliquid

inreactorpressure

vessel

(m)

Fig. 22. Reactor pressure vessel collapsed level at nominal

conditions.

0 .2 .4 .6 .8 1 1.2 1.4 1.6

X 104

Time (s)

-.40

-.20

0

.20

.40

.60

.80

LiquidvelocitythroughhotpackageofS

G1(m/s)

XXX power:75MW

X X X

X

XX

X

XX

XX

X

X X X X X X X

YYY power:150MW

Y Y Y YY

Y

Y

YY Y

Y

Y

Y

Y

Y Y Y Y

ZZZ power:300MW

Z

ZZZZZZZZZZZZ

Z

Z

Z

ZZ

Z

Fig. 23. Liquid velocity through the pipes of steam generator at

different power levels.

inventory that changes from 30 to 40% of the initial

value. It should be noted that negative value for liquid

velocity does not imply directly the occurrence of

CCFL but gives an idea for the potential occurrence of

such phenomenon. The search for CCFL conditions

should be based upon specific transient calculation

of licensing or design interest that was not within thepurpose of the present activity.

As mentioned above, the reflux condensation phe-

nomenon is one of the effective heat removal mech-

anisms during SBLOCA (Kawanishi et al., 1991)

in PWR. The break size effect on reflux condensa-

tion is important, as confirmed from the present ac-

tivity.

From the analyses of VVER-1000 SBLOCA tran-

sients originated by break areas between 11.6 and

5.8%, the reflux condensation phenomenon is not ob-

served. The ultimate reason is that in those conditionsthe heat exchange between primary and secondary

sides of SGs is lower than the thermal power removal

via the break. However, in the calculation of SBLOCA

originated by a 3% area, condensation in the upper

rows of horizontal tubes is calculated.

From the calculation of the SBLOCA scenario

originated by the 1.1% break area, flow reversal and,

therefore the potential for reflux condensation, ap-

pears evident at the time when 60% of mass inventory

is observed. Fig. 24 shows the liquid and gas velocity

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0 .5 1 1.5 2 2.5 3 3.5

X 104Time (s)

-.20

0

.20

.40

.60

.80

1.00

1.20

1.40

Liquidandgasvelocity(m/s)

XXX Liquid

X

XX X

X

XX X X X X X X

YYY Gas

Y

YY Y

Y

Y

Y

Y

Y

Y

Y

YY

effect of reflux condensation

Fig. 24. Liquid and gas velocity at pipes of steam generator in a

VVER-1000.

in the inlet side of the upper row of horizontal tubes.

Negative values for liquid velocity are indication for

the potential occurrence of the reflux condensation

phenomenon.

6. Conclusions

A wide range investigation of natural circulation

in VVER-1000 has been performed within a devoted

research activity documented in the present paper. This

included:

The use of experimental data (e.g., derived from

PSB and ISB facilities);

The use of the RELAP5 system code;

The use of a qualified VVER-1000 nodalisation and

of procedures to attain the qualification;

The use of the NPP plant data (in the process ofnodalisation qualification);

The use of the (fast Fourier transform based method)

FFTBM to quantify the accuracy or the error that

characterizes the comparison between predicted and

measured sets of data;

The use of the NCFM to evaluate the natural circu-

lation performance of the VVER-1000 NPP.

Selected thermalhydraulic phenomena expected in

case of natural circulation in VVER-1000 geometry

are studied by considering both fluid draining from

the cold leg and SBLOCA scenarios.

The consideration of buoyancy effect showed that

by increasing (or decreasing) the average distancebetween the heat sink and the heat source, consti-

tuted by the steam generator tubes and the core in

the VVER-1000 loop respectively, the mass flow rate

through the loop is increased (or decreased). It has

also been calculated that the stability performance

of the entire systems is also substantially affected

by this geometric parameter. The core power effect

upon an established natural circulation scenario has

also been characterized and compared with the ef-

fect derived in the case of Western PWRs, with main

reference to the single-phase regions (DAuria et al.,

1991).The influence of the presence of the loop seal, i.e.,

the U-shaped pipe between the steam generator outlet

and the pump suction in the primary loop has been in-

vestigated. Its noticeable influence upon both the natu-

ral circulation flow-rate and the stability performance

of the system has been quantified.

The application of the NCFM showed that the nat-

ural circulation performance of the VVER-1000 nu-

clear power plants is equivalent or better than the nat-

ural circulation performance of Western PWRs, espe-

cially for small values of the primary system massinventory.

Natural circulation in VVER-1000 systems has also

been analysed during SBLOCA transients originated

by different break area. The effects of ECCS including

HPIS, LPIS and accumulators on the SBLOCA have

been evaluated and their relevance has been character-

ized in quantitative terms. In case of SBLOCA, it was

found that the overall system is less prone to insta-

bility than in the case when the coolant draining pro-

cedure is applied. This derives from the presence of

the driving force in case of SBLOCA scenarios origi-nated by the pressure difference across the break. The

presence of emergency systems also makes the sys-

tem more stable with respect to the case when those

systems are not present.

The potential for the occurrence of reflux conden-

sation in VVER-1000 conditions has been studied. At

a preliminary level it was found that conditions for

reflux condensations are not established when break

area has a value larger than 3% of the area of the cold

leg.

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Acknowledgements

The authors gratefully thank to Dr. G.M. Galassi

(University of Pisa, Italy) and Dr. N. Fil (Gidropress,Russia) for the review of the paper and the fruitful

discussions had when performing the present activity.

One of the authors (S.K. Mousavian) appreciates the

Ministry of Sciences, Researches and Technology of

Iran and to the AEOI for supporting of this work.

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