CRP on Natural Circulation Phenomena, Modelling and Reliability

of Passive Safety Systems that Utilize Natural Circulation

10-13 September, 2007, IAEA, Vienna   

Modelling of natural circulation phenomena

in VVER-440 reactors

 

 P. Matejovič, M. Bachratý

VVER-440/V213 design

• 2nd generation of soviet design PWRs of medium power

• incorporated most of design requirements of PWRs built at the same time (e.g. 3 x 100% ECCS)

• constructed in Russia (2), Ukraine (2), CEC (12+2), Finland (2)

• 2 units still under construction (Mochovce 3&4 NPP)

• rather inefficient, but robust and conservative design with large T-H safety margin

• Loviisa NPP upgraded for severe accidents (IVR, PARs)

VVER-440 geometry of primary system:

Natural circulation is influenced by:

• horizontal SG => driving head for the natural circulation is rather small

• six loops configuration

• loop seals in both, hot and cold legs

• large primary and secondary side coolant inventories

PACTEL facility, new configuration

• Volumetric scale 1:305;

• Basic elevations preserved in full scale;

• Reduced number of loops (3 instead of 6);

• Widely used for LOCAs, SG boil-off , etc.;

“Large” diameter PACTEL steam generator

VVER-440 horizontal SG

VVER-440 SG – horizontal cross-section

SG tubing

Natural circulation during boil-off transients

   Main Goals of LOF-10:

 

     to study study the SG behavior in VVER-440 reactor geometry during a loss-of-feedwater transient.

   to test the ability of thermal-hydraulic computer codes to analyse this kind of phenomena

0.

500.

1000.

1500.

2000.

2500.

3000.

0 10 20 30 40 50 60 70 80

Rows [-]

Heat transfer area [m2]

Heat transfer area versus number of rows for VVER-440 steam generators

LOF-10 boil-off experiment:

 

   the experiment started from steady-state conditions with forced circulation in single loop (1000 s)

RCP was switched off and the FW injection was interrupted

    Core heating power was 75 kW, what corresponds to 1.7% power of the reference reactor

65

55

50

5

35

30

15

25

20

3192

165

200

292

300

392

4245

1

260

7070

200

300

100

65

192

5

71

RELAP5-3D nodalization of the PACTEL used for LOF-10

129109

600

620 FW

607

180

102

100

192

160 18

2

185

108

622

142

140

110

130

601

605

606

610 630

111 - 124

183

103

190

624

621

144

146

150

162 16

3

167

166

Fig. 1.1. Pressure in pressurizer.

69.

70.

71.

72.

73.

74.

75.

76.

77.

78.

79.

0 2000 4000 6000 8000 10000 12000 14000 16000

Time [s]

Pre

ssu

re [

Ba

r]

RELAP

PACTEL

PACTEL: LOF 10 RELAP5-3D

Fig. 1.3. Mass flow rate through CL No. 3.

0.

0.5

1.

1.5

2.

2.5

3.

3.5

4.

4.5

5.

5.5

0 2000 4000 6000 8000 10000 12000 14000 16000

Time [s]

Flo

w [

kg/s

]

RELAP

PACTEL

PACTEL: LOF 10 RELAP5-3D

Fig. 1.5. Temperature in cold leg No.3.

235.

240.

245.

250.

255.

260.

265.

270.

275.

280.

0 2000 4000 6000 8000 10000 12000 14000 16000

Time [s]

Tem

pe

ratu

re [

C]

RELAP

PACTEL

PACTEL: LOF 10 RELAP5-3D

Fig. 1.8. Temperature in RV head.

240.

245.

250.

255.

260.

265.

270.

275.

280.

285.

290.

295.

0 2000 4000 6000 8000 10000 12000 14000 16000

Time [s]

Tem

pe

ratu

re [

C]

RELAP

PACTEL

PACTEL: LOF 10 RELAP5-3D

Fig. 1.9. Pressure in SG3.

37.

38.

39.

40.

41.

42.

43.

0 2000 4000 6000 8000 10000 12000 14000 16000

Time [s]

Pre

ssu

re [

Ba

r]

RELAP

PACTEL

PACTEL: LOF 10 RELAP5-3D

Fig. 1.10. Level in SG 3.

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0 2000 4000 6000 8000 10000 12000 14000 16000

Time [s]

Le

vel [

m]

RELAP

PACTEL

PACTEL: LOF 10 RELAP5-3D

hot

collector

cold

collector

Calculated natural circulation flow pattern in SG tubing at t = 2000 s

Conclusions:

    reasonable results were obtained with RELAP5-3D

necessary condition: sufficiently fine nodalisation should be used with large number of SG tube layers

    practical limitations: 6-loop arrangement with 78 layers of heat exchange tubes in 1 SG = > compromises are necessary

An example of improperly designed facility