NEW OPERATIONAL EXPERIENCES WITH NPP TEMELIN FEED WATER

DISTRIBUTION SYSTEM

L. Junek, R. Krautschneider Institute of Applied Mechanics Brno Ltd., Brno, Czech Republic

The reconstructed unique feed water distribution system is installed on NPP Temelin. Feed water is distributed vertically inside the tube bundle. This solution has of course a risk that “cold” feed water can in some occasions flow to the bottom part of steam generator and cause thermal influence to the vessel. The hypothesis of bottom part “undercooling” has been confirmed by newly installed measurements. New analytical and numerical analyses were done to check whether the feed water distribution system is according to the project. Combined with checking the feed water distribution system also the steam production analyses were performed. The original design and reconstructed feed water system on NPP Temelin is shown in Fig. 1. In reconstructed design the feed water is brought partly inside the tube bundle (2/3 of feed water amount) and partly above the perforated sheet (1/3 of feed water amount).

Fig. 1. Feed water system on NPP Temelin, original project (left) and reconstructed (right)

Newly installed temperature measurements were placed inside SG (16 thermocouples measuring the media temperature) and also on the outer surface at the bottom part of SG (16 thermocouples measuring the temperature on the outer surface of the SG’s vessel).

Measurements inside SG

To be able to measure the temperature on the secondary side of SG along the high direction, there were placed 16 thermocouples inside. Thermocouples were put on 4 rods on different highs, so on each rod there were 4 thermocouples. See Fig. 2. Two rods were placed on each side of SG’s hot leg, and one rod was placed on the central symmetry axis behind hot leg and behind cold leg. Rods

holding thermocouples were mounted to perforated sheet above the tube bundle. On each rod one of the thermocouples was placed as close to the bottom surface of SG as possible.

Fig. 2. Thermocouples placement inside the SG

Measurements on the outside

On the SG’s outer surface thermocouples were also mounted. The placement of 16 thermocouples is shown in Fig. 3a-b. Eleven thermocouples were placed under the various segments of SG, two thermocouples (No. 3, 14) were placed close to draining sockets, and other three thermocouples (No. 7, 8, 9) were placed around the surface in radial direction.

In Fig. 3a is also shown the position of the thermocouple YB10-40T004 which was the only thermocouple placed at the bottom of SG until the placement of new thermocouples all along the SG’s bottom part. Thermal changes on thermocouple YB10-40T004 were the first evidence of “undercooling” of SG’s bottom part.

Fig. 3a. Thermocouples placement on SG’s outer surface

Fig. 3b. Thermocouples placement on SG’s outer surface

Evaluation of newly installed measurements [3] Measurements from newly installed thermocouples inside SG and also on its outer surface, were evaluated in detail to know in what power levels and under which conditions the effect of “undercooling” is occurring. The temperature history in selected time interval is shown in following figures (Fig. 4 and 5), where can be seen how is the effect of “undercooling” occurring when the power level is changing. The temperature fluctuations caught by the thermocouple inside, very near to SG’s bottom surface can be seen in Fig. 4.

3

1 2 4

5

610111213

14

15 16

9

6 7 8

Fig. 4. Temperature measurements (in time) inside SG – behind Cold Leg

Fig. 5. Temperature measurements (in time) on the outer surface of SG’s vessel

The temperature on the SG’s bottom part (outer surface) along the vessel for different power levels is shown in Fig. 6. Again the 30% power level is showing the highest “undercooling”.

30% power level

0% power level

0% power level

30% power level

Tem

pera

ture

[C]

time

time

Tem

pera

ture

[C]

Temperature fluctuations

Fig. 6. Temperature on the SG’s bottom part (outer surface) along the vessel for different power

levels According to evaluation of previous measurements and especially based on evaluation of newly installed measurements, it’s obvious that on certain lower power levels (especially 30%) the effect of “undercooling” of the SG’s bottom part occurs. Numerical analysis of feed water distribution system [1] To verify whether the reconstructed feed water distribution system is according to project, numerical analyses were performed. All CFD (Computational Fluid Dynamics) analyses were done in Ansys CFX™ system. The feed water distribution system on NPP Temelin is shown in Fig. 7. There are 16 distribution pipes( ∅89x4.5mm), 6 pipes are placed on one side from SG’s hot leg, and 10 pipes are placed on other side. There are various number of holes in these distribution pipes. Holes are either ∅12mm or ∅36mm(pipes 6 and 7). The detail of holes in distribution pipes is in Fig. 8. Some holes are at the position above the perforated sheet, and some holes are at the position inside the tube bundle. The placement of NPP Temelin’s feed water distribution system inside the simplified SG’s model is in Fig. 9.

Position along SG [mm]

Tem

pera

ture

[C]

CL

HL

30% power level

Fig. 7. Feed water distribution system on NPP Temelin

Fig. 8. Feed water distribution system on NPP Temelin – detail of holes inside distribution pipes

1

6

7

16

Fig. 9. Placement of feed water distribution system inside the simplified SG’s model

The division to segments, and placement of feed water distribution pipes to various segments of SG is shown in Fig. 10.

Fig. 10. Placement of feed water distribution pipes to various segments of SG.

Distribution pipes No.

SG’s segments

Fig. 11. Placement of feed water distribution system inside the simplified SG’s model – media model Flow rates for individual distribution pipes are in Table 1. Flow rate for each pipe is the sum of flow above perforated sheet and flow inside tube bundle. In Table 2 there is comparison of flow rates between project design and numerical CFD evaluation. From both tables is evident, that differences between project and CFD evaluation are not important, and so the current state is according to project.

Table 1 Flow rates for individual distribution pipes

Pipe No.

Flow rate - project

Flow rate – CFD

Absolute difference

Relative difference

[kg/s] [kg/s] [kg/s] [%] 1 20.4 20.6 -0.2 -1.12 20.4 20.4 0.0 -0.13 20.4 20.3 0.1 0.34 20.4 19.9 0.5 2.45 30.4 30.3 0.1 0.36 38 38.5 -0.5 -1.47 38 38.2 -0.2 -0.58 27 25.7 1.3 4.79 21 20.6 0.4 2.1

10 23 22.9 0.1 0.411 22 23.1 -1.1 -5.212 25 24.7 0.3 1.113 25 24.0 1.0 3.914 25 24.7 0.3 1.115 25 25.1 -0.1 -0.416 30 31.7 -1.7 -5.7

Total 411 411.0 0.0

Table 2 Feed water distribution system’s project and CFD evaluation comparison

project project CFD CFD Total flow rate 411 kg/s 100 % 411 kg/s 100 % In tube bundle 260 kg/s 63.3 % 262 kg/s 63.8 % Above perforated sheet 151 kg/s 36.7 % 149 kg/s 36.2 %

The example of calculated velocity field contours is shown in the next two figures. The section through the holes above the perforated sheet is in Fig. 12a, and the section through the holes inside the tube bundle is in Fig. 12b.

Fig. 12a. Contours of velocity-section through the holes above the perforated sheet

Fig. 12b. Contours of velocity-section through the holes inside the tube bundle

Thermal calculations [2] After confirmation that the feed water distribution system is according to reconstruction design project, some thermal calculations were done. In these calculations the tube bundle’s steam production was evaluated for different power levels. In Fig. 13 there is chart showing the steam production together with feed water amount for all SG’s segments.

0.0

30.2

25.125.125.125.1

45.2

48.3

30.6

20.520.5

0.0

29.0

44.646.8

20.2

24.7

40.838.238.2

20.520.5

15.012.613.013.414.0

35.3

40.9

20.221.320.6

0.0

10.0

20.0

30.0

40.0

50.0

60.0

12345678910111213141516

sekce

[kg/

s] GnvGp

HK

SK

HL

CL

Fig. 13. Steam amount(red) and feed water amount(blue) comparison for each segment of SG –

nominal 100% power level Thermal hydraulics analyses [4] The same time as the new measurements were installed inside SG, and also on the outer surface of SG’s vessel, some preliminary CFD thermal hydraulics numerical calculations were done. These numerical calculations were done in Ansys CFX™ system and also showed that some “undercooling” of SG’s bottom part occurs. In Fig. 14 there is the model of SG’s tube bundle divided to different parts, because the tube bundle is very inhomogeneous thermal source. The simplified thermal hydraulic numerical model is shown in Fig. 15. Contours of volume fraction and contours of temperature on the numerical CFD model for nominal 100% power level are shown in following figures (Fig. 16 to 19). Other power levels, especially 30% power level which is most important from the view of “undercooling”, are solved currently. Thermal results from CFD numerical calculations were transferred to structural model in ANSYS to perform stress calculations for later assessment from the view of fatigue and life time prediction. Contours of equivalent stress on ANSYS structural model are shown in Fig. 20.

segment

100%

Fig. 14. SG’s tube bundle model

Fig. 15. CFD numerical model

Fig. 16. Contours of Volume Fraction (Water Vapor)

Fig. 17. Contours of Volume Fraction (Water Vapor) – central section

Fig. 18. Contours of temperature (Water) – section along Hot Leg

Fig. 19. Contours of temperature (Water) – Hot Leg transversal section

Fig. 20. Contours of equivalent stress (Tresca) on structural model

Conclusion According to some signs of “undercooling” on bottom parts of steam generators on NPP Temelin caught by previously installed measurements, there were installed new thermocouples inside SG and also some new thermocouples were installed on the outer surface of SG’s vessel. In connection to this “undercooling” phenomenon the validation of the current feed water distribution system inside SG was done. The calculation results showed that the current state is according to the reconstruction project. Later some needed thermal calculations were performed for different power levels. At the same time with installing new thermocouples inside SG and also at the outer surface of the bottom part of SG, some preliminary numerical CFD calculations were performed. Results of these CFD thermal hydraulics calculations were in agreement with later evaluated temperature measurements. The most significant “undercooling” was observed when changing the power level to 30%.

References

1. Krautschneider, R.: „Feed water system reconstruction assessment on SG WWER 1000 MW, 2.1. a) Validation of feed water amount brought to SG’s hot leg, validation of feed water system design project“, IAM Brno Report 4421/08, Brno 2008

2. Krautschneider, R.: „Feed water system reconstruction assessment on SG WWER 1000 MW, 2.1.

b) Steam amount calculations for various power levels“, IAM Brno Report 4481/09, Brno 2009

3. Junek, L.: Feed water system reconstruction assessment on SG WWER 1000 MW, a) Latest temperature measurements evaluation on SG4 in year 2009“, IAM Report 4565/09, 2009

4. Krautschneider, R.: Feed water system reconstruction assessment on SG WWER 1000 MW, SG

hot leg’s thermal analyses – influence of cold media at the bottom of SG“, IAM Report 4584/09, 2009