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P. K. VijayanReactor Engineering Division,

Bhabha Atomic Research Centre, Mumbai, India

INPRO-CP on Advanced Water Cooled Reactors, Vienna, Austria, 29th October, 2009

Thermal Stratification Studies in Pools with Immersed Heat Exchangers

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CONTENTS• Relevance of thermal stratification to AWCRs• Thermal stratification issues• Phenomenological investigations• Thermal stratification in Isolation condenser system • Isolation condenser system simulation• Description of the ICS Integral Test Loop• Experimental investigations• Simulation of ICS with system codes• Discussion of the results• Further planned work• Summary

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Introduction & Relevance

• Immersed heat exchangers in large pools are used in several advanced reactor systems for decay heat removal

• They are used either for direct core cooling or for cooling of steam generators

• In AHWR the isolation condenser system is used for decay heat removal in the event of station blackout.

• In PHWR, the PDHRS employs a pool with immersed heat HXs on the secondary side of the SG

• Thermal stratification where the low density hot fluid stays above the high density cold fluid is possible in such systems

outletPrimary

inlet

dow

ncom

er

Steam

Feed water

Level

PHWR-700 PDHRS schematic

Isolation Condenser in BWR

IC

Condensate

Steam

Cooling tankReactorVessel

BARCBARCBARCBARC Passive Safety Feature

Passive Core Decay Heat Removal by ICs immersed in Gravity Driven Water Pool

Isolation Condenser System in AHWR

IC

ICS

Isolation Condensers (ICs) in AHWR• ICs are Vertical Heat Exchangers submerged

in Gravity Driven Water Pool (GDWP)- Steam condenses in vertical pipes of ICs

rejecting heat to GDWP.- Condensate returns by gravity to steam

drum.- Designed to remove 6% FP

Layout of ICs

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Thermal Stratification Issues

• Thermal stratification reduces the driving force for heat transfer affecting the heat exchanger performance.

• It hampers full utilization of the loop inventory• In case of water pool in concrete tanks, there is

a possibility that concrete may exceed limiting temperature

• Boiling is not desirable for pools within the containment

BARCBARCBARCBARC Planned Studies on Thermal Stratification

• Phenomenological investigations in simple systems to assess the performance of CFD codes

• Prediction of thermal stratification phenomenon in IC pool with CFD code

• Integral tests in ITL

• Integral simulation of MHTS & ICS along with IC pool

BARCBARCBARCBARC Thermal Stratification in a Water Tank with a Single Immersed Strip Heater

• Experimental investigation – Flow visualization with aluminium particles, PIV for velocity distribution & thermocouples at different elevations for stratification•The rising boundary layer gets reflected downward at the free surface, rises again due to the buoyancy and then flows along the free surface till it hits the boundary wall.•Secondary loops observed adjacent to boundary layer • Vortex shedding observed at the free surface at high heat flux• Stable stratification observed with low density water remaining at top. Measured temperatures

Experimental set up

(a) Low power (b) High power (c) Heater lower endFlow visualization by Aluminum particles

Velocity distribution by using PIV system

0 2 4 6 8 10

30

32

34

36

Tem

pera

ture

, o C

Thermocouple Locations

Experiment

BARCBARCBARCBARC CFD Simulation of Thermal Stratification in a Water Tank with a Single Immersed Heater

• The problem has been analysed using codes TRIO_U and PHOENICS • Stable stratification predicted with high temperature water remaining at the top

0 1 2 3 4 5 6 7 8 9 1029

30

31

32

33

34

35

36

37

Tem

pera

ture

, o C

Thermocouple Locations

Experiment PHOENICS TRIO_U

Predicted temperature distribution at 1800 s using TRIO_U

Measured and predicted temperatures

TRIO_U Model

Heated Surface

q”=3125w/m2

Free surface  

q”=h(Tw

‐Ta

)

Adiabatic 

Surface 

Adiabatic 

Surface 

Adiabatic Surface 

TRIO_U nodalisation (96000 elements)

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Simulation of IC Pool of AHWR• CFD simulation with PHOENICS code• One eighth symmetry sector with two ICs simulated• Time varying heat flux simulating decay power

specified for the tube outer surface• Cartesian grids were used• Free surface heat transfer accounted with adiabatic

side boundaries• Pool level is constant• Single-phase pool without boiling

Temperature Contour after 6 hours of IC operation

Temperature Contour at topmost layer after 3 days of IC operation

Temperature vs. height after 6 hours

of IC operation Temperature vs

height after 3 days of IC operation

Thermal stratification is significant in the short term but negligible after 3 days of operation

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Isolation Condenser Performance Testing in ITLITL simulates the ICS, along with other systems such as MHTS, ECCS and FWS

IC

SD

GD

WP

FCS

BFST

QOV

HEADER

PBC

SFP

JC SB

AA

3-D Isometric of ITL

FWC

V

SDPCVICQOV4

Designed based on Power to volume scaling.

Main objective is generation of database for ICS performance and IC pool stratification

Apart from thermal stratification studies measurement of heat transfer coefficient was an additional objective

Photograph of IC pool

BARCBARCBARCBARC Instrumentation for thermal stratification in pool

800

mm

300 300

300

Top Inlet Header of IC

Slots for immersing strip

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• Passive decay heat removal by IC following station blackout has been simulated.

• Degradation in IC performance has been investigated, which can be due to- Reduction in pool level (due to boil-off on IC-pool side) resulting in exposure of IC tubes- Reduction in MHT pressure resulting in reduction of driving temperature difference for heat transfer from IC primary to secondary side.

• Experiments conducted at different powers (i.e.. 3%, 4%, 5% and 6% FP). The power is kept constant throughout the duration of the experiment.

• For each power two initial water levels were studied in the IC-pool e.g. 0.83 m and 1.75m.

Scenario considered for experiments in ITL

- Steady state achieved at a given power.- SFP trips and SDPCV & FWCV close. - When SD pressure reaches 80 bar IC active valve

opens fully- Test repeated for different initial IC pool level

Experiments Carried out in ITL

0 20 40 60 80 100

0

2

4

6

8

10

12

14

16

40

5060708090

Matrix2

Safe region

Failure region

% o

f Non

-con

dens

able

s(IC

circ

uit)

GD

WP

Wat

er

Tem

pera

ture

(o C)

% Height Exposure of IC Tubes

-1.000

1.125

3.250

5.375

7.500

9.625

11.75

13.88

16.00

• Reliability of ICS performance has been estimated using the system code RELAP5/MOD3.2.• Degrading factors considered: • Non-condensable gases in IC tubes• Increased Temperature in the IC pool• Level decrease in IC pool

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Variation of IC-pool thermocouples temp. at 75 kW and 1.75 m initial pool level

0 20000 40000 6000020

40

60

80

100

120

140

160

Pool Level - 1.75 mLevel of IC Top header - 1.01 mLevel of IC Bottom header - 0.3 m

IC P

ool T

empe

ratu

re -

o C

Time - s

TIS1541A (thermocouple level - 1.6 m) TIS1541C (thermocouple level - 1.2 m) TIS1541G (thermocouple level - 0.6 m)

0 5000 10000 15000 20000 2500020

40

60

80

100

120

140

160

Pool Level - 1.75 mLevel of IC Top header - 1.01 mLevel of IC Bottom header - 0.3 m

TIS1541H(B2) thermocouple level - 0.6 m

IC P

ool T

empe

ratu

re -

o C

Time - s

Variation of IC-pool thermocouples temp. at 75 kW and 0.83 m initial pool level

0 20000 40000 600000

20

40

60

80

100

Lev

el -

m

SD Pressure

Pres

sure

- ba

r

Time - s

0.0

0.5

1.0

1.5

2.0IC Active valve opens

IC Pool level

Variation of SD pressure and IC-pool level at 75 kW and 1.75 m initial pool level

LevelPressure

0 5000 10000 15000 20000 250000

20

40

60

80

100

120

140

Leve

l - m

SD Pressure

Pres

sure

- ba

r

Time - s

0.00

0.25

0.50

0.75

1.00

IC Pool level

Variation of SD pressure and IC-pool level at 75 kW and 0.83 m initial pool level

LevelPressure

Experimental Results at 3% FP• The water level falls due to boil-off on IC- pool side.

• The thermocouples near the top of the IC- pool (e.g. at 1.6 m and 1.2 m height) show a temperature rise as soon as the IC active valve opens. The temperatures drop suddenly because thermocouples near the top gets exposed to atmosphere due to fall in IC-pool level

•The thermocouples near the bottom of the IC-pool (e.g. at 0.6 m height) show a delayed rise in temperature, indicating thermal stratification

No degradation

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Stratification in IC pool

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80

20

40

60

80

100

120

Power - 75 kWInitial IC pool level - 1.75 m

Tem

pera

ture

in IC

poo

l - o C

Elevation from IC pool bottom - m

6000 s 8000 s 10000 s 10500 s 11000 s 11500 s

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Variation of SD pressure and IC-pool level at 128 kW and 1.75 m initial pool level

0 10000 20000 30000 40000 500000

20

40

60

80

100

120

140

Lev

el -

m

SD Pressure

Pres

sure

- ba

r

Time - s

0.0

0.5

1.0

1.5

2.0

IC Pool level

LevelPressure

0 10000 20000 30000 400000

20

40

60

80

100

120

Lev

el -

m

SD Pressure

Pres

sure

- ba

r

Time - s

0.0

0.5

1.0

1.5

2.0

IC Pool level

Variation of SD pressure and IC-pool level at 154 kW and 1.75 m initial pool level

Level

Pressure

Variation of SD pressure and IC-pool level at 128 kW and 0.83 m initial pool level

0 5000 10000 15000 200000

20

40

60

80

100

120

140

Leve

l - m

SD Pressure

Pres

sure

- ba

r

Time - s

0.00

0.25

0.50

0.75

1.00

IC Pool level

Level

Pressure

0 5000 10000 150000

20

40

60

80

100

120

Lev

el -

m

SD PressurePr

essu

re -

bar

Time - s

0.00

0.25

0.50

0.75

1.00

IC Pool level

Variation of SD pressure and IC-pool level at 154 kW and 0.83 m initial pool level

Level

Pressure

Experimental Results at other Powers

• The rise of pressure towards the end of experiment is more visible at higher powers, indicating degradation in IC performance.

• With increase of power the minimum pressure observed during an experiment increases but the pool level at which degradation is observed does not change much for the same initial IC-pool level.

Pressure increasing indicating degradation

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Pool Level at which IC performance degrades

6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 00 .0 0

0 .2 5

0 .5 0

0 .7 5

1 .0 0

1 .2 5

IC D

egra

datio

n le

vel -

m

P o w e r - k W

In it ia l IC -p o o l le v e l o f 0 .8 3 m In it ia l IC -p o o l le v e l o f 1 .7 5 m

• In all the experiments the degradation of IC performance is indicated by increase in MHT pressure towards the end of an experiment.

• Degradation level is the level corresponding to the minimum pressure observed during a particular experiment.

Degradation of IC Performance

Minimum MHTS pressure observed during experimentation.

6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 00

1 0

2 0

3 0

4 0

5 0

Min

MH

TS P

ress

ure

- bar

P o w e r - k W

In it ia l IC -p o o l le v e l o f 0 .8 3 m In it ia l IC -p o o l le v e l o f 1 .7 5 m

BARCBARCBARCBARC Simulation of Thermal Stratification in IC Pool

Challenges in simulation of Thermal stratification in IC poolThe boundary conditions within the MHT system is well known. However, the boundary condition on the outside of the immersed IC tubes is required for the simulation of thermal stratification in the pool using CFD codes.

The heat transfer boundary condition on the outside of the tubes is varying with time and is not known apriori

Boiling occurs on the outside surface of the IC tubes. Away from the tubes single-phase condition prevails

Condensation occurs above the top of the IC tube.

Mixing of two-phase fluid with single-phase pool water above the IC tubes

Simulation of free surface and heat transfer from free surface are required

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Our Approach Simulation of integral performance requires coupling of a CFD code with system code. Since we do not have such a code, the simulation of the test is planned in two stages

- Using system code (RELAP5/MOD3.2) obtain the temperature distribution along the outside surface of the IC tubes immersed in the pool as a function of time. For this calculation, the initial pool temperature needs to be specified.

- Using this time varying temperature distribution as the boundary condition, compute the thermal stratification phenomenon in the IC using a CFD code

Simulation of Thermal Stratification in IC Pool

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2 204

702

701

6 05

1 7

60 4 6 03

8 02

1 8

707

706

705

704

703

801

80 3

36 05 2 403

27 02

2 601

27 01

36 01

36 0236 0336 04

44

2703

2 4052 404

4 3

4 2

2501

3 501

5 0

36 06

36 07

36 08

36 09

3701

3 9

22 05

3 502

2 402

2 401

41

40

2301

S IN G LE JU N C TIO N (59

6 01

6 02

V A LV E JU N C T IO N (2 )

T IM E D E P E N D E N T JU N C T IO N (1)

T IM E D E P E N D E N T V O L U M E (2)

H E A T S TR U C T U R E (12

8 05

4 05

16

1 5

50 1

80 44 04

4 03

22 0433

8 07

8 064 02

4 01

10 1

19

8 08 1 4

2 01

1 013 01

S IN G LE -V O LU M E (63)

M U L TI-V O L U M E P IP E

21 1101

2 201

3 8

9 01

2202

2203

20

1001

Nodalization of MHTS and ICS

ICS Performance Simulation• Computations were carried out with RELAP5 code using the nodalisation

• Apart from MHTS, the IC tubes and the pool were simulated.

• Simulation calculations were also carried out for AHWR.

Measured and predicted pressure for ITL & AHWR with the same nodalisation

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Pressure Level

Simulation of IC performane at 75 kW and 1.75 m initial pool level

0 10000 20000 30000 40000 50000 600000

10

20

30

40

50

60

70

80

90

IC P

ool L

evel

- m

Pre

ssur

e - b

ar

Time - s

SD Pressure (Expt.) SD Pressure (RELAP5)

0.0

0.5

1.0

1.5

2.0

IC Pool Level (Expt.) IC Pool Level (RELAP5)

0 10000 20000 30000 40000 50000 60000

20

40

60

80

100

120

thermocouple at level - 1.4 m (Expt.) thermocouple at level - 0.6 m (Expt.) temperature of pool at level - 1.4 m (RELAP5) temperature of pool at level - 0.6 m (RELAP5)

IC P

ool T

empe

ratu

re -

o C

Time - s

1.2

1.2

Simulation of IC pool temperatures at 75 kW and 1.75 m initial pool level

0 5000 10000 15000 200000

50

100

150

Pool Level - 0.83 mLevel of IC Top header - 1.05 mLevel of IC Bottom header - 0.3 m

IC

Poo

l Tem

pera

ture

- o C

Time - s

thermocouple temperature at level - 0.6 m (Expt.) IC Pool temperature at level - 0.6 m (RELAP5)

Simulation of IC pool temperature at 75 kW and 0.83 m initial pool level

0 5000 10000 15000 20000

20

40

60

80

100

IC P

ool L

evel

- m

SD Pressure (Expt.) SD Pressure (RELAP5)

Pres

sure

- ba

r

Time - s

0.0

0.2

0.4

0.6

0.8

1.0

IC Pool level(Expt.) IC Pool level(RELAP5)

Simulation of IC performance at 75 kW and 0.83 m initial pool level

Level

Pressure

RELAP5 Prediction vs. Data• RELAP5 simulates the depressurization rate and IC-pool level very closely.

• The thermocouple temperatures near the top of the IC-pool (e.g. at 1.6 m and 1.2 m height) is very well simulated by RELAP5.

•But, the thermocouple temperatures near the bottom of the IC-pool (e.g. at 0.6 m height) is not well simulated by RELAP5,due to one dimensional nature of the code.

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Summary• Scaled Isolation Condenser performance tested up to 6% of FP.• Although thermal stratification is observed, it does not significantly

degrade the heat transfer to pool if IC tubes are submerged.• Significant boiling and level decrease is observed.• IC performance degrades when IC tubes are nearly 45% exposed for

initial IC pool level of 1.75m.• Maximum pool temperature observed ~ 110oC. The pool surface

temperature is slightly above 100oC. Concrete temperature limit of 65oC could be an issue.

• RELAP5 code adequately simulates the MHTS performance.• Due to 1-D nature of the code, the thermal stratification is not

adequately simulated.

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Further Planned Work

• Simulation of thermal stratification phenomenon with CFD code

• Refined calculation with RELAP5 code

• Test in ITL with decay power simulation

• Simulation of this test with RELAP5 code as well as CFD code

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Thank you