P. K. Verma, Vikas Jain, A. K. NayakP. K. Vijayan and R. K. SinhaReactor Engineering Division

Bhabha Atomic Research Centre Trombay, Mumbai

Indiapkverma@barc.gov.in

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Many new reactor like AHWR, ESBWR incorporate large pools as passive heat sinks.

These large pools are employed for various purposes like decay heat removal and containment cooling.

Heat transfer to large pools with immersed heat exchangers pose a challenge to thermal-hydraulic design: Thermal Stratification.

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Indian innovative reactor AHWR also employs large water pool at high elevation in form of GDWP – Gravity Water Driven Pool

GDWP serves as a heat sink for Isolation condenser system, Passive containment cooling system as well as long-term emergency core cooling system.

During SBO, core decay heat is removed through Isolation condensers submerged in GDWP.

It is desired to have the decay heat removal during SBO using ICS to GDWP for a period of three days.

However, over a period of time, pool may undergo thermal stratification that degrades the further heat transfer by natural convection. As a result of it there may be local boiling and concrete wall of GDWP may be subjected to high temperature.

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CORE

COOLANT CHANNEL

STEAM TO TURBINE

STEAM DRUM

FEED WATER

DOWN COMER

RISER

HEADER

ISOLATION CONDENSERS

GRAVITY DRIVEN WATER POOL(GDWP)

TAIL PIPE

CONDENSATE RETURN LINE

PASSIVE VALVE

ACTIVE VALVE

TOP HEADER

BOTTOM HEADER

FEEDER

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Temperature contours in a thermally stratified pool

Topmost layer of the pool after 3 day simulation

*Vijayan P. K. 2010. Presentation on “Reducing Thermal Stratification and Boiling in Pools with Immersed Heat Exchangers”, INPRO-CP on Advanced Water Cooled Reactors, Vienna, Austria, 4-5

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Incorporation of shrouds in the pool around IC is conceptualized – to guide the flow and promote the mixing so that entire inventory of pool may participate in heat removal.

As the pool very large in comparison to the size of IC: use of multiple shrouds is envisaged.

First, a five shroud configuration is considered for analysis. Subsequently, the effect of increasing and decreasing the number of shrouds is considered.

A RELAP5/MOD3.2 analysis of GDWP with multiple shrouds is prepared.

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Top view of one sector with shrouds

A typical arrangement of shrouds in the pool( cross section view of one sector with shrouds)

Schematic of the pool (without shrouds)

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8

Result for without shroud configuration

1. Heat flow from the IC to pool is modeled as a uniform heat flux source at the centre of the pool.

2. Time variation of the heat flux follows the decay heat.3. Role of concrete structure of GDWP in heat transfer

and thermal inertia is not considered in view of conservatism.

4. Conduction across the shrouds is neglected.5. Inter-shroud regions are considered as pipe with same

flow area and appropriate hydraulic diameter to simulate the frictional pressure drops.

6. Geometry is too large so local effects are neglected and one dimensional simulation is carried out .

Assumptions

Five shroud arrangement

Seven shroud arrangement

Three shroud arrangement9

0 50000 100000 150000 200000 250000 300000300

310

320

330

340

350

360

370

Tem

per

ature

(K

)

Time (s)

Inlet to IC section Outlet from IC section

0 50000 100000 150000 200000 250000 300000-500

0

500

1000

1500

2000

Mas

s flow

rat

e (k

g/s

)

Time (s)

201000000 204000000 215000000

0 50000 100000 150000 200000 250000 300000300

310

320

330

340

350

360

370

Tem

per

atur (K

)

Time (s)

1st 3rd 5th

105000 110000 115000 120000 125000 130000 135000320

325

330

335

340

345

350

Tem

per

atur (K

)

Time (s)

1st 3rd 5th

Average Shroud Temperature

Result for five shroud configuration

10

0 50000 100000 150000 200000 250000 300000300

310

320

330

340

350

360

370

Tem

per

ature

(K

)

Time (s)

Inlet to IC Section Outlet from IC Section

0 50000 100000 150000 200000 250000 300000-500

0

500

1000

1500

2000

Mas

s F

low

rat

e (k

g/s

)

Time (s)

201000000 210000000 216000000

0 50000 100000 150000 200000 250000 300000300

310

320

330

340

350

360

370

Tem

per

atur

(K)

Time (s)

1st 3rd 5th 7th

10000 11000 12000 13000 14000 15000 16000 17000 18000 19000 20000300

305

310

315

320

Tem

per

atur (K

)

Time (s)

1st 3rd 5th 7th

Effect of increasing number of shroud to Seven

Average Shroud Temperature11

0 50000 100000 150000 200000 250000 300000300

310

320

330

340

350

360

370

2000 4000 6000 8000 10000300

301

302

303

304

305

306

307

308

309

310

Tem

per

ature

(K

)

Time (s)

Inlet to IC section outlet from IC section

0 50000 100000 150000 200000 250000 300000-500

0

500

1000

1500

2000

Mas

s flow

rat

e (k

g/s

)

Time (s)

201000000 203000000 215000000

0 50000 100000 150000 200000 250000 300000300

310

320

330

340

350

360

370

Tem

per

atue

(oK

)

Time (s)

1st 3rd

20000 21000 22000 23000 24000 25000310

311

312

313

314

315

Tem

per

ature

(oK

)

Time (s)

1st 3rd

Average Shroud Temperature

Effect of decreasing number of shroud to Three

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For three shroud configuration flow rate is large and consistent as compare to five and seven shrouds arrangements.

Continuous flow is obtained in three shroud configuration

Peak fluid temperature is less in three shroud configuration

• Based on this analysis, it is proposed to take up a coupled calculation of MHTS and ICS with GDWP instead of using decay heat as a boundary condition.

• Subsequently, it is planned to take up a detailed CFD calculation to capture the local flow and temperature conditions near IC. 13

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Table 1: Decay Power supplied

Time (s) P/P0

0 01 0.06880

10 0.04650100 0.028601000 0.01580

10000 0.00847100000 0.00417259200 0.00320

50000 100000 150000 200000 250000105

106

107

108

Pow

er (W

)

Time (s)

Power dessiped through single IC

Heat taken by one compartment of GDWP

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[1] Sinha, R.K., Kakodkar, A., 2006. Design and development of the AHWR—the Indian thorium fuelled innovative nuclear reactor. Nuclear Engineering and Design, Volume 236, Issues 7-8, 683-700.

[2] Vijayan P. K. 2010. Presentation on “Reducing Thermal Stratification and Boiling in Pools with Immersed Heat Exchangers”, INPRO-CP on Advanced Water Cooled Reactors, , 4-5,

[3] Kang M. G. 2002. Technical note on “Thermal mixing in a water tank during heating process”, International Journal of Heat and Mass Transfer 45, 4361–4366

[4] Zhao H. , Peterson Per F. 2010. “An Overview of Modeling Methods for Thermal Mixing and Stratification in Large Enclosures for Reactor Safety Analysis”. The 8th International Topical Meeting on Nuclear Thermal-Hydraulics, Operation and Safety (NUTHOS-8) ,

[5] Fletcher, C.D., Schultz, R.R., 1995. RELAP5/Mod3.2 Code Manual, NUREG/CR-5535, .

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