Analysis of Operator Response to Station · PDF fileAnalysis of Operator Response to Station...

Analysis of Operator Response to Station Analysis of Operator Response to Station

BlackOutBlackOut

Alex MATEVAlex MATEV

Eskom Research & Innovation CentreEskom Research & Innovation Centre

IntroductionIntroduction

MIT Nov/Dec 004

Pretoria

Johannesburg

Bloemfontein

Cape TownEast London

Port Elizabeth

Durban

Maputo

MOZAMBIQUE

LESOTHO

NAMIBIA

TT

TTTT

TT

TTT

TH

H

H

G

GN

H

SWAZILAND

BOTSWANA

ESKOM POWER STATIONST


Coal fired87.3%

Nuclear6.8%

Imported4.1%

Pumped Storage1.4%

Hydroelectric0.4%

Koeberg Nuclear Power Station SitePBMR Module Control Buiding

Turbine Hall

Unit 1 & 2 Nuclear Reactor Containment

PBMR

ACP1ACP2

Koeberg Nuclear Power Station Site

PBMR Module Control Buiding

Turbine Hall

Unit 1 & 2 Nuclear Reactor Containment

PBMR


Station BlackOut (SBO): Simultaneous loss of all offsite and onsite AC power sources

SBO results in:• Loss of primary charging pumps providing RCP sealing water• Loss of Component Cooling Water causes loss of RCP thermal

shield cooling

Safety concern:• Failure of RCP seals due to their overheating • Safety Injection unavailability due to SBO • Unmitigated LOCA, inadequate core cooling, fuel failure

Reactor Coolant PumpsReactor Coolant Pumps’’ Seals DesignSeals Design

RCP Seals DesignRCP Seals Design

Seal #1 - Main seal • Designed for pressure drop of nominal RCS pressure• Leakage within 0.68 - 1.02 [m3/h]: most to leak-off line, rest to seal #2• Leak-off from No.1 seal is returned to charging pumps suction side.

Seal #2 – To provide backpressure on seal #1 • Designed for full RCS pressure when Seal #1 fails; limits then RCS

leakage within 1.8 to 2.7 [m3/h]• Normal leakage about 11 [dm3/h]: most to leak-off line, rest to #3 seal.

Seal #3 – To provide backpressure on seal #2• Designed to limit leakage to the containment• Leakage directed to containment sump• Not a pressure boundary seal, does not play a role in

limiting RCS leakage following SBO

RCP SealsRCP Seals’’ CoolingCooling

Normal Operating Conditions: 1) RCP seal injection flow (from RCS charging pumps)• Seal injection flow acts as a buffer to prevent reactor coolant from entering the pump seal and bearing section.• A portion of the seal injection flows down the pump shaft and into RCS, the remainder flows up through the seals system

2) RCP thermal barrier cooled by Component Cooling Water• Heat exchanger to cool the incoming reactor coolant before it enters the RCP bearing and seals• Reservoir of cool water: When sealing flow lost, it takes several minutes to leak cool water through RCP seals before hot reactor coolant fills the volume and approaches bearing and seal #1

Abnormal Operating Conditions: At least one cooling system should be restored within several minutes

RCP Seals Leakage ModelsRCP Seals Leakage Models

RCP seal leakage rate: Dependent on RCS pressure and seals’ material temperature

Timing After Loss of All RCP Seal Cooling

Greater than 120 minutes0 – 13 minutes 13 – 120 minutes

RCS Pressure less than 11.8 MPa

RCS Pressureless than 11.8 MPa

Probability Flow [m3/h]

Probability Flow[m3/h]



1.0 4.769 0.79 4.769 0.79 4.769 0.396 4.769

0.01 17.26 0.01 17.26 0.005 17.26

0.1975 41.33 0.1975 41.33 0.099 41.33

0.0025 109 0.0025 109 0.50 109

RCP Seals Leakage ModelsRCP Seals Leakage Models

Average RCP Seals Leakage Flow Rates as Function of Time

Time interval after SBO Average leakage flow rate per Reactor Coolant Pump

0 – 13 minutes 4.769 [m3/h]

13 – 120 minutes 12.375 [m3/h]

Greater than 120 minutes,with RCS pressure < 11.8 MPa

12.375 [m3/h]

Greater than 120 minutes,with RCS pressure > 11.8 MPa

60.566 [m3/h]

Note: Above RCP seals’ leakage rates correspond to primary coolant conditions at cold leg, RCP discharge side, at nominal reactor power (T=277 oC, P=16 MPa

Operator Response to Station Black OutOperator Response to Station Black Out

Plant recovery from SBO: • Only possible by restoring AC power • In the meantime: Minimize RCS inventory loss

Objective (as defined by US NRC):Ensure SBO Coping Time of 4 to 8 hours

Operator actions:1) Limit loss of RCS coolant:

Close primary PORV, letdown 2) Limit loss of SG coolant:

Isolate SG normal feed water, SG blow-down, close Main Steam Isolation Valves (MSIVs)

3) Use turbine-driven AFW pump to restore level in SG 4) Open SG relief valves to establish P=1.3 MPa

RCS RCS CooldownCooldown Benefits and ConstraintsBenefits and Constraints

Benefits: • Reduced rate of RCS coolant leakage through RCP seals postpones fuel overheating• Cooler fluid leaking through RCP seals reduces heat load on seals• Discharge of borated coolant from hydro-accumulators into RCS increases shutdown margin (SDM)

Constraints:1) Possible purging of hydro-accumulators nitrogen into RCS: To prevent it, keep RCS pressure above 0.6 MPa by closing SG steam relief valve

2) Deep cooldown can cause core recriticality: Possible when low ShutDown Margin (e.g. fail to insert rods, little Xe135)

RCS RCS CooldownCooldown Benefits and ConstraintsBenefits and Constraints

Constraints (CONT’D):3) Maintain SG level: Close SG steam relief valve if level too lowStop feedwater to SG if level too high

Simulation Results

Case #0: SBO without SG DepressurizationAssumptions:• Initial plant state: Nominal full power• Only SG#1 available for depressurization• Turbine-driven AFW pump flow only to SG#1: 80 [m3/h] when

secondary side pressure P=7.03 [MPa] or higher• Steam dump to atmosphere only via SG safety valves (POPEN=7.4

[MPa]• All MSIVs closed at time T=TSBO+10 min• Operator controls secondary coolant level in SG#1 by opening

AFW valve at SG level < -0.9 [m] and closing it at level > 0.2 [m]

Simulation Results

Cases #1 to #3: SBO With SG DepressurizationAssumptions:• Initial plant state: Nominal full power• Only SG#1 available for depressurization• Turbine-driven AFW pump flow only to SG#1:

Case Number

SG pressure7.45 MPa

SG pressure1.5 MPa

SG pressure1.1 MPa

No. 1 80 [m3/h] 60 [m3/h] 60 [m3/h]

No. 2 80 [m3/h] 50 [m3/h] 50 [m3/h]

No. 3 80 [m3/h] 40 [m3/h] 40 [m3/h]

Note: For SG steam pressures in between above values, pump affinity laws used to determine AFW flow as function of SG steam pressure

Simulation Results - Assumptions

Cases #1 to #3: SBO With SG DepressurizationAssumptions:All MSIVs closed at time T=TSBO+10 min• Steam dump to atmosphere only via SG safety valves

(POPEN=7.4 [MPa] until time T=TSBO+15 min• Operator opens at time T=TSBO+15 min SG#1 relief valve and

dumps steam to establish SG secondary side steam pressurePSG=1.3±0.2 [MPa]

• Operator closes SG#1 relief valve and stops steam dump every time the SG level becomes lower than -1.2 [m]

• Operator controls secondary coolant level in SG#1 by opening AFW valve at SG level < -0.9 [m] and closing it at level > 0.2 [m]

Primary Coolant Pressure

Station Black Out Without Depressurization Of Secondary SideAFW Flow Rate 80 [m3/h] to SG#1 Only

Fig. 0.1. Primary Coolant Pressure

6.5

7

7.5

8

8.5

9

9.5

10

10.5

11

11.5

12

12.5

13

13.5

14

14.5

15

15.5

16

16.5

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Pres

sure

[MPa

]

Primary Coolant Pressure

Station Black Out With Steam Generator #1 DepressurizationDifferent Auxiliary Feedwater Flows to SG #1 at Steam Pressure 1.3 MPa

Fig.1.1. Primary Coolant Pressure

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Pres

sure

[MPa

]

40 m3/h50 m3/h60 m3/h

Primary Coolant Injected by Hydro-Accumulators


Fig.1.1A. Coolant Injected by Accumulators into RCS

0

5

10

15

20

25

30

35

40

45

50

55

60

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Mas

s [1

000*

kg]

40 m3/h50 m3/h60 m3/h

Reactor Coolant System Inventory

Station Black Out Without Depressurization Of Secondary SideAFW Flow Rate 80 [m3/h] to SG#1 Only Fig.0.2. RCS Primary Coolant Inventory

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Mas

s [1

000*

kg]

Reactor Coolant System Inventory


Fig.1.2. RCS Coolant Inventory

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Mas

s [1

000*

kg]

40 m3/h50 m3/h60 m3/h

Reactor Coolant Average Temperatures in Loop#1 (SG depressurized) and Loops#2,3 (SGs not depressurized)


Fig.0.3. Primary Coolant Average Temperatures

285286287288289290291292293294295296297298299300301302303304305306

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Tem

pera

ture

[deg

C]

Loop#1Loops#2,#3

Reactor Coolant Average Temperatures in Loop#1 (SG depressurized)


Fig.1.3. Primary Coolant Average Temperature in RCS Loop#1

187

197

207

217

227

237

247

257

267

277

287

297

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Tem

pera

ture

[deg

C]

40 m3/h50 m3/h60 m3/h

Reactor Coolant Average Temperatures in Loops#2,3 (SGs not depressurized)

Station Black Out With Steam Generator #1 DepressurizationDifferent Auxiliary Feedwater Flows to SG #1 at Steam Pressure 1.3 MPa Fig.1.3A. Primary Coolant Average Temperature in RCS Loops #2 and #3

155

165

175

185

195

205

215

225

235

245

255

265

275

285

295

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Tem

pera

ture

[deg

C]

40 m3/h50 m3/h60 m3/h

Reactor Pressure Vessel Level Measurement

Reactor Pressure Vessel Level Measurement:“Hot-calibrated” for normal operating conditions: Primary coolant

average temperature TAVG = 295.9 [oC] and pressure P= 15.5 [MPa].

( )[ ]( ) 681.3

*

**972.10

,,

,

−−

−=

∑calgcalfRPV

icalgii

RPV L

LL

ρρ

ρρ

Value of RPVIndicated Level

Corresponds to

7.291 [m] Reactor vessel completely full of water up to the main flange

5.241 [m] Steam-liquid mixture level at outlet nozzle centerline

3.682 [m] Steam-liquid mixture level at top of fuel pellets stack

0.0 [m] Steam-liquid mixture level at bottom of fuel pellets stack

-3.681 [m] Reactor vessel completely empty of liquid



Fig.0.4. Liquid Level in Reactor Pressure Vessel

0

0.614

1.228

1.842

2.456

3.07

3.684

4.298

4.912

5.526

6.14

6.754

7.368

7.982

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Leve

l [m

]



Fig.1.4. Coolant Level in RPV (Hot-Calibrated)

0

0.614

1.228

1.842

2.456

3.07

3.684

4.298

4.912

5.526

6.14

6.754

7.368

7.982

8.596

9.21

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Leve

l [m

] 40 m3/h50 m3/h60 m3/h

Total Amount of Primary Coolant Leakage


Fig.0.5. Total Mass of Primary Coolant Lost Through RCP Seals

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Mas

s [1

000*

kg]

Total Amount of Primary Coolant Leakage


Fig.1.5. Coolant Lost Through RCP Seals

0102030405060708090

100110120130140150160170180190200

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Mas

s [1

000*

kg]

40 m3/h50 m3/h60 m3/h

Secondary Side Pressure in SG#1 (Depressurized)


Fig.0.6. SG#1 Steam Pressure

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Pres

sure

[MPa

]

Secondary Side Pressure in SG#1 (Depressurized)


Fig.1.6. Steam Pressure in SG#1 (Depressurized)

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Pres

sure

[MPa

]

40 m3/h50 m3/h60 m3/h

Secondary Side Pressure in SG#2,3 (Non-Depressurized)


Fig.0.7. SG#2 Steam Pressure

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Pres

sure

[MPa

]

Secondary Side Pressure in SG#2,3 (Non-Depressurized)


Fig.1.7. Steam Pressure in SG#2 (Non-Depressurized)

4.5

5

5.5

6

6.5

7

7.5

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Pres

sure

[MPa

]

40 m3/h50 m3/h60 m3/h

Secondary Coolant Inventory in SG#1 (Depressurized)

Station Black Out Without Depressurization Of Secondary SideAFW Flow Rate 80 [cub.m/h] to SG#1

Fig.0.8. SG#1 Secondary Coolant Inventory

45

50

55

60

65

70

75

80

85

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Mas

s [1

000*

kg]

Secondary Coolant Inventory in SG#1 (Depressurized)


Fig.1.8. Secondary Coolant Inventory in SG#1 (Depressurized)

44

48

52

56

60

64

68

72

76

80

84

88

92

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Mas

s [1

000*

kg]

40 m3/h50 m3/h60 m3/h

Secondary Coolant Inventory in SG#2,3 (Non-Depressurized)


Fig.0.9. SG#2, #3 Secondary Coolant Inventory

6

10

14

18

22

26

30

34

38

42

46

50

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Mas

s [1

000*

kg]

Secondary Coolant Inventory in SG#2,3 (Non-Depressurized)


Fig.1.9. Secondary Coolant Inventory in SG#2 (Non-Depressurized)

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Mas

s [1

000*

kg]

40 m3/h50 m3/h60 m3/h

Peak Fuel Cladding Temperature

Station Black Out Without Depressurization Of Secondary SideAFW Flow Rate 80 [m3/h] to SG#1 Only Fig.0.10. Peak Cladding Temperature

523

623

723

823

923

1023

1123

1223

1323

1423

1523

1623

1723

1823

1923

2023

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Tem

pera

ture

[K]

Peak Fuel Cladding Temperature


Fig.1.10. Peak Fuel Cladding Temperature

373

473

573

673

773

873

973

1073

1173

1273

1373

1473

1573

1673

1773

1873

1973

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Tem

pera

ture

[K]

40 m3/h50 m3/hhttemp

Maximum Steam Temperature at Core Exit


Fig.0.11. Steam Temperature at Exit from Core Hot Channel

523

573

623

673

723

773

823

873

923

973

1023

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Time [hours]

Tem

pera

ture

[K]

Maximum Steam Temperature at Core Exit


Fig.1.11. Temperature of Steam at Core Exit

373

423

473

523

573

623

673

723

773

823

873

923

973

1023

1073

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Time [hours]

Tem

pera

ture

[K]

40 m3/h50 m3/h60 m3/h

Conclusions

• SBO causes a loss of reactor coolant pumps’ (RCP) seal injection flow and pump thermal barrier cooling.

• SBO considerably increases the risk of RCP seal failure that results in an unmitigated LOCA and large-scale fuel failure

• Any reduction of leakage through the RCP seals will extend the time to fuel damage and increase the time to restore AC power

• “SBO Coping Time” is defined as the time until the peak fuel cladding temperature exceeds 650 [oC]

• Safety Objective: Achieve “SBO Coping time at least 4 to 8 hours”• SBO Without SG Depressurization: “SBO Coping Time” is 6 hours• SBO With SG Depressurization to cool down the reactor system –

extends “SBO Coping Time” from 6 to 16 hours• Parametric studies done with different AFW pump flows have

shown that AFW flow of 40 [m3/h] to only one SG kept at steam pressure of 1.3 [MPa] yields SBO Coping Time of 16 hours, i.e. it is sufficient to meet US NRC requirement: “SBO Coping Time at least 4 to 8 hours”

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