safety analysis of severe accident of spent fuel pool

Safety Analysis of Severe Accident of Spent Fuel Pool

Zheng HuangRoyal Institute of Technology (KTH), Nuclear Power Safety Division

The 8th meeting of EMUG

Outline

• Introduction

• Modeling

• Accident analysis

• Passive cooling system

• Concluding remarks

Introduction

• The safety and risk assessment for beyond design basis accident (BDBA) in SFP is increasingly concerned after Fukushima accident.

• SFP design for PWR

• Representative initiating events:

� loss of cooling

� Loss of coolant (partial / complete)

Modeling

n01n02n03

n04

n05n06n07n08

n09n10

Active region

Top rack

Supporting plate

i05i04i03i02i01

Core nodalization

(a) radial (b) axial

Modeling

Hydraulic CV nodalization

Bottom of the pool

Bypass

Top of the pool

Atmosphere

Accident analysis

Calculation matrix

4 cases were selected, regarding the decay power level and initial water level.

Case Decay power (MW) Initial water level (m)

1 5.0 6.7

2 3.5 6.7

3 3.5 4.0(*)

4 2.0 1.8

(*) 4.0m is the elevation of the top of the fuel assembly.

Accident analysis

Transient results

Accumulated mass of released hydrogen

0 1 2 3 40

300

600

900

1200

1500

Hyd

roge

n m

ass

(kg)

Time (day)

case1: 5.0MW, 6.7m case2: 3.5MW, 6.7m case3: 3.5MW, 4.0m case4: 2.0MW, 1.8m

0.0 0.5 1.0 1.5 2.00.1

1

10

100

1000

Hyd

roge

n re

leas

e m

ass

(kg)

Time (day)

(a) For all cases (b) Closeup of case 6

Accident analysis

Transient results

(a) Iodine Release Mass (b) Cesium Release Mass

0 1 2 3 40.00E+000

2.00E-013

4.00E-013

1.00E-0092.00E-009

Iodi

ne R

elea

se M

ass

(kg)

Time (day)


-1 0 1 2 3 40

40

80

120

Cae

sium

Rel

ease

Mas

s (k

g)

Time (day)


Accident analysis

Transient results

(a) Case 1 (b) Case 6

Cladding temperature of ring 1 of the core

0 2.4 2.5 2.6 2.7 2.8 2.9 3.0

0

500

1000

1500

2000

2500

Rin

g 1#

Fue

l Cla

ddin

g T

empe

ratu

re (

K)

Time (day)

COR-TCL_103 COR-TCL_104 COR-TCL_105 COR-TCL_106 COR-TCL_107 COR-TCL_108 COR-TCL_109 COR-TCL_110

0.0 0.5 1.0 1.5 2.0

0

500

1000

1500

2000

2500

Rin

g 1#

Fue

l Cla

ddin

g T

empe

ratu

re (

K)

Time (day)

COR-TCL_103 COR-TCL_104 COR-TCL_105 COR-TCL_106 COR-TCL_107 COR-TCL_108 COR-TCL_109 COR-TCL_110

Accident analysis

Transient results

(a) Case 1 (b) Case 6

0 1 2 30

2

4

6

8

Wat

er L

evel

(m

)

Time (day)

water level fuel top

0.0 0.5 1.0 1.5 2.0

0.8

1.2

1.6

3.95

4.00

4.05

4.10

Wat

er L

evel

(m

)

Time (day)

water level fuel top

Water level in the SFP

Passive cooling system

Objectives and system

maintain the cooling under the SBO scenario up to at least 72 hrs

HXs

Fuel assemblies

Spent fuel pool

Isolation valve

Cooling tank

Condenser

Schematics of the passive cooling system (PCS)


Coupling model

• The transient responses of the SFP and natural circulation loop (NCL) are calculated simultaneously by coupling MELCOR and RELAP.

• SFP: MELCOR

• NCL: RELAP

HXs

Fuel assemblies

Spent fuel pool

Isolation valve

Cooling tank

Condenser

MELCOR

RELAP


Coupling model

Fuel assemblies

Spent fuel pool

HXsIsolation valve

Cooling tank

Condenser

MELCOR RELAP

Heat sink

TSFP

WPCS

Variables to be exchanged:

• Temperature of the SFP (TSFP)

• Heat removal power of the PCS (WPCS)

Scheme of the coupling model


Coupling model

• Data communication: Named Pipes mechanism

A named pipe is a named, one-way or duplex pipe for communication between the pipe server and one or more pipe clients, which is one of the methods of inter-process communication (IPC).

CF00100 ‘Ttank' FUN1 5 300.0

CF00110 1.0 0.0 TIME

CF00111 1.0 0.0 CVH-TLIQ.200

CF00112 0.0 0.0 TIME

CF00113 0.0 0.0 TIME

CF00114 0.0 0.0 TIME


Coupling model

• Synchronization: Semaphore

• During the calculation, one code will be forced to halt and wait if its current time is larger than counterpart’s until it is surpassed.

• The data is obtained by interpolating the two most neighboring values.

1

1

2 3

4

5 6

7

8 9

Code B

Code A

t

t


Model of passive cooling system

121

122

201

124

HX

Cooling tank

Downcomer

700

Environment

900

SFP

Riser

Heat structure

HX

HX

123

Heat structure

... …

Condenser

300

202

Expansiontank

Component Parameters ValuesRiser Diameter 0.45 m

Length 10.0 mDowncomer Diameter 0.40 m

Length 10.0 mHX/Condenser Diameter of tube 0.034

Total heat transfer area 300 m2

Tube material Stainless steelCooling tank Depth 7.5 m

Nominal Volume 1000 m3


Calculation case

• The initial water level of SFP is 5.0m;

• The decay heat power if 3.0MW;

• The initial temperature of the loop and SFP is 30.0℃;

• The PCS is actuated at the time 0.0 sec;

• The fluid in the NCL of the PCS is initially stagnant.


Simulation results

Water level (compared with the case without PCS)

0 1 2 3 4 5

1

2

3

4

5

W ith PCS W ithout PCS

Wat

er le

vel i

n th

e S

FP

(m

)

T im e (day)


Simulation results

Water level (compared with the case without PCS)

0 1 2 3 4 5

1

2

3

4

5

W ith PCS W ithout PCS W ith PCS

(double heat transfer area)

Wat

er le

vel i

n th

e S

FP

(m

)

T im e (day)


Simulation results

0 1 2 3 4 520

40

60

80

100T

empe

ratu

re (

o C)

Time (day)

Spent fuel pool Cooling tank of the PCS

Temperature of SFP and PCS cooling tank


Simulation results

0 1 2 3 4 5

0.0

0.6

1.2

1.8

2.4

Hea

t rem

oval

pow

er o

f the

PC

S (

MW

)

Time (day)0 1 2 3 4 5

0

10

20

30

40

50

60

Mas

s flo

w r

ate

of th

e na

tura

l circ

ulat

ion

(kg/

s)Time (day)

(a) PCS heat removal power (b) Mass flow rate of NC of the PCS

Concluding remarks

• Simulation of a severe accident of the spent fuel pool of a prototypical was carried out. Generally, the calculation results are physically reasonable.

• To cope with the SBO, a passive cooling system design featuring a natural circulation loop is proposed, and evaluated by using coupling MELCOR and RELAP model. Results show that the PCS is able to effectively remove the heat and delay the early exposure of the fuel assemblies.

• However, due to the decrease of the temperature difference, the efficiency of the

passive system decreases in the long term period. Further measures can be taken to enhance its performance:

� Enlarge the heat transfer area of the HX and the condenser;

� Increase the vertical height of the natural circulation loop;

� Enlarge the pipe diameter to reduce the flow resistance;

� Increase the volume of the cooling tank;

� Refill and cool down the cooling tank water of PCS;

Thanks for your attention!

safety analysis of severe accident of spent fuel pool

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