Safety Analysis of Severe Accident of Spent Fuel Pool ...
Transcript of 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
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
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)
case1: 5.0MW, 6.7m case3: 3.5MW, 6.7m case4: 3.5MW, 4.0m case6: 2.0MW, 1.8m
-1 0 1 2 3 40
40
80
120
Cae
sium
Rel
ease
Mas
s (k
g)
Time (day)
case1: 5.0MW, 6.7m case2: 3.5MW, 6.7m case3: 3.5MW, 4.0m case4: 2.0MW, 1.8m
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)
Passive cooling system
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
Passive cooling system
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
Passive cooling system
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
Passive cooling system
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
Passive cooling system
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
Passive cooling system
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.
Passive cooling system
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)
Passive cooling system
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)
Passive cooling system
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
Passive cooling system
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;