Safety Analysis of Severe Accident of Spent Fuel Pool
Zheng Huang
Royal Institute of Technology (KTH), Nuclear Power Safety Division
The 8 th 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
n01 n02 n03 n04 n05 n06 n07 n08 n09 n10
Active region Top rack
Supporting plate
i05 i04 i03 i01 i02
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 4
0 300 600 900 1200 1500
Hydrogen mass (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 2.0
0.1 1 10 100 1000
Hydrogen release mass (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 4
0.00E+000 2.00E-013 4.00E-013 1.00E-009 2.00E-009
Iodine Release Mass (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 4
0 40 80 120
Caesium Release Mass (kg)
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
Ring 1# Fuel Cladding Temperature (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
Ring 1# Fuel Cladding Temperature (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 3
0 2 4 6 8
Water Level (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
Water Level (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
HXs
Isolation 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 H
X H
X
123 Heat structure
... …
Condenser 300 202
Expansion tank
Component Parameters Values
Riser Diameter 0.45 m
Length 10.0 m
Downcomer Diameter 0.40 m
Length 10.0 m
HX/Condenser Diameter of tube 0.034 Total heat transfer area 300 m2
Tube material Stainless steel
Cooling 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 P C S W ith o u t P C S
Water level in the SFP (m)
T im e (d a y)
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 P C S W ith o u t P C S W ith P C S
(d o u b le h e a t tra n sfe r a re a )
Water level in the SFP (m)
T im e (d a y)
Passive cooling system
Simulation results
0 1 2 3 4 5
20 40 60 80 100
Temperature (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
Heat removal power of the PCS (MW)
Time (day)
0 1 2 3 4 5
0 10 20 30 40 50 60
Mass flow rate of the natural circulation (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;