PRACTICAL EXAMPLES OF THE ANALYSIS OF SEVERE ACCIDENTS

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PRACTICAL EXAMPLES OF THE ANALYSIS OF SEVERE
ACCIDENTS
Regional Workshop on Evaluation of Specific
Preventative and Mitigative Accident Management
Strategies
Presented Dr. Chris Allison
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Outline

• Analysis of SAs
• Bundle boiloff influence of SA models
• Bundle quench
• CORA-13 PWR severe oxidation transient during
  reflood
• TMI-2

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Bundle Boiloff

• Two identical bundles
• 32 rods in 6X6 array 0.91 m height
• Boildown transient
• High decay heat 58.5 Kw (2.0 Kw/m per rod)
• One bundle modeled using RELAP5 heat structure
  1D heat conduction only
• One bundle modeled using SCDAP fuel rod component
  2D heat conduction, oxidation, ballooning and
  rupture, material liquefaction

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Influence of SA models starting below 1500 K
Fuel rod temperature above midpoint
SCDAP predicted temperatures
RELAP
RELAP predicted temperatures
Time (s)
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Oxidation heat generation comparable to decay heat
Power - Kw
Decay heat
Oxidation heat generation
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Oxidation limited by Zr relocation
Maximum bundle temperature - K
Maximum temperature
Hydrogen production g/s
Hydrogen production
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Axial temperature distribution
Temperature - K
U-Zr-O relocation
Dryout
Bottom
Top
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Oxidation front starts above midpoint
H2 generation rate g/s
Zr melt relocation
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Ballooning and rupture occurs near 1000 K
Hoop Strain
Temperature
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Zr-O-U Relocation to lower portion of bundle
Fuel outer radius including frozen crust
Temperature
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CORA-13 PWR Quench

• Electrically heated PWR bundle
• 25 rods (16 fuel rods, 7 heated fuel rod
  simulators, 2 Ag-In-Cd control rods)
• 1.00 m heated length
• Constant steam/argon flow

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Oxidation heat generation during reflood gtgt
electrical heating
Power - Kw
Note Electrical power shutdown prior to quench
Oxidation heat generation
Quench
Decay heat
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Oxidation during reflood results in temperature
excursion and renewed melting
Maximum bundle temperature - K
Maximum temperature
Hydrogen production g/s
Hydrogen production
Quench
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Axial temperature distribution
Renewed heating in upper bundle due to reflood
Temperature - K
Bottom
Top
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Oxidation of liquid U-O-Zr signficant during
reflood
H2 generation rate g/s
Zr melt relocation
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Ballooning and rupture occurs near 1200 K
Hoop Strain
Temperature
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Zr-O-U Relocation to lower portion of bundle
U-Zr-O freezing
Ballooning
Fuel outer radius including frozen crust
Temperature
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TMI-2

• The TMI-2 problem is described in the
  SCDAP/RELAP5/MOD3.2 reference manual (Volume V)
• General description (Section 5.5)
• Input model description (Appendix A.11)
• TMI-2 sample problem on CD includes
• Restart plot file
• Sample input file (restarting after B-pump
  transient and formation of initial molten pool)
• Sample plot input file

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TMI-2 Core Nodalization
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Calculated peak core temperatures and pressures
for TMI-2
Temperature
Pressure
Core uncovery
B-pump Transient
ECCS Injection
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Rapid Zircaloy oxidation resulted in initial
liquefaction and relocation of core metals
Liquefaction of UO2 and ZrO2
Melting of Zr
Control rod melt relocation, onset of rapid
oxidation
Fuel temperatures
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B-Pump Transient resulted in sharp increase in
oxidation in middle of core
Oxidation rate
B-pump Transient
Peak core temperature
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B-Pump transient cooled lower portion of core
Axial nodes 3-5
Fuel temperatures
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Molten (U-Zr)-O2 pool continued to grow after
water injection
B-pump Transient
Molten pool radius in core
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Molten (U-Zr)-O2 relocates into LP after ECCS
injection
Temp. of melt in LP
Melt relocation into LP
Height of debris in LP
ECCS Injection