PRACTICAL EXAMPLES OF THE ANALYSIS OF SEVERE

ACCIDENTS

Presented Dr. Chris Allison

Regional Workshop on

Evaluation of Specific Preventative

and Mitigative Accident

Management Strategies

Outline

• Analysis of SAs– Bundle boiloff – influence of SA models– Bundle quench

• CORA-13 – PWR – severe oxidation transient during reflood

– TMI-2

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

Axial Power Distribution

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1

Height (m)

Axi

al P

ow

er D

istr

ibu

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Influence of SA models starting below 1500 K

RELAPRELAP predicted temperatures

SCDAP predicted temperatures

Fuel rod temperature

above midpoint

Time (s)

Oxidation heat generation comparable to decay heat

Decay heat

Oxidation heat generation

Power - Kw

Oxidation limited by Zr relocation

Maximum temperature Hydrogen production – g/s

Maximum bundle temperature - K

Hydrogen production

Axial temperature distribution

Bottom Top

Temperature - K

U-Zr-O relocation

Dryout

Oxidation front starts above midpoint

H2 generation rate – g/s

Zr melt relocation

Ballooning and rupture occurs near 1000 K

Hoop Strain Temperature

Zr-O-U Relocation to lower portion of bundle

Fuel outer radius including frozen crust

Temperature

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

Oxidation heat generation during reflood >> electrical heating

Decay heat

Oxidation heat generation

Power - Kw

Quench

Note: Electrical power shutdown prior

to quench

Oxidation during reflood results in temperature excursion and renewed melting

Maximum temperature Hydrogen production – g/s

Maximum bundle temperature - K

Hydrogen production

Quench

Axial temperature distribution

Bottom Top

Temperature - KRenewed heating in upper bundle due to reflood

Oxidation of liquid U-O-Zr signficant during reflood

H2 generation rate – g/s

Zr melt relocation

Ballooning and rupture occurs near 1200 K

Hoop Strain Temperature

Zr-O-U Relocation to lower portion of bundle

Fuel outer radius including frozen crust

Temperature

Ballooning

U-Zr-O freezing

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

TMI-2 Core Nodalization

Calculated peak core temperatures and pressures for TMI-2

B-pump Transient

Core uncovery

ECCS Injection

Temperature

Pressure

Rapid Zircaloy oxidation resulted in initial liquefaction and relocation of core metals

Fuel temperatures

Liquefaction of UO2 and ZrO2

Melting of Zr

Control rod melt relocation, onset of rapid oxidation

B-Pump Transient resulted in sharp increase in oxidation in middle of core

Peak core temperature

Oxidation rate

B-pump Transient

B-Pump transient cooled lower portion of core

Fuel temperatures Axial nodes 3-5

Molten {(U-Zr)-O2} pool continued to grow after water injection

B-pump Transient

Molten pool radius in core

Molten (U-Zr)-O2 relocates into LP after ECCS injection

ECCS Injection

Melt relocation into LP

Temp. of melt in LP

Height of debris in LP