Investigations of Zirconium Fires Investigations of Zirconium Fires during Spent Fuel Pool LOCAsduring Spent Fuel Pool LOCAs

Samuel G. DurbinSamuel G. Durbin

Eric R. LindgrenEric R. Lindgren

Funded byFunded byUS Nuclear Regulatory CommissionUS Nuclear Regulatory Commission

JCN# Y6758JCN# Y6758

OutlineOutline

•• OverviewOverview

•• ObjectiveObjective

•• Experimental ApproachExperimental Approach

•• HardwareHardware

2

•• ExperimentsExperiments

–– Single, full length assembly with “hot” neighbor BCSingle, full length assembly with “hot” neighbor BC

–– 11××4 arrangement 4 arrangement –– Central “hot” assembly with cold Central “hot” assembly with cold neighborsneighbors

•• SummarySummary

OverviewOverview

•• Validate severe accident codes Validate severe accident codes for whole pool LOCA analysesfor whole pool LOCA analyses

•• Phased experimental approachPhased experimental approach

–– Study physical phenomena Study physical phenomena separatelyseparately

3

•• Provide input parameters to Provide input parameters to accident codesaccident codes

–– Examine nature of Zircaloy fires Examine nature of Zircaloy fires in prototypic assembliesin prototypic assemblies

•• Validate predictive capabilityValidate predictive capability

•• Develop mitigation strategies Develop mitigation strategies

BWR Spent Fuel Pool

SFP Experiment ObjectivesSFP Experiment Objectives

•• Provide Provide full scalefull scale thermal hydraulic and zirconium ignition data thermal hydraulic and zirconium ignition data –– Prototypic componentsPrototypic components

•• Eliminate scaling argumentsEliminate scaling arguments•• Represents fuel design intricaciesRepresents fuel design intricacies

–– Gas flow conditions Gas flow conditions

4

•• Spent fuel pool Spent fuel pool completecomplete loss of coolant accident (LOCA)loss of coolant accident (LOCA)•• Dry cast storage performanceDry cast storage performance•• Air ingress during late stages of core meltAir ingress during late stages of core melt--downdown

–– Code validationCode validation•• Closely coupled into experimental designClosely coupled into experimental design

–– Improve whole pool source term simulations for safety and Improve whole pool source term simulations for safety and consequence analysesconsequence analyses

Actual HardwareActual Hardware•• Prototypic 9Prototypic 9××9 BWR hardware9 BWR hardware

–– Full length, prototypic 9Full length, prototypic 9××9 BWR 9 BWR componentscomponents

–– Electric heater rods with ZrElectric heater rods with Zr--2 2 claddingcladding

Upper tie plate

Water Rod Outlet

5Nose piece &

debris catcherBWR channel, water tubes

& spacers

Water Rod Inlet

Integral Effects TestsIntegral Effects Tests(Single, Full Length)(Single, Full Length)

•• Single, full length, prototypic 9Single, full length, prototypic 9××9 BWR assembly9 BWR assembly

–– 5000 W simulating a 100 day old assembly5000 W simulating a 100 day old assembly

–– 74 Zr heater rods74 Zr heater rods

•• Eight partial lengthEight partial length

–– Prototypic hardware:Prototypic hardware:

•• Upper & lower tie platesUpper & lower tie plates

6

pp ppp p

•• Seven spacersSeven spacers

•• Water tubes and channel boxWater tubes and channel box

•• Single pool rack cellSingle pool rack cell

–– MeasurementsMeasurements

•• Temp profiles: Axial and radialTemp profiles: Axial and radial

•• Induced flow: Induced flow: Effect of ignition on flow Effect of ignition on flow

•• OO22 concentration: Determine depletionconcentration: Determine depletion

•• Nature of fire: Nature of fire: Initiation location & axial burn rateInitiation location & axial burn rate

Full Scale ZircoloyFull Scale Zircoloy(Ignition Test Instrumentation)(Ignition Test Instrumentation)

Cross section above partial length rods

3.02

3.66 m

7

0.61

1.22

1.83

2.44

O2 Sensor

Light Pipe

Thermocouple

Cross section of full bundle

0

Full Length ZrFull Length Zr(Ignition Results)(Ignition Results)

•• Assembly power 5000 WAssembly power 5000 W–– Situational equivalent of a cluster Situational equivalent of a cluster

of 100 dayof 100 day--old assemblies old assemblies (hot neighbor BC)(hot neighbor BC)

•• Thermocouples failed after Thermocouples failed after ignitionignition

–– Sharp transition to breakaway Sharp transition to breakaway oxidationoxidation

–– Oxygen depletionOxygen depletion300

700

1100

1500

1900

2300

Tem

per

atu

re (

K)

Data PCTMELCOR

8

0

5

10

15

20

25

0 2 4 6 8 10 12Time (hr)

O2

Con

cen

trat

ion

(%

)

–– Requires breakaway reaction Requires breakaway reaction kineticskinetics

•• Interesting dynamics on burnInteresting dynamics on burn--front movementfront movement

–– Usually downward to follow Usually downward to follow oxygen and fresh Zroxygen and fresh Zr

–– Late phase ignition above the Late phase ignition above the initial ignition locationinitial ignition location

–– Saw tooth pattern on code max Saw tooth pattern on code max temperature response (ignition temperature response (ignition front movement to new node)front movement to new node)

DataMELCOR

3000 2 4 6 8 10 12

Time (hr)

PostmortemPostmortem

9

Full Length Ignition Movie

Zircoloy Short Stack DesignZircoloy Short Stack Design•• Simulate “hot” center Simulate “hot” center

assembly in 1 assembly in 1 ×× 4 4 arrangementarrangement

–– Unpowered Zircaloy Unpowered Zircaloy peripheral assembliesperipheral assemblies

–– “cold neighbor” BC“cold neighbor” BC

10

•• Bottom and top tie Bottom and top tie plates allow flowplates allow flow

–– Bundle and annular flow Bundle and annular flow rates and temperature rates and temperature independently controlledindependently controlled

•• Prototypic commercial Prototypic commercial pool rackpool rack

Inlet Condition ControlInlet Condition Control•• Simulate segment of full length assembliesSimulate segment of full length assemblies

–– Temperature and flow histories independently controlledTemperature and flow histories independently controlled

•• Based on previous experimental and modeling resultsBased on previous experimental and modeling results

Center North West South East Mass Flo w Contro ller Air Heater(s) Destination

1 1A an d 1B Center b undle2 2 Center annu lu s3 3 North b und le

11

AH

1A

AH

3

AH

4

AH

5

AH

6

AH

7

AH

8

AH

2

MF 6MF 7

MF 8

MF 1MF 2

MF 3

MF 4MF 5

Air in M

anifo

ld

3 3 No t b u d e

4 4North and W est

annu lu s5 5 W est bund le6 6 Eas t bun dle

7 7South an d East

annu lu s8 8 So uth b und le

Bundle

Annulus

AH

1B

North

West

Sout

h

East

BundleFlow cross section

700

1100

1500

1900

2300

Tem

per

atu

re (

K)

Zircaloy 1 Zircaloy 1 ×× 44 IgnitionIgnition(Ignition Results)(Ignition Results)

•• “Hot” center assembly in 1 “Hot” center assembly in 1 ×× 4 4 arrangementarrangement

–– Equivalent of 15 dayEquivalent of 15 day--old fuel old fuel surrounded by background surrounded by background assemblies (cold neighbor BC)assemblies (cold neighbor BC)

–– Air flow rates and temperatures Air flow rates and temperatures independently controlledindependently controlled

–– Strong radial heat transfer to unStrong radial heat transfer to un--

Data PCTMELCOR

Center

Peripherals

12

3000 2 4 6 8

Time (hrs)

0

5

10

15

20

25

0 2 4 6 8Time (hrs)

O2

Con

cen

trat

ion

(%

)

ggpowered peripheral assembliespowered peripheral assemblies

•• Characterizing peripheral rod Characterizing peripheral rod emissivity importantemissivity important

–– Investigated sensitivity to reaction Investigated sensitivity to reaction kineticskinetics

•• Increased heat rate and Increased heat rate and oxygen consumption prior to oxygen consumption prior to sharp breakaway oxidation sharp breakaway oxidation behavior behavior

•• Possible connection to phase Possible connection to phase change in Zr at 1090 Kchange in Zr at 1090 K

DataMELCOR

Short StackShort Stack(Ignition Test)(Ignition Test)

13

1×4 Ignition Movie

PWR Whole Pool Partial LOCAPWR Whole Pool Partial LOCA

•• MELCOR calculations MELCOR calculations for uniform pool loadingfor uniform pool loading

•• Lowest powered Lowest powered assembly in study assembly in study potentially more potentially more vulnerablevulnerable

Water Level at 50% height of heated length

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vulnerablevulnerable–– Less steam generated to Less steam generated to

cool upper part of cool upper part of assembliesassemblies

•• Partial LOCAs may lead Partial LOCAs may lead to earlier Zr ignitionto earlier Zr ignition–– Less coolable for water Less coolable for water

levels below ~50% of levels below ~50% of heated lengthheated length

K.C. Wagner and R.O. Gauntt, “Mitigation of Spent Fuel Pool Loss-of-Coolant Inventory Accidents andExtension of Reference Plant Analyses to Other Spent Fuel Pools, Sandia Letter Report,” Nov. 2006

Low Power (7.4 kW) Case

Spent Fuel Pool ConfigurationsSpent Fuel Pool Configurations

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•• LowLow--density racking least vulnerabledensity racking least vulnerable

•• HighHigh--density racking with density racking with interspersed high and low powered interspersed high and low powered assemblies is best practice for pools assemblies is best practice for pools near capacitynear capacity

K.C. Wagner and R.O. Gauntt, “Mitigation of Spent Fuel Pool Loss-of-Coolant Inventory Accidents andExtension of Reference Plant Analyses to Other Spent Fuel Pools, Sandia Letter Report,” Nov. 2006

BWR SummaryBWR Summary

•• Integral ignition testsIntegral ignition tests

–– FullFull--Scale Assembly Ignition Test:Scale Assembly Ignition Test:

•• Represents uniform distribution of 100 day old Represents uniform distribution of 100 day old assembliesassemblies

•• Prototypic heatPrototypic heat--up and ignition response up and ignition response

16

•• Breakaway phenomena importantBreakaway phenomena important–– Evident in both temperature and OEvident in both temperature and O22 measurementsmeasurements

–– 11××4 Ignition Test:4 Ignition Test:

•• Represents 15 day old assembly with old neighborsRepresents 15 day old assembly with old neighbors

•• Confirmed delayed ignitionConfirmed delayed ignition–– Potential for radial propagation Potential for radial propagation

–– Showed importance of radial heat transferShowed importance of radial heat transfer