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Vg# 1
MELCOR Best Practices - An Accident Sequence Walkthrough
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company,for the United States Department of Energy’s National Nuclear Security Administration
under contract DE-AC04-94AL85000.
Larry Humphries, K. Wagner, J. Jun, R. Gauntt (SNL)
and
Hossein Esmaili (NRC)
Vg# 2
Best Practices adopted for NRC SOARCA Program
• State of the Art Reactor Consequence Analyses (SOARCA)
– Realistic evaluation of severe accident progression, radiological releases and offsite consequences• Provide a more accurate assessment of potential offsite consequences
– Focus on a spectrum of scenarios most likely to contribute to release and subsequent offsite consequences, using a risk-informed approach
• SOARCA expert review panel meeting– Solicit discussion from experts
• Experts also supplied additional recommendations– Discussion of base case approach on some key complex
and uncertain events for MELCOR calculations– Best Practices distilled from these findings
• Model defaults updated as result of this work• Uncertainty recognized as important
– Separate task evaluting uncertainty using uncertainty engine
•Introduction•BWR•PWR•Other Best Practices
Vg# 3
BWR Topics for Consideration
1. Thermal response and seizure of cycling S/RVs during late phase of in-vessel fuel damage
2. Criteria for representing mechanical failure of highly-oxidized but erect fuel assemblies
3. Volatile fission product speciation
4. Structural (non-radioactive) aerosol generation
5. Aerosol deposition on reactor and containment surfaces
6. Debris heat transfer in reactor vessel lower plenum
7. RPV failure mode and criteria in heads with varied and multiple penetrations
8. Hydrogen combustion and ignition
9. Debris spreading and Mark I shell melt-through
10. Under-cutting of reactor pedestal wall via long-term molten corium-concrete interaction
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 4
BWR Walk-through: BWR/4 Mark I LT-SBO
• Total loss of off-site power
• RCIC operation on batteries for 8 hrs
• Manual S/RV control available, but partial depressurization not reflected in the example calculation
• Containment venting not available due to loss of power
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 5
BWR/4 Reactor Vessel Model
LOOP A
FL387
FL383
FL385
LOOP B
FL384
FL382
FL614
CSTCV360
Steam Dome
CV320Lower Plenum
FL304
CV310Downcomer
CV310Downcomer
CV
383
CV
385
CV
384
CV
382
FL386
FL312FL312
CV
300
CV
300
FL310FL311
FL356
FL3
57
FL370 CV391Turbine / Hotwell
(time independent)
InboardMSIVs
FL371
FWPump
CV392Feedwater source(time independent)
CRDPump
RCIC
Core Spray(Loop 1)
CV600
FL372
FL702
FL706
FL606
CV370 (Steam Lines)
FL399(RV Breach)
CV350 Steam Separators
SVs(2 valves)
FL9
10(W
etw
ell
Ven
t)
to drywellatmosphere
CV200Drywell
CV220
CV901Environment
FL359 - FL369
FL358
FL653
RHR-2 RHR-1
HPSW
FL6
48
FL6
49
FL652
SPRSR01
CV652
CV
651
CV
650
CV
700
ReactorBuildingRWCU return
(not modeled)
CV374 CV372
FL611
FL376 FL374
CV220
CV653FL656 (LPCI)
FL655(supp.poolcooling)
FL657(LPCI)
FL654(supp.poolcooling)
CV394(time
independentkeep-full at
steady state)
S/RV (typical)11 parallel valves
TurbineBuilding
FL6
00
FL6
13
CV611
FL351
FL699
FL391(RWCUbreak)
FL6
51
FL6
50
A CB D
CV355Steam Dryers
FL355FL352
FL704
SPRSR02CV631
HPCI
FL6
33
FL631
FL626
FL634
CV345 Shroud Dome
FL345
• BWR/4 Reactor Vessel Model
– Important features & components
– Nodalization
• Reactor Coolant System
• Cavity Model– Elevation
View
– Aerial View
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 6
0
100
200
300
400
500
600
700
0 5 10 15 20 25 30
time [hr]
Wat
er L
evel
[in
]
Top of Jet Pumps
Lower core plate failure:debris relocation
into lower head
DowncomerCore Region
Top of Active Fuel
Bottom of Active Fuel
Automatic operation of RCIC
Base of downcomer
RPV lower head failure
LT-SBO: Early Thermal-Hydraulic Response
Sequence characteristics:
• 0 – 8 hrs: RCIC operates to maintain water level
• Controlled depress. does not occur to maintain RCIC steam supply
0
200
400
600
800
1000
1200
0 5 10 15 20 25 30
time [hr]
Pre
ssu
re [
psi
]
Containment Failure(drywell shell melt -through)
S/RV seizes open
Local failure of lowercore support plate(initial debris relocationinto lower head)
Cycling of lowest setpoint S/RV
Cooling from automaticcycles of RCIC
RPV lower head failure
1
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 7
S/RV Seizure at High Temperature
• Automatic S/RV actuation after battery depletion
• Typically a single cycling valve
• Demand frequency: One cycle every 3-4 min at onset of core damage
• Steam/H2 discharge with variable temperatures
• Gas temperature exceeds 1000 K close to time of lower core plate failure
500
600
700
800
900
1000
1100
1200
1300
1400
1500
0 5 10 15 20 25 30
time [hr]
Tem
per
atu
re [
K]
Core Exit Gas
Gases discharged through S/RV(s)
S/RV seizes (10 cycles over 1000K) 1
•Introduction•BWR
–LTSBO–Nodalization
1.S/RV1.Seizure @ high
temperature2.Valve designs
1.depressurization rate
2.FP transport
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 8
Valve Design Affects Response
• BWR/5 and /6: Spring-actuated, direct-acting relief valves [Crosby or Dikkers]
– Disk opens to position proportional to pressure against spring
– Model: Seize in last position based on residual lifetime
• 60 min @ 1000 K
• 30 min @ 1500 K
• BWR/3 and /4: 2- or 3-Stage Target Rock Pilot-Operated relief valves
– Valve disk cycles between full-open & full-closed
– Model: Seize in open position at 10th cycle above 1000 K
PBAPS (3-stage TR) GGNS
•Introduction•BWR
–LTSBO–Nodalization
1.S/RV1.Seizure @ high te
mperature2.Valve designs
1.depressurization rate
2.FP transport
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 9
0
0.2
0.4
0.6
0.8
1
1.2
-2 -1 0 1 2 3 4 5 6
time [hr]
Fra
ctio
n o
f F
ull
-Op
en P
osi
tio
nLowest Setpoint SRV
Stem Position
time [hr]
Depressurization Rate Depends on Valve Position at Seizure
0
200
400
600
800
1000
1200
1400
-2 -1 0 1 2 3 4 5 6
time [hr]
Pre
ssu
re [
psi
]
GGNS (Dikkers spring-actuated)
PBAPS (3-stage T-R)
Reactor Vessel Pressure
SRV seizure
•Introduction•BWR
–LTSBO–Nodalization
1.S/RV1.Seizure @ high te
mperature2.Valve designs
1.depressurization rate
2.FP transport
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 10
Pressure History Influences Fission Product Transport / Deposition
0
0.1
0.2
0.3
0.4
-2 -1 0 1 2 3 4 5 6
time [hr]
Fra
ctio
n o
f In
itia
l C
ore
In
ven
tory
PBAPS
Iodine Released from RCS(to suppression pool via open SRV)
SRV seizure
GGNS
[Release fraction normalized to zero at time of SRV seizure]
GGNS
Vessel breach
PBAPS
RPV dryout
•Introduction•BWR
–LTSBO–Nodalization
1.S/RV1.Seizure @ high te
mperature2.Valve designs
1.depressurization rate
2.FP transport
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 11
Core Response to Oxidation Transient
500
1000
1500
2000
2500
10 11 12 13 14 15 16 17 18
time [hr]
Tem
per
atu
re [
K]
Top of Core (axial level 15)
Middle of Core (axial level 12)
Bottom of Core (axial level 8)
Ring 1
Water level in shroud below
2/3 core height
Fuel assembly'failure' at this location(conversion to particulate debris)
2
•Introduction•BWR
–LTSBO–Nodalization
1.S/RV
2.Fuel rod Failure1.Core Oxidation2.Damage function3.Failure model4.New input
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 12
Damage Function for Local Collapse of Fuel Rods
• Mechanical failure of fuel rods assumed to result from combination of:
– Loss of intact, unoxidized clad materal
– Thermal stress
• Molten Zr ‘breaks out’ from ZrO2 shell at 2400 K
• Standing fuel rods collapse (forming particulate debris) based on a cumulative damage function
– Concept: Swelling, thermal expansion and mechanical stresses increase with temperature. Insults to fuel integrity build with time at temperature.
•Introduction•BWR
–LTSBO–Nodalization
1.S/RV
2.Fuel rod Failure1.Core Oxidation2.Damage function3.Failure model4.New input
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 13
Fuel Degradation Modeling
• Molten metallic Zr breakout temperature (2400K)
• Fuel rod collapse– Time-at-temperature damage
function
– Similar to MAAP model
– Eliminates single temperature criterion
ZrO2 oxideShell
Oxidizing ZrMetal held underOxide shell
Release of Molten Zr (2400K)
•Introduction•BWR
–LTSBO–Nodalization
1.S/RV
2.Fuel rod Failure1.Core Oxidation2.Damage function3.Failure model4.New input
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 14
Fuel Collapse Model Implementation
• The logic has been implemented within the code (MELCOR 2.1 & 1.8.6)
– Require new input records to activate the logic (set of CFs no longer needed)
– Different input format between two versions of the code
• Added input record: COR_ROD (CORROD in 1.8.6)
– Requires two fields• (1) IRODDAMAGE: tabular function name for the residual
lifetime of fuel as a function of cladding temperature (tabular function number (integer) in 1.8.6)
• (2) RCLADTHICKNESS: minimum un-oxdized clad thickness under which the rod collapse model supplants the default temperature based criterion (default = 1.0E-4 m)
•Introduction•BWR
–LTSBO–Nodalization
1.S/RV
2.Fuel rod Failure1.Core Oxidation2.Damage function3.Failure model4.New input
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 15
The Phebus Experiment Facility
• Irradiated fuel heated in test package by Phebus driver core
– Fuel heatup
– Zr oxidation, H2
– Fission product release
• Circuit (700 C) transports FP through steam generator tube
– Deposits in circuit and SG
• Containment receives FP gas and aerosol
– Settling
– Iodine chemistry
3
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
–Phebus Facility–Validation–Vapor Pressures–FPT1 Deposition–Booth
Parameters
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 16
Validation of Fission Product Release Models
VERCORS 4
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2000 4000 6000 8000 10000 12000
time (sec)
Rel
ease
Fra
ctio
n
0
500
1000
1500
2000
2500
3000
Tem
per
atu
re (
K)
ORNL-BoothdataCORSOR-MTemperature
VERCORS 4
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2000 4000 6000 8000 10000 12000
time (sec)
Rel
ease
Fra
ctio
n
0
500
1000
1500
2000
2500
3000
Tem
per
atu
re
ORNL BoothCORSOR MI dataCOR-TFU.102
VERCORS 4
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2000 4000 6000 8000 10000 12000
time (sec)
Rel
ease
Fra
ctio
n
0
500
1000
1500
2000
2500
3000
Tem
per
atu
re (
K)
ORNL BoothCORSOR MTe dataTemperature
VERCORS 4
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2000 4000 6000 8000 10000 12000
time (sec)
Rel
ease
Fra
ctio
n
0
500
1000
1500
2000
2500
3000
Tem
per
atu
re (
K)
ORNL BoothCORSOR MMo data
Temperature
Cs Release I Release
Te Release Mo Release
Temperature•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
–Phebus Facility–Validation–Vapor Pressures–FPT1 Deposition–Booth
Parameters
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 17
Vapor Pressures of Some Important Species
• Molybdenum vapor pressure extremely low
• Cs2MoO4 considerably higher, but…
• Less volatile than CsOH or CsI
• MELCOR treatment– Cs and Mo treated as
Cs2MoO4 with respect to volatility
– CsI left unchanged
1000 1500 2000 2500 3000 35001 10 4
1 10 3
0.01
0.1
1
1010
0.0001
P Cs Ti atm
P CsI Ti atm
P CsMo Ti atm
P Mo Ti atm
3.5 1031.025 103 Ti
K
Mo
Cs2MoO4
CsI
CsOH
fuelk
fuel
kv A
RT
P
D
Num )
0(
DVapor transport rate
vap
or
pre
ssu
re -
atm
Temperature – K
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
–Phebus Facility–Validation–Vapor Pressures–FPT1 Deposition–Booth
Parameters
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 18
Cs Distribution as Fraction of Bundle Inventory: ORNL-Booth (modified)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
5000 7500 10000 12500 15000 17500 20000
time [sec]
fra
cti
on
of
inv
en
tory
bundleplenumhot legsteam generatorcold legcontainmentTotal
- Plenum
- Hot Leg
- Stm Gen
- Containment
- total
FPT-1FinalDist
FPT-1 Deposition using Modified ORNL-Booth Release Model
• Distribution of transported fission products– Predictions versus experiment
– Performance reasonable for application
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
–Phebus Facility–Validation–Vapor Pressures–FPT1 Deposition–Booth
Parameters
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 19
CORSOR-Booth ORNL-Booth Adjusted
ORNL-Booth Diffusion coeff. Do 2.5x10-7 m2/sec 1x10-6 m2/sec 1x10-6 m2/sec
Activation Energy Q 3.814x105 joule/mole 3.814x105 joule/mole 3.814x105 joule/mole
Grain radius, a 6 m 6 m 6 m Class Scale Factors --- --- ---
Class 1 (Xe) 1 1 1 Class 2 (Cs) 1 1 1 Class 3 (Ba) 3.3x10-3 4x10-4 4x10-4
Class 4 (I) 1 0.64 0.64 Class 5 (Te) 1 0.64 0.64 Class 6 (Ru) 1x10-4 4x10-4 0.0025 Class 7 (Mo) 0.001 0.0625 0.2 Class 8 (Ce) 3.34x10-5 4x10-8 4x10-8 Class 9 (La) 1x10-4 4x10-8 4x10-8 Class 10 (U) 1x10-4 3.6x10-7 3.2x10-4
Class 11 (Cd) 0.05 0.25 .25 Class 12 (Sn) 0.05 0.16 .16
Booth Parameters for Different Data Fits
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
–Phebus Facility–Validation–Vapor Pressures–FPT1 Deposition–Booth
Parameters
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 20
Release of Structural Aerosols
• For BWRs, principal source is tin (alloy material in Zircaloy)
– Approx. 70+ MT of Zircaloy in fuel clad + canister
– 1.45% of which is Sn
• Direct experimental measurements are very limited, but general observations from Phebus tests suggest:
– Sn levels greatly reduced in unoxidized Zr
– No Sn found in remnants of ZrO2
– Total quantity of Sn on downstream surfaces roughly half of total available mass
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol–Background–Modeling
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 21
Release of Structural Aerosols-- Modeling Approach (BWRs) --
• Create special RN class to track released mass separately from fission product Sn
• Invoke ‘non-fuel’ release model in COR Package
– Associate new RN class with releases from core components with Zr and ZrO2
– Release rates scaled from CORSOR model for FP Sn 1
10
100
1000
10000
100000
0 1 2 3 4 5time (hr)
Ma
ss
[kg
]5
195
505
Sn as Fission Product
CORSOR-Booth
CORSOR-Booth
16,700
CORSOR-M
Sn from Zircaloy
CORSOR-M 34,600
Birchley FPT-1 multiplier
CORSOR multiplier = 1.0
2
CORSOR-Booth
CORSOR-M
3200%
1550%
47%
18%
ReleaseMass (kg)
Percent ofSn in Zirc
• Sensitivity calculations performed to determine appropriate release rate scalar
– Results for full-scale plant model compared to similar work by Birchley at PSI
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol–Background–Modeling
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 22
MELCOR Aerosol MechanicsMAEROS
• MAEROS sectional model of Gelbard– 10 sections [.1 - 50 m]– Condensed FP vapor sourced into
smallest section• Particles grow in size
– Agglomeration– Water condensation
• Particle fallout by gravitational settling• Particle deposition processes
– Thermophoresis– Diffusiophoresis– Brownian motion
• BWR structural aerosol release (Sn) from Zr cladding and canister
– Significant aerosol mass– Affects agglomeration, growth and
fallout• Cs chemisorption in RCS modeled
– Iodine from CsI revolatilizes when reheated
0.1 1 10 100
aerosol diameter [m]
sect
ion
mas
s
0.1 1 10 100
aerosol diameter [m]
sect
ion
mas
s
Aerosol size distribution evolves in time, depending on sources, agglomeration and removal processes
Time 1
Time 2
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition–MAEROS
Aerosol mechanics
–Deposition–Pool Scrubbing
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 23
Aerosol Deposition on Reactor and Containment Surfaces
• All deposition and retention mechanisms available in MELCOR are active in all regions of plant models
– Settling, phoretic processes, chemisorption, etc.• Special features added to address mechanisms not captured by
default models– “Filters” with filtration efficiencies designed to reflect:
• Impaction losses on elbows and surfaces of long-length piping upstream of rupture location in LOCAs
• Vapor scrubbing in water pools for species other than iodine
• Reactor, containment and auxiliary building surfaces are represented in detail
– High level of nodalization: proper temperature distributions and competing transport pathways
– Sub-divide complex structures into linked but separate surfaces to properly reflect orientation
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition–MAEROS Aeroso
l mechanics–Deposition–Pool Scrubbing
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 24
Enhanced Pool Scrubbing Model
• Current SPARC90 pool scrubbing model – Fission product decontamination calculated
• aerosol Particles• currently, Iodine is the only vapor that is scrubbed
• Removal of CsOH and CsI vapors– Typically enter the pool at high temperature in vapor form– Would deposit on the bubble/water surfaces and be scrubbed– Cooling offered by the pool would condense the vapor species to form
aerosol particles– Treatment for the scrubbing of these vapor species now available in
MELCOR 1.8.6 and 2.1• Usage
– MELCOR 1.8.6: IBUBT or IBUBF field on FLnnn02 must be 2• e.g., FL10002 0 0 2 2
– MELCOR 2.1: ‘AllScrubbing’ (or 2) accepted as a vaild input for IBUBT or IBUBF on FL_JSW
• e.g., FL_JSW 0 AllScrubbing AllScrubbing
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition–MAEROS Aeroso
l mechanics–Deposition–Pool Scrubbing
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 25
Debris Mass and Composition in Lower Head
0
20
40
60
80
100
120
140
160
180
10 15 20 25 30 35 40
time [hr]
Deb
ris
Mas
s [M
T]
Failure of Lower Head
Failure of LowerCore Plate
UO2
ZrO2
Unoxidized steel
Zr
SS
0
20
40
60
80
100
120
140
160
180
15 15.5 16 16.5 17
time [hr]
Deb
ris
Mas
s [M
T] UO2
ZrO2
Zr
SS
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP–Debris mass in LH–Debris & LH Tempe
ratures–Falling debris quenc
h–Stable debris in LH–HT to LH
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 26
Debris and Lower Head Temperature
500
700
900
1100
1300
1500
1700
1900
2100
2300
2500
10 15 20 25 30 35 40
time [hr]
Tem
per
atu
re [
K]
Lower head dryout
Lower head wall Inner surfaceOuter surface
Ring 1Early 'dribble' of Inconel
Lower core plate failure(bulk of core debris pours into lower head)
Creep rupture of hemispherical head
Debristemperature
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP–Debris mass in LH–Debris & LH
Temperatures–Falling debris quenc
h–Stable debris in LH–HT to LH
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 27
MELCOR Framework for Debris-Coolant Heat Transfer in Lower Head
• Step 1: “Falling Debris Quench”– Parametric model of fragmentation and cooling of a
molten jet pouring into pool of water
– Free parameters
• Vfall (effective fall velocity)
• Heat transfer coefficient
• Dpart (Dh of final particles)
– Dpart and HTC developed from FARO data
– Vfall selected to mimic end-state temperature of debris in deep-pool FARO tests.
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP–Debris mass in LH–Debris & LH Tempe
ratures–Falling debris
quench–Stable debris in LH–HT to LH
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 28
MELCOR Framework (continued)
• Step 2: Stable Debris Bed Heat Transfer
– Stable debris bed cooling limited by 1-D Lipinsky CCFL correlation
• Historically limited heat transfer to uppermost region of debris bed
– Coarse nodalization required to expose entire debris bed to water
• Proposed approach: restore detailed nodalization and disable1-D CCFL to reflect lateral in-flow of water from adjacent ‘rings’ of the debris bed.
– Permits calculation of debris temperature distribution
– Permits more accurate representation of heat transfer to control rod guide tubes
Top Guide &Upper tie plate
Lower tie plate& nose pieces
105
101
205
201
305
301
405
401
505
501
He
igh
t o
f Fue
l Ass
em
blie
s
He
ated
Le
ngth
Lower Core Plate &"Elephant's Foot"
Co
ntro
l rod
guid
es t
ubes
BWR/4 Core Nodalization
106 206 306 406 506
117 217 317 417 517
104 204 304 404 504
103 203 303 403 503
102 202 302 402 502
108
208
308
408
508
107
207
307
407
507
109
209
309
409
509
110
210
310
410
510
111
211
311
411
511
112
212
312
412
512
113
213
313
413
513
114
214
314
414
514
115
215
315
415
515
116
216
316
416
516
Pe
netr
atio
n -
R1
CRD stub tubes
Pe
netr
atio
n -
R2
Pe
netr
atio
n -
R3
Pe
netr
atio
n -
R4
Pe
netr
atio
n -
R5
Lower head
TAF
BAF
0.0 m
3.086 m
3.619 m
4.152 m
4.685 m
5.218 m
5.494 m
5.875 m
6.256 m
6.637 m
7.018 m
7.399 m
7.780 m
8.161 m
8.542 m
8.923 m
9.304 m
9.669 m
CV
514
CV
524
CV
534
CV
544
CV
554
CV345Shroud Dome
CV320Lower
Plenum
CVH Nodalization accompanyingCOR Nodalization
CV
111
CV
121
CV
131
CV
141
CV
151
CV
511
CV
521
CV
531
CV
541
CV
551
CV
512
CV
522
CV
532
CV
542
CV
552
CV
112
CV
122
CV
132
CV
142
CV
152
CV
513
CV
523
CV
533
CV
543
CV
553
CV
113
CV
123
CV
133
CV
143
CV
153
CV
114
CV
124
CV
134
CV
144
CV
154
CV
515
CV
525
CV
535
CV
545
CV
555
CV
115
CV
125
CV
135
CV
145
CV
155•Introduction
•BWR– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP–Debris mass in LH–Debris & LH Tempe
ratures–Falling debris quenc
h–Stable debris in LH–HT to LH
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 29
MELCOR Framework (concluded)
• Step 3: Heat Transfer to (and failure of) Lower Head
– New 2-D curved head model in MELCOR 1.8.6
– Solid debris heat conduction to vessel wall
– Heat transfer coefficient between debris & head sensitive to debris temperature and morphology
– Creep rupture of hemispherical head based on Larson-Miller parameter and life-fraction rule applied to a 1-D mechanical model
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP–Debris mass in LH–Debris & LH Tempe
ratures–Falling debris quenc
h–Stable debris in LH–HT to LH
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 30
Effect of Penetrations on Failure Criteria
• Penetration failure can be represented with the following tools:
– Failure criteria specified via user-defined control function– Distinct temperature of lumped parameter steel mass in
contact with debris and inner surface of lower head
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
–Modeling–Justification
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 31
Baseline Analyses will Not Exercise Penetration Failure Model
• Reasons:
– Experimental/analytical work for BWR penetrations does not conclusively demonstrate high probability of failure at a time that significantly precedes creep rupture of the head
– Melt penetration into penetration/stub tube structure does not necessarily result in debris ejection from RPV
– Lumped-parameter penetration model in MELCOR does not account for complexities of melt penetration into structure and local changes in debris state
– MELCOR sensitivity studies with active model (using reasonable range
of penetration masses) indicate penetration failure has small impact on event chronology
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
–Modeling–Justification
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 32
Hydrogen Combustion & Ignition
• MELCOR (HECTR) combustion models will be active in all calculations
– Apply default criteria for steam inerting, combustion efficiency, flame speed, etc.
– One non-default option: • Time required for a flame to propagate to neighboring
control volume specified on an individual CV basis
• Ignition criteria
– Use default for sequences with well-defined ignition sources (generally all cases with active ac power)
– Defer ignition until vessel breach for total loss of power scenarios.
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 33
Lateral Debris Spreading in Mark I Drywell
• Potential for early drywell failure often dominated by drywell shell melt-through in Mark I containment
– Not a factor in some Mark I plants due to deep sumps or curbs
• Modeling approach follows basic conclusions of NRC issue resolution
– Potential for failure dominated by lateral debris mobility
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
–Mark I drywell–Debris spreading–Contact with dry
well liner
10.MCCI
•PWR•Other Best Practices
Vg# 34
Debris Spreading / Shell Melt-through Criteria
• Debris mobility tied to debris temperature and static head (height differential between neighboring areas):
– Overflow not allowed if Tdebris < Tsolidus
– Above solidus:• CAV0 to CAV1: 0.5 m when Tdebris > Tsolidus
0.15 m when Tdebris = Tliquidus
• CAV1 to CAV2: 0.5 m when Tdebris > Tsolidus
0.10 m when Tdebris = Tliquidus
• Spreading rate expressed in terms of transit time across single CAV
– When Tdebris = Tliquidus: 10 min for CAV1 30 min for CAV2– When Tdebris = Tsolidus: infinite
• 5 min delay to shell failure after debris contact with T > 1811 K
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
–Mark I drywell–Debris spreading–Contact with dry
well liner
10.MCCI
•PWR•Other Best Practices
Vg# 35
Debris Spreading & Contact with DW Liner
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
20 30 40 50
time [hr]
Deb
ris
Dep
th [
m]
In-pedestal(CAV 0)
90o sector outside pedestal doorway
(CAV 1)
Balance of drywell floor (CAV 2)
CAV 2CAV 1CAV 0
0
500
1000
1500
2000
2500
20 30 40 50
time [hr]
Tem
per
atu
re [
K]
In-pedestal (CAV 0)
90o sector outside pedestal doorway
(CAV 1)
Balance of drywellfloor (CAV 2)
T-ablation(1750K)
(S)
(L)
Time delay between Vessel Breach and DW
melt-through:14 min
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
–Mark I drywell–Debris spreading–Contact with
drywell liner
10.MCCI
•PWR•Other Best Practices
Vg# 36
Corium Crust to Water Heat Flux
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 0.1 0.2 0.3 0.4 0.5
Time (hr)
Hea
t Flu
x (
kW/m
**2)
1*K2*K
5*K10*K
20*K50*K
100*K
MCCI Modeling
• Corium assumed to be well mixed (default)
• Enhanced effective corium thermal conductivity (10x)
– produces 1 to 5 MW/m2 heat flux
– Accounts for cracks and fissures and crust failure
– Consistent with interpretation of MACE tests
d d
concrete
corium
water
Range observed In MACE Tests
•Introduction•BWR
– LTSBO– Nodalization
1.S/RV
2.Fuel rod Failure
3.Volatile FP speciation
4.Structural aerosol
5.Aerosol Deposition
6.Debris HTF in LP
7.RPV failure with penetrations
8.H2 combustion
9.Debris Spreading (Cavity)
10.MCCI
•PWR•Other Best Practices
Vg# 37
Summary of Main Points for PWR Discussion
• Pump seal leakage and blowout• RCS natural circulation treatment• Core plate failure
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.RCS natural circulation
3.Core plate failure
•Other Best Practices
• Safety relief valve cycling and failure• Fission product release, speciation, and volatility• Fuel degradation and relocation treatment• Debris/coolant heat transfer• Vessel head failure and debris ejection• Hydrogen combustion• MCCI
Issues Specific to PWR reactors
Issues Previously discussed (Treated the Same as for BWR reactors)
Vg# 38
Plant and NSS Nodalizations
• Plant Buildings– Containment
• Elevation View
• Aerial view
– Other Buildings
• Detailed nodalizations of RCS and Core
– Capture important 2-D effects
– Natural circulation patterns
• Core
• RCS
• Steam generators
• Loop seals
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.RCS natural circulation
3.Core plate failure
•Other Best Practices
Vg# 39
Walkthrough of Station Blackout Accident in a PWR
• Short term station blackout
• Loss of ac power
• No feedwater injection
• No ECCS
• Leaking pump seals
• Key modeling issues identified in walkthrough
•Introduction•BWR•PWR
– Nodalization– SBO
• SBO definition• Initiation to SG
dryout• SG dryout• SG dryout to pu
mp seal failure• Core uncovery
to hotleg failure
• Core Waterlevel
1.Pump seals
2.RCS natural circulation
3.Core plate failure
•Other Best Practices
Vg# 40
Primary and Secondary PressuresSTSBO - No Mitigation
0
2
4
6
8
10
12
14
16
18
0 1 2 3 4 5 6 7 8
Time (hr)
Pre
ss
ure
(M
Pa
)
Pressurizer
SG A
SG C
SG B
Hot leg C creep rupture
SG dryout
Station Blackout High Pressure PWR SequenceAccident Initiation – SG dryout
• Initial full loop RCS water circulation removes energy
• Main coolant pump seals leak water
• Pressurizer safety valve cycling stops
•Introduction•BWR•PWR
– Nodalization– SBO
• SBO definition• Initiation to SG
dryout• SG dryout• SG dryout to pu
mp seal failure• Core uncovery
to hotleg failure
• Core Waterlevel
1.Pump seals
2.RCS natural circulation
3.Core plate failure
•Other Best Practices
Vg# 41
Steam Generator Secondary Water Accident Initiation – SG dryout
• Full loop RCS natural circulation period
• Good decay heat removal
• Secondary dry at ~1.2 hr
• Primary RCS pressurization follows
SG Downcomer LevelsSTSBO
0
2
4
6
8
10
12
14
0 0.5 1 1.5 2 2.5 3
Time (hr)
Lev
el (
m)
SG A
SG B
SG C
Bottom of the downcomer
•Introduction•BWR•PWR
– Nodalization– SBO
• SBO definition• Initiation to SG
dryout• SG dryout• SG dryout to pu
mp seal failure• Core uncovery
to hotleg failure
• Core Waterlevel
1.Pump seals
2.RCS natural circulation
3.Core plate failure
•Other Best Practices
Vg# 42
Station Blackout High Pressure PWR SequenceSG dryout – Pump Seal Failure
• SG dryout starts RCS re-pressurization to relief valve setpoint• RCS becomes steam-filled challenging pump seals
– Seal blowout at 2.8 hrs• Seal failure increases coolant mass loss rate• Cycling relief valve
– Same treatment as BWR SRV
0
2
4
6
8
10
12
14
16
18
0 1 2 3 4 5 6 7 8
Time (hr)
Pre
ss
ure
(M
Pa
)Pressurizer
SG A
SG C
SG B
Hot leg C creep rupture
SG dryout
pump sealfailure leads todepressurizationcycling
reliefvalve
PumpSeal
failure
•Introduction•BWR•PWR
– Nodalization– SBO
• SBO definition• Initiation to SG
dryout• SG dryout• SG dryout to
pump seal failure
• Core uncovery to hotleg failure
• Core Waterlevel
1.Pump seals
2.RCS natural circulation
3.Core plate failure
•Other Best Practices
Vg# 43
Station Blackout High Pressure PWR SequenceCore Uncovery – Hotleg failure
• Coolant loss and low core water level leads to RCS depressurization
• Core damage phase
• PWR valves less susceptible to high temperature conditions
Primary and Secondary PressuresSTSBO - No Mitigation
0
2
4
6
8
10
12
14
16
18
0 1 2 3 4 5 6 7 8
Time (hr)
Pre
ss
ure
(M
Pa
)Pressurizer
SG A
SG C
SG B
Hot leg C creep rupture
SG dryout
pump seal failure leads todepressurization
PumpSeal
failure
•Introduction•BWR•PWR
– Nodalization– SBO
• SBO definition• Initiation to SG
dryout• SG dryout• SG dryout to pu
mp seal failure• Core uncovery
to hotleg failure• Core
Waterlevel
1.Pump seals
2.RCS natural circulation
3.Core plate failure
•Other Best Practices
Vg# 44
Core Water Level
• Hot leg and SG natural circulation• Hot leg failure depressurizes vessel
– Accumulators dump• Partial core quench and second vessel boildown• Core damage and hydrogen generation as water in core falls
Vessel Water LevelSTSBO
-4
-2
0
2
4
6
8
10
0 1 2 3 4 5 6 7 8
Time (hr)
Tw
o-P
has
e L
evel
(m
)
Accumulators
BAF
TAF
Lower head
Vessel top
Vessel failure
•Introduction•BWR•PWR
– Nodalization– SBO
• SBO definition• Initiation to SG
dryout• SG dryout• SG dryout to pu
mp seal failure• Core uncovery
to hotleg failure
• Core Waterlevel
1.Pump seals
2.RCS natural circulation
3.Core plate failure
•Other Best Practices
Vg# 45
Seal LeakageSTSBO - No Mitigation
0
20
40
60
80
100
120
140
160
180
0 1 2 3 4 5 6 7 8
Time (hr)
Flo
w R
ate
(gp
m)
Total Seal LeakageSeals fail
Pump Seal Leakage
• Model based on Rhodes analysis of leakage and likelihood and degree of seal failure
– Seals initially leak on loss of site power and back pressure• 21 GPM
– Saturation conditions in RCS (high temperature) produces seal failure• Failure can range between 170 and 250 GPM
• Assume:– 21 GPM initially– 170 GPM at saturation
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.RCS natural circulation
3.Core plate failure
•Other Best Practices
Vg# 46
Natural Circulation Modeling
• SCDAP/RELAP5 Studies from mid-1980’s to present
• COMMIX CFD, 1987
• 1/7th-Scale Westinghouse Test, 1989-1993
• Fluent CFD Work, 2003 to present– Numerical CFD extends work
• 1/7th-scale
• Full-scale
• Westinghouse designs
• CE designs
• SCDAP/RELAP5 SGTI analysis– FLUENT support
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.RCS natural circulation
1.Background2.Assessment3.MELCOR mo
deling4.Hotleg modeli
ng5.Tube flow mo
deling6.Other modeli
ng considerations
3.Core plate failure
•Other Best Practices
Vg# 47
• 1/7th Westinghouse Assessment
– Steady state tests
– Safety valve cycles
– Hot leg fission product heating
– Hydrogen binding
• Comparison to experiment and SCDAP/RELAP5 (where available)
– In-vessel
– Hot leg
– Steam generator
Natural Circulation ModelingMELCOR Approach
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.RCS natural circulation
1.Background2.Assessment3.MELCOR mo
deling4.Hotleg modeli
ng5.Tube flow mo
deling6.Other modeli
ng considerations
3.Core plate failure
•Other Best Practices
Vg# 48
• MELCOR vessel and RCS models developed from SCDAP/RELAP5 natural circulation models
– 5 ring vessel with 2-D core and upper plenum
– Geometry and loss factors from RCS
– Zion, Surry, and Calvert Cliffs SCDAP/RELAP5 models
– New modeling approach to hot leg and steam generator natural circulation flows
• SCDAP/RELAP5 renodalizes model when natural circulation conditions are expected
– Special 2-D hot leg and steam generator model
• Application of MELCOR includes calculation of source term beyond RCS failure
– S/R5 used to predict timing and location of creep rupture failure and not subsequent events
Natural Circulation ModelingMELCOR Approach4
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.RCS natural circulation
1.Background2.Assessment3.MELCOR
modeling4.Hotleg modeli
ng5.Tube flow mo
deling6.Other modeli
ng considerations
3.Core plate failure
•Other Best Practices
Vg# 49
• Hot leg counter-current natural circulation tuned to a Froude Number correlation using results from FLUENT CFD analysis
where
g acceleration due to gravity.
Q volumetric flow rate in a horizontal duct
average fluid density (ρ)
Δρdensity difference between the two fluids
CD hot leg discharge coefficient
Natural Circulation ModelingMELCOR Approach4
Q = CD
[ g (Δρ / ρ ) D5
] 1/2
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.RCS natural circulation
1.Background2.Assessment3.MELCOR mo
deling4.Hotleg
modeling5.Tube flow mo
deling6.Other modeli
ng considerations
3.Core plate failure
•Other Best Practices
Vg# 50
• Steam generator tube to hot leg flow ratio tuned results from the FLUENT CFD analysis
• Inlet plenum subdivided into 3 regions for hot, mixed, and cold regions from plume analyses
• Steam generator mixing fractions based on FLUENT CFD analysis
– M-ratio(steam generator tube to hot leg flow ratio) = 2
Natural Circulation ModelingMELCOR Approach4
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.RCS natural circulation
1.Background2.Assessment3.MELCOR mo
deling4.Hotleg modeli
ng5.Tube flow
modeling6.Other modeli
ng considerations
3.Core plate failure
•Other Best Practices
Vg# 51
• Explicit modeling of structures in hot leg and steam generator– Convective heat transfer
• Augmented in hot leg based on FLUENT turbulence evaluations
– Gas to structure radiative exchange in the hot leg and steam generator
– Ambient heat loss through the piping and insulation
• Individual modeling of relief valves– When the valves are lumped, it creates a very large flow that
non-physically disrupts natural circulation flow patterns and the timing of the valve openings
• Creep rupture modeling– Hot leg nozzle carbon safe zone
– Hot leg piping
– Surge line
– Steam generator inlet tubes
Natural Circulation ModelingMELCOR Approach4
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.RCS natural circulation
1.Background2.Assessment3.MELCOR mo
deling4.Hotleg modeli
ng5.Tube flow mo
deling6.Other
modeling considerations
3.Core plate failure
•Other Best Practices
Vg# 52
Westinghouse PWR Core Plate
• Weight of core material mass• Engineering stress formulae used (e.g. Roark)• Failure based on exceeding yield stress at
temperature• Sequential failure of multiple supporting
structures treated
CL
Lower core plate
Flow mixer plate
Core plate Support Columns
1
Level
5
4
3
2
1
6
Ring 2 3 4 5
PLATEG (Level 6)
PLATEG (Level 5)
COLUMN (Level 4)
PLATE (Level 3)
Lower core plate
Flow diffuser plate
Core plate support columns
Core support forging
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.RCS natural circulation
3.Core plate failure
•Other Best Practices
Vg# 53
Other MELCOR Best Practices
• Some standardize some non-default input– Porosity of particulate debris
• CORZjj01 PORDP 0.4
• Some Numeric in Nature– SC-4401(3); Maximum number of iterations permitted
before solution is repeated with a decreased (subcycle) timestep.
• Some enable some previously non-default models– RN1002 – enable Hygroscopic model– FLnnnFF – KFLSH=1 enables flashing model
• Some new models activated– FLnnn02 IBUBF & IBUBT
• -1 Vapor heat transfer in pools for RCS FLs• +2 SPARC scrubbing in pools for spargers, quencher,
vents, and BWR downcomers.
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default Templa
tes– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 54
MELCOR Default Templates
! The following records updates the default by individual packageCOR_DFT 1.86CAV_DFT 1.86RN1_DFT 1.86HS_DFT 1.86CVH_DFT 1.86
! This record restore original defaults all at onceEXEC_GLOBAL_DFT 1.86
• New defaults enabled automatically in 2.1
• M1.8.6 defaults enabled as follows:
! The following records updates the default by individual packageCORDEFAULT 2.0CAVDEFAULT 2.0RN1DEFAULT 2.0HSDEFAULT 2.0CVHDEFAULT 2.0
! This record restore updates the default all at onceDEFAULT 2.0
• New defaults disabled automatically in 1.8.6
• M2.1 defaults enabled as follows:
Note: See User Guide for list of those default items changed in M2.0 default template
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default
Templates– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 55
Other Common Best Practices
Item Record Field Value(s) Description
1. BUR000 IACTV 0 (Active) Burn package activation
2. BUR1xx
(xx = CV)
IGNTR 86 for CVs where ignition is to be prohibited.
Apply to RCS control volumes to preclude combustion.
3. BUR1xx
(xx = CV)
TFRAC 1.0 Time fraction of burn before propagation to neighboring CV is allowed. Value of 1.0 means a flame must travel the radius of the control volume before propagating to its neighbor.
4. FLnnn0T ZBJT0,
ZTJT0
ZTJT0 = ZBJT0 + z
(For axial containment flow paths only)
To insure that MELCOR properly estimates vertical burn propagation in containment, drywell, reactor building, and auxiliary building, it is necessary to define “vertical” flow path “from” and “to” elevations with a small dZ. If the “from” and “to” elevations are set equal (which has been historical practice to ensure complete vertical pool drainage), the MELCOR burn package uses criteria for horizontal burn propagation.
5. FLnnnFF KFLSH 1 Calculate superheated pool flashing for all liquid LOCA connections to initially dry containment regions. KFLSH activates the model. Activate RN1Ikkk as needed for impact into specified heat structures.
6. FLnnn02 IBUBF & IBUBT -1
+2
Vapor heat transfer in pools for RCS FLs.
SPARC scrubbing in pools for spargers, quencher, vents, and BWR downcomers.
7. RN2FLTXX00 FPVAPOR Various geometric values MELCOR SPARC pool scrubbing model was modified to scrub all gaseous RN classes for
8. COR00001 DRGAP 0.0 Thickness of gas gap between fuel pellets and cladding set 0.0 to account for swelling of operating fuel.
9. COR00001A ILHTYP
ILHTRN
0
BWR =0, PWR =1
Lower head is a hemisphere
Transition is at RCOR (BWR)
or RVES (PWR)
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default Templa
tes– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 56
Other Common Best Practices
Item Record Field Value(s) Description
10. COR00009 HDBPN
HDBLH
MDHMPO
MDHMPM
TPFAIL
CDISPN
100 W/m2-K
100 W/m2-K
‘MODEL’
‘MODEL’
9999 K
1.0
This record activates the internal molten pool to lower head heat transfer models and provides reasonable solid debris to lower head heat transfer coefficient.
11. COR00012 HDBH2O
VFALL
2000 W/m2-K
0.01 m/s
HTC in-vessel falling debris to pool (W/m2-K)
Velocity of falling debris (m/s). ). Perhaps not correct for shallow pools and not necessary in deep pools since adoption of no 1-D CCFL limitation via the one-dimensional Lipinski model.
12. CORCR0 IAICON 2 For PWRs only
Activate control rod release model, 2 = Model is active, vaporization is allowed from both candling material and conglomerate.
13. CORZjj01 PORDP 0.4 Porosity of particulate debris
14. CORijj04 DHYPD Core - 0.01 m
LP - 0.002 m
Particulate debris equivalent diameter (LP values for DHYPD, HDBH2O, VFALL tuned to get appropriate end-of-pour debris temperature. 2mm based on FAERO fragmented debris size). Perhaps not correct for shallow pools.
15. CORZjjNS TNSMAX 1520 K
1700 K
Control blades failure temperature (BWR)
Core top guide failure temperature (BWR)
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default Templa
tes– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 57
Other Common Best Practices
Item Record Field Value(s) Description
16. CORijjDX FBYXSS Calculated. For BWRs only. Fraction of lower head COR cells normally displaced by control rod guide tubes should be ‘excluded’ from volume available to particulate debris. Volume recovered when tubes (as supporting structure) fails.
17. SC-1131(2) TRDFAI 2800 K Fuel rod collapse temperature (addressed with CORijjFCL records)
18. SC-1141 (2) GAMBRK 0.20 kg/m-s Molten Zr breakout flowrate parameter to yield 2 mm/s as evidenced in CORA experiments
19. SC-1701 (1) 0.01 Open volume fraction for subnode blockage criterion. This is the default setting.
20. SC-4401(3) XPASMX 15 Maximum number of iterations permitted before solution is repeated with a decreased (subcycle) timestep.
21. DCHNEMnn00 ELMNAM
ELMMAS
Use ORIGEN results for core, if available.
Elemental fission product mass at shutdown for calculation of decay heat.
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default Templa
tes– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 58
Other Common Best Practices
Item Record Field Value(s) Description
22. DCHNEMnnmm DCHEAT Use pre-combined methodology for Cs, I, and
Mo
Elemental fission product decay heat per unit mass (based on shutdown RN inventory).Define specific decay heat for CsI (Class 16) as 0.51155 of value for Class 2 (Cs) plus 0.48845 of value for Class 4 (I).
Define specific decay heat for Cs2MoO4 (Class 17) as 0.7348 of
value for Class 2 (Cs) plus 0.2652 of value for Class 7 (Mo).
If ORIGEN results are not available for the core, perform an input deck with BE burn-up and cycle history. Redistribute RN mass as follows,Class 2 initial mass represents the NUREG-1465 Cs gap mass not already included in Class 16.Class 4 initial mass is empty (10-6 kg)Class 7 initial mass is remaining Mo mass not included in Class 17.Class 16 has all I and an appropriate amount of Cs mass for CsI stoichiometry.Class 17 has the remaining Cs not included in Classes 2 and 16 plus Mo for Cs2MoO4 stochiometry.
23. DCHCLSnnn0,
DCHCLSnnnm
RDCNAM,
CLSELM
New RN
definitions for
Classes 1-12, 16-18
If ORIGEN results are available, synthesize ORIGEN data to define a single representative element for each class with decay heat data that reflects decay heat for all elements within the class (DCHNEMxxxx input.) Redefine each class to include only the representative element.
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default Templa
tes– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 59
Other Common Best Practices
Item Record Field Value(s) Description
24. DCHDEFCLS0 DEFCLS 13, 14, 15 Specifies that MELCOR DCH default classes are to be used.
25. DCHCLNORM CLSNRM ‘No’ when ORIGEN results are available.
‘Yes’ when MELCOR is used to estimate initial
inventories.
New ORIGEN input for elements/classes defines the total core decay heat.
Otherwise, let MELCOR normalize the elemental decay heats to the rated power.
Do not use RN1DCHNORM. Default behavior normalizes Class 10 (Uranium).
26. HSccccc400 &
HSccccc600
CPFPL
CPFAL
See discussion Minimum value of CVH pool fraction such that heat transfer is calculated to Pool/Atmosphere. For heat structures within the RPV, use 0.9. For PWR SG Tubes, use 0.1. All other structures modeled use default value of 0.5.
27. HSccccc401
HSccccc601
EMISWL
RMODL
PATHL
0.27
EQUIV-BAND
0.1 m
Mean emissivity of SS type 316
Equivalent band radiation model.
Nominal optical distance in steam (m).
For SS heat structures within the reactor vessel and those being monitored for creep-rupture failure.
28. HSDGccccc0 ISRCHS
ISDIST
GASNAM
HS #
1
SS
Heat structure for application of degas model.
Degassing model requires 1 mesh.
Name of released gas.
For SS boundary structures modeled with the HS package that are coupled to the core.
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default Templa
tes– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 60
Other Common Best Practices
Item Record Field Value(s) Description
29. HSDGccccc1 RHOSRC
HTRSRC
TEMPL
TEMPU
7930 kg/m3
2.68x105 J/kg
1695 K
1705 K
Gas source density.
Gas source heat of reaction.
Lower temperature for degassing.
Upper temperature for degassing.
For SS boundary structures modeled with the HS package that are coupled to the core.
30. MPMATxxxx MLT 2800 K
2800 K
Uranium-dioxide
Zirconium-oxide
Because of the interactions between materials, liquefaction can occur at temperatures significantly below the melt point. The interaction between ZrO2 and UO2 results in a mixture that is
fluid at above about 2800 K (compared to the melting temperatures of
3113 K and 2990 K, respectively, for the pure materials). Similarly, although pure B4C melts
at 2620 K, interaction with steel produces a mixture that is fluid at above about 1700 K.
31. RN1001 NUMSEC
NUMCMP
NUMCLS
10
2
20 (PWR)18 (BWR)
Default
Default
For BWR & PWR: 16 = CsI, 17 = Cs2MoO4
Now Class 17 includes default settings for Cs2MoO4.
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default Templa
tes– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 61
Other Common Best Practices
Item Record Field Value(s) Description
32. BWR structural tin release RN/DCH data for RN Class 18
For BWR: RN Class 18 = SnO2 (non-radioactive)
Define SnO2 (DCHCLSnnn0)
18 = ‘SnO2’
SnO2decay heats (DCHNEMnn00)
0 W/kg (no decay heat)
SC(7110) vapor pressures
SnO2: Log10(P(mm Hg)) = 15400/T + 8.15
SC(7111) diffusion coefficients
SnO2: Sigma = 3.617, E/K = 97
SC(7120) elem./compound molecular weights
Sn: MW = 150.7 kg/kg-mole
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default Templa
tes– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 62
Other Common Best Practices
Item Record Field Value(s) Description
33. PWR control rod RN data for RN Classes 18, 19, and 20
For PWR RN Class 18 = Ag, 19 = In, 20 = CdDefine Ag, In, Cd (DCHCLSnnn0)
18 = ‘Ag-CR’, 19 = ‘In-CR’, 20 = ‘Cd-CR’
Ag, In, Cd decay heats (DCHNEMnn00)
0 W/kg (no decay heat)
SC(7110) vapor pressures
Ag: Log10(P(mm Hg)) = 1000/T + 1.26x104 + 7.989
In: Log10(P(mm Hg)) = 400/T + 1.27x105 + 8.284
Cd: Log10(P(mm Hg)) = 500/T + 5.31x103 + 7.99
SC(7111) diffusion coefficients
Ag: Sigma = 3.48, E/K = 1300
In: Sigma = 3.61, E/K = 2160
Cd: Sigma = 3.46, E/K = 1760
SC(7120) elem./compound molecular weights
Ag: MW = 107.8 kg/kg-mole
In: MW = 114.8 kg/kg-mole
Cd: MW = 112.4 kg/kg-mole
34. RNCA100 ICAON 1 (Active) Chemisorption model is active (default).
35. RN1002 IHYGRO 1 (Active) Hygroscopic model activation. (RNACOND set to default, 0 = condensation of water onto all aerosols.)
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default Templa
tes– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 63
Other Common Best Practices
Item Record Field Value(s) Description
36. RNCRCLxx
SC7100
ICRMT / ICLSS / FRAC
(2) Zr
(3) ZrO2
(4) steel
(5) steel ox.
(6) B4C
2 / 18 / 0.0145
3 / 18 / 0.0145
0.1
1.0
0.0
0.0
0.0
For BWRs, apply the non-fuel release model. Assign aerosol generated from Zr and ZrO2 to RN Class 18 (SnO2). The mass will be added as a
non-radioactive mass to this class. The fraction of material mass available for release as an aerosol from these materials is 0.0145 (Sn fraction in Zirc-2 and -4.)
Note: must also add input for the release rate (SC7103) for RN Class 18. Values should be identical to those used (default) for Class 12 (fission product Sn).
Multipliers for various structural material types
37. RNFPNijjXX NINP
RINP1
RINP2
Use ORIGEN results, if available.
NINP = RN Class, RINP1 = mass, RINP2 = axial peaking factor. Distributes mass based on distribution developed with ORIGEN.
If ORIGEN results are unavailable, NINP = 0, RINP1 = axial peaking factor, RINP1 = radial peaking factor. Where,
ij RINP1 * RINP2 = 1.
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default Templa
tes– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10
Vg# 64
Other Common Best Practices
Item Record Field Value(s) Description
38. RNGAPijjnn NINP
RINP1
RINP2
1 (Xe) = 0.05
2 (Cs) = 1.00
3 (Ba) = 0.01
5 (Te) = 0.05
16 (CsI) = 0.05
Where, NUREG-1465 recommends the following gap quantities,Xe = 5%Cs = 5%Ba = 1%Te = 5%
39. RN2FLTXX00 FPVAPOR Various geometric values For all flow paths entering pools via quenchers or spargers, specify the flow path to scrub all gaseous RN classes.
•Introduction•BWR•PWR
– Nodalization– SBO
1.Pump seals
2.Loop seal
3.RCS natural circulation
4.Core plate failure
•Other Best Practices
– Overview– Default
Templates– Tables 1– Tables 2– Tables 3– Tables 4– Tables 5– Tables 6– Tables 7– Tables 8– Tables 9– Tables 10