Post-Test Calculation and Uncertainty Analysis of the Experiment QUENCH-07 with the System Code
ATHLET-CD
H. Austregesilo, Ch. Bals, K.Trambauer
Gesellschaft für Anlagen- und Reaktorsicherheit (GRS), Germany
Workshop on Evaluation of Uncertainties in Relation to Severe Accidents and Level 2 Probabilistic Safety Analysis
Aix-en-Provence, November 7-9, 2005
7-9 November 2005CSNI Workshop on Evaluation of Uncertainties - Aix-en-Provence 2
Topics
Introduction:– Short description of the code ATHLET-CD
– Short description of the QUENCH facility and of test QUENCH-07
Post-Test Calculation of QUENCH-07– Input data
– Main results of reference calculation
Sensitivity analysis– Methodology
– Main results
Conclusions
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ATHLET-CDAnalysis of Thermal-Hydraulics of Leaks and Transients
with Core Degradation
Mechanistic code for beyond design basis and severe accidents
Assessment of accident management systems and procedures
Simulation of processes in primary and secondary coolant systems:– Loss of coolant, core heat-up, degradation, melting, relocation
– Release and transport of fission products and aerosols
– Mechanical and thermal loads of reactor pressure vessel
Calculation of source terms for containment analyses
Status of development: core damage before gross relocation
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ATHLET-CD: Main Modules (I)
RCS Thermal-Hydraulics (ATHLET):– two-fluid modelling– additional balance equations for non-condensable gases– one-dimensional heat conduction within structures– control simulation module for the description of control,
protection and balance-of-plant systems
Core Degradation (ECORE):– simulation of fuel and control rods, as well as BWR core structures– mechanical fuel rod behaviour, including thermal expansion,
ballooning and cladding rupture– cladding oxidation (parabolic rate equations)– melting and relocation of cladding Zircaloy, absorber rods and guide tubes,
fuel and oxidized cladding
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ATHLET-CD: Main Modules (II)
Fission Products and Aerosol Release (FIPREM):– fission products release and diffusion inside the grain– up to 24 elements or release groups considered
Fission Product and Aerosol Transport (SOPHAEROS):– deposition, transport and agglomeration processes
– gas phase chemistry
Heat-up and Melting within Debris Bed (MESOCO):– 2D balance equations with 3 components: particles, melt and gases
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QUENCH test section and bundle cross section (FZKA 6412)
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Main Features of Test QUENCH-07
First experiment with a boron carbide absorber rod in the bundle
Main objectives:– Determination of the impact of a B4C absorber rod on a pre-oxidized
LWR fuel rod bundle at high temperatures:
Impact of absorber rod failure on fuel rod degradation
Impact of absorber rod on bundle behaviour during cooldown
– Information on gas generation after failure of absorber rod cladding and guide tube due to oxidation of B4C , in particular additional H2 generation and release of CO, CO2, CH4.
– Additional heat-up due to the oxidation of B4C and its contribution to the temperature escalation of fuel rods
– Information on the B4C - stainless steel - Zry interactions
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Boundary conditions for QUENCH-07
ATHLET−CD Post−Test Calculation of QUENCH−07
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Time (s)
0
5
10
15
20
Pow
er (
kW),
Mas
s F
low
Rat
e (g
/s)
Electric powersteam flow
Heat−up Oxidation Cooldown
Transient
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Nodalization of QUENCH test section for ATHLET-CD
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
BUNDLEBYPASS
CROSSFLOW
JACKETIN
JACKETTUBE
JACKETOUT
JACKETAR
TOPJACIN
TOPJACTUBE
TOPJACOUT
TOPJACH2O
OUTERLP
SHROUD
SHRTOP
TOPHS
OUTERWALL
OUTERTOP1
OUTERTOP2
GRID1
GRID2
GRID3
GRID4
GRID5
ROD2 ROD3
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Input Data for ATHLET-CD
Basis: standard data sets used for the calculation of previous QUENCH experiments, specially for QUENCH-06 (ISP-45)
Modelling options as recommended in the code User’s Manual, except calculation of Zr oxidation at temperatures above 1773K (correlation of Prater-Courtright instead of Urbanic-Heidrick)
Steam/argon inlet temperatures: Temperatures measured by thermocouple T511 minus 100K
External resistance per heated rod: 4 mΩ
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Calculated and measured axial temperature profiles
ATHLET−CD Post−Test Calculation of QUENCH−07
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
Elevation (m)
600
800
1000
1200
1400
1600
1800
Tem
pera
ture
(K
)
Rod2 calc.TFS2 exp. Rod3 calc.TFS5 exp. SHROUD calc.TSH exp.
ATHLET−CD Post−Test Calculation of QUENCH−07
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
Elevation (m)
600
800
1000
1200
1400
1600
1800
Tem
pera
ture
(K
)
Rod2 calc.TFS2 exp. Rod3 calc.TFS5 exp. SHROUD calc.TSH exp.
Oxidation phase (t = 2700 s) Start of transient phase (t = 3150 s)
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Test section temperatures at elevation 950 mm
code reproduces satisfactorily thermal behaviour of test bundle
clad temperatures at the end of first heat-up phase are underestimated
good agreement with respect to the start of temperature escalation due to oxidation
code overestimates shroud temperature excursion due to oxidation at the final heat-up phase
shroud failure was not reproduced by the code
ATHLET−CD Post−Test Calculation of QUENCH−07
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Time (s)
0
500
1000
1500
2000
2500
Tem
pera
ture
(K
)
ROD2, calc. TFS 2/13, 0.95m, exp.ROD3, calc. TFS 5/13, 0.95m, exp.TIT A/13, 0.95m, exp.SHROUD TT/2 Node 13, cal.TSH 13/90, 0.95m, exp.
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Integral H2 production
good agreement up to start of oxidation escalation
code strongly underestimates H2production during quench
calculation did not consider:- oxidation of external shroud surface- oxidation of Mo-electrodes
(about 22 g H2)- possible cracking of oxide layer
due to thermal shock
total amount of H2 generation:calc. : 92 gexp. : 177 g (130 g for simulated components)
H2 mass due to CR oxidation:calc.: 8.7 gexp.: 8 g
ATHLET−CD Post−Test Calculation of QUENCH−07
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Time (s)
0
50
100
150
200
Mas
s (g
)
acc. H2 out calc.H2−rods calc.H2−CR (Zr+B4C) calc.intl.H2 exp.
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Combined Sensitivity Analysis
Application of some features of the GRS methodology for code uncertainty analyses:
– Simultaneous variation of uncertain parameters
– Statistical evaluation of the code results
– Application of the Wilks formula:
number of code calculations independent of number of input and output parameters
number of code calculations only depends only on the desired statistical tolerance limits (e.g. for a two-sided statistical tolerance, a minimum of 93 calculations are required for a 95% probability and a 95% confidence level)
Support software SUSA:– input of uncertain parameters and probability distributions– automatic generation of ATHLET-CD input data sets (total: 100)– statistical evaluation of code results
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Input parameters for sensitivity analysis (I)
index parameter input data unit distribution min. value
max.value
reference calculation
discrete options
1 External heater rod resistance WHRES0 mΩ uniform 3.6 4.4 4.0
2 Deviation of steam inlet temperature to T511 DELTA K uniform -80 -120 -100
3 Factor for conductivity of shroud insulation FSUSA1 - uniform 0.8 1.2 1.0
4 Factor for heat capacity of shroud insulation FSUSA2 - uniform 0.8 1.2 1.0
5 Factor for conductivity of heater rods FSUSA3 - uniform 0.8 1.2 1.0
6 Factor for heat capacity of heater rods FSUSA4 - uniform 0.8 1.2 1.0
7 Factor for argon conductivity (top of shroud) FSUSA5 - uniform 0.5 2.0 1.0
8 Nodalization at top of bundle PAR08 - discrete 1 3 1 1: node length 10 cm2: node length 5cm3: node length 3.3cm
9 Correlation for Zr oxidation IOXMOD - discrete 15 19 15 15: Cathcart + Prater/Courtright16: Cathcart + Urbanic/Heidrick19:Leistikov + Prater/Courtright
10 Correlation for B4C –SS interaction IB4CSS - discrete 3 7 7 3:JAERI 5:Belovsky 7:Nagase
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Input parameters for sensitivity analysis (II)
index parameter input data
unit distribution min. value
max.value
reference calculation
discrete options
11 Correlation for B4C oxidation ICRB4C - discrete 1 7 7 1:mod Steinbrück5:Steinbrück7:VERDI/BOX data
12 Factor for HTC due to convection to vapour FSUSA6 - uniform 0.8 1.2 1.0
13 HTC to argon (jacket tube) PAR13 W/m2K uniform 10 20 15
14 HTC to air (containment) PAR14 W/m2K uniform 5 20 10
15 Limit of steam availability for oxidation red. OXXLIM - log. uniform 0.1 0.5 0.1
16 Limitation of protective oxide layer HROXLM mm uniform 0.1 0.3 0.2
17 Fraction of bundle area assigned to bypass flow
PAR17 - uniform 0.01 0.1 0.05
18 Gap conductivity in the heated rods HTCGAP W/m2K uniform 500 1500 500
19 Emissivity of heated rods EPS - normal σ = 0.1
0.6 1.0 0.8
20 Factor for surface area due to porosity FAREA - uniform 1.0 3.0 2.0
21 Melt temperature of absorber guide tube CRTAM K uniform 1423 1523 1473
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Uncertainty and Sensitivity Analysis of Test QUENCH 07 with ATHLET-CD 1.1LTwo-sided tolerance limits
Sample Size = 100, BETA = 0.95, GAMMA = 0.95
400
800
1200
1600
2000
2400
2800
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Time (s)
Cla
ddin
g te
mpe
ratu
re o
uter
ring
at 9
50 m
m (K
)
calc min calc max TIT A/13 exp calc medians
Cladding temperature of outer ring at elevation 950 mm
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Total generated H2 mass
Uncertainty and Sensitivity Analysis of Test QUENCH 07 with ATHLET-CD 1.1LTwo-sided tolerance limits
Sample Size = 100, BETA = 0.95, GAMMA = 0.95
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Time (s)
Tota
l gen
erat
ed H
2 m
ass
(kg)
calc mincalc maxH2 expcalc medians
130 g
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Sensitivity Coefficients
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10 12 14 16 18 20 22 24
Index of Parameter
Tim
e po
int o
f tem
pera
ture
esc
alat
ion
at 9
50m
m
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10 12 14 16 18 20 22 24
Index of Parameter
Tota
l gen
erat
ed H
2 m
ass
Total generated H2 massTime point of temperature escalation
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Sensitivity coefficients of main uncertainty parameters
Sensitivity coefficientsIndex Description Range of
variation Temperature escalation
Total H2 mass H2 due to B4C
1 electrical resistance ± 10% 0.436 -0.322 -0.324
3 conductivity of shroud insulation ± 20% 0.360 -0.304 -0.174
5 conductivity of fuel pellets ± 20% 0.324 -0.093 -0.049
7 Ar conductivity in top of shroud ± 50% 0.178 -0.261 -0.094
8 nodalization upper part of bundle 3.3-10 cm -0.095 0.139 0.215
9 correlation for Zr oxidation 15,16,19 0.195 0.134 0.005
11 correlation for B4C oxidation 1,5,7 0.151 -0.051 -0.047
12 HTC convection to cooling gas ± 20% 0.488 -0.453 -0.547
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Main findings of the sensitivity analysis (I)
Main sources of code uncertainties:– convective heat transfer between cladding and steam-argon mixture
– input value for the external electrical resistance (controls the actual power generated within rods)
– input of material properties, mainly the thermalconductivity of test section components and cooling gases
Non-prototypic aspects of the test facility have a considerable influence on code results
Small variations of input parameters can affect considerably the good agreement between experiment and prediction obtained with the reference calculation
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Main findings of the sensitivity analysis (II)
Influence of chosen nodalization not so strong as expected a priori.Finer nodalization in the upper part of test bundle leads to:
– increased cladding temperatures– earlier start of temperature escalation– higher hydrogen production
Choice of correlations for Zr oxidation plays a secondary role on code uncertainties
Main uncertainty parameters affect mostly the H2 generation before cooldown. Their influence on H2 production during quench is considerably reduced.
Modelling of B4C-SS interaction and B4C oxidation does not affect significantly the calculated global thermal behaviour nor total H2production
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Summary and Conclusions (I)
Post-test calculation of QUENCH-07 showed a good overall agreement with experimental results concerning thermal-hydraulic behaviour of test bundle
Combined sensitivity analysis provided additional information about the influence of several code input parameters and modelling options on the simulation of main phenomena observed experimentally
Results indicate that the main experimental measurements lay within the uncertainty range of the calculated data, except for the increased hydrogen production after cooldown initiation:
. Potential need for further modelling improvement
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Summary and Conclusions (II)
Main contributors to the uncertainty of code results:– heat transfer coefficient due to forced convection to steam/argon– thermal conductivity of shroud insulation– input value for the external heater rod resistance
Uncertainties on modelling of B4C oxidation do not affect significantly the total calculated H2 release rates
This study was a first step towards a more general application of the GRS uncertainty methodology for the simulation of severe accidents
Further steps shall include similar analyses for in-pile experiments (e.g. PHÉBUS) and for the TMI-2 accident, in order to extend:
– the range of phenomena covered by the simulation
– the database needed for the determination of uncertainty parameters