Post on 12-Mar-2018
Insights from the Peach Bottom SOARCA
Uncertainty Analysis
S. Tina Ghosh and Edward L. Fuller, USNRC
Kyle W. Ross and Randall Gauntt, Sandia National Laboratories
IAEA Workshop on Advances in Understanding the
Progression of Severe Accidents in Boiling Water Reactors
Vienna, Austria
17-21 July 2017
Outline
• Goals and approach
• Description of PB SOARCA LTSBO scenario
• Selected parameters and bases
• Results
• Phenomenological insights from MELCOR
analyses
• Conclusions
2
3
Goals of the
Uncertainty Analysis
• Develop insight into overall sensitivity of
SOARCA results to uncertainty in inputs
• Identify most influential input parameters for
releases and consequences
• Demonstrate uncertainty analysis methodology
Approach
• Investigated parameter uncertainty
• Focus on epistemic (state-of-knowledge)
uncertainty in input parameters for MELCOR
and MACCS simulations
• Peach Bottom, unmitigated, long-term station
blackout scenario chosen
• Scenario definition not changed after Fukushima
4
5
Approach (continued)
• Key uncertain input parameters were identified
• Uncertainty in these parameters propagated in
two steps using Monte Carlo and Latin
Hypercube (LHS) sampling: – A set of source terms generated using MELCOR model
– A distribution of consequence results generated using
MACCS model
• Three epistemic sample sets of 300 generated
to complete a corresponding number of
individual code runs (Monte Carlo
“realizations”) to evaluate the influence of the
uncertainty on the estimated outcome
Approach (continued)
6
• Results reported included:
– Analysis of source term releases, in particular, cesium and
iodine release over time
– Distribution of latent cancer fatality risk, with three dose
threshold models
– Description of most influential uncertain parameters in study
• Tools used to analyze results include statistical
regression-based methods as well as scatter plots and
phenomenological investigation of individual realizations
of interest
• Guidance solicited from SOARCA peer reviewers on the
uncertainty analysis plan documenting the approach,
chosen parameters and distributions
Process for Choosing
Parameters and Distributions
• Core team of staff from SNL and NRC with
expertise in probability and statistics, uncertainty
analysis, and MELCOR and MACCS modeling
for SOARCA
• Subject matter experts (SMEs) provided support
in reviews of data and parameters
• Approach is based on a formalized PIRT
(phenomena identification, and ranking table)
process.
7
Process (continued)
• Focus on:
– confirming that the parameter representations
appropriately reflect key sources of
uncertainty, and
– ensuring model parameter representations
(i.e., probability distributions) are reasonable
and have a defensible technical basis.
8
Process (continued)
• Attempt to obtain contribution from uncertainty
across the spectrum of phenomena operative in
the analyses, through a balanced depth and
breadth of coverage
• A subset of possible uncertain parameters is
proposed that cover the range of phenomena
across the stages of a severe accident.
9
10
Probabilistic Uncertainty analysis is an iterative process
Scenario Selection current MELCOR model/software
current WinMACCS model/software
Inputs & Parameter
Selection informal expert elicitation from SMEs
& Peer Review
Uncertainty Analysis
Framework MELCOR Uncertainty Engine
MELCOR
MELMACCS
WinMACCS/LHS
Uncertainty Analysis partial rank regression correlation
step-wise rank regression correlation
scatter plots
Single Realization phenomenological analysis
Stability Analysis # realization stability
random seed stability
alt. modeling cases
Iterate Range of validity
Stability of analysis
Alternative model scenarios
SRV Stochastic
SRV Thermal
MSL Creep Rupture
Sensitivity Analysis
Address Bugs or Modeling Errors
Analysis Complete
Event Times for PB SOARCA
Unmitigated LTSBO (NUREG-CR-7110)
Event Time (hr)
Loss of all onsite and offsite AC power (SBO) 0.0
Low-level 2 and RCIC actuation signal 10 minutes
Operators manually open SRV to depressurize RPV 1.0
RPV pressure drops below LPI setpoint (400 psig) 1.2
Battery depletion and immediate SRV re-closure 4.0
Steam line floods and RCIC flow terminates 5.2
SRV sticks open because of excessive cycling 8.2
Downcomer water level reaches top of active fuel 8.4
First hydrogen production 8.9
First fuel-cladding gap release 9.1
First channel box failure 9.3
Water level reaches bottom of lower core plate 9.3
First localized failure of lower core plate 9.6
First core cell collapse due to time at temperature 9.8
Large-scale relocation of core debris to lower plenum 10.5
Lower head dries out 13.3
Lower head failure 19.7
Drywell head flange leakage begins 19.9
Hydrogen burns initiated in reactor building 20.0
Refueling bay to environment blowout panels open 20.0
Hydrogen burns initiated in refueling bay 20.0
Drywell shell melt-through initiated 20.0
Hydrogen burns initiated in lower reactor building 20.1
Railroad access opens due to overpressure 20.1
Refueling bay roof fails due to overpressure 20.2
Calculation terminated 48.0
11
Table 5-1 Timing of Key Events for LTSBO
RPV Pressure
(PB SOARCA LTSBO)
12
0
200
400
600
800
1000
1200
1400
0 2 4 6 8 10 12 14 16 18 20 22 24
Pre
ssu
re [
psig
]
time [hr]
RPV Pressure
Batteries exhaust
- SRV recloses
Operator manually
opens 1 SRV SRV seizes open
Lower head
failure
Initial debris
relocation intolower head
RPV water level
(PB SOARCA LTSBO)
13
0
100
200
300
400
500
600
700
800
0 2 4 6 8 10 12 14 16 18 20 22 24
Tw
o P
ha
se
Mix
ture
Le
ve
l [i
n]
time (hr)
In-Shroud
Downcomer
RPV Water Level
Automatic RCICactuation
Operator takes manualcontrol of RCIC
RCIC steamline floods
Large scale debrisrelocation into
lower head
+5 to +35"
Batteries exhaust- SRV recloses
Main Steam Line Nozzle
Top of Active Fuel (TAF)
Bottom of Active Fuel (BAF)
SRV sticks open
Containment Pressure
(PB SOARCA LTSBO)
14
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50
Pre
ssu
re [
psig
]
time [hr]
Containment Pressure
DW liner melt-through(head flange leak recloses)
SRV sticks open
DW head flangeleakage begins
Debris relocationinto lower head
LTSBO Source Term to
the Environment
15
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0 6 12 18 24 30 36 42 48
Fra
cti
on
of
Init
ial
Co
re I
nve
nto
ry
time [hr]
Fission Products in Environment
NG
I & Te
Cd
Ba
Sn
Ce
La
Ru
Mo
Containment failure
Cs
see next figure for detail in this range
LTSBO iodine fission
product distribution
16
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 6 12 18 24 30 36 42 48
Fra
cti
on
of
Init
ial
Co
re I
nven
tory
time [hr]
Iodine Distribution
Release toenv. (2.0%)
Captured in Suppression Pool
Deposited/Airbornewithin RPV
Drywell (negligible)
Releasedfrom fuel
LTSBO cesium fission
product distribution
17
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 6 12 18 24 30 36 42 48
Fra
cti
on
of
Init
ial C
ore
In
ven
tory
time [hr]
Cesium Distribution
Release toenvironment (0.5%)
Captured in Suppression Pool
Deposited/Airbornewithin RPV and steam lines
Drywell (negligible)
Releasedfrom fuel
18
Speciation of Cesium and
Iodine • Based on Phebus program
findings
– Iodine treated as CsI
– Cs treated as CsI and
Cs2MoO4
• Cs2MoO4 considerably less
volatile than CsOH or CsI
– Affects retention in RCS and
long term revaporization
• Uncertainty analysis
exploring alternative
balance of speciation
– I2, CsOH, CsI and Cs2MoO4
1000 1500 2000 2500 3000 35001 10
4
1 103
0.01
0.1
1
1010
0.0001
P Cs Ti
atm
P CsI Ti
atm
P CsMo Ti
atm
P Mo Ti
atm
3.5 103
1.025 103
Ti
K
Mo
Cs2MoO4
CsI
CsOH
va
po
r p
res
su
re -
atm
Temperature – K
SRVLAM – SRV stochastic failure to
reclose (use PB IPE distribution)
19
20
BWR SRV Seizure Modeling (from PB SOARCA report)
10
in
6 in
Pilot Valve
Assembly
In severe accident conditions, high
temperature gases well exceed
design conditions
Top ~ 600K
TSA > 800 to 1100K
cycles for hours
Seizure and sticking open eventually
occurs
excessive cycling
thermal deformation
partial or full open
Valve behavior important to accident
progression 0
200
400
600
800
1000
1200
1400
0 2 4 6 8 10 12 14 16 18 20 22 24
Pre
ssu
re [
ps
ia]
time [hr]
RPV Pressure
Batteries exhaust- SRV recloses
Operator manuallyopens 1 SRV SRV sticks open
Lower headfailure
Large scale debrisrelocation intolower head
Modes of Valve
Seizure
• Excessive cycling
• Differential
thermal
expansion
• Material
deformation
SRV Thermal Failure Modeling (from PB SOARCA report)
• In severe accident conditions, high temperature
gases well exceed design conditions – Top ~ 600K
– TSA > 800 to 1100K
– cycles for hours
• SOARCA Best Estimate Boundary
Conditions: – Gas flow through the valve is based on a cycle period of
45 seconds with an open cycle duration of 5 seconds
– Gas velocity during an open cycle is constant during a
single five second cycle, but increases from 420 m/s to
500 m/s as the gas temperature increases from 600 K
to 1100 K
– Gas temperature as a function of time. Approximately
100 K temperature changes between the beginning and
end of a cycle were neglected. The mid-point values
were used
– The steam and hydrogen mole fractions were specified
as a function time 21
Approx. 3-in
6 in
Valve stem sleeve
Valve stemValve disc
Ring piston
600
650
700
750
800
850
900
950
1000
1050
1100
600 620 640 660 680 700
time [min]
Tem
pera
ture
[K
]
Gas Temp
Valve Stem: Centerline of 3-in diam SS cylinder
Valve Body: Outer surface of 1-in thick CS cylinder
Valve Body: Mid-point of 750 lb CS cylinder
~140K~20K
SRVOAFRAC – SRV open area fraction
22
SLCRFRAC – Main steam line creep
rupture area fraction
23
24
Modeling Melt
Progression Stages
U O2
Z r O2
2 9 0 0 K3 1 0 0 K
s o lid
liq u id
2 p h a s e
2 9 0 0 K
U O2
/Z r O2
Q u a s i B in a r y E q u ilib r iu m D ia g r a m
U Zr
O
UO 2 ZrO2(3 1 0 0 K )
(2 8 0 0 K )
ZrO(2 2 0 0 K )
(2 9 0 0 K )
2 2 5 0 K
3 1 0 0 K
2 6 7 3 K
s o lid
liq u id2 p h a s e
Z r ( O )U O 2
Z r ( O ) /U O2 E q u ilib r iu m P h a s e D ia g r a m
2 1 5 0 K
2 9 0 0 K
Z rZ r O2
s o lid
liq u id2 p h a s e
Z r /Z r O 2 Q u a s i B in a r y E q u ilib r iu m P h a s e D ia g r a m
Z r ( O )
T > 1000K
rapid
oxidation
UO2-
ZrO2
liquefac-
tion at
2800K
molten Zr
Breakout
2400K
2800K Zr metal melt at 2150K
UO2 dissolution in Zr metal ~20%
Zr melt breakout ~2400K
Loss of rod geometry ~2600K
UO2-ZrO2 liquefaction ~2800K
Parameters uncertain
oxidizing
Molten Zr
T > 2100K
Cumulative distribution function for flow area
resulting from drywell liner failure for PB
unmitigated LTSBO Replicate 1, with samples
identified that have a surge of water from the
wetwell during depressurization of the drywell
25
Railroad Doors
• Railroad doors opening
creates a chimney effect
leading to increased
releases to the environment;
actual opening area is not as
important as whether they
are open or closed
• This effect was noted in the
SOARCA best estimate and
has been noted in previous
MELCOR studies.
26
CV412(Refueling Bay)
CV411
CV409
(195' SE
Quadrant)CV407
(195' NE
Quadrant)
CV405
(165' SE
Quadrant)
CV404
(165' North Half)
CV403
(135' South Half)
CV402
(135' North Half)
FL407
(open hatch)
FL423
FL902(DW liner shear)
FL422
(open hatch)
CV401 (Torus Rm)
FL425 (blowout panels)
CV
90
3
(En
viro
nm
en
t)
CV401
FL414
(open hatch)
FL017(DW nom leakage)
FL401 (open floor grating)FL402 (open grating)
FL404 (E)
FL408 (E)
FL409 (W)
CV410
(195' SW
Quadrant)
FL416 FL417
Dryer - Separator Storage Pit
divides the SE/SW Quadrants
CV408
(195' NW
Quadrant)
FL415
Spent Fuel Pool Volume
included in CV412
which divides
NE/NW Quadrants of 195'
FL403 (W)
Section A-A
CV570
(Equip Access
Airlock)
FL445FL446
N
A
A
B
B
CV
90
1
(En
viro
nm
en
t)
FL424 (nom. leakage)
FL421 (roof failure)
FL
90
3
CV406
(165' SW
Quadrant)
FL410
FL904
Drywell Liner
Melt-
Through
MELCOR Uncertain Parameters
27
Sequence Issues
• Battery duration
• SRV stochastic failure rate,
thermal seizure criteria, and
open area fraction
In-Vessel Accident Progression
• Main steam line (MSL) creep
rupture open area fraction
• Zircaloy melt breakout
temperature
• Molten clad drainage rate
• Fuel failure criterion
• Debris radial relocation time
constants
Ex-vessel Accident Progression
• Debris lateral relocation time
constants
Containment & building behavior
• Drywell liner failure flow area
• Drywell head flange leakage
parameters
• Hydrogen ignition criteria
(where flammable)
• Railroad doors open fraction
Fission Product release,
transport, and deposition
• Cesium and Iodine chemical
forms
• Aerosol deposition parameters
28
Cesium Release Fractions
for the Three Categories of
SRV Failures Observed (one
horse hair for each individual
realization)
• Stochastic stuck-open SRV failures
• SRV seizures at high temperature
• SRV seizures leading to main
steam line creep rupture
• Some individual realizations singled
out for closer examination in the
report
Influences strongly affecting timing and
magnitude of environmental releases
• Whether an SRV sticks open before or after the onset of core
damage, with higher releases if SRV sticks open after core
damage
• Whether a MSL creep rupture occurs, with higher releases if MSL
rupture occurs (partial suppression pool bypass)
• The amount of cesium chemisorbed as CsOH onto the stainless
steel of RPV internals (more chemisorption means less cesium
release to the environment)
• Whether core debris relocates from the RPV to the reactor cavity
all at once or over an extended period of time (relocation all at
once leads to lower releases to the environment,
• The degree of oxidation, primarily fuel-cladding oxidation,
occurring in-vessel with greater oxidation resulting in larger
releases, and
• Whether a surge of water from the wetwell up onto the drywell
floor occurs at drywell liner melt-through (draining of surge water
into reactor building leads to larger releases).
29
Sampled parameters strongly
influencing the magnitude of the
fission product releases
• The expected number of cycles an SRV can undergo before
failing to reclose (i.e., remain in the fully open position),
• The chemical form of cesium (i.e., the amount of cesium as CsOH
as opposed to Cs2MoO4),
• The size of the breach in the drywell liner resulting from core
debris contacting and melting through the liner,
• The fractional open area of an SRV after it has failed to reseat
because of overheating,
• The time-at-temperature criterion specified for loss of “intact” fuel
rod geometry, and
• The temperature at which oxidized cladding mechanically fails.
30
Sampled parameters strongly
influencing the timing of the fission
product releases
• When the RCIC system fails
• When a SRV fails to reseat,
• The open fraction of a SRV after it fails to reseat given a
thermally-induced failure.
• The time to station battery failure
• The number of cycles an SRV can undergo before failing to
reclose (i.e., remain in the fully open position)
• The fractional open area of a SRV after it has failed to
reseat because of overheating
31
Conclusions
• The use of more advanced non-linear regression techniques
proved to be advantageous because they capture interaction
effects and non-monotonic effects missed by the linear rank
regression technique.
• Interaction effects among variables and non-monotonic effects
are common in complex systems, such as nuclear power plant
systems and environmental factors during and after a severe
accident.
• The use of select single-realization analyses in this
uncertainty analysis proved useful in validating the results of
the statistical regression analyses through phenomenological
explanations.
• A major determinant of source term magnitude is whether the
SRV sticks open before or after the onset of core damage.
– Compounding this effect is whether or not MSL creep rupture occurs,
which leads to higher environmental releases and consequences.
32
Questions?
33
Acronyms
BWR Boiling water reactor
DW Dry well
LCF Latent cancer fatality
LHS Latin Hypercube Sampling
LNT Linear no threshold
LTSBO Long-term station blackout
MSL Main steam line
ORNL Oak Ridge National Laboratory
PIRT Phenomena Identification and Ranking Table
RN Radionuclide
RPV Reactor Pressure Vessel
SME Subject matter expert
SNL Sandia National Laboratories
SOARCA State-of-the-Art Reactor Consequence Analyses
SRV Safety relief valve
SST1 Siting study source term 1
UA Uncertainty Analysis
34
BACKUP SLIDES (PARAMETER DISTRIBUTIONS)
35
BATTDUR – Battery Duration
36
SC1141(2) – Molten clad drainage rate
37
FFC – Fuel failure criterion
38
FFC – Fuel failure criterion (continued)
39
Radial debris relocation time
constants – RDSTC (solid)
40
Radial debris relocation time constants – RDMTC
(liquid)
41
Drywell head flange gap
area versus pressure
42
FL904A – Drywell liner failure flow area
43
Water level in containment for Realization 170
of the Peach Bottom unmitigated LTSBO
Replicate 1, an SRV Stochastic failure (water
drains into reactor building from DW)
44
Chemical form of Cesium
• Chemical form of Cs as CsOH (present in bins
1, 2, and 4) can have an important effect
limiting the mass available for release to the
environment due to chemisorption for high
temperature scenarios like MSL creep rupture
conditions
• CsOH can slightly increase releases for lower
temperature accident progression events like
stochastic failure of SRV
45
CHEMFORM – Iodine and cesium
fraction
46
Parameter Distribution
CHEMFORM: Five alternative combinations of RN
classes 2, 4, 16, and 17 (CsOH, I2, CsI, and Cs2MoO4)
Note the fraction cesium below represents the
distribution of 'residual' cesium which is the mass of
cesium remaining after first reacting with the amount of
iodine assumed to form CsI.
Discrete distribution
Combination #1 = 0.125
Combination #2 = 0.125
Combination #3 = 0.125
Combination #4 = 0.125
Combination #5 = 0.500
Five Alternatives Species (MELCOR RN Class)
CsOH (2) I2 (4) CsI (16) Cs2MO4 (17)
Combination #1 fraction iodine -- 0.03 0.97 --
fraction cesium 1 -- -- 0
Combination #2 fraction iodine -- 0.002 0.998
fraction cesium 0.5 -- -- 0.5
Combination #3 fraction iodine -- 0.00298 0.99702 --
fraction cesium 0 -- -- 1
Combination #4 fraction iodine -- 0.0757 0.9243 --
fraction cesium 0.5 -- -- 0.5
Combination #5 fraction iodine -- 0.0277 0.9723 --
fraction cesium 0 -- -- 1
SOARCA estimate Fraction iodine -- 0.0 1.0 --
Fraction cesium 0.0 -- -- 1.0
RRIDRFRAC, RODRFRAC – Railroad
door open fraction
47
H2IGNC – Hydrogen ignition criteria
48
RHONOM – Particle density
49