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A Pilot Study on The Best-Estimate Thermal-Hydraulic Analysis Methodology
Applied For a Probabilistic Safety Assessment
PSAM9
Integrated Safety Assessment Division, KAERI
Ho-Gon LIM/Joon-Eon YANG
KAERIKAERI 2
Contents
Introduction
OECD/NEA Safety Margin Action Plan
Procedure For Uncertainty Quantification – Accident Sequence Selection
– Identification Of Important Parameters
– Calculation for Nominal Plant Condition
– Calculation to Get the Distribution of The Event Sequence
– Quantification of Failure Probability of The Event Sequence
Conclusion And Future Works
KAERIKAERI 3
Introduction
A thermal/hydraulic (T/H) calculation and an expert judgement are used to develop event trees (ET).
Conservative approach was implemented to compensate the uncertainty.
– The conservative approach can distort the results of PSA (shadoweffect).
Recently, the use of best-estimate T/H code is emphasized
– ASME PRA Standard
There is a concern about hot to deal with the uncertainty caused by to the best-estimate analysis
– We proposed a procedure to quantify the uncertainty due to the best-estimate analysis
KAERIKAERI 4
OECD/NEA Safety Margin Action Plan
STATESmallLOCA
GSLOCA
Long TermCooling
LTC
Feed andBleed
Operation
FB
SecondaryHeat
Removal
SHR
LPSI
LPSI
OperatorAction for
LPSIInjection
O/A
HPSI
HPSI
ReactorSub-
Criticality
RT
OK
ToATWS
CD
CD
OK
CD
CD
OK
CD
CD
Conditionalsuccess/
failure
CSF
Calculate the distribution of safety criteria (PCT, RCS pressure)Compare to Safety Limit (ex. 2200 oF) and calculate exceeding probabilityAssign success/failure probability of a event scenario using new event tree heading
KAERIKAERI 5
Procedure for Uncertainty Quantification
Construct the ET based on the Nominal Plant Data
– Eliminate conservative bias
Screening– Fatal failure or obvious success
• Failure of RHR
• Normal shutdown procedure in ET
– Insignificant accident scenario• F(ETi)< ε
Identification of Parameters – Important parameters affecting
accident progression• Code and plant specific parameters
Calculation based on Random Sampling
– 10CFR50.46 alternative• Response surface construction
– 100 random sampling in the present study• Sampling method is not fixed yet.
Feedback to existing ETs or conditional failure probability quantification
KAERIKAERI 6
Accident sequence Selection(1/2)
Anticipated Transient Without Scram (ATWS)
Characteristics of a selected scenario– Composed of successful turbine trip, favourable exposure
time, PSV reseat, and so on
– An event sequence with the largest frequency
– Large uncertainty exist due to reactivity feedback and the pressure relieving characteristics
– The unfavourable exposure time (UET) is determined based on the nominal plant data
KAERIKAERI 7
Accident sequence Selection(2)
ATWS first scenario #1 in Ulchin 3&4 in Korea
GATWS
AnticipatedTransients
WithoutScram
T_TRIP
Turbine Trip
MTC
FavorableModerate
Temp.Coefficient
PSV
PressurizerSafety Valve
Reseat
CSGTR
ConsequentialSGTR
AFW
DeliverAuxiliary
Feedwater
SR1
SteamRemovalvia TBVsor ADVs
SR2
SteamRemoval
via MSSVs
EBOR
EmergencyBoration
SDC
EstablishShutdown
Cooling
MSHR
MaintainSecondary
HeatRemoval
BD
BleedRCS
HPI
HPSISInjection
HPR
HPSISRecirculation
CSR
RecirculationCooling
SSDC
Switch toShutdown
Cooling
G-U3-IE-ATWS
GSCGTOP
GMHTOP-N
GCSRCTOPGHSRGTOP
GHSIGTOPGSDOL
GMXEBOR
GMSADV1OF4
GMHTOP-N
GCSRCTOPGMXSSDC
GHSRGTOPGHSIGTOP
GSDOLGMXEBOR
GMSMSSVALLO
GCSRCTOPGMXSSDC
GHSRGTOPGHSIGTOP
GSDOE
GAFTOP-N
GCSRCTOP GMXSSDCGHSRGTOP
GHSIGTOPGSDOE
sgi
PSVGCSRCTOP
GHSRGTOP-LMLGHSIHTOP-ML
G-U3-AT-SQ34
GSCGTOP
GMHTOP-N
GCSRCTOPGHSRGTOP
GHSIGTOPGSDOL
GMXEBOR
GMSADV1OF4
GMHTOP-N
GCSRCTOPGMXSSDC
GHSRGTOPGHSIGTOP
GSDOLGMXEBOR
GMSMSSVALLO
GCSRCTOPGMXSSDC
GHSRGTOPGHSIGTOP
GSDOE
GAFTOP-N
GCSRCTOP GMXSSDCGHSRGTOP
GHSIGTOPGSDOE
sgi
PSVGCSRCTOP
GHSRGTOP-LMLGHSIHTOP-ML
G-U3-AT-SQ34
SEQ#
STATE
STATE
FREQ
1 OK 2 OK 3 OK 4 CD 5 CD 6 CD 7 CD 8 CD 1.092E-07 9 OK 10 OK 11 OK 12 CD 13 CD 14 CD 15 CD 16 CD 17 OK 18 OK 19 CD 20 CD 21 CD 22 CD 23 OK 24 OK 25 CD 26 CD 27 CD 28 CD 29 to CSGTR 30 OK 31 CD 2.489E-10 32 CD 1.962E-10 33 CD 1.704E-10 34 CD 2.24E-07 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68
KAERIKAERI 8
IDENTIFICATION OF THE IMPORTANT PARAMETERS (1/2)
It is necessary to know the parent distribution effectively under the given calculation resources
General Procedure for Parameter Selection– Identification of important phenomena in the accident
scenario
– Identification of parameters in code describing the important phenomena
– Ranking their importance in terms of ΔPCT or Δpop
In the Present analysis– Simplified phenomena identification and parameter selection
procedures
– No ranking process
KAERIKAERI 9
IDENTIFICATION OF THE IMPORTANT PARAMETERS (2/2)
phenomena parameter variable distribution Selection
Dist. 2σ min max Nominal(mean)
opening set pressure(bar) Normal 2.76 171.03 175.17 172.41 O
PSV discharge rate(kg/s) Normal 10.79 57.96 79.55 68.75 O
Fuel-Clad conductivity XClad-coolant conductivity Xinitial coolant temperature X
Pressurizer solid state pressurizer water level (meter) 0.67 -0.381 0.9652 0(50%) OAux. capacity 500 gpm bound X
delay time 45 second bound XAux. temp.(K) 22.22 277.44 321.89 293 O
Convective heat transfer coeff. Xturbine trip time XMSSV flow rate 134.61 722.75 991.97 857.34 X
MSSV set pressure 1.03 85.52 87.59 86.21 OMSIV closing
Time 5 x
Heat transfer to secondary side
Reactivity feedback
RCS pressure relieving valve
capacity
KAERIKAERI 10
Nominal Event Calculation (1/3)
Event history for nominal plant stateTime (sec) Event
0.0 Loss of main feedwater (representative ATWS IE)
42.8 RCS trip set point by PZR high pressure(164 bar)
53.5 Auxiliary Feed water actuation set point by S/G low level
63.8 S/G dryout
72.01 PZR Safety valve opening set pressure
81.9 First PZR blowdown pressure
83.72Main steam line isolation valve (MSIV) closing set point by low
pressure
86.0 Second opening of PZR safety valve
88.72 Complete closing of MSIV
95.1 PZR solid state
98.5 AFW start
~100 Hot leg saturation pressure
115.1 RCS peak pressure
159. MSSV set point by S/G high pressure
1000.0 Calculation terminated
KAERIKAERI 11
Nominal Event Calculation (2/3)
Temporal Behavior of plant main parameter
0 200 400 600 800 10000
500
1000
1500
2000
2500
3000
Rea
ctor
Pow
er(M
W)
T ime(second)
Thermal Power
0 200 400 600 800 1000
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Rea
ctiv
ity(d
ollo
r)
T ime(second)
Reactivity total doppler moderator
0 200 400 600 800 100040
80
120
160
200
240
280
Pre
ssur
e(X
0.1
Mpa
)
Time(second)
Pressure Pressurizer S/G
0 200 400 600 800 10000.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
PZR
Lev
el
Time(second)
pressurizer Level
• Reactor Power • Reactivity
• RCS & S/G pressure • PZR level
KAERIKAERI 12
Nominal Event Calculation (3/3)
Determination of UET (Unfavorable Exposure Time) according to the fuel burn-up
0 100 200 300 400
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
Pre
ssur
e
Time(second)
a pcm/oF b pcm/oF c pcm/oF
Pressure Limit(3200 psi)
KAERIKAERI 13
Calculation to Get the Distribution of The Event Sequence (1/2)
100 sampling calculation for the selected parameters based on their probability density function)
95/95 approach based on Wilk’s formula is not applied
The calculation by the nominal data is covered well by the sampling calculation results
There is a bifurcation in the calculation, which has frequently appeared in a BE T/H calculation
0 20 40 60 80 100 120 140
14
16
18
20
22
24
26
28
Pressure by Norm inal Data
Pre
ssur
e (M
pa)
T im e(second)
KAERIKAERI 14
Calculation to Get the Distribution of The Event Sequence (1/2)
The distribution has a camelback shape due to a bifurcation of the calculation
The failure probability of the event sequence is given as 0.55 approximately
22 23 24 25 260
5
10
15
20
25
30
Rel
ativ
e Fr
eque
ncy
Pressure(Mpa)
RCS pressure distribution (Number of run = 93)
21 22 23 24 25 260.0
0.2
0.4
0.6
0.8
1.0
Cum
ulat
ive
Prob
abili
ty
Pressure(Mpa)
RCS Pressure Limit
KAERIKAERI 15
Quantification of the Event Sequence Failure Probability
In a PSA, the frequency of success scenario is not quantified – Conservativeness of success scenario
Event Scenario frequency
The frequency of success sequence is 6.183E-6– The failure frequency is 3.4E-6
⎟⎟⎠
⎞⎜⎜⎝
⎛⋅⋅= ∏ ∏
−
= −−
mn
j
n
mnkkjESES
SSpffi
1
iESf
jS
ESf
: Frequency of the event sequence, i
: The failure event of system, j : Initiating event frequency
KAERIKAERI 16
Conclusion & future work
Conclusion– Best-estimate thermal-hydraulic approach to calculate the
uncertainty of an event sequence is implemented
– ATWS was selected as a pilot study
– The RCS pressure was used for the core damage criterion
– Although all the important parameters was not identified and used in the calculation, we demonstrated the possibility that the conditional failure probability can be calculated through the best-estimate T/H calculation
Future Work– Determination of sampling method and number of sampling
– Application for whole event trees considered in a PSA model