Recent VVER-440 severe accident analyses with
MELCOR 1.8.6
Petr Vokáč, [email protected]
NRI Řež plc
2nd EMUG meeting, 1st March 2010, Praha SÚJB
Overview
∙ Base case model, pre-processing for specific nodalization
∙ First simulations of a shutdown accident scenario
∙ First simulations of IVR strategy
∙ Conclusions
∙ Linux/Unix issues — code portability, post-processing
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
VVER-440 Base Case Input Model for MELCOR 1.8.6
∙ already presented in detail at MCAP 2009:
– mixed BWR (for the core) and PWR (primary&sec. circ.) input model
– crossflow from the VVER-440 fuel follower channel to the bypass at the main
core region
∙ it uses minimalistic approach in order to:
– allow fast running simulations
– demonstrate the capability of the code and the input model to simulate severe
accident of a VVER-440
– allow regular testing on various accident scenarios including station blackout,
LOCAs, . . .
Pre-processing for specific nodalization
Objective: to keep consistency of input models with different nodalization
∙ Core input models are pre-processed in Python
∙ Other parts of the input deck are split into small blocks:
– generic — common parts to all input models
– parts specific to particular nodalization
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 1 of 17
∙ Everything is joined together using R*I*F command
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 2 of 17
First simulations of a shutdown accident scenario — Overdraining
Scenario: “Overdraining during the preparation to remove the reactor lid” was
selected from the existing PSA-1 according to criteria:
∙ large relative frequency
∙ preliminary estimation of the source term: large but not catastrophic
Initial plant state:
∙ about 5 days after shutdown; reactor is at atmospheric pressure, cooled by natural
circulation in two primary loops and secondary heat removal in “water-water”
mode; control rod drivers are being dismantled
Accident initiating event:
∙ at postulated time “zero” of the scenario, a draining pump is started to decrease
water level in the RPV below the reactor lid separation elevation to allow the lid
removal
∙ it is assumed that due to the wrong RPV water level measurement, the water
level in RPV drops below the hot leg nozzles, it leads to the loss of natural
circulation in the primary and to the loss of the heat transfer to secondary
(though secondary heat exchangers are still operable); then the draining pump is
stopped and draining pipes closed
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 3 of 17
Overdraining: Model adaptation
∙ Core input models from the “Base Case” without any change
∙ Three loop primary model :
– triple loop isolated from secondary and closed by the main isolation valves at
cold legs
– double loop on natural circulation
– single loop with pressuriser, it is connected to a coolant sink representing the
draining pump (coolant removal flow rate was set to maximum allowed
according to the operator guidance)
– pressuriser is open through the drained quench tank to SG boxes
– additional flow path through the reactor lid simulates the dismantled control
rod drives
∙ The secondary circuit input model is limited to heat exchanger connected to the
double primary loop on natural circulation
∙ Containment and Reactor Hall input models are the same as in the Base Case
∙ Modified initial conditions and the decay heat for shutdown conditions
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 4 of 17
Overdraining: Improved nodalization of flow paths in the loops on natural circulation
13.7 VVER-440/213, DRAIN, Model 10 06
12.3
11.0
cv231
cv225
cv232
cv073
9.7
cv290
8.3
cv220
cv280
cv270
cv071
cv260
7.0
5.6
cv210
cv017
cv010
4.3
3.0
1.6
cv015
cv014
cv012
First ring CVHs
Other core volumes not shown
cv250
cv020
0.3 Time = -204.9 s = -0.06 h, Plot record 400
∙ the hot leg nozzle is simulated by 10 small flow paths stacked vertically - to allow
smooth flow rate decrease with decrease of water level in RPV — very important
nodalization change
∙ the hot leg loop seal simulated by two flowpath to allow the counter-current flow
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 5 of 17
Overdraining: Simulation results
∙ the RPV water level at the hot leg nozzle elevation: ∼1.1 h top, ∼1.6 h bottom
∙ the coolant in RVP starts to heat-up (∼1.4 h) and then it starts to boil (∼2.2 h)
∙ the increase of coolant temperature causes the coolant volume increase and the
partial water level recovery in RPV
∙ boiling in RPV causes recovery of natural circulation in the double
loop, it is two phase circulation and it is sufficient to remove residual power from
the core (it is assumed that secondary coolant is kept at constant temperature by
a heat exchanger)
∙ the loss of coolant from primary through openings is negligible — boiling is not
intensive and in some calculations (sensitivity studies) temperature in RPV even
decreased few degrees below saturation (this occurs at ∼6 h)
∙ sensitivity study indicates that the complete loss of natural circulation
occurs only when the draining pump is left operating for almost one
hour after the start of the boiling in RPV — this is very unlikely situation
— the core damage is predicted at ∼17 h
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 6 of 17
Overdraining: Simulation results
Sensitivity study MD2: draining for 2 h, increased pressure drop on MCP
Stabilization of flow pattern in the double primary loop occurs after 6 h.
Coolant temperature in RPV is below saturation. Liquid coolant just overflows the
hot leg nozzle bottom. Decay heat is completely removed from the core to the
secondary.
13.7 VVER-440/213, DRAIN, Model 10 06 MD2
12.3
cv250
11.0
cv231
cv225
cv232
cv073
9.7
cv290
8.3
cv220
cv280
cv270
cv071
cv260
7.0
5.6
cv210
cv017
cv010
4.3
3.0
1.6
cv015
cv014
cv012
cv020
0.3 Time = 22700.1 s = 06.31 h, Plot record 1248
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 7 of 17
Overdraining: Uncertainties of simulation results
∙ the residual heat removal exchanger on secondary side: how will it behave after
loss and recovery of natural circulation in the primary loops?
∙ flow friction coefficients on loops with natural circulation; mainly on MCPs, loop
seals, SG tubes . . .
sensitivity study did not change the conclusions about the natural circulation
recovery
∙ should the primary side of SGs be drained? when?
start of the water level decrease in the hot collector of the drained loop ∼1.66 h, in
the cold collector of the drained loop ∼1.7 h
water level at the top of SG tubes in the drained loop ∼1.75 h, at the bottom of
SG tubes ∼2.68 h (if the drainage continued at this time)
start of the water level decrease in the hot collector of the double loop ∼2.91 h (if
drainage continued at this time)
∙ the size of release path in the reactor lid has negligible influence on results — the
loss of coolant is related only to the boiling rate
⇒ uncertainties are large; recovery of natural circulation should be confirmed by
a detailed TH code simulation
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 8 of 17
IVR: Development of VVER-440 input model
Assumption: the VVER-440/213 cavity can be modified for IVR without any
restriction
Objective: develop MELCOR input model for the IVR strategy
(it is not intended to verify whether IVR is successful or not)
Base case input modification steps:
1. (input model 10_02)
changes to allow draining of the bubble tower to the reactor cavity
RPV thermal isolation removed
modified cavity CVH model to cool RPV efficiently (two volumes)
2. (input model 10_05)
modification of the lower plenum COR nodalization
(two CVH volumes in the cavity)
3. (input model 10_07)
modification of the lower plenum CVH nodalization - not successful
three CVH volumes in the cavity
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 9 of 17
1. IVR with the base case model
VVER-440/213, IVR, Model 10 02
T [K]
∙ reference scenario: station blackout with the
RCS depressurisation:
BRU-A opened at 10 min,
OVKO and PORV-1 at 2 h.
Coolant mass in LP (cv020) <1 t at ∼9.2 h,
RPV fails at ∼9.5 h.
2400
2300
2200
2100
2000
1900
1800
1700
1600
1500
(warp=0.23, min(warp)=0.16; high pressure SBO min(warp)=1.29)
1400
∙ IVR scenario:
the bubble tower is drained at 2.22 h,
at ∼3.2 h water level in the cavity is above
the elliptical LH.
Coolant mass in LP (cv020) <1 t at ∼9.7 h,
RPV fails at ∼12 h.
1300
1200
1100
1000
900
800
700
600
500
400
(warp=0.25, min(warp)=0.19)
Time = 34000.2 s = 09.44 h, Plot record 505
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 10 of 17
1. IVR with the base case model
Comparison of temperature of particulate debris in the node 101 with temperature of
the RPV wall in the central ring. Temperature difference depends on amount of
coolant in the lower plenum (cv020).
VVER-440/213 M186YT NOIVR 10-02
3000
2500
20
2000
15
1500
10
1000
25
COR-TPD 101
COR-TLH#1
COR-TLH#8
CVH-MASS cv020
20
2000
15
1500
10
1000
5
500
5
500
0
7
7.5
8
8.5
9
Time [h]
9.5
10
10.5
Scenario without the cavity flooding
0
11
7
8
9
10
Time [h]
11
Scenario with IVR
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 11 of 17
12
13
CVH-MASS [t]
COR-TPD 101
COR-TLH#1
COR-TLH#8
CVH-MASS cv020
2500
Temperature [K]
VVER-440/213 M186YT IVR 10-02
25
CVH-MASS [t]
Temperature [K]
3000
2. IVR: Improved COR nodalization in the lower plenum
VVER-440/213, IVR, Model 10 05-r04
VVER-440/213, IVR, Model 10 05
T [K]
26
3200
25
3100
24
3000
23
2800
2900
2700
22
2600
2500
21
2400
20
19
18
17
2300
2200
2100
2000
16
1900
15
1800
14
1700
13
1600
1500
12
1400
11
10
09
08
07
1300
06
05
04
03
02
01
900
1200
1100
1000
800
700
1
2
3
Time = 100800.0 s = 28.00 h, Plot record 1290
4
600
5
500
400
Color key:
fu
cl
cn
ss
ns
pb
pd
mb1 mp1 mb2 mp2
Time = 100800.0 s = 28.00 h, Plot record 1290
Core and RPV wall temperatures
Core state
Simulation normal end at 28 h (110 h CPU, warp=0.25, min(warp)=0.1)
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 12 of 17
2. IVR: Improved COR nodalization in the lower plenum
The whole lower plenum (below the original followers fuel bottom) is still in one
CVH node (cv020).
Coolant in cv020 still influences heat transfer between the debris and the RPV wall.
It is just not so clearly visible as compared to the simple (base case) COR
nodalization.
VVER-440/213 M186YT IVR 10-05-r04
VVER-440/213 M186YT IVR 10-05-r04
2200
16
2000
14
3000
16
14
2500
1800
12
12
1000
6
COR-TPD.202
COR-TLH#1
COR-TLH#8
CVH-MASS cv020
800
4
600
2000
10
8
1500
6
1000
2
500
400
4
COR-TPD.505
COR-TLH#1
COR-TLH#8
CVH-MASS cv020
2
0
5
10
15
20
25
30
0
5
10
Time [h]
Ring 2
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
15
20
Time [h]
Ring 5
Slide 13 of 17
25
30
CVH-MASS [t]
8
1200
Temperature [K]
10
1400
CVH-MASS [t]
Temperature [K]
1600
3. IVR: Improved CVH nodalization in the lower plenum
13.7
VVER-440/213, IVR, Model 10 07
cv020 was split to six new volumes:
in each core ring there is a small cvh
volume adjacent to the RPV wall
cv075
11.7
cv073
9.7
cv072
cv071
7.7
cv017
cv018
cv027
cv028
cv037
cv038
cv047
cv048
cv010
5.8
cv015
cv016
cv025
cv026
cv035
cv036
cv045
cv046
cv013
cv014
cv023
cv024
cv033
cv034
cv043
cv044
cv011
cv012
cv021
cv022
cv031
cv032
cv041
cv042
cv502
3.8
1.8
cv005
cv020
cv501
cv001
cv002
cv003
cv004
-0.2
-2.1
cv201
-4.1
-6.1
Time = -1200.0 s = -0.33 h, Plot record 0
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 14 of 17
3. IVR: Improved CVH nodalization in the lower plenum
∙ Coolant from the small CVH volumes is forced away on debris/melt arrival (some
2-3 kg of liquid coolant remains).
It seems that temperature difference between RPV wall and debris is not
influenced by the remaining coolant in cv020.
∙ Calculations failed at ∼10 h due to a sudden temperature increase of vapour in
new small CVH volumes. This event is connected to the removal of the remaining
liquid coolant from the volume.
∙ Calculations with the more detailed CVH nodalization were very slow (∼10 h of
scenario consumed almost 7 days of CPU on Core 2 [email protected] GHz, warp=0.06)
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 15 of 17
Conclusions
(for both IVR and shutdown scenarios)
∙ prediction of vessel failure with the input model containing just one CVH volume
for the whole LP is not correct
∙ attempts to split LP to more CVH volumes failed:
– simulations are too slow
– simulations always fail — some problem with coolant remaining in small CVH
volumes filled with melt and debris
∙ coolant boiling at low pressure slows down the simulations
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 16 of 17
Implementation of MELCOR on UNIX based workstations
∙ presented VVER-440 simulations were calculated and evaluated on Linux (64bit
kernel) with optimised (-O1) MELCOR 1.8.6.YT executable compiled with Intel
fortran 10.0:
– no problems with optimised executable experienced
– only -O0 (no optimisation) recommended by SNL (why?)
∙ post-processing tools developed in fortran, Python (PyGTK, PYX), GNUPlot
∙ compile script converted to Makefile (without dependencies of include files)
∙ alternative platform tested: Mac Mini with Mac OS X 10.6
– it allows to run the same GNU tools as on Linux
– MELCOR plotfile is binary compatible on Linux, Mac, Windows
⇒ the same pre/post-processing tools can be used on Mac and Linux
– However Intel fortran for Mac costs $700. Is it worth to buy this compiler
just to make -O0 executable?
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010
Slide 17 of 17
Thank you for your attention
Petr Vokáč ([email protected]): Recent VVER-440 severe accident analyses with MELCOR 1.8.6
2nd EMUG, Praha SÚJB, 1st March 2010