020-060

May 2-3, 2013

Reliability, Safety and Management Engineering and Software Development Services

Objectives;

Variants of the nodalizations considered;

Conditions and assumptions;

Cases analyzed, initial and boundary conditions;

Important parameters for the sensitivity studies;

Summary results from the sensitivity studies;

Issues identified during the sensitivity studies;

Conclusions;

References.

Main objectives of the presentation are as follows:

o

To discuss the results from sensitivity studies which are performed in order to be analyzed WWER-1000 plant model response during severe accident progression when three different MELCOR 2.1 nodalizations for the core region are used. These three nodalizations are respectively with one, five and thirty hydrodynamic volumes in the reactor core region;

o

To discuss the results from sensitivity studies which are performed in order to be analyzed WWER-1000 plant model response during severe accident progression when two different MELCOR 2.1 nodalizations for the steam generators secondary side are used. These two nodalizations are respectively with one and three hydrodynamic volumes for the

steam generators secondary side;

o

To represent and discuss various MELCOR 2.1 and/or plant model issues which have been identified during the performance of the sensitivity

studies.

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The nodalization schemes for the reactor pressure vessel and reactor

internals, the primary loops and the steam generators secondary side are

shown on Figure 1, Figure 2 and Figure 3 respectively .

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010-020 010-020

010-020

CV013 CV035

CV046

CV032 CV037

CV034 CV055 CV050

CV027

CV054 CV014

CV026

CV025

CV033 CV024

CV053 CV043 CV023

CV056

CV044

CV022 CV012

CV047

CV042 CV016

CV057

CV045 CV017

CV052 CV036

CV015 CVX00

CVX09 (CV110*)

CV040

X=1,2,3,4

035-060 034-060

033-060 032-060

031-060 027-060

057-050

x09-010 (110-010*) 29,440

035-040

CVX09 (CV110*)

16.899

18.519 060-070

070-050

18.419

040-060 24.930

020-040 22.928

020-060

CV020

CV040

CV070

CV050

CV091

CV060 CV010

22.928 047-060

CV032

033-034 031-032 034-035 032-033 035-040

23.728

CV031

CV033

091-092

057-060 047-050

CV034

CV035

CV092

020-031 020-040

26.753

020-032 020-033

020-060

020-034 020-035

050-091 24.647

040-050 040-060 040-050

070-092

037-060

CV010

CV060

CV070

031-050 032-050

020-012 033-050

034-050

16.899 035-050

050-x00

020-022 017-050

24.930

x09-010 (110-010*)

CVX00 29,440

CVX09 (CV110*)

16.899

26.953 26.953

020-032 030-060

18.519

020-040

020-030 060-070

070-050

18.419

060-070

020-042 027-050 24.930

040-050

020-060

040-060

29,440

030-050

CV030

CV020

CV040

020-052

CV070

CV050

CV060 CV010

18.519 030-040

050-x00

x09-010 (110-010*)

22.928

CV020

017-060 037-050

050-x00 CVX00

o Figure 1: Reactor pressure vessel and reactor internals nodalizations (for all the cases 5 radial rings in the COR package are modeled)

x05-x06

x00-x04 25.275

CVx01 CVx05

32,17

x24-x25

x06-x07 x26-x27

x07-x08 x23-x24

22,18 CVx23

x20-x21 CVx02CVx03

x21-x22 x27-x28

109-110*

26.125

23.475

x04-x05

CVx25

CVx08

CVx27

CVx00

24.000 30,128

29.140 x28-x05

20,215 20,215

CVx06

32,17

29.140

CVx04

CVx09

30,128

27,938 27,938

24.325

x25-x05

x01-x02

CVx28

CVx20 x02-x23

CVx26

CVx07 23.900

x08-x09 x02-x26

CVx21 CVx22

CVx24

25.700

20,640 x02-x03

CV110

x22-x05 x02-x20

21,065 23.900

x09-010 (110-010*) 25.700 050-x00

28,0395 28.7565 29.5545

CV050

CV010

CV458 CV458

110-458 109-458

X=1,2,3,4

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o Figure 3: Steam generators secondary side nodalizations

CVX28

CVX24 CVX27

CVX01 CVX22

CVX02

CVX25

FLX03

CVX20 FLX05

FLX04 CVX23

X=5,6,7,8 CVX21

CVX26 Riser

Steam Dome

Downcomer CVX00

CVx12

x00-x10

CVx10

x00-x10

X=5,6,7,8

x10-x12

CVx10 CVx12 x02-x06

29.155 30.380 31.930

35.275

34.695 35.275

30.380

27.930

34.695

31.930

27.930

For the performance of the sensitivity studies, the following conditions and assumptions take place:

o Subversion 5026 of MELCOR 2.1 has been used for the calculations;

o For the flow paths which connect the reactor lower plenum to the reactor core, flow paths inside the core, flow path to the reactor core bypass and the artificial flow paths between the core and the core shroud, the flow blockage option has been activated (FL_BLK). The default value of the minimum

porosity which is used in calculation of the flow resistance is chosen. The exception of that assumption is a case with double ended break of a main circulation pipeline (case 3B from Table 2) for which this coefficient is set to 0.5 in order to avoid failure of MELCOR 2.1;

o No safety systems are available with the exception of the case with reactor pressure vessel bottom break (as initiating event);

o HS degasing option is activated (HS_DG) – for the steel structures inside the vessel;

o No operator actions are considered.

For the purposes of sensitivity studies, the following cases have been selected:

o Total loss of power supply to the unit – For WWER-1000 (V320) this transient is characterized by high primary pressure and relatively slow primary water inventory decrease. Input decks with 1, 5 and 30 volumes in the reactor core are used. One and three volumes for the steam generators secondary side are separately used;

o Double ended break of a main circulation pipeline with DN 2x850 mm (cold leg break) – this case is characterized by the most dynamic change in the primary side parameters and very fast degradation of the core in case of unavailability of safety systems. Input decks with 1, 5 and 30 volumes in the reactor core are used. One volume for the steam generators secondary side is used because the secondary side influence on the severe accident progression for that case is not expected to be significant;

o Reactor pressure vessel partial break with DN 1130 mm (cross section area around 1.0 m²) with active and passive safety systems availability – this case is analyzed in order to assess the core blockage behavior for 1, 5 and 30 volumes in the reactor core region with safety systems availability. One volume for the steam generators secondary side is used, because the secondary side influence on the severe accident progression for that case is not expected to be

significant;

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o Middle LOCA of a cold circulation pipeline (close to the reactor vessel) with DN60 mm – this case is analyzed in order to assess the reactor installation response in case of severe accident progression as a result of middle LOCA.

Input decks with 1, 5 and 30 volumes in the reactor core are used. One and three volumes for the steam generators secondary side are separately used;

o Primary to secondary LOCA with DN 43 mm (PRISE 43 mm) – this case is specifically analyzed in order to be assessed the secondary side influence on the primary parameters in severe accident conditions.

Summary information for the cases which have been selected for the sensitivity studies is presented in Table 1.

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Case Case Case

Case Initial Initial Initial Initial Condition ConditionCondition Condition

Safety systems Safety systems Safety systems Safety systems

availability availabilityavailability

availability VariantVariantVariantVariant

Number of Number of Number of Number of Core/SG Core/SG Core/SG Core/SG volumes volumes volumes volumes

1. Station blackout Nominal Power No

1A 1V/1V

2A 5V/1V

3A 30V/1V

4A 30V/3V

2. LB LOCA with

DN2x850 mm Nominal Power No

1B 1V/1V

2B 5V/1V

3B 30V/1V

3. RPV bottom partial failure with DN 1130

mm

Nominal Power

Yes (one train from each safety

system)

1C 1V/1V

2C 5V/1V

3C 30V/1V

4. MB LOCA with DN60

mm Nominal Power No

1D 1V/1V

2D 5V/1V

3D 30V/1V

4D 30V/3V

5. PRISE with DN43 mm

Nominal Power No 1E 30V/1V

2E 30V/3V

Table 1: Summary information for the cases analyzed

For the sensitivity studies performed, the following parameters during the severe accident progression are analyzed:

o CPU time consumption (tCPU) – up to the moment of vessel breach;

o Total amount of hydrogen generated in the reactor core (COR-DMH2-TOT);

o Onset of gap release (tGR);

o Time when the maximal fuel cladding temperature reaches 1200^oC (tCD);

o Onset of reactor core degradation (tDEG);

o Onset of UO₂ relocation into the reactor lower plenum (tMUO2);

o Total mass of UO₂ relocated into the lower plenum before the vessel breach (MUO2);

o Onset of the reactor pressure vessel breach (tVBR);

o Total amount of molten material ejected to the reactor cavity (COR-MEJEC- TOT);

o Maximal temperature of the hot legs (Thl_max) – important for station

blackout scenarios due to the creep rupture phenomenon (it is assessed 30 min before the vessel breach because HPME/DCH is concurrent to the creep rupture phenomenon);

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CaseCase

CaseCase VariantVariantVariantVariant

Number Number Number Number

of of of of Core/SG Core/SG Core/SG Core/SG volumes volumes volumes volumes

COR COR COR COR---- DMH2DMH2 DMH2DMH2----

TOT TOT TOT TOT [kg][kg][kg]

[kg]

tCPU tCPU tCPU tCPU [h][h]

[h][h]

tStZr tStZrtStZr tStZr [s][s]

[s][s]

tGR tGRtGR tGR [s][s]

[s][s]

tCD tCD tCD tCD [s][s][s]

[s]

tDEG tDEG tDEG tDEG [s][s]

[s][s]

1. Station blackout

1A 1V/1V 582 10.8 10500 10946 12500 15430

2A 5V/1V 912912912912 11.3 11500 11691 15000 24435244352443524435

3A 30V/1V 760 16.9 11000 11347 13000 15844

4A 30V/3V 668 17.5 10500 10727 12500 14997

2. LB LOCA with DN2x850

mm

1B 1V/1V 114 10.1 5 40 160 800

2B 5V/1V 149 10.0 5 45 160 800

3B 30V/1V 131 19.8 5 47 240 720

3. RPV bottom partial failure with DN 1130

mm

1C 1V/1V 95 108 6 48 175 797

2C 5V/1V 208 157 6 48 175 807

3C 30V/1V 2.3 331* 120 278 NONONONO NONONONO 4. MB LOCA

with DN60 mm

1D 1V/1V 273 11.2 2000 1800 2500 2826

2D 5V/1V 263 9.8 2000 2000 2500 3341

3D 30V/1V 295 16.5 2000 2077 2500 3113

4D 30V/3V 246 17.4 2000 2038 2500 3067

5. PRISE with DN43 mm

1E 30V/1V 454 22.3 8000 8417 9500 10576

2E 30V/3V 401 22.9 8000 8242 9500 10271

Table 2: Summary results from the sensitivity studies performed – part 1

*Calculated to the 24-th h (accident progression time) after the accident initiation

Case CaseCase

Case VariantVariantVariantVariant

Number of Number of Number of Number of Core/SG Core/SG Core/SG Core/SG volumes volumesvolumes volumes

tMUO2 tMUO2 tMUO2 tMUO2

[s][s]

[s][s]

MUO2MUO2MUO2 MUO2 [tons]

[tons]

[tons]

[tons]

Thl_max Thl_max Thl_max Thl_max

**

**

**

**

[K]

[K]

[K]

[K]

tVBRtVBR tVBRtVBR [s][s]

[s][s]

CORCOR CORCOR---- MEJEC MEJEC MEJEC MEJEC----

TOT TOTTOT TOT [tons]

[tons]

[tons]

[tons]

1. Station blackout

1A 1V/1V 15684 69 1260126012601260 25236 195 2A 5V/1V 24490244902449024490 52 1428142814281428 25615 218

3A 30V/1V 16389 63 1015 22351 222

4A 30V/3V 15946 53 994 20525 194

2. LB LOCA with DN2x850 mm

1B 1V/1V 1940 77 Not needed 8166 154

2B 5V/1V 2470 74 ^{Not needed} 9024 178

3B 30V/1V 2480 78 Not needed 7983 157

3. RPV bottom partial failure with DN 1130

mm

1C 1V/1V 797 83 ^{Not needed} 22350223502235022350 144 2C 5V/1V 9208 76 Not needed 40500405004050040500 114 3C 30V/1V NO NO Not needed NONONONO NONONONO

4. MB LOCA with DN60 mm

1D 1V/1V 5704 70 ^{Not needed} 12845 186

2D 5V/1V 4290 77 Not needed 14936 173

3D 30V/1V 5923 72 ^{Not needed} 12057 156

4D 30V/3V 5506 76 Not needed 13258 162

5. PRISE with 1E 30V/1V 10698 66 ^{Not needed} 17972 194

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o Figure 4: Total amount of the hydrogen generated for the in- vessel phase of severe accident (cases A, B,)

0 100 200 300 400 500 600 700 800 900 1000

0 10000 20000 30000 40000 50000 60000

Mass [kg]

Time [s]

COR-DMH2-TOT_1A COR-DMH2-TOT_2A COR-DMH2-TOT_3A COR-DMH2-TOT_4A

0 20 40 60 80 100 120 140 160

0 10000 20000 30000 40000 50000

Mass [kg]

Time [s]

COR-DMH2-TOT_1B COR-DMH2-TOT_2B COR-DMH2-TOT_3B

o Figure 5: Total amount of the hydrogen generated for the in- vessel phase of severe accident (cases D, E)

0 50 100 150 200 250 300

0 20000 40000 60000 80000 100000

Mass [kg]

Time [s]

COR-DMH2-TOT_1D COR-DMH2-TOT_2D COR-DMH2-TOT_3D COR-DMH2-TOT_4D

0 50 100 150 200 250 300 350 400 450 500

0 20000 40000 60000 80000 100000

Mass [kg]

Time [s]

COR-DMH2-TOT_3E COR-DMH2-TOT_4E

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o Figure 6: Total amount of the hydrogen generated – RELAP/SCDAP – Mass (tons) vs. Time (s) [2]

o Figure 7: Maximal gas temperature in the core – station blackout cases

500 1000 1500 2000 2500 3000

10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000

Temperature[K]

Time [s]

CFVALU_2357_1V1V CFVALU_2357_5V1V CFVALU_2357_30V1V CFVALU_2357_30V3V

0 500 1000 1500 2000 2500 3000

0 10000 20000 30000 40000 50000 60000

Temperature[K]

Time [s]

CFVALU_2357_1V1V CFVALU_2357_5V1V CFVALU_2357_30V1V CFVALU_2357_30V3V

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o Figure 8: Hot legs gas temperatures (left) and hot legs metal temperature (right)

400 600 800 1000 1200 1400 1600 1800

0 10000 20000 30000 40000 50000 60000

Temperature[K]

Time [s]

CVH-TVAP_400_1V1V CVH-TVAP_400_5V1V CVH-TVAP_400_30V1V

CVH-TVAP_400_30V3V ⁴⁰⁰

600 800 1000 1200 1400 1600

0 10000 20000 30000 40000 50000 60000

Temperature[K]

Time [s]

HS-TEMP_1V1V HS-TEMP_5V1V HS-TEMP_30V1V HS-TEMP_30V3V

Issue 1: Related to station blackout scenario (5 volumes in the core region):

When nodalization scheme with 5 volumes in the core is used, then the core degradation starts immediately after the vessel breach or very late after that moment which is not physically accurate. This leads to initial ejection of a very small amount of molten material (mainly steel – about 500 kg) and the peak of the molten corium ejection starts about 4000 s after the vessel breach (see COR- MEJEC-TOT_2B_1 on Figure 9). The root cause of that issue might be related to erroneous activation of the lower head mechanical model (by the code or due to user mistakes). It is not clear so far what the exact reason is.

This issue has been overcome by the following ways:

o The lower head mechanical model is switched off by the sensitivity coefficient 1600 (3) which is set to 1.0E10. When the molten core material in the lower plenum reaches around 50-70 tons and there is molten material into the lower plenum region below the barrel perforated bottom (around 2.5 t), then SC

1600 (3) is set back to its default value of 1.0E3. The lower head fails and the molten corium ejection seems to be quite reasonable (see COR-MEJEC-

TOT_2B_2 on Figure 9). For Variant 2A (from Table 2) this approach has been applied;

o Nodalization scheme with 30 hydrodynamic volumes in the core – for that nodalization this issue does not appear at all.

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Depending on the time step this issue might cause additional problems such as:

o Reactor vessel breach with a very small amount of corium ejected – 1.0E-38 kg followed by a large corium ejection several hundred seconds after the vessel breach;

o Generation of error massage: Cavity Overfilled (an attempt to be ejected physically impossible amount of molten material – around 900 tons). This used to happen for the MELCOR 2.1 subversions before subversion 5026 for our WWER-1000 plant model specifically.

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o Figure 9: Mass of molten corium ejected – left hand side (close plan)

0 200 400 600 800 1000 1200 1400 1600 1800 2000

20000 22000 24000 26000 28000 30000

Mass [kg]

Time [s]

COR-MEJEC-TOT_2B_1 COR-MEJEC-TOT_2B_2

0 50000 100000 150000 200000 250000

20000 30000 40000 50000 60000

Mass [kg]

Time [s]

COR-MEJEC-TOT_2B_1 COR-MEJEC-TOT_2B_2

Issue 2: Related to station blackout scenario (5 and 30 volumes in the core region):

When nodalization scheme with 5 or 30 volumes in the core is used, then, at the moment of the reactor pressure vessel breach, the primary pressure oscillation between 14 MPa and 20 MPa occurred (see CVH-P_20_A on Figure 10). It is

followed by a message for lowed head failure due to overpressure. This issue has been overcome by the following ways:

o 500-1000 s before the vessel breach the sensitivity coefficient 1505 (1) was set to 1.0 (core blockage porosity coefficient) (see CVH-P_20_B on Figure 10);

o When 3 volumes for the steam generators are used then this issue does not appear at all.

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o Figure 10: Reactor lower head pressure before and shortly after the vessel breach

13 14 15 16 17 18 19 20

22200 22250 22300 22350 22400

Pressure [MPa]

Time [s]

CVH-P_20_A CVH-P_20_B

Issue 3: Related to LB LOCA with DN 2x850 mm (30 volumes in the core region)

o When nodalization scheme with 30 volumes in the core is used then a very large temperature peak around 10 000 K in a volume from the core region occurs and therefore MELCOR 2.1 fails (see CFVALU_2357_0.05 on Figure 11).

This issue is overcome by setting sensitivity coefficient 1505 (1) to 0.5. Values of that coefficient equal to 0.05 (by default), 0.1 and 0.25 lead to high

temperature peak in the core and code failure.

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o Figure 11: Maximal gas temperatures in the core for Issue 3 (right) – blockage porosity 0.05, 0.25 and 0.5

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

600 800 1000 1200 1400

Temperature[K]

Time [s]

CFVALU_2357_0.05 CFVALU_2357_0.25 CFVALU_2357_0.5

Issue 4: Related to RPV bottom partial failure with DN 1130 mm (all variants);

For that initiating event, quite unexpected results have been obtained. For the variant with 1 and 5 hydrodynamic volumes in the core, the reactor vessel breach occurs about 22350 s and 40500 s after the accident initiation respectively (see Figure 12 (left hand side)). For the case with 30 volumes, core degradation does not occur at all (to ensure that, the calculation is extended to 24h) (see Figure 12 (right hand side)). At that moment, no reasonable explanation for these results has been found.

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o Figure 12: Mass of molten corium ejected (left) and maximal fuel cladding temperature (right) for Issue 4

0 20 40 60 80 100 120 140 160

0 50000 100000 150000 200000 250000 300000

Mass [tons]

Time [s]

COR-MEJEC-TOT_1C COR-MEJEC-TOT_2C

0 500 1000 1500 2000 2500 3000

0 20000 40000 60000 80000 100000

Temperature[K]

Time [s]

CFVALU_9017_1C CFVALU_9017_2C CFVALU_9017_3C

be drawn:

o The general progression of the severe accident for most of the cases considered is reasonable. The exceptions are the cases with RPV bottom break (with DN 1130 mm). The obtained results are quite different and it is difficult to draw a clear

conclusion about them. This causes great uncertainty in the containment event trees development for these cases;

o Most of the issues which have been identified are related to the nodalization schemes with 5 and 30 hydrodynamic volumes into the core region;

o The variants with 1 hydrodynamic volume in the core have not caused any issues and the code run quite stable. This variant leads to the lowest amount of hydrogen

generated for the in-vessel phase of the severe accident in case of station blackout scenario (case 1A). According to the results for the same initiating event which are generated by RELAP/SCDAP, the total amount of hydrogen generated for that phase is around 800 kg (see Figure 6). So the lowest amount of hydrogen for that case may or may not be physically accurate. As it can be seen from Table 2, the COR-DMH2- TOT parameter for cases 2A ÷ 4A is significantly higher than in case 1A which can be explained by the possibility for in-core circulation of water-steam mixture and the different core blockage behavior (more volumes and more flow paths available in the core for 5 and 30 volumes nodalizations). For case 2A, the largest value for the COR- DMH2-TOT parameter comes from the latest core relocation into the lower head and

o For LB LOCA with DN 2x850 mm, MB LOCA with DN 60 mm and PRISE 43 mm, the total amount of hydrogen generated into the core during the in-vessel phase does not differ significantly for the nodallizations with 1, 5 and 30 volumes in the core. Variant 3B is characterized by physically impossible gas temperature behavior in the core when the minimal blockage fraction is less than 0.5;

o For the cases 1A and 1B, the maximal hot legs metal temperatures are significantly higher than in cases 1C and 1D. This result is important when one considers creep rupture phenomenon which is more likely to occur for the hot legs than for the pressurizer surge line or SG tube (in the cases where none of the loop seals is cleared);

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1)

Actualization of PSA level 2 for KNPP (Kozloduy NPP, Bulgaria) units 5 and 6 (full power and shut-down), Risk Engineering Ltd (project is in

progress);

2)

Accident Management Guidelines and Procedures: Improvement of the Emergency Documentation for Ukrainian NPP, Risk Engineering Ltd.

(project is in progress)

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Sofia 1618 Bulgaria

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