vver-440simulationsusingmelcor2.2 ......cv016 cv018 cv021 cv023 cv025 cv027 cv022 cv024 cv026 cv028...

VVER-440 simulations using MELCOR 2.2:degraded core reflood and boric acid

transport in the primary

Petr Vokáč, [email protected]ÚJV Řež, a.s.

10th EMUG, 25.-27. 4. 2018, University of Zagreb,Zagreb, Croatia

VVER-440 simulations using MELCOR 2.2

Contents

∙ Input model conversion M1.8.6 → M2.2

∙ Comparison of test calculations M1.8.6 vs. M2.2

∙ Boric acid transport model for MELCOR 2.2

∙ Degradation of borated steel from control elements

∙ Evaluation of degraded core reflood

∙ Conclusions

1


Input model conversion M1.8.6 → M2.2

Using Reader: conversion finally successfull with MELCOR version 2.2 (release02-22-2017)

The last problem identified:

In the MELCOR 1.8.6 input distinction of canister types CN and CB was not doneexplicitly (split is done by M1.8.6 internaly using sensitivity coefficients 1501).

In MELCOR 2.2 mass input for both CN and CB is required. However previousversion of MELGEN 2.x failed with converted input model by segmentation faultand it did not indicate where the problem is.

In the current model: xmcnzr = xmcbzr= xmcnzr/2.0 from 1.8.6., default value of sensitivity coefficients 1501 is 0.5

2


Comparison of M1.8.6 vs. M2.2 with converted inputLarge break LOCA scenario:

∙ reasonable agreement for core and RCS

∙ larger differences in results for containment(MELCOR 2.2 pressure too high)

– probably due to the Bug 1848 in release 02-22-2017 ?– not analysed in detail yet (analyses in 2017 were fo-

cused on reflood and boric acid transport)

∙ problems with occasional CVH (coupled with COR) tem-perature going to 10000 K (reported Bug 1946)

400

600

800

1000

1200

1400

0 2 4 6 8 10 12 14

200

400

600

800

1000

1200

Tem

pera

ture

[K]

Tem

pera

ture

[o C

]

Time [min]

M2.2M1.8.6

Criterion 550oC

Temperature at the core exit

0

50

100

150

200

250

300

350

0 20 40 60 80 100 120

Bre

ak fl

ow in

tegr

al [t

]

Time [min]

M2.2M1.8.6

Integral of the break flow

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60 70 80

Mas

s flo

w in

tegr

al [t

]

Time [s]

M2.2M1.8.6

Integral of flow from HA

0 0.5

1 1.5

2 2.5

3 3.5

4 4.5

0 1 2 3 4 5 6

Mas

s [t]

Time [h]

M2.2M1.8.6

Mass of ZrO2 in the RPV3

https://melzilla.sandia.gov/show_bug.cgi?id=1848



Boric acid transport model

∙ transport in liquid coolant — user defined fission product class BAC withmolar mass 61.8 g/mol, declared as “radioactive”. Initial inventory specified– for primary RCS volumes (including hydroaccumulators)– and for the volume representing trays of the containment pressure

suppression system.

∙ additional removal processes from the pool:– removal from the pool due to oversaturation– transport from pool to atmosphere due to intensive boiling

Data on boric acid propertities are based on literature review, mainly on:

∙ P. Wang, J.J. Kosinski, M.M. Lencka, A. Anderko and R.D. Springer: Thermodynamic modeling of boric acid and selected metalborate systems. Pure Appl. Chem., Vol. 85, No. 11, pp. 2117–2144, 2013. http://dx.doi.org/10.1351/PAC-CON-12-07-09

∙ A. Bruggeman, J. Braet, F. Smaers and P.De Regge: Separation of Boric Acid from PWR Waste by Volatilization During Evapo-ration. Separation Science and Technology, 1997 32:1-4, 737-757, http://dx.doi.org/10.1080/01496399708003227

4

http://dx.doi.org/10.1351/PAC-CON-12-07-09

http://dx.doi.org/10.1080/01496399708003227


Removal due to oversaturationSaturation concentration is temperature dependent — lookup table setup based onapproximation of literature data (in comparison with correlation used in MAAP5):

0

20

40

60

80

100

0 20 40 60 80 100 120 140 160

Equ

ival

ent m

ass

H3B

O3

[g/1

00g(

H2O

)]

Temperature [°C]

From literature dataMAAP5 correlation

Δ𝑚𝐷𝑒𝑝𝐵𝐴𝐶 = 𝑚𝑆𝑎𝑡

𝐵𝐴𝐶 − 𝑚𝐵𝐴𝐶 for 𝑚𝑆𝑎𝑡𝐵𝐴𝐶 < 𝑚𝐵𝐴𝐶

Δ𝑚𝐷𝑒𝑝𝐵𝐴𝐶 = 0 for 𝑚𝑆𝑎𝑡

𝐵𝐴𝐶 ≥ 𝑚𝐵𝐴𝐶

Removal rate calculated using control functions and negative source in selectedcontrol volumes.

5


Removal due to rapid coolant evaporation

H3BO3(aq) → H3BO3(g)

Transport rate is proportional to the boiling ratewith distribution coefficient dependent again onthe boiling rate, e.g.:

𝐷 = 𝐶𝑔

𝐶𝑤=

(︂𝜚𝑔

𝜚𝑤

)︂0.9 0.001

0.01

0.1

1

0 50 100 150 200 250 300 350 400

D=

Cg/

Cw

Temperature [°C]

Pool→steam distribution coefficient 𝐷 in boilingconditions.

In the current input model 𝐷 is taken constant (0.01 or 0.001 for large break LOCA).

6


Removal due to rapid coolant evaporation (cont.)

Removal rate is calculated using control functions.

Boiling rate has to be calculated from:

∙ change of pool mass in the control volume

∙ inlet and outlet of pool through all flow paths

Mass of pool evaporated, Δ𝑚𝑣𝑎𝑝, in the time interval between 𝑡𝑖−1 and 𝑡𝑖−1 can becalculated:

Δ𝑚𝑣𝑎𝑝 = 𝑚(𝑡𝑖) − (𝑚(𝑡𝑖−1) + Δ𝑚𝑓𝑙𝑜𝑤) for 𝑚(𝑡𝑖) < (𝑚(𝑡𝑖−1) + Δ𝑚𝑓𝑙𝑜𝑤)Δ𝑚𝑣𝑎𝑝 = 0 for 𝑚(𝑡𝑖) ≥ (𝑚(𝑡𝑖−1) + Δ𝑚𝑓𝑙𝑜𝑤)

where Δ𝑚𝑓𝑙𝑜𝑤 is mass of pool entering the volume in this time interval.

Δ𝑚𝑉 𝐴𝑃𝐵𝐴𝐶 = 𝐷 · 𝑚𝐵𝐴𝐶 · Δ𝑚𝑣𝑎𝑝

𝑚(𝑡𝑖−1)

7


Removal implementation

Total removal rate is calculated:

�̇�𝐵𝐴𝐶 = Δ𝑚𝐷𝐸𝑃𝐵𝐴𝐶 + Δ𝑚𝑉 𝐴𝑃

𝐵𝐴𝐶

EXEC-DT (⇒ �̇�𝐵𝐴𝐶 ≤ 0)

Removal is implemented using fission product source to the pool in the controlvolume.

Input RN1_AS, with negative source rate.

Enhancement of RN1_AS proposed as a bug 1927.

8



Testing of the boric acid transport model

Large break LOCA scenario with blackout, variants:

00 — converted 1.8.6 input model

01 — BAC class added

02 — like 01 but removal from pool due to oversaturation

03 — like 01 but removal from pool with steam 𝐷 = 0.001

04 — like 03 but 𝐷 = 0.01

05 — both removal processes, 𝐷 = 0.01

06 — both removal processes, 𝐷 = 0.001

9

VVER-440 simulations using MELCOR 2.2Testing of the boric acid transport model

0

20

40

60

80

100

120

140

160

180

0 0.5 1 1.5 2 2.5

CO

R-D

MH

2-T

OT

[kg]

Time [h]

01020304050600

Hydrogen produced

27.09

27.092

27.094

27.096

27.098

27.1

27.102

27.104

27.106

27.108

27.11

27.112

0 5 10 15 20 25 30

H3B

O3

tota

l mas

s [t]

Time [min]

010203040506

Total mass of H3BO3

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.5 1 1.5 2 2.5

Mas

s H

3BO

3 in

RP

V p

ool [

t]

Time [h]

010203040506

Mass of H3BO3 in RPV pool

-15000

-10000

-5000

0

5000

10000

15000

20000

25000

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

Con

cent

ratio

n [g

(H3B

O3)

/kg(

H2O

)]

Time [h]

010203040506

Concentration of H3BO3 in lower plenum10

VVER-440 simulations using MELCOR 2.2Degradation of borated steel from control elements

1 2 3 4 501020304050607

08091011

12

13

14

15

16

17

18

192021

22

23

24

25

26

27

Time = -1200.0 s = -0.33 h, Plot record 1

VVER-440/213, LLOCA, Model 11 13 (Melcor2.2)

Initial state

1 2 3 4 501020304050607

08091011

12

13

14

15

16

17

18

192021

22

23

24

25

26

27

Time = 1120.1 s = 00.31 h, Plot record 893


Melting of borated steel

1 2 3 4 501020304050607

08091011

12

13

14

15

16

17

18

192021

22

23

24

25

26

27

Time = 1360.0 s = 00.38 h, Plot record 897


Just before fuel geometry collapse

fu cl cn ss ns pb pd mb1 mp1 mb2 mp2 flc flb hs

Why does NS melt to MB1 instead of MB2?What does it held molten steel in the bypass after canister failure? Reported as a bug 1957.Should I put control elements into separate ring(s)?11



Reflood calculationsReflood calculations with converted model were not successfull — quenching of thecore not efficient even in cases when it should be (early reflood with large injectionrate) ⇒ review of the input model:

∙ opening height of channel↔bypass radial cross flow flowpaths (blockage optionchannel-box) — increased for the whole height of connected volumes

∙ channel↔channel radial cross flow flowpaths in the upper core added. Eachflowpath is opened by control function on canister failure.

∙ refined COR axial nodalization in the upper core: 5 → 10 axial levels in thefuel region. CVH/FL nodalization kept the same.

∙ refined COR radial nodalization: 4 → 5 or 6 rings including CVH/FL for eachring.

⇒ results looks better

(anyway validation on QUENCH11 is planned to be done in 2018 to gain moreconfidence)

12


Reflood calculations — refined nodalization

1 2 3 4 5 6 7010203040506070809

10111213

14

15

16

17

18

19

20

21222324

25

26

27

28

29

30

31

32

33

34


VVER-440/213, LLOCA, Model 11 17 6R (Melcor2.2) Variant 01

Six ring core nodalization

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

cv020

cv011

cv013

cv015

cv017

cv012

cv014

cv016

cv018

cv021

cv023

cv025

cv027

cv022

cv024

cv026

cv028

cv031

cv033

cv035

cv037

cv032

cv034

cv036

cv038

cv041

cv043

cv045

cv047

cv042

cv044

cv046

cv048

cv051

cv053

cv055

cv057

cv052

cv054

cv056

cv058

cv061

cv063

cv065

cv067

cv062

cv064

cv066

cv068

cv001

cv002

cv003

cv004

cv005

cv006

cv071

cv072

cv073

cv075


VVER-440/213, LLOCA, Model 11 17 (Melcor2.2) R6 Variant 01

CVH in RPV nodalization

13


Reflood calculationsScenario:

∙ large break LOCA on a cold leg, blackout

∙ alternative core cooling recovery at certain time

Reflood with the flow rate 0.3 m3/s of clean water at 55∘C was assumed. The coolant isdistributed by equal portion both below and above the core. For 312 fuel assemblies in theupper part of the core and 126 rods in the assembly it corresponds to about 8 g/(s·rod)(4 g/(s·rod) from above the core — coolant injected below the core is lost to the break).

Variants with different time of injection start:

11 — at 400 s (6.7 min), i.e.: shortly after the criterion 550∘C on the core exit is met.

21 — at 600 s (10 min), shortly before the onset of rapid cladding oxidation.

31 — at 800 s (13.3 min), shortly before the onset of steel components melting.

41 — at 1100 s (18.3 min), during the melting of steel components.

51 — at 1300 s (21.7 min), shortly before the first loss of fuel rods geometry.

61 — at 1400 s (23.3 min), after fuel relocation in the limited part of the core.

(based on timing of the calculation with the converted input)14

VVER-440 simulations using MELCOR 2.2Reflood calculations — base case without reflood

0

50

100

150

200

250

300

0 100 200 300 400 500 600

FL-

I-H

2O-M

FLO

W [t

]

Time [min]

4R1864R2205R1865R2206R1866R220

Integral of the break outflow

400

600

800

1000

1200

1400

1600

0 5 10 15 20 25

200

400

600

800

1000

1200

1400

Tem

pera

ture

[K]

Tem

pera

ture

[o C

]

Time [min]

4R1864R2205R1865R2206R1866R220

Cr. 550oC

Core outlet temperature

0

50

100

150

200

250

300

350

10 20 30 40 50 60 70 80 90 100

CO

R-D

MH

2 [k

g]

Time [min]

4R1864R2205R1865R2206R1866R220

Hydrogen production

0

0.1

0.2

0.3

0.4

0.5

-1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5

War

p [1

]

Time [h]

4R1864R2205R1865R2206R1866R220

CPU demand15

VVER-440 simulations using MELCOR 2.2Reflood calculations — maximum clading temperature

11,21: Temperature increase after reflood at the top fuel node in the peripheral ring

300

400

500

600

700

800

900

1000

0 10 20 30 40 50 60

MA

X C

OR

-TC

L [K

]

Time [min]

4R1864R2205R1865R2206R1866R220

11 (reflood at 400 s)

200

400

600

800

1000

1200

1400

1600

10 15 20 25 30 35 40 45 50 55 60

MA

X C

OR

-TC

L [K

]

Time [min]

4R1864R2205R1865R2206R1866R220


200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

0 20 40 60 80 100 120 140 160

MA

X C

OR

-TC

L [K

]

Time [min]

4R1864R2205R1865R2206R1866R220


200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

12 14 16 18 20 22 24 26 28 30

MA

X C

OR

-TC

L [K

]

Time [min]

4R1864R2205R1865R2206R1866R220

31 zoom16

VVER-440 simulations using MELCOR 2.2Reflood calculations — hydrogen production

0

0.5

1

1.5

2

2.5

3

3.5

10 11 12 13 14 15 16 17 18

CO

R-D

MH

2 [k

g]

Time [min]

4R1864R2205R1865R2206R1866R220


0

5

10

15

20

25

30

35

40

45

13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18

CO

R-D

MH

2 [k

g]

Time [min]

4R1864R2205R1865R2206R1866R220


20

40

60

80

100

120

140

160

18 20 22 24 26 28 30

CO

R-D

MH

2 [k

g]

Time [min]

4R1864R2205R1865R2206R1866R220


40

60

80

100

120

140

160

180

200

20 22 24 26 28 30

CO

R-D

MH

2 [k

g]

Time [min]

4R1864R2205R1865R2206R1866R220

51 (reflood at 1300 s)17

VVER-440 simulations using MELCOR 2.251 vs 61 — Hydrogen production

40

60

80

100

120

140

160

180

200

20 22 24 26 28 30

CO

R-D

MH

2 [k

g]

Time [min]

4R1864R2205R1865R2206R1866R220

60

80

100

120

140

160

180

200

220

240

260

20 25 30 35 40 45 50 55 60

CO

R-D

MH

2 [k

g]

Time [min]

4R1864R2205R1865R2206R1866R220

51 vs 61 — Max COR-TSCV

0

500

1000

1500

2000

2500

20 40 60 80 100 120

MA

X C

OR

-TS

VC

[K]

Time [min]

4R1864R2205R1865R2206R1866R220


0

500

1000

1500

2000

2500

20 40 60 80 100 120

MA

X C

OR

-TS

VC

[K]

Time [min]

4R1864R2205R1865R2206R1866R220

61 (reflood at 1400 s)18

VVER-440 simulations using MELCOR 2.2M22 Total core energy

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

CO

R-E

NE

RG

Y-T

OT

[GJ]

Time [h]

11213141516101

6 ring model

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

CO

R-E

NE

RG

Y-T

OT

[GJ]

Time [h]

11213141516101

5 ring model

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

CO

R-E

NE

RG

Y-T

OT

[GJ]

Time [h]

11213141516101

4 ring model

0

20

40

60

80

100

120

140

160

0 1 2 3 4 5

CO

R-E

NE

RG

Y-T

OT

[GJ]

Time [h]

11213141516101

4 ring, the first converted model19

VVER-440 simulations using MELCOR 2.2M22 hydrogen production

0

50

100

150

200

250

10 15 20 25 30 35 40

CO

R-D

MH

2-T

OT

[kg]

Time [min]

11213141516101

6 ring model

0

50

100

150

200

250

10 15 20 25 30 35 40

CO

R-D

MH

2-T

OT

[kg]

Time [min]

11213141516101

5 ring model

0

50

100

150

200

250

10 15 20 25 30 35 40

CO

R-D

MH

2-T

OT

[kg]

Time [min]

11213141516101

4 ring model

0

20

40

60

80

100

120

140

160

10 15 20 25 30 35 40

CO

R-D

MH

2-T

OT

[kg]

Time [min]

11213141516101

4 ring, the first converted model20


Conclusions

∙ input model successfuly converted M1.8.6→M2.2

∙ results:– comparable for core and RCS– overestimated pressure in the containment in M2.2

(waiting for updated MELCOR release)

∙ simulations of core reflood more successfull with M2.2 (M1.8.6 too slow)

∙ boric acid transport model implemented using user defined FP class and lot ofCFs — not numerically stable and reliable⇒ enhacement of MELCOR code requested

∙ non-physical behaviour of molten steel from control elementsobserved ⇒ ?

21

vver-440simulationsusingmelcor2.2 ......cv016 cv018 cv021 cv023 cv025 cv027 cv022 cv024 cv026 cv028...

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