VVER-440 simulations using MELCOR 2.2: degraded core reflood and boric acid transport in the primary

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VVER-440 simulations using MELCOR 2.2:

degraded core reflood and boric acid transport in the primary

Petr Vokáč, petr.vokac@ujv.cz ÚJV Řež, a.s.

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

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VVER-440 simulations using MELCOR 2.2

Input model conversion M1.8.6 → M2.2

Using Reader: conversion finally successfull with MELCOR version 2.2 (release 02-22-2017)

The last problem identified:

In the MELCOR 1.8.6 input distinction of canister types CN and CB was not done explicitly (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 previous version of MELGEN 2.x failed with converted input model by segmentation fault and 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

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VVER-440 simulations using MELCOR 2.2

Comparison of M1.8.6 vs. M2.2 with converted input

Large 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) temperature 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

Temperature [K] Temperature [oC]

Time [min]

M2.2 M1.8.6 Criterion 550^oC

Temperature at the core exit

0 50 100 150 200 250 300 350

0 20 40 60 80 100 120

Break flow integral [t]

Time [min]

M2.2 M1.8.6

0 20 40 60 80 100 120 140 160

0 10 20 30 40 50 60 70 80

Mass flow integral [t]

Time [s]

M2.2 M1.8.6

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

0 1 2 3 4 5 6

Mass [t]

Time [h]

M2.2 M1.8.6

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VVER-440 simulations using MELCOR 2.2

Boric acid transport model

∙ transport in liquid coolant — user defined fission product class BAC with molar 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 metal borate 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

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VVER-440 simulations using MELCOR 2.2

Removal due to oversaturation

Saturation concentration is temperature dependent — lookup table setup based on approximation 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 Equivalent mass H3BO3 [g/100g(H2O)]

Temperature [°C]

From literature data MAAP5 correlation

Δ𝑚^𝐷𝑒𝑝_𝐵𝐴𝐶 = 𝑚^𝑆𝑎𝑡_𝐵𝐴𝐶 − 𝑚_𝐵𝐴𝐶 for 𝑚^𝑆𝑎𝑡_𝐵𝐴𝐶 < 𝑚_𝐵𝐴𝐶 Δ𝑚^𝐷𝑒𝑝_𝐵𝐴𝐶 = 0 for 𝑚^𝑆𝑎𝑡_𝐵𝐴𝐶 ≥ 𝑚_𝐵𝐴𝐶

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

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VVER-440 simulations using MELCOR 2.2

Removal due to rapid coolant evaporation

H₃BO₃(aq) → H₃BO₃(g)

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

𝐷 = 𝐶_𝑔 𝐶_𝑤 =

(︂ 𝜚_𝑔 𝜚_𝑤

)︂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 boiling conditions.

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

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VVER-440 simulations using MELCOR 2.2

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, Δ𝑚_𝑣𝑎𝑝, in the time interval between 𝑡_𝑖−1 and 𝑡_𝑖−1 can be calculated:

Δ𝑚_𝑣𝑎𝑝 = 𝑚(𝑡_𝑖)− (𝑚(𝑡_𝑖−1) + Δ𝑚_{𝑓 𝑙𝑜𝑤}) for 𝑚(𝑡_𝑖) < (𝑚(𝑡_𝑖−1) + Δ𝑚_{𝑓 𝑙𝑜𝑤}) Δ𝑚_𝑣𝑎𝑝 = 0 for 𝑚(𝑡_𝑖) ≥ (𝑚(𝑡_𝑖−1) + Δ𝑚_{𝑓 𝑙𝑜𝑤}) where Δ𝑚_{𝑓 𝑙𝑜𝑤} is mass of pool entering the volume in this time interval.

Δ𝑚^{𝑉 𝐴𝑃}_𝐵𝐴𝐶 = 𝐷 · 𝑚_𝐵𝐴𝐶 · Δ𝑚_𝑣𝑎𝑝 𝑚(𝑡_𝑖−1)

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VVER-440 simulations using MELCOR 2.2

Removal implementation

Total removal rate is calculated:

˙

𝑚_𝐵𝐴𝐶 = Δ𝑚^𝐷𝐸𝑃_𝐵𝐴𝐶 + Δ𝑚^{𝑉 𝐴𝑃}_𝐵𝐴𝐶

EXEC-DT (⇒ 𝑚˙ _𝐵𝐴𝐶 ≤ 0)

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

Input RN1_AS, with negative source rate.

Enhancement of RN1_AS proposed as a bug 1927.

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VVER-440 simulations using MELCOR 2.2

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

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VVER-440 simulations using MELCOR 2.2

Testing 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

COR-DMH2-TOT [kg]

Time [h]

01 0203 04 0506 00

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

H3BO3 total mass [t]

Time [min]

01 0203 04 05 06

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

Mass H3BO3 in RPV pool [t]

Time [h]

01 0203 04 0506

-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

Concentration [g(H3BO3)/kg(H2O)]

Time [h]

01 0203 04 0506

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VVER-440 simulations using MELCOR 2.2

Degradation of borated steel from control elements

1 2 3 4 5

0102 0304 05 06 07 0809 10 11 12 13 14 15 16 17 18 19 2021 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 5

0102 0304 05 06 07 0809 1011 12 13 14 15 16 17 18 19 2021 22 23 24 25 26 27

Time = 1120.1 s = 00.31 h, Plot record 893 VVER-440/213, LLOCA, Model 11 13 (Melcor2.2)

Melting of borated steel

1 2 3 4 5

0102 0304 05 06 07 0809 1011 12 13 14 15 16 17 18 19 2021 22 23 24 25 26 27

Time = 1360.0 s = 00.38 h, Plot record 897 VVER-440/213, LLOCA, Model 11 13 (Melcor2.2)

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.

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VVER-440 simulations using MELCOR 2.2

Reflood calculations

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

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

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

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

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

⇒ results looks better

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

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VVER-440 simulations using MELCOR 2.2

Reflood calculations — refined nodalization

1 2 3 4 5 6 7

0102 0304 0506 07 08 09 1011 12 13 14 15 16 17 18 19 20 21 2223 24 25 26 27 28 29 30 31 32 33 34

Time = -3600.0 s = -1.00 h, Plot record 1

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

Time = -3600.0 s = -1.00 h, Plot record 1

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

CVH in RPV nodalization

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VVER-440 simulations using MELCOR 2.2

Reflood calculations

Scenario:

∙ 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 is distributed by equal portion both below and above the core. For 312 fuel assemblies in the upper 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.

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VVER-440 simulations using MELCOR 2.2

Reflood calculations — base case without reflood

0 50 100 150 200 250 300

0 100 200 300 400 500 600

FL-I-H2O-MFLOW [t]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

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

Temperature [K] Temperature [oC]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220 Cr. 550^oC

Core outlet temperature

0 50 100 150 200 250 300 350

10 20 30 40 50 60 70 80 90 100

COR-DMH2 [kg]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

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

Warp [1]

Time [h]

4R186 4R220 5R186 5R220 6R186 6R220

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VVER-440 simulations using MELCOR 2.2

Reflood 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

MAX COR-TCL [K]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

11 (reflood at 400 s)

200 400 600 800 1000 1200 1400 1600

10 15 20 25 30 35 40 45 50 55 60

MAX COR-TCL [K]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

0 20 40 60 80 100 120 140 160

MAX COR-TCL [K]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

12 14 16 18 20 22 24 26 28 30

MAX COR-TCL [K]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

16

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VVER-440 simulations using MELCOR 2.2

Reflood calculations — hydrogen production

0 0.5 1 1.5 2 2.5 3 3.5

10 11 12 13 14 15 16 17 18

COR-DMH2 [kg]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

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

COR-DMH2 [kg]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

20 40 60 80 100 120 140 160

18 20 22 24 26 28 30

COR-DMH2 [kg] 4R186

4R220 5R186 5R220 6R186 6R220

40 60 80 100 120 140 160 180 200

20 22 24 26 28 30

COR-DMH2 [kg] 4R186

4R220 5R186 5R220 6R186 6R220

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VVER-440 simulations using MELCOR 2.2

51 vs 61 — Hydrogen production

40 60 80 100 120 140 160 180 200

20 22 24 26 28 30

COR-DMH2 [kg]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

60 80 100 120 140 160 180 200 220 240 260

20 25 30 35 40 45 50 55 60

COR-DMH2 [kg]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

51 vs 61 — Max COR-TSCV

0 500 1000 1500 2000 2500

20 40 60 80 100 120

MAX COR-TSVC [K]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

0 500 1000 1500 2000 2500

20 40 60 80 100 120

MAX COR-TSVC [K]

Time [min]

4R186 4R220 5R186 5R220 6R186 6R220

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VVER-440 simulations using MELCOR 2.2

M22 Total core energy

0 20 40 60 80 100 120 140

0 0.5 1 1.5 2

COR-ENERGY-TOT [GJ]

Time [h]

11 2131 41 5161 01

6 ring model

0 20 40 60 80 100 120 140

0 0.5 1 1.5 2

COR-ENERGY-TOT [GJ]

Time [h]

11 2131 41 5161 01

5 ring model

0 20 40 60 80 100 120 140

0 0.5 1 1.5 2

COR-ENERGY-TOT [GJ]

11 2131 41 5161 01

0 20 40 60 80 100 120 140 160

0 1 2 3 4 5

COR-ENERGY-TOT [GJ]

11 2131 41 5161 01

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VVER-440 simulations using MELCOR 2.2

M22 hydrogen production

0 50 100 150 200 250

10 15 20 25 30 35 40

COR-DMH2-TOT [kg]

Time [min]

11 2131 41 5161 01

6 ring model

0 50 100 150 200 250

10 15 20 25 30 35 40

COR-DMH2-TOT [kg]

Time [min]

11 2131 41 5161 01

5 ring model

0 50 100 150 200 250

10 15 20 25 30 35 40

COR-DMH2-TOT [kg]

Time [min]

11 2131 41 5161 01

0 20 40 60 80 100 120 140 160

10 15 20 25 30 35 40

COR-DMH2-TOT [kg]

Time [min]

11 2131 41 5161 01

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VVER-440 simulations using MELCOR 2.2

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 of CFs — not numerically stable and reliable

⇒ enhacement of MELCOR code requested

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

VVER-440 simulations using MELCOR 2.2: degraded core reflood and boric acid transport in the primary

VVER-440 simulations using MELCOR 2.2:

degraded core reflood and boric acid transport in the primary

VVER-440 simulations using MELCOR 2.2

Contents

VVER-440 simulations using MELCOR 2.2

Input model conversion M1.8.6 → M2.2

VVER-440 simulations using MELCOR 2.2

Comparison of M1.8.6 vs. M2.2 with converted input

VVER-440 simulations using MELCOR 2.2

Boric acid transport model

VVER-440 simulations using MELCOR 2.2

Removal due to oversaturation

VVER-440 simulations using MELCOR 2.2

Removal due to rapid coolant evaporation

VVER-440 simulations using MELCOR 2.2

Removal due to rapid coolant evaporation (cont.)

VVER-440 simulations using MELCOR 2.2

Removal implementation

VVER-440 simulations using MELCOR 2.2

Testing of the boric acid transport model

VVER-440 simulations using MELCOR 2.2

Testing of the boric acid transport model

VVER-440 simulations using MELCOR 2.2

Degradation of borated steel from control elements

VVER-440 simulations using MELCOR 2.2

Reflood calculations

VVER-440 simulations using MELCOR 2.2

Reflood calculations — refined nodalization

VVER-440 simulations using MELCOR 2.2

Reflood calculations

VVER-440 simulations using MELCOR 2.2

Reflood calculations — base case without reflood

VVER-440 simulations using MELCOR 2.2

Reflood calculations — maximum clading temperature

VVER-440 simulations using MELCOR 2.2

Reflood calculations — hydrogen production

VVER-440 simulations using MELCOR 2.2

51 vs 61 — Hydrogen production

51 vs 61 — Max COR-TSCV

VVER-440 simulations using MELCOR 2.2

M22 Total core energy

VVER-440 simulations using MELCOR 2.2

M22 hydrogen production

VVER-440 simulations using MELCOR 2.2

Conclusions