ermsar 2012, cologne march 21 – 23, 2012 main results of the istc project #3876 “thermo-...

ERMSAR 2012, Cologne March 21 – 23, 2012

Main results of the ISTC Project #3876 “Thermo-Hydraulics of U-Zr-O Molten Pool under Oxidising

Conditions in Multi-Scale Approach (Crucible - Bundle - Reactor Scales)”, (THOMAS)

M.S. Veshchunov, V.V. Chudanov, A.E. Aksenova, A.V. Boldyrev, V.A. Pervichko, V.E. Shestak

Nuclear Safety Institute (IBRAE)

Russian Academy of Sciences


ISTC Project THOMASGeneral Information

Duration: October 2008 — December 2011

Leading Institution: IBRAE (Moscow)

Collaborators: KIT (Karlsruhe)

ITU (Karlsruhe)

IRSN (Cadarache)

CEA (Cadarache)

IVS (Trnava)


3

Project Objectives• On the base of analysis of available test data from small and large

scale experiments, to develop a mechanistic description of U-Zr-O molten pool behaviour in oxidising conditions

• For this purpose, to carry out a tight coupling of the two advanced numerical tools developed within the previous ISTC Project #2936: the SVECHA physico-chemical (molten pool oxidation) model and the 2D thermo-hydraulic code CONV

• This will allow extension of thermal hydraulic consideration of oxidised melt from small scales (crucible tests) up to a large scale (reactor pressure vessel), including an intermediate scale corresponding to molten pools in bundle tests


4

UO2 effective fuel rod

Zr(O) effective cladding

(U-Zr-O) melt

(Zr,U)O2

oxide

vslug

Fusion front

zUO2 effective

fuel rod

Zr(O) effective cladding

(U-Zr-O) melt

(Zr,U)O2

oxide

vslug

Fusion front

zCalculates:• U-Zr-O melt composition and average

temperature• U-Zr-O melt oxidation and bulk ceramic

precipitates formation• UO2 pellet dissolution• (Zr,U)O2 peripheral crust thickness and

temperature distribution • Melt blockage relocationValidated against:• FZK crucible tests• CORA (melt relocation)• Phebus FPT 0&1 (molten pool oxidation)

2-d Model for Molten Pool Oxidation and Relocation Previous ISTC Project #2936


5

2-d Model for Molten Pool Oxidation Verification against FZK Crucible Tests

on ZrO2 dissolution by molten Zr

0 500 1000 1500 2000

2,5

3,0

3,5

4,0

4,5

5,0

5,5

2275

2300

2325

2350

2375

2400

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

Wal

l thi

ckne

ss,

mm

Bot

tom

thi

ckne

ss,

mm

Time, s

Tem

pera

ture

, K

Tmelt

= 2373KT

wall= +1K

Tbott

= -1K

Vol

ume

frac

tion

of p

reci

pita

tes

Corrosion-erosion mechanism: depending on oxygen flux matches at the solid/melt interface, the peripheral oxide layer (crust) can grow (“corrosion”) or dissolve by corium melt (“erosion”).


6

• Heat flux matches• Heat balance• Mass flux matches• Mass balances

2-d Molten Pool Oxidation Model

Melt Boundary layerOxide crust

VS

FeO

Additional layers

Supplied with additional layers (Vessel Steel, FeO) and corresponding flux matches at new interfaces for consideration of vessel wall corrosion kinetics

+


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Vessel Steel (VS) Oxidation Model

• Parabolic correlation from METCOR tests (15 Kh2NMFA vessel steel) under unlimited steam supply

• “Oxygen starvation” regime: oxygen flux Jox through the crust becomes rate controlling during relatively long period of interactions 0 4 8 12 16

Time (h)

0

0.2

0.4

0.6

0.8

1

Th

ick

ne

ss

(m

m)

D issoution exp.

D issoution calc.

O xide exp.

O xide calc.

T=1273 K

• Jox has to be calculated from the solution of the oxygen diffusion problem in the multi-layered system


8

U-Zr-O Corium Melt - Steel Oxidation Model (HTLQ)

• HTLQ calculates the solid phases thicknesses, temperatures and O fluxes• HTLQ has to be coupled with 2-D thermo-hydraulic code through the heat flux

and oxygen mass flux at the melt-solid interface

C, T

r

Tmelt Tint

Tox

TS

Tout

COCO(t,r)

FeO

Boundary mesh

Heat flux

Oxygen flux

(U,Zr)O2

CU, CZr

Melt bulk

Jdif


9

Tox/crust < Teut 1600 KOxide thickness Corrosion depth

Tox/crust > Teut 1600 KOxide thickness ( 200 μm) <<

Corrosion depth ( 5mm)

METCOR observations


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“Flowering” mechanismLow temperatures High temperatures

Melt

Crust

FeO

Initial position of Fe boundary

Compressive stresses

Tensile stresses

Cracks and tears Melt

Eutectics

VS

Formation of FeO/crust eutectic melt and its extrusion (or “drainage”) through the crust

Melt

Crust

FeO

VS


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METCOR tests

Eutectic T1600 K


12

HTLQ Numerical Calculations

TS < 1600 K TS > 1600 K

0 5000 10000 15000 20000 25000Tim e (s)

0

2

4

6

8

10

Th

ick

ne

ss

(m

m)

Periphery crust

S teel ox ide

C orrosion depth

0 5000 10000 15000 20000 25000Tim e (s)

0

5

10

15

20

25

Th

ick

ne

ss

(m

m)

P eriphery crust

S teel ox ide

C orrosion depth

Steam

VSTS

Growth of corrosion depth with time


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Comparison with on-line measurements

Calculation runs On-line measurements#1 (Low temperature)Oxide/crust temp.: TS 1200 K

Heat flux: F 1 MW/m2

Corrosion rate: R 0.1 mm/h

MCP-2 test (regime 2)Oxide/crust temp.: TS 1220 K

Heat flux: F 0.81 MW/m2


#2 (High temperature)Oxide/crust temp.: TS 1800 K


Corrosion rate: R 5 mm/h

MCP-2 test (regime 9)Oxide/crust temp.: TS 1640 K




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Model predictions

0 50 100 150 200 250Time (h)

0

40

80

120

160

200

Th

ickn

es

s (m

m)

O x. F lux (m ol m -2 s -1)0.05

0.1

0.5

1.0

0 50 100 150 200 250Tim e (h)

0

40

80

120

160

200

Th

ickn

es

s (m

m)

H e a t Flu x (W m -2)10 5

2 10 5

3 10 5

4 10 5

Variation of vessel steel wall thickness with time at TS > 1600 K

Outer surface temperature: 273 K

Heat flux from melt: 2×105 Wm-2

Outer surface temperature: 273 K

Oxygen flux from melt: 0.5 molem-2s-

1


1515

Modification of CONV2D

Initialization of the initial and boundary conditions CONV2d:

Base calculation cycle

Heat conductivity block (convection + diffusion)

Hydrodynamics block (calculation of the velocities and pressure)

Output of the results (2d-temperature fields, heat fluxes, 1d characteristics)

Transformation of data to format of the melt-steel oxidation 1-D module

melt-steel oxidation 1-D module

Transformation of data to CONV2D formatModification of

the boundary conditions block for O2

Development of advection-diffusion

block for O2

Adaptation for two type of geometries:

reactor case &experimental

facility

Adaptation of turbulence model for the O2 transferin the reactor case

A set of the output parameters

was extended

- Modified blocks

- New blocks


1616

Steel Vessel

Crust

MeltLAVA Test Apparatus at FZK

PRV

CONV2D Adaptation to Different Geometries

9045

0


17

t = 5 s t = 35 s

Temperature

Calculation ResultsSmall-scale (crucible)

Oxygen


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Calculation ResultsLarge-scale (RPV)

r = 2.0775 m

Initial melt temperature: 2773 K

Heat source: 1 MW/m3

Initial oxygen concentration: 105 mole m-3

Oxygen flux to the melt: 1 mole m-2 s-1

Outer wall surface temperature: 100ºC

0 10 20 30 40 50Time (min)

2200

2400

2600

2800

3000

Me

lt T

emp

era

ture

(K

)

Position5°

45°

90°

0 10 20 30 40 50Time (min)

800

1200

1600

2000

2400

Ste

el T

emp

era

ture

(K

)

Position5°

45°

90°

0 10 20 30 40 50Time (min)

0

4

8

12

Cru

st

Th

ick

ne

ss

(mm

)

Position

5°

45°

90°

0 10 20 30 40 50Time (min)

0

40

80

120

160

200

Ste

el

Th

ick

ne

ss (

mm

)

Position

5°

45°

90°

(at interface) (near wall)


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Conclusions (1)• The model for U-Zr-O molten pool oxidation (developed within the

previous ISTC Project #2936) was upgraded for simultaneous consideration of vessel steel (VS) corrosion by the corium melt; on this base, the new oxidation/corrosion module HTLQ was developed

• The model allows interpretation of the METCOR tests observations and qualitatively describes VS corrosion kinetics observed in low- and high-temperature regimes

• The oxygen advection-diffusion block in the thermal-hydraulic code CONV2D was developed; adaptation of the turbulence model of CONV2D for solving the oxygen transport problem in the reactor case was carried out


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Conclusions (2)• The oxidation/corrosion module HTLQ was implemented in CONV2D;

the coupled code was thoroughly tested and verified, and then applied to simulation of corium retention after melt relocation into RPV

• Calculation results indicate that in-vessel retention by cooling the outside vessel wall with water might be ineffective, owing to physico-chemical dissolution of solid ceramic crust (at melt/wall interface) that prevents vessel walls from direct physico-chemical attack of the corium melt, and wall thinning (melting through) in the lack of crust up to a few cm

• This important conclusion suggests further, more thorough investigation of the crust physico-chemical stability under conditions of oxidized corium convection in RPV with residual heat generation in the melt and external wall cooling by water

ermsar 2012, cologne march 21 – 23, 2012 main results of the istc project #3876 “thermo-...

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