In-vessel prospective corium modelling in the MAAP … - In-vessel... · In-vessel prospective corium modelling in the MAAP_EDF code ... PWR-like reactor application ... No refractory

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May16-18, 2017

Warsaw, Poland

8TH CONFERENCE

ON SEVERE ACCIDENT

RESEARCH

ERMSAR 2017

In-vessel prospective corium modelling

in the MAAP_EDF code

Nikolaï Bakouta, M.Torkhani, A. Le Belguet

EDF R&D – PERICLES

Nuclear Safety and Fuel Cycle Group

ERMSAR 2017, Warsaw, May 16-18, 2017

Outline

Context

Introduction of the modelling

Summary description

Comprehensive description

PWR-like reactor application

Recent experiments

Envisaged long-term developments

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Context

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Qualitative data

• phase compositions

• proprieties

Experimental programs

• prototypic (MASCA, CORDEB)

• simulant (SIMECO, LIVE)

Analytical studies

• thermochemistry

• liquid/solid interface

conditions


In-vessel corium model developed at EDF

Focused on the coupling of physico-chemistry and thermal-hydraulics

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Light metal

Oxide

Heavy metal

Light metal goes down

Heavy metal rises up

Transient behavior of the pool


In-vessel corium model developed at EDF

Implemented in MAAP alongside with the native modelling

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Steel (Fe, Ni, Cr)

Mass transfer

Boundary condition

Tliq=f(M) ≈2700K

Boundary condition

Tfuison_steel=1800K

Light metal

Oxide

Heavy metal

Out-of-equilibrium

In-equilibrium

Solid debris

Focusing effect

Crust

EPRI’s in-vessel modelling EDF’s in-vessel modelling


Summary description

Whole pool associated with a set of 0D immiscible layers exchanging heat and

mass

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Mass transfer regarding :

• Corium relocation

• Melting of internal structures and vessel

• Inversion of stratification

Energy balance written in enthalpy related to :

• Decay heat

• Convective and radiative heat transfer

• Mass exchange


Convective heat flux

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At the interface i: where

For the liquid layers beneath

the crust: Ti=Tliquidus of the pool.

Nusselt correlations associated with hi


Inversion of stratification

Observed during experiments with sub-oxidized prototypic corium (MASCA, CORDEB).

Methodology elaborated by CEA on the basis of Reynolds analogy of mass and heat transfer to calculate the

transient uranium concentration (concentration of other species is assumed to be proportional).

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Validated on the MASCA-RWC experiment (50 kg) with the diffusion coefficient of uranium Dm=2.10-8m2/s

estimated using the Stokes-Einstein formula.


Composition of phases

Originally proposed by IRSN, a modelling for the system U-Zr-O-Steel fitting MASCA

experiments under certain conditions:

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Concentration of species is independent of the temperature

Ratio U/Zr remains constant

Oxygen reacts primarily forming UO2, then ZrO2

Oxides Fe, Ni and Cr are not considered

Steel is considered as a separate element

Enthalpy of chemical reactions is negligible

A straight "tie line" passes through the intersection points on the U-Zr-Steel plan (point B)

and U-Zr-O plan (point A)

Intersection of the "tie line" with the miscibility gap boundary gives the concentration of

phases: the point C1 corresponds to the oxide phase and the point C2 to the metal phase


Modelling of crusts

Plain front of melting/solidification. The side crust is composed of several segments while the upper crust is represented by

an individual layer.

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Steady-state conduction flux assumed at crust interfaces :

The mass variation (melting/solidification):


Boundary conditions

For the layers beneath the crust, boundary conditions for the convective flux Qconv is assumed to be equal to Tliquidus of the pool obtained with the native MAAP routine TDEBRI.

TDEBRI was recently evaluated against NucleaToolbox software in the frame of EDF/CEA benchmarking exercise

involving gradual mixing of steel and sub-oxidized corium.

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Tliquidus of the pool


Improvements of modelling

Ongoing developments regarding the state equation (EOS) and properties of the corium

pool :

Implement an alternative for TDEBRI routine

State equation of the pool T=f(H,M)

Liquidus and solidus temperatures

Pool melting/solidification

Middle term developments concerning solid debris behavior :

Incorporation of the solid debris into liquid pool

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IVR conditions, massive corium spreading

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(A) Initial state – a two-layer (oxide and light metal) corium pool

(B) Beginning of vessel melting, initial formation of the steel layer, maximum heat flux to the vessel

(C) Opposite external heat flux appears over the vessel heating

(D) Internal heat flux is decreased due to the steel layer thickening



IVR conditions, progressive corium spreading

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(A) Initial state – a two-layer (heavy metal and oxide) pool with smaller contact area with the vessel

(B) Slowly growing steel layer

(C) High temperature of the oxide layer leads to a higher heat flux to the vessel

(D) Vessel failure


Recent experiments

CORDEB experiments findings : steel gradually passes through the crust, resulting in the

inversion of the pool stratification.

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Schematic presentation of the steel/crust interaction in CORDEB experiments

(image courtesy of NITI).

The phenomenon which, in the reactor conditions, could lead to thinning of the steel layer and the Focusing

Effect increasing.

So far, in the EDF modelling, the upper crust is not chemically reacting with steel.


Recent experiments

AP experiments (NITI) focused on investigation of a crust formation on a steel specimen immerged in the metal/oxide pool

and on the metal/crust interaction at the bottom of the crucible.

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1. Metal edge

2. Shrinkage cavity

3. Metal

4. Oxide

5. Metal bridge

6. Metal lens

7. Steel specimen

Oxide/metal ingot and immerged specimen used in AP5 experiment (image courtesy of NITI).

Findings : No refractory crust can form between the vessel and the light metal layer

The refractory crust formed on the reactor vessel in front of oxide layer can persist contacting the molten metal


Envisaged long-term developments

Crusts :

Initial formation of the light metal layer without crust between the vessel

Chemical interaction of the light metal and the upper crust resulting in metal mass transfer towards the pool

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Current crust model Intended crust model


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Thank you for your attention

Documents

In-vessel prospective corium modelling in the MAAP … - In-vessel... · In-vessel prospective corium modelling in the MAAP_EDF code ... PWR-like reactor application ... No refractory