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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ERMSAR 2017, Warsaw, May 16-18, 2017
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
ERMSAR 2017, Warsaw, May 16-18, 2017
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
ERMSAR 2017, Warsaw, May 16-18, 2017
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
ERMSAR 2017, Warsaw, May 16-18, 2017
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
ERMSAR 2017, Warsaw, May 16-18, 2017
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
ERMSAR 2017, Warsaw, May 16-18, 2017
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.
ERMSAR 2017, Warsaw, May 16-18, 2017
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
ERMSAR 2017, Warsaw, May 16-18, 2017
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):
ERMSAR 2017, Warsaw, May 16-18, 2017
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
ERMSAR 2017, Warsaw, May 16-18, 2017
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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ERMSAR 2017, Warsaw, May 16-18, 2017
PWR-like reactor application
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
ERMSAR 2017, Warsaw, May 16-18, 2017
PWR-like reactor application
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
ERMSAR 2017, Warsaw, May 16-18, 2017
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.
ERMSAR 2017, Warsaw, May 16-18, 2017
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
ERMSAR 2017, Warsaw, May 16-18, 2017
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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