1. Benchmark VVER1000 MCCI Reactor Test Case
Antoaneta Stefanova and Pavlin Groudev
Institute for Nuclear Research and Nuclear Energy,
Bulgarian Academy of Sciences
Sofia, Bulgaria

2. Introduction
The VVER-1000 reference nuclear power plant was chosen as SARNET2 benchmark MCCI test-case.
The initial conditions for MCCI test are taken after SBO scenario calculated with ASTEC version 1.3R2.
MCCI test calculations with and without water injection.
Finaly 7 organizations were performed the VVER MCCI test calculations in dry conditions and 2 organisations (INRNE and IRSN) were performed calculations with water injection.

3. Geometry of initial cavity
and concrete basemat
The reactor pit is cylindrical.
Basemat axial thickness =3.6 m.
Cavity height =2.35 m (including metal layer).
Cavity radius = m.
Cavity floor area = m2 (It is included to the area of cylindrical pit and corridor to the isolating door. The area of cylindrical part is m2.)
Concrete composition and other features
Concrete composition
Content, %
H2OCHEM
1.775
CO2
6.761
SiO2
47.36
Fe2O3
2.01
Al2O3
1.755
CAO
20.03
MGO
1.135 Fe
16.17
H2OEVAP
3.0
Density, kg/m3
2600
Solidus temperature (K)
1420
Liquidus temperature (K)
1820
Radiation emissivity
0.63
Ablation temperature (K)
1773
Concrete type:
Siliceous type, rather low gas content (4.8% H2O, 6.8% CO2)
High iron fraction: 16.2% (compared to 6.15% in PWR

4. Initial conditions for MCCI test after SBO scenario
The SBO scenario have been performed with the following assumptions
In order to calculate the Station Blackout scenario it was accepted the following assumptions:
Failure of secondary side BRU-A valves, as they are electrically driven (BRU-K valves are failed also in standard SBO scenario). It will be used SG SVs – they will start to open at 85 bars and closing at 71 bars.
Pressurizer valve open after reaching its set point for opening and stuck open – for faster melting of reactor core.
Failure of all HAs.
Initial conditions for MCCI test
The total mass of kg corium slump transferred to the cavity is observed at sec. The temperature of ejected corium is 2879 K.

5. Performed calculations/Melt-through time (axial ablation = 3.6 m)
Received Results
Organizations
Country
Used Code
Performed calculations/Melt-through time (axial ablation = 3.6 m)
INRNE
Bulgaria
ASTEC
Dry cavity – s
water injection – s
IRSN
France
Dry cavity – s
water injection – s
GRS
Germany
Dry cavity s EI
Dry cavity – performed calculations ( s)/melt-through time (57h)
NUBIKI
Hungary
Dry cavity s
KIT
WECHSL
Dry cavity – s TU
Dry cavity – s

6. Main assumptions Partner Assumtion GRS IRSN NUBIKI INRNE EI KIT TUS
1. Bulk convective heat transfer correlation
Oxide layer/concrete
Surface Renewal Model
t bottom=2730s
t side=194s
BALI correlation+
HSLAG=1000
(W/m2/K)
Depending on the existing gas flow
BALI correlation+
Metal layer/concrete
t bottom=50.4s
t side=516s
t top=50.4s
BALI correlation +
Pool/atmospher e
t top=194s
BALI convection correlation
Oxide/Metal
1600 W/(m2K) in metal layer
80 W/(m2K) in oxide layer
Greene correlation
Greene correlation
Based on Werle exp.

7. Main assumptions - 2 Partner Assumtion GRS IRSN NUBIKI INRNE EI KIT
TUS
2. Crust presence
Metal/ crust no
yes
Oxcide/ crust
3.Pool/ crust interface temperatur e
Tsol
Tsolidification=0.8Tliq+
0.2Tsol
Oxide
Metal:
Tsolidification=1.0Tsol
Tsolidification=0.7Tliq+
0.3Tsol
Tsol≤ Tint≤ Tliq
4. Emissivitie s
Environme nt
0.8 -
Pool
5.Solidus and liquidus temperatur es
Calculated by GEMINI02 with NUCLEA09 data base
Calculated by GEMINI02 with NUCLEA09 data base
Input data
6. Configurat ion evolution criterion
static stratified pool configuratio n with metal layer below the oxide layer
bHS = pool is homogeneous and stratification is checked;
bSH = pool is stratified and homogenisation is checked
bHS = pool is homogeneous and stratification is checked;
bSH = pool is stratified and homogenisation is checked
bHS = pool is homogeneous and stratification is checked;
bHS = pool is homogeneous and stratification is checked;
Not used
bHS = pool is homogeneous and stratification is checked;
bSH = pool is stratified and homogenisation is checked

8. Main assumptions - 3 Partner Assumtion GRS IRSN NUBIKI INRNE EI KIT
TUS
7. Cavity erosion algorithm
Cavity boundary noding
150 nodes
Pool is divided into segments
8. Quenching models
Water ingression
Crust permeability K = 3.10-
Crust permeability K = 3.10-
Melt eruption
Proportionality factor E = 0.08
9. Initial assumed pool configuration
Initial stratified configuration with metal layer below
Initial stratified configuration with oxide layer below
Initial stratified configuration with metal layer below
Initial stratified configuration with oxide layer below
Initial homogeneous configuration

9. Comparison of calculated results for dry cavity condition
Accepted initial stratification with metal above the oxide;
Till the metal layer is above there is prononsed radial ablation;
After aproximately 3h pool change to homogeneous that equlize radial and axial ablation;
Secondary stratification with metal below appears at 12.6h after MCCI onset, which results in faster axial erosion till the melt- throught time at 27h.

10. Comparison of calculated results for dry cavity condition
Accepted initial stratification with metal above the oxide;
Till the metal layer is above there is prononsed radial ablation;
After the 2h pool change to homogeneous that equlize radial and axial ablation;
Secondary stratification with metal below appears at 13h after MCCI onset, which results in faster axial erosion till the melt-throught time at 25h.

11. Comparison of calculated results for dry cavity condition
Fixed stratified pool configuration with metal layer below the oxide layer;
Fixed decay heat partition (10% of decay heat is released in the metal layer and 90% is released in the oxide layer);
Axial ablation is larger than the radial ablation but due to bad heat transfer assumed between oxide and metal in the calculations the difference between axial and radial ablation is only small;
Melt-through appears later (at 48h).

12. Comparison of calculated results for dry cavity condition
Initial stratification with metal layer above the oxide layer;
Switch from stratified to homogeneous configuration appears very soon (at 2100s);
Homogeneous phase prolongs very short time (1.5h) due to considerably higher value of the coefficient bHS =0.1, that contributes to higher threshold value of Jg for change from homogeneous to stratified pool (at 7600s);
Melt-throught appears later at 57h.

13. Comparison of calculated results for dry cavity condition
Initial stratification with metal layer bellow the oxide layer;
Switch from stratified to homogeneous configuration appears very soon (at 2772s).The radial erosion accelerates;
Homogeneous phase prolongs rather long (18h) due to considerably lower value of bHS (0.054 – BALISE threshold value corresponding to complete mixing;
Melt-through apears at 26.3h.

14. Comparison of calculated results for dry cavity condition
Homogeneous configuration. There is no pool segregation all the time;
Heat transfer in the model depends on the existing gas flow and eventual crust formation;
Melt-thought appears later (at approximately 55.2h).

15. Comparison of calculated results for dry cavity condition
Initial stratification with metal above the oxide;
Till the metal layer is above there is prononsed radial ablation;
After aproximately 3h pool change to homogeneous that equlize radial and axial ablation;
Secondary stratification with metal below appears at 20.2h after MCCI onset, which results in faster axial erosion till the melt-throught time at h.

16. Comparison of calculated results for dry cavity condition
During the period of pool homogenisation mass of the metal layer is equal to zero.
Metals in the pool decrease due to metal oxidation reactions with the passed through gases: water vapor and carbon dioxide.
In GRS calculation it is supported considerably higher mass of the metal layer in comparison with the other calculations due to not all content of generated gas pass through the metal layer which is below the oxide layer all the time.
There is no metal layer assumed in KIT model.

17. Comparison of calculated results for dry cavity condition
In GRS calculation a static pool configuration is considered. The pool is supposed to be stratified all the time with the metal layer below the oxide layer. However, it is assumed bad heat transfer between oxide and metal layerrs in GRS calculation
In KIT(WECHSL) calculation pool is homogeneous and the metal phase is homogeneously dispersed in the form of droplets.
All other MEDICIS\ASTECv2 calculations assume GREEN=1 for the oxide\metal heat transfer.

18. General Conclusions
Seven calculations without quenching have been done with different MCCI computer codes:
-six calculations have been done with MEDICIS/ASTECv2 code
-one calculation with WECHSL code;
Two calculations (IRSN and INRNE) with MEDICIS\ASTECv2 have been done with water injection initiated from the beginning till the end of calculation. Water flow is 66kg/s and water temperature is 60ºC at atmosphere pressure;
In ASTEC calculations due to the high iron content in concrete and the rather low gas content, which contribute to maintain the pool stratification with metal below until the end of the calculations fast axial ablation and an early melt-through have been observed even in case of water injection. It can be also explained by the impact of high oxide/metal convective heat transfer coefficient (except in GRS calculation).

19. General Conclusions
The low impact of corium quenching by water injection as obtained in IRSN and INRNE calculation (see table below) is due to:
-Focussing of decay energy downwards in final stratified phase;
-Low concrete gas content limiting the efficiency of melt eruption;
-Water ingression determined by crust permeability = m2
INRNE
Bulgaria
MEDICIS\ASTECv2
Dry cavity – 27.3h
Water injection – 34.2h
IRSN
France
Dry cavity – 25.5h
Water injection – 30.9h

20. Thank you for
your attention !