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

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.

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 = 2.906 m. Cavity floor area = 31.47 m2 (It is included to the area of cylindrical pit and corridor to the isolating door. The area of cylindrical part is 26.53 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

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 77 074 kg corium slump transferred to the cavity is observed at 21 897 sec. The temperature of ejected corium is 2879 K.

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 – 98279.84 s water injection – 123286.1 s IRSN France Dry cavity – 91828.48 s water injection – 111293.6 s GRS Germany Dry cavity - 172850 s EI Dry cavity – performed calculations (86307.81 s)/melt-through time (57h) NUBIKI Hungary Dry cavity - 94845s KIT WECHSL Dry cavity – 198728s TU Dry cavity – 114580 s

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.

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 = 0.027 - pool is homogeneous and stratification is checked; bSH = 0.1 - pool is stratified and homogenisation is checked bHS = 0.054 - pool is homogeneous and stratification is checked; bSH = 10. - pool is stratified and homogenisation is checked bHS = 0.05 - pool is homogeneous and stratification is checked; bHS = 0.1 - pool is homogeneous and stratification is checked; Not used bHS = 0.035 - pool is homogeneous and stratification is checked; bSH = 0.5 - pool is stratified and homogenisation is checked

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

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.

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.

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).

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.

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.

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).

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 31.8h.

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.

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.

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).

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 = 3.10-11 m2 INRNE Bulgaria MEDICIS\ASTECv2 Dry cavity – 27.3h Water injection – 34.2h IRSN France Dry cavity – 25.5h Water injection – 30.9h

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