ERMSAR 2012, Cologne March 21 – 23, 2012 Analysis of Corium Behavior in the Lower Plenum of the Reactor Vessel during a Severe Accident Rae-Joon Park, Kyoung-Ho Kang, Kwang-Il Ahn, Seong-Wan Hong Korea Atomic Energy Research Institute

ERMSAR 2012, Cologne March 21 – 23, 2012  The strategy of the APR1400 for severe accident mitigation aims at retaining molten core in-vessel first (IVR-ERVC: In-Vessel corium Retention through External Reactor Vessel Cooling) and ex-vessel cooling of corium second in case the reactor vessel fails, reinforcing the principle of defense-in-depth.  IVR-ERVC was adopted as one of severe accident management strategies. In IVR-ERVC condition, the cavity will be flooded by the SCP and the BAMP to the hot leg penetration bottom elevation. Background - IVR-ERVC: Active system (No passive) & non severe accident design feature in the APR1400 Schematic Diagram of the APR1400(Advanced Power Reactor)

ERMSAR 2012, Cologne March 21 – 23, 2012  IVR-ERVC Evaluation Method  To determine the thermal load from the corium pool to the outer reactor vessel : Important of corium behavior in the lower plenum  To determine the maximum heat removal rate of CHF on the outer reactor vessel.  To decide the thermal margin by comparison of the thermal load with CHF for IVR-ERVC achievement  It is very important to analyze the corium behavior in the lower plenum to determine the thermal load for IVR-ERVC evaluation of the APR1400. Background

ERMSAR 2012, Cologne March 21 – 23, 2012  Possibility of Melt Pool Layer Inversion: MASCA experimental results:  When sufficient amount of non oxidized zirconium (Zr) is available, then metallic uranium (U) migrates to the metallic layer.  The density increase of the metallic layer can lead to inverse stratification with an additional heavy metal layer below the oxidic pool.  Thinning of the top metal layer can increase the risk of the focusing effect. Original Two-Layer FormationThree Layer Formation after layer Inversion Background

ERMSAR 2012, Cologne March 21 – 23, 2012  Objective: Analysis of corium behavior in the lower plenum to determine the thermal load for IVR-ERVC evaluation of the APR1400  Contents - To decide molten pool configuration in the lower plenum - To determine the heat load to the outer reactor vessel using ASTEC computer code Objective

ERMSAR 2012, Cologne March 21 – 23, 2012  Determination of Initial Melt Pool Condition in the LP using SCDAP/RELAP5 ■ Two Dominant Sequences for the APR1400: SBLOCA, TLFW from Level I PSA results ■ SCDAP/RELAP5 Nasalization (*) SBLOCA: Small Break Loss Of Coolant Accident (*) TLFW: Total Loss of Feed Water

ERMSAR 2012, Cologne March 21 – 23, 2012  Melt Pool Condition in the Lower Plenum: SCDAP/RELAP5 Results  Zr oxidation rate (Cn): - Molar ratio - ZrO 2 /(Zr + ZrO 2 )  U/Zr ratio: Molar ratio TLFWSBLOCA Corium Mass (ton) UO 2 Mass (ton) (Total 120 ton) ZrO 2 Mass (ton) Zr Mass (ton) (Total 34 ton) Stainless Steel Mass (ton) 50.0 B 4 C mass (ton) 1.4 Corium Temperature (K) 2,9002,983 Zirconium Oxidation Fraction (Cn) U/Zr Ratio ■ The SCDAP/RELAP5 results such as the mass and the temperature of melt compositions were used as an input for the thermodynamic calculations.

ERMSAR 2012, Cologne March 21 – 23, 2012  Determination of Corium Composition using GEMINI Code  Mass distribution of the individual melt components was obtained.  Mass fraction of the each melt component which involved in the metallic layer and oxidic layer was determined. TLFWSBLOCA Metallic LayerOxidic LayerMetallic LayerOxidic Layer B C Cr Fe Ni O U Zr

ERMSAR 2012, Cologne March 21 – 23, 2012  Generals of Layer Inversion in the Corium Pool  The thermodynamic calculations are aimed to determine the composition of a U-Zr-Fe-O mixture at thermodynamic equilibrium for a given temperature.  Major parameters controlling the layer inversion: - U/Zr ratio - Zr oxidation fraction (C n ) - Mass of UO 2 and steel - Carbon content in the corium pool  The less Zr is oxidized, the higher mass of metal that can stratify below the oxidic pool.  For a given mass of UO 2, when the mass of Zr increases then it favors the dissolution of UO 2 and the transfer of U in the steel layer.

ERMSAR 2012, Cologne March 21 – 23, 2012  Evaluation of Layer Inversion in the Corium  Using the density evaluation graphs, the mass of metallic layer which is heavier than the oxidic layer can be calculated.  In addition to this iron mass relocated below the oxidic pool, the U and the part of Zr listed in the GEMINI calculation result can stratify below the oxidic layer.  The mass of Zr which stratify below the oxidic layer was calculated by assuming that the mass fraction of U is fixed at 0.4 among the total mass of heavy metallic layer below the oxidic layer.  Total mass of heavy metallic layer below the oxidic layer can be obtained by summing the Fe, the U, and the Zr in two severe accident sequences of the APR1400.

ERMSAR 2012, Cologne March 21 – 23, 2012  Final Melt Pool Configuration Two-Layer Formation in the SBLOCA Three-Layer Formation in the TLFW

ERMSAR 2012, Cologne March 21 – 23, 2012  Evaluation Results on Layer Inversion in the APR1400  Melt pool configurations were different in the SBLOCA and the TLFW of the APR1400.  In case of SBLOCA, two layer where U/Zr ratio and initial melt pool temperature were relatively higher, layer inversion phenomena can be precluded, which results in two-layer formation.  In case of TLFW, however, layer inversion occurs, which results in three-layer formation.  Final melt pool configuration is input for corium behavior analysis using ASTEC computer code.

ERMSAR 2012, Cologne March 21 – 23,  ASTEC Input for Two layer Formation Case of SBLOCA in the APR1400 Reactor vessel head geometry Inner diameter:4.74 m Wall thickness:0.17 m Initial power of decay heat41.3 MW Corium masses of components UO tSteel50 t ZrO tZr8.7 t Corium oxidation degree60% Initial temperature of corium Metal layer:2,200 K Oxidic layer:2,983 K Boundary conditions of the outer vessel surface: ERVC condition T amb =393 K HTC = 2  10 4 W/m 2 K ASTEC Input

ERMSAR 2012, Cologne March 21 – 23, Corium Configuration (Two-Layer Formation of SBLOCA sequence) Used ASTEC modules: ICARE Modelled components: Lower plenum (component LOWERPLE), Number of spatial meshes: 80 (10 - in axial direction, 8 - in radial direction) Layers of corium: Oxide (lower layer), Metal (upper layer) Used models: COND- Thermal conduction EXCHLOWE - Exchanges between corium and LP wall CONV, CONVLOWE - Convective heat exchange DECALOWE- Decanting toward corium layers Vessel rupture criteria: FUSION and MECHANIC  Used ASTEC Model for APR1400 Lower Plenum Used ASTEC Model

ERMSAR 2012, Cologne March 21 – 23, Corium Mass Corium Temperature Preliminary ASTEC Results

ERMSAR 2012, Cologne March 21 – 23, Vessel GeometryLower Head Vessel Temperature Preliminary ASTEC Results

ERMSAR 2012, Cologne March 21 – 23, 2012  Initial melt pool configurations were determined using SCDAP/RELAP5 and GEMINI results for two dominant severe accident sequences in the APR1400.  Melt pool configurations were different in the SBLOCA and the TLFW  Where U/Zr ratio and initial melt pool temperature were relatively higher, layer inversion can be precluded, which results in two-layer formation in the SBLOCA.  However, layer inversion occurs, which results in three-layer formation in the TLFW. Conclusions 17

ERMSAR 2012, Cologne March 21 – 23, 2012  ASTEC results predict the corium temperature, the lower head vessel temperature, and the reactor vessel geometry change as a function of time in two-layer formation case of SBLOCA, which is preliminary results.  More detailed analysis of the main parameter effects on the corium behavior in the lower plenum is necessary to determine the initial and boundary conditions for the IVR-ERVC evaluation in the APR1400, in particular, for three-layer formation case of the TLFW.  Comparisons of present results with others are necessary to verify the present results and to apply to the actual APR1400 IVR-ERVC evaluation. Conclusions 18

ERMSAR 2012, Cologne March 21 – 23, Thank you for your attention! Toward the Robust and Resilient Nuclear System for the Highly Improbable Event