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.  

   

M M

M

Cavity

Containment Building

HVT

IRWST IRWSTM

M

Aux. Building

SCP (5000 gpm)

BAMP (200 gpm)

CVCS

RCS

M

M M

M

M

SteamGenerator

SteamGenerator

ReactorVessel

Reactor CavityFlooding System

External Reactor VesselCooling System

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 Formation Three 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

- ZrO2/(Zr + ZrO2)

U/Zr ratio: Molar ratio

TLFW SBLOCA

Corium Mass (ton) 194.5 171.1

UO2 Mass (ton)(Total 120 ton) 113.2 99.6

ZrO2 Mass (ton) 18.2 13.4

Zr Mass (ton)(Total 34 ton) 11.7 6.7

Stainless Steel Mass (ton) 50.0 50.0

B4C mass (ton) 1.4 1.4

Corium Temperature (K) 2,900 2,983

Zirconium Oxidation Fraction (Cn) 54.0 60.0

U/Zr Ratio 1.5 2.0

■ 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.

TLFW SBLOCA

Metallic Layer Oxidic Layer Metallic Layer Oxidic Layer

B 1.01 0.09 0.73 0.12

C 0.30 0.00 0.01 0.00

Cr 8.76 0.24 2.33 0.10

Fe 35.88 0.12 26.35 0.15

Ni 3.92 0.08 3.16 0.09

O 0.30 17.84 0.23 14.10

U 14.65 85.14 12.64 73.51

Zr 6.05 19.12 3.96 12.61

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 (Cn)

- Mass of UO2 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 UO2, when the mass of Zr increases then it favors

the dissolution of UO2 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, 2012 13

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 heat 41.3 MW

Corium masses of componentsUO2 99.6 t Steel 50 t

ZrO2 13.4 t Zr 8.7 t

Corium oxidation degree 60%

Initial temperature of corium Metal layer: 2,200 K

Oxidic layer: 2,983 K

Boundary conditions of the outer vessel surface: ERVC condition

Tamb = 393 K

HTC = 2104 W/m2K

ASTEC Input

ERMSAR 2012, Cologne March 21 – 23, 2012 14

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, 2012 15

Corium Mass Corium Temperature

Preliminary ASTEC Results

ERMSAR 2012, Cologne March 21 – 23, 2012 16

Vessel Geometry Lower 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, 2012 19

Thank you for your attention!

Toward the Robust and Resilient Nuclear System for the Highly Improbable Event