ermsar 2012, cologne march 21 – 23, 2012 analysis of corium behavior in the lower plenum of the...
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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