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ERMSAR 2012, Cologne March 21 – 23, 2012
Experimental and computational studies of the coolability of heap-like and cylindrical debris beds
E. Takasuo, S. Holmström, T. Kinnunen, P.H. Pankakoski, V. Hovi, M. Ilvonen (VTT Technical Research Centre of Finland)
S. Rahman, M. Bürger, M. Buck, G. Pohlner (Institute of Nuclear Technology and Energy Systems, University of Stuttgart)
IKE
ERMSAR 2012, Cologne March 21 – 23, 2012
Introduction
Ex-vessel debris coolability is a key issue at the Finnish and Swedish BWRs
– Melt pours from the RPV to a deep water pool (flooded lower drywell of the containment)
– Porous debris bed is formed as a result of melt solidification, fragmentation and settling of the particles
Coolability mainly depends on the debris bed configuration
– Can be highly complex: porosity, particle size and morphology, overall geometry
Depends on the melt discharge scenario and interactions with the pool
2
ERMSAR 2012, Cologne March 21 – 23, 2012
Scope of the present studies
Experimental studies
– Measurements of dryout heat flux in two representative debris bed geometries
Simulations
– Prediction of dryout heat flux in the experiments (in an already established debris bed) code validation
– Quenching analysis during debris bed formation in a plant scale scenario
3
ERMSAR 2012, Cologne March 21 – 23, 2012
Experimental activities at VTT
The COOLOCE (Coolability of Cone) test apparatus replaced the STYX facility in 2009
– Conical (heap-like) and cylindrical test beds
– Test series have been run for both geometries for a range of pressure
Objective is to clarify the effect of geometry (lateral flooding vs. height) and provide new data for simulation code validation
4
STYX with downcomers (2008)
ERMSAR 2012, Cologne March 21 – 23, 2012
Conical test bed
The heating arrangement (electrical resistance heaters) and thermocouples
The conical particle bed filled with particles (spherical ceramic beads, ø 0.8 - 1.0 mm) held in shape by a net
ERMSAR 2012, Cologne March 21 – 23, 2012
Cylindrical test bed
The heating arrangement (electrical resistance heaters) and thermocouples
The cylindrical particle bed filled with particles (spherical ceramic beads, ø 0.8 - 1.0 mm), porosity ~ 38%
ERMSAR 2012, Cologne March 21 – 23, 2012
Experimental results
It was found that the coolability of the conical debris bed is improved by 50%-60% compared to the cylindrical bed in case the beds are equal in height
However, if the beds have equal radius and volume (flat-shaped cylinder) the coolability of the conical bed is poorer by about 50%
– The effect of the increased height (and thermal loading near the tip of the cone) is greater than the effect of lateral flooding
7
ERMSAR 2012, Cologne March 21 – 23, 2012
MEWA and JEMI codes
MEWA 2D and JEMI are developed by IKE (University of Stuttgart) specifically for severe accident analysis
The different stages of melt and debris coolability can be evaluated with the coupling of MEWA to JEMI
– Melt breakup and jet quenching
– Particle settling
– Initial quenching of the debris bed
– Fully quenched debris bed coolability
8
ERMSAR 2012, Cologne March 21 – 23, 2012
Comparison of experiments and MEWA simulations
9
ERMSAR 2012, Cologne March 21 – 23, 2012
MEWA results: Cone and tall cylinder
10
Particle temperature in post-dryout conditions
ERMSAR 2012, Cologne March 21 – 23, 2012
MEWA results: Cone and flat-shaped cylinder
The beds have equal radius, i.e. the scaling corresponds to reactor scenarios
11
ERMSAR 2012, Cologne March 21 – 23, 2012
Debris quenching simulations
Quenching of initially hot debris was modeled by MEWA
– Simultaneous settling and quenching of hot particles that form a conical bed in the water pool
– Initial particle temperatures from jet breakup and particle mixing calculations with JEMI
– More realistic compared to earlier approaches that deal with already established, initially hot debris bed
– Postulated accident with a melt mass of 185 tons with the discharge rate of 0.157 m3/s
12
ERMSAR 2012, Cologne March 21 – 23, 2012
JEMI/MEWA simulation results
Simulations with realistic initial conditions
– Cooling is supported by quenching of the lateral region during settling
– Cool down of particles is observed when they reside at the surface of the debris bed
– The quenching is fast enough so that the heat-up due to decay heat does not yield temperatures beyond 2000 K
– The cooling of the upper parts by gas flow is effective because of the fast quenching of the lower parts
Simulations with uniform initial temperature
– Quenching of the lower bed regions occurs slower
– The cooling of upper regions by gas flow is not effective due to slow water infiltration in the lower parts
– Melting temperature (>2800 K) is reached in large parts of the bed already after 2800 s
13
ERMSAR 2012, Cologne March 21 – 23, 2012
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]0.02
Superficial Liq. Vel. [m/s]Time t = 20.012s
14
Quenching during build-up
ERMSAR 2012, Cologne March 21 – 23, 2012 15
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]0.1
Superficial Liq. Vel. [m/s]Time t = 70.094s
Quenching during build-up
ERMSAR 2012, Cologne March 21 – 23, 2012 16
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]0.01
Superficial Liq. Vel. [m/s]Time t = 120.15s
Quenching during build-up
ERMSAR 2012, Cologne March 21 – 23, 2012 17
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]0.02
Superficial Liq. Vel. [m/s]Time t = 190.25s
Quenching during build-up
ERMSAR 2012, Cologne March 21 – 23, 2012 18
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]0.005
Superficial Liq. Vel. [m/s]Time t = 600.57s
Quenching during build-up
ERMSAR 2012, Cologne March 21 – 23, 2012 19
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]0.01
Superficial Liq. Vel. [m/s]Time t = 1245.7s
Quenching during build-up
ERMSAR 2012, Cologne March 21 – 23, 2012 20
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]0.003
Superficial Liq. Vel. [m/s]Time t = 2401s
Quenching during build-up
ERMSAR 2012, Cologne March 21 – 23, 2012 21
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]0.001
Superficial Liq. Vel. [m/s]Time t = 3854.1s
Quenching during build-up
ERMSAR 2012, Cologne March 21 – 23, 2012 22
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]
0.3
Superficial Liq. Vel. [m/s]
Time t = 1.5259E-005s
Quenching of an established bed
ERMSAR 2012, Cologne March 21 – 23, 2012 23
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]
0.01
Superficial Liq. Vel. [m/s]
Time t = 190.21s
Quenching of an established bed
ERMSAR 2012, Cologne March 21 – 23, 2012 24
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]
0.009
Superficial Liq. Vel. [m/s]
Time t = 600.89s
Quenching of an established bed
ERMSAR 2012, Cologne March 21 – 23, 2012 25
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
200400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]
0.01
Superficial Liq. Vel. [m/s]
Time t = 2403.6s
Quenching of an established bed
ERMSAR 2012, Cologne March 21 – 23, 2012
-4 -3 -2 -1 0 1 2 3 4
Radius [m]
0
1
2
3
Hei
gh
t [m
]
200400600800
1,0001,2001,4001,6001,8002,0002,2002,4002,6002,800
Particle Temperature [K]
0.008
Superficial Liq. Vel. [m/s]
Time t = 2804.1s
26
Quenching of an established bed
ERMSAR 2012, Cologne March 21 – 23, 2012
MEWA simulations: Maximum temperature
27
CASE1: Quenching during build-up CASE2: Uniform initial temperature
ERMSAR 2012, Cologne March 21 – 23, 2012
Possibilities of 3D modeling
3D approach facilitates the modeling of complex geometries and/or internal inhomogeneity, pool model may be included
Demonstration calculations of the dryout behavior of an established debris bed have been conducted by PORFLO
– Multi-purpose 2-phase 3D solver developed at VTT
– Models suitable for porous beds have been included
The code is capable of capturing the main processes of debris bed dryout
– Development is still on-going, suitability for coolability prediction has to be verified for various cases
28
ERMSAR 2012, Cologne March 21 – 23, 2012
PORFLO results
Liquid saturation in post-dryout conditions for the conical and cylindrical beds
29
ERMSAR 2012, Cologne March 21 – 23, 2012
Summary
New experimental data of the effect of lateral flooding and debris bed height on coolability has been obtained
– Poorer coolability for the conical bed due to greater height
The MEWA simulation results agree very well with the measured dryout power
Preliminary 3D calculations with PORFLO suggest that full 3D approach could be feasible for coolability analyses
Simulations with quenching during debris bed build-up suggest improved coolability margins compared to cases with already established hot debris bed
30