INTRODUCTION

FLOW AND SOLIDIFICATION OF CORIUM IN THE VULCANO FACILITY
Christophe JOURNEAU, Eric BOCCACCIO, Claude JEGOU, Pascal PILUSO, Gérard COGNET
Commissariat à l’Energie Atomique, CEA / Cadarache,
Dept. of Thermalhydraulics and Physics, F13108 St Paul lez Durance, France
ABSTRACT
In the hypothetical case of a nuclear reactor severe accident, the reactor core could melt and form a mixture, called corium, of highly
refractory oxides (UO2, ZrO2), steel in a metallic or oxidised form and of the substrate decomposition products, which could eventually
flow out of the vessel. The VULCANO facility is dedicated to experiments with prototypic corium (including depleted UO2). This
paper is devoted to corium spreading experiments, in which the effects of flow and solidification are studied. Due to the complex
behavior of corium in the solidification range, an interdisciplinary approach has been used relating thermodynamics of multicomponent
mixtures, rheological models of silicic semisolid materials, high temperature heat transfer, free-surface flow with temperaturedependant properties. This is coupled with the use of specific high temperature instrumentation requiring in-situ cross calibration.
Eleven high temperature spreading tests have been performed and analyzed. The main experimental results are the good spreadability
of corium, the effects of cooling rates on the microstructure, the occurrence of a large thermal contact resistance at the corium-substrate
interface, the presence of a rheological boundary layer at the surface. Furthermore, the experiments showed the role of the gaseous
inclusions in the melt. These tests with prototypic material have been used to validate spreading models and codes.
INTRODUCTION
Nuclear fission energy provides about 80 % of the electricity
supply in France and 33 % in the European Union. It has several
important strategic benefits: independence from fossil fuels,
long-term security of primary energy supply and zero emission
of greenhouse-effect gases. However, despite these advantages,
there is an opposition to nuclear energy from sectors of the
public in the European countries. Consequently, this implies the
improvement of knowledge with the view to simultaneously
increasing safety and maintaining competitiveness.
Since the Three Mile Island accident in 1979 [1] and especially
since the Chernobyl accident in 1986 [2], it is clear that one of
the key points to increase safety is that any reasonably credible
accident must be controlled within the reactor containment with
no off-site consequences.
In this context, for some years now, the CEA has undertaken a
large program [3] on severe accidents aimed at their mastering
in both existing and future power plants.
For the European Pressurized water Reactor (EPR) project, core
melt accident is one of the severe accidents to be mastered under
the new safety approach for the reactor design. Therefore, a
dedicated area of about 175 m² [4] has been devoted to the
spreading of the molten mixture called corium - essentially
composed of uranium and zirconium dioxides and of more or
less oxidized steel - that would result from the hypothetical
melting down of the core, melting through the reactor vessel and
mixing with a sacrificial concrete in the reactor pit. The role of
spreading is to reduce the surface thermal load due to the
radioactive decay heat.
To study corium spreading, a series of experiments have been
performed in Europe. The CORINE facility [5] has been built
for low temperature analytic experiments. The Scaled Simulant
Spreading Experiments (S3C) uses simulants at temperatures up
to 1400 K [6]. In the KATS facility [7] alumina above 2000 K is
used as a simulant. Simulant spreading experiments have also
been performed for volcanological applications [8].
Spreading experiments using prototypic corium, i.e. melt
containing depleted uranium dioxide with compositions
prototypic of a hypothetic severe accident scenario, have been
performed in the CARLA [9], FARO [10] and VULCANO [11]
facilities.
This paper presents the VULCANO facility and the main results
from the spreading tests that have been conducted there. They
have been obtained through an interdisciplinary approach
linking heat transfer, fluid mechanics, physico-chemistry,
materials science and thermodynamics. Rather than focusing on
specific experiments or techniques, this paper synthesizes the
various data and results obtained in this test series.
THE VULCANO FACILITY
The VULCANO facility [11] is mainly composed of a furnace
[12] and a test section, which is thoroughly instrumented. Posttest analyses, which are a necessary part of prototypic corium
tests, will also be described. An interdisciplinary experimental
team operates this facility.
The furnace
To realize prototypic material experiments, with acceptable
safety conditions, the furnace must be able to melt oxidic
mixtures of various compositions (UO2, ZrO2, SiO2, FeOx ) with
the possible addition of metals at temperatures between 2000
and 3200 K. It shall have the capacity to melt and to pour 100
kilograms continuously and at low pouring rates (0.1 to 1 L/s). A
study of the candidate technologies leads to the choice of a
transferred plasma-arc furnace.
rise of surface thermocouples. The geometrical calibration (see
Figure 2) is obtained by positioning a checkerboard on the
spreading section and correlating the pixel position with the
vertices co-ordinates.
Figure 2: Checkerboard used to calibrate the “fish-eye”
camera before test VE-U1.
Figure 1 : The VULCANO furnace.
Two plasma torches are ignited by an electrical short circuit.
The main arc is then created and transferred between these two
torches having opposite polarity (see Figure 1). The electrode
tips are made of graphite. The plasmagenic gas is of argon
and/or nitrogen plus, in some cases, corium fumes. The
maximum available power is around 600 kW ( 1000 A – 600 V).
Powders to melt are inserted in the cylindrical rotating cavity
(400 mm diameter – 500 mm long). A self-crucible of zirconia is
realized in the furnace by the centrifugation (between 150 and
300 rpm) and heating of the powder. The furnace external
surfaces are water-cooled while the inner surface of the load is
subject to the radiation from the plasma arc (temperatures
between 10 000 and 20 000 K). Radiation heats the surface,
then, convection and mainly conduction transfer the heat to the
inner layers. The corium powders are then loaded and molten.
Two successive phases of loading and melting are used in order
to increase the furnace effective capacity by compensating for
the powder low density.
The heating process is controlled by optical pyrometry and inboard thermocouples. When a sufficient quantity of corium has
been molten, the arc power is reduced and the cathode is
withdrawn. The furnace is then tilted so that the melt pours out
in the test section. The plasma arc is maintained during the
pouring operation, in order to prevent a too large cooling.
Test sections
The VULCANO-E spreading test section consists of a spreading
plane. Refractory bricks (zirconia or magnesia) or a steel plate
have been used as substrate. Two geometries have yet been
used: spreading in an open square or in a 19° angular sector
materialized by refractory magnesia bricks.
Instrumentation
The test section is mounted on a weighing scale in order to
measure the pouring flow rate.
Temperature measurements are performed by thermocouples (K,
N, S and C – tungsten-rhenium – types), bichromatic pyrometers
and infrared thermography [13].
Spreading progression and front velocity is measured with
geometrically calibrated cameras [14] and compared with the
The infrared camera is in-situ cross-calibrated [13] with a
bichromatic pyrometer, the aiming point of which is positioned
on the thermographic images. The least square fitted correlation
is then used to convert the thermographic images in temperature
maps. This calibration takes into account simultaneously the
intrinsic calibration of the device and the corium surface
emissivity in the considered infrared bandwidth .
Post-mortem analyses
After the test, the cooled corium is examined. Its shape is
measured and samples are taken for chemical and material postmortem analyses (see e.g. [15]).
Chemical analyses are made on crushed samples using a Philips
1404 X-Ray Fluorescence (XRF) analyzer. X-Ray Diffraction
(XRD) analyses are performed with a Siemens D5000
diffractometer. Resin-wrapped samples are observed using
optical microscopes (Olympus PME3 and Polyvar MET) and a
Stereoscan Cambridge S360 Scanning Electron Microscope
(SEM). Local composition is estimated with an Oxford ISIS 300
Energy Dispersive Spectrometry (EDS) microanalysis. For the
spreading experiments, the element composition is relatively
constant over the spread volume, but there are important
variations against height in terms of compositions and
microstructures. The chemical analyses are compared with
thermodynamic computations using the GEMINI2 software and
the TDBCR databases[17]. The main findings of these analyses
will be reported in a later section.
THE SPREADING TESTS
Simulant tests on 2D sections
Two tests have been performed with zirconia-alumina melts that
were spread on a 2D square test section. During test VR-18,
which was the first spreading test at the VULCANO facility on
May 23, 1996, 6.5 kg of 55%wt zirconia, 45%wt alumina have
spread over a 450x525 mm spreading section. Figure 3 shows
the dissymmetric final shape of the spread.
Simulant tests on 19° angular sector
Five tests have been performed with simulant materials on a 19°
angular sector, in order to model, with limited masses
axisymmetric spreading. The melts simulated corium-concrete
mixtures by replacing mole-wise uranium by hafnium. The main
characteristics of these tests are reported in the following table.
Simulant test with metal/oxide melt
Figure 3: Postmortem view of the VR-18 spread
(unbounded 2D spreading)
During the next test, VR-19, 23 kg of a zirconia-31%wt alumina
mixture totally filled the 0.24 m² spreading area. This concluded
the technological tests of the VULCANO facility.
An experiment (test VE-06) has been performed in which
metallic iron has been added to the oxidic melt. The spread
composition was around 5% Fe, 52%w HfO2, 14%w ZrO2,
27%w Fe2SiO4, 2%w CaO, simulating the mixture of 71% of a
mixed metal-oxide corium with 29% of ferrosiliceous concrete.
34 kg of melt were spread over a final length of 45 cm. The
molten iron remained in droplets which formed an emulsion
with the oxidic liquid and did not sediment although the
temperature remained above the iron melting point for 3 minutes
and these oxides were less dense than iron.
Table 1: Physical and chemical characteristics of simulant spreading tests on θ=19° angular sector.
Uncertainties are around ± 70 K for the temperatures and around ± 20% for pouring rates.
Test
Composition (in wt %)
VE-01
50%HfO2, 10%ZrO2,
34%Al6Si2O13, 6% Al2CaSi2O8
58% HfO2, 10%ZrO2,
31%Al6Si2O13, 1%CaO
40% HfO2, 5%ZrO2,
39%Fe2SiO4, 16%Fe3O4
61% HfO2, 11%ZrO2,
21% Fe2SiO4, 4% Al2O3, 3% CaO
33% HfO2, 22%ZrO2, 22%SiO2,
22%FeO, 1% CaO
VE-02
VE-03
VE-04
VE-07
Mass
(kg)
12
21
15
12
17
Flow rate
(L/s)
0.1
discontinuous
0.1
discontinuous
0.1
continuous
0.7
continuous
0.5
continuous
Pouring
Temp. (K)
2070
Main results
Small spreading: 22 cm
Thickness 40 mm
Accumulation
Thickness 40-100 mm
Spreading length 30 cm
Thickness at front 20 mm
Accumulation
No spreading
Spreading length 55 cm
Compact homogeneous structure
2270
2070
2270
2420
VE-U1 Corium Spreading
VE-U1
160
140
Spread Length (cm)
The first spreading experiment with UO2, VE-U1, was
performed in the VULCANO facility on December 2, 1997. The
corium composition (in mass percentages: 44% UO2, 23% ZrO2,
21% SiO2, 12% FeOx) was prototypic of corium discharge from
the reactor pit after ablation of sacrificial ferro-siliceous
concrete. The solidification range of this mixture is of more than
900 K with a liquidus temperature of 2250 K. 38.8 kg of corium,
at an initial temperature of 2090±100 K, spread over 1.2 meters.
Figure 5 presents a view of the spread, taken when the front was
at 0.9 m from inlet. The flow length evolution (Figure 4) has
been obtained from the analysis of video images and is
consistent with the time of passage recorded on surface
thermocouples.
120
100
80
area-averaged length
Actual final length
THEMA (low flow rate)
THEMA (high flow rate)
60
40
20
0
0
5
10
15
20
25
30
35
40
45
Time (s)
Figure 4: Progression of corium length as measured from
the video images, compared to the values calculated by
THEMA spreading code (see below)
The measurement has been interrupted after 15 s when part
of the spread was out of the video image field.
VE-U5
Figure 5: Front view of the flowingVE-U1 spread
(taken with the camera calibrated in Figure 2).
VE-U3
The VE-U3 test used a mixture with a smaller concrete content,
having the following composition, in mass percentage: 63 %
UO2, 22% ZrO2, 8% SiO2, 7% FeO, i.e. being richer in UO2 than
VE-U1 corium and thus 150 K more refractory. 15.6 kg of
corium, poured at an initial temperature of 2400 ± 60 K with a
flow rate of 0.3 L/s, have spread over 33 cm with an average
height of 32 mm.
In this test, two successive spreading phases were realised with a
corium of the following initial composition: 46% UO2, 10 %w
ZrO2, 24%w Fe O, 20% SiO2. A first part of the corium flow was
poured on the spreading section while a second part was
maintained temporarily in a crucible, where an exothermic
reaction with metallic zirconium provided chemical heating. As
the crucible door melted, a second flow occurred which mixed
with the already stopped first spread (visible in bright on Figure
6). In total, 36 kg were spread at a mass flow rate of 2 kg/s with
an initial temperature around 2100 K.
This test showed the ability of corium to progress when a second
load is poured even after the initial progression had stopped.
Table 2: Physical and chemical characteristics of prototypic corium spreading tests.
Uncertainties are around ± 70 K for the temperatures and around ± 20% for pouring rates.
Test
VE-U1
VE-U3
VE-U5
Composition (in wt %)
Mass
(kg)
38.8
44% UO2, 23% ZrO2, 21% SiO2,
12% FeOx
63 % UO2, 22% ZrO2, 8% SiO2,
15.6
7% FeO
46% UO2, 10 %w ZrO2, 24%w FeO, 36
20% SiO2 + zirconium metal
Flow rate
(L/s)
0.6
Pouring
Temp. (K)
2090
0.3
2400
0.5
2100
Main results
Spreading over 1.2 m
Several tongues. Large voids
Spreading over 33 cm
Small porosity
Successive spreads
Analysis in progress…
1/ 8
 g ⋅G 3  1/ 2
U = 
 t
 3⋅ν 
The characteristic time for solidification is estimated as the time
necessary to remove sufficient heat as to make the corium
practically immoveable (the immovability enthalpy is
approximated as the average of the enthalpies at the liquidus and
solidus temperature) considering heat losses by radiation (at the
inlet temperature) and convection ( with Nu=0.0023 Pe).
The final average height order of magnitude δs is given, for 1D
spreading by the following expression:
Figure 6: VE-U5 corium spread over a steel plate
(The molten metallic gate of the crucible is visible as a bright
spot in the upstream of the flow).
MAJOR RESULTS
These tests have shown that spreading of corium-concrete
mixtures is quite efficient, even at low flow rates and with
composition containing up to 22%wt of silica (i.e. viscous
liquids). They also enabled us to validate and improve our
model of the rapid cooling processes taking place during the
spreading transient.
Validation of spreading models and codes
One of the major objectives of this series of tests was to validate
spreading models and codes for severe accident management.
Dinh et al. [6] have proposed a simplified model based on a
scaling approach. They assume that corium flows as an
isothermal viscid hydrodynamic fluid for the time needed to cool
its front down to an immobilization temperature.
A
characteristic spreading velocity can be estimated with
Huppert’s model [17] for gravity-viscous spreading :


1/ 2

V 3/ 4[qrad + qconv −qvol ] 

δ s =Maxδ cap,
1/16


1/ 2
7 /16 g 
G
ρ
m ⋅∆H sold




 3⋅ν 


This approximate method gives for the above-mentioned test,
the order of magnitude final corium height within a ratio of 2.
Eberlé [18] has developed the THEMA 1D software that
integrates the conservation of mass, energy and momentum over
verticals of the flow. This code has been satisfactorily validated
against the VULCANO spreading experiments. For example,
Figure 4 presents the spreading-length evolution computed for
VE-U1 considering two hypothesis for the evolution of inlet
flow (which is indirectly known since the mass in the inlet is
weighed with that on the spreading section). Currently, the main
obstacle to a precise validation is due to the large uncertainties
still present on the corium physical properties. These
uncertainties can lead to final spreading lengths between 0.6 and
2 m for VE-U1 – for an experimental average value of 1.13 m.
Therefore, a validation has been made with low temperature
simulants, the properties of which are well known [5].
More refined codes modelling the 2D behaviour of the flow
have also been developed. The 2-dimensional CROCO [19] and
CORFLOW [20] codes have been also satisfactorily validated
against the VE-U1 spreading experiment.
Sensitivity analysis have been performed and showed that the
spreading length, for VE-U1, was mainly controlled by the flow
rate, especially during the beginning of the spreading, the
viscosity law, the temperature difference between inlet and
liquidus temperatures and the corium thermal conductivity. The
substratum temperatures during the time of the flow are mostly
correlated to the thermal contact interface and the substrate
conductivity. It must be stressed that there are still large
uncertainties on material data for these mixtures and on
temperature measurements ( ± 70 K).
Microstructure and thermodynamic modeling
The chemical analyses of the spread melts indicate that there is
no sign of heterogeneity within the flow, which has a somehow
constant element composition. Furthermore, solidification does
not occur in the form of a dendritic mush growing from the cold
region, but rather as a suspension of solids in the remaining
liquid. This configuration is thus similar to that of semisolid
alloys [21].
Notwithstanding the chemical element uniform repartition, the
microstructure and the phases in presence vary greatly
depending on the vertical position. These effects [15] have been
explained by the variations in cooling rate, and for very thin
layers (< 100 µm), the oxidation by air. In the spread lowermost
layer (less than 1 mm thick in our cases), which, as it will be
developed after, was thermally insulated by a large thermal
contact resistance, the solidification was close to thermodynamic
equilibrium. A tracer of equilibrium below 1900 K is the
formation of silicates of hafnium, zirconium, and/or uranium,
including, at least in VE-U1, a compound close to chernobylite
[2], the zirconium-uranium silicate observed in Chernobyl
“lavas”. In the bulk of the spread, solidification is quicker (>10
K.s-1) and diffusion in the solid phase had not enough time to
occur. In some cases (e.g. VE-07), the upper interface is
characterized by dendritic particles.
The post-mortem analyses of these tests made with mixtures of a
large number of constituents ( at least 5 major chemical
elements) have also contributed to the validation of the
thermodynamic databases and have lead to the determination of
thresholds between the different types of solidification.
Physical properties
As shown by the spreading-code sensitivity-analysis, physical
properties can greatly affect the spreading of corium. But, there
are a lot of uncertainties in estimating the properties of a mixture
of a multicomponent melt and solid particles at high
temperatures.
For the estimation of specific mass, we assume, following
Nelson & Carmichael [23], that the partial molar volume of all
the liquid phase constituent have no compositional dependence.
The evolution of the corium rheology during solidification can
be estimated using the methodology of Ramacciotti et al. [24].
Since cooling is mainly due to radiation, there is a moving
cooling front inside the height of the corium. As solidification
and physical properties are linked to temperature, there is not
only a moving thermal boundary layer, but also a compositional
boundary layer and a rheological boundary layer [22]. Viscosity
varying very sharply with solid fraction, there is an exponential
increase in viscosity in the rheological boundary layer, with e.g.
a characteristic length of 0.3-0.5 mm for VE-U1 test.
Nevertheless, the surface remains above the solidus temperature
during the spreading phase and thus, there is no solid crust on
the surface, but rather a viscous skin. This rheological boundary
layer has the visible effect, in these flows, of folding the surface
(see e.g. Figure 7) as volcanic lavas of the ropy pahoehoe type
[25]. This effect has been observed on more than 10 tests having
a wide range of compositions.
On the opposite, during spreads of UO2-ZrO2 corium, with a
solidification range lower than 100 K, that were performed at
the FARO facility [10], solid crusts has been observed floating
on the melt. In both cases, there is a strong coupling between
heat transfer, thermodynamics and fluid dynamics which affects
the flow of corium flow.
Corium-Substrate Interface
During these tests, the substrate has only been locally attacked
by corium (mainly near the inlet of the spreading plane). Post
test analyses show mainly a penetration of corium in the brick
pores with little chemical reaction. The outcome would be
different in longer term experiments for which residual heat is
simulated. Indeed in this type of experiment, dissolution of the
crucible has been observed at VULCANO.
The heat flux at the corium-substrate interface can be estimated
using an inverse conduction technique [26] from temperature
measured at various depths inside the substrate. For instance,
during the VE-U1 flow, the downward heat flux entering the
zirconia bricks was estimated around 200 kW/m², whereas the
radiated flux at the free surface was around 900 kW/m². These
values must be compared to the heat flux that is expected in case
of perfect contact between two large conducting bodies:
 b ⋅ T + bsub ⋅ Tsub

b
− Tsub 
q = sub ⋅  cor cor
bcor + bsub
πt 

In the case of VE-U1, the flux would have been of 500 kW/m²
after 5 s of contact. This means that a large thermal contact
resistance must have been present. It is estimated around 36 mK.m²/W and is attributed to the shrinkage at solidification of
the lower crust [27].
Gases
One of the main differences observed in these tests between
simulant and prototypic material is the presence of large voids in
the VE-U1 and VE-U5 spreads. It must be noted that for
simulant test VE-07, very close in composition and operating
conditions to test VE-U1 (actually it was the rehearsal of VE-U1
with hafnia instead of zirconia), only a small porosity was
observed.
These voids cannot be only attributed to air flowing from the
substrate pores, since it also occurred when the substrate was
made of steel (VE-U5) so that gases cannot come from the
thermal expansion of the air trapped in the substrate pores.
Figure 7 shows a view of VE-U5 spreads. Part of the crust have
been destroyed by a gas pressure below it. Under the portion of
crust that remained intact, grains mainly made of U3O8 and SiO2
have been observed. The inner side of the crust had a structure
reminding convection cells.
qconv
qrad
qv
t
T
U
V
δs
δcap
∆H
ν
ρm
Figure 7: View of the VE-U5 corium spreads showing
holes at the surface.
Several evidences are consistent with an hypothesis of oxygen
exchange processes during the experiment leading to the local
formation of volatile UO3 and SiO. Further analyses are
underway to investigate this hypothesis and assess its
consequence on corium physical properties and behavior.
An associated difference between prototypic and simulant melts
is that many of the simulant melt displayed uncracked surfaces
whereas in our prototypic material tests, the corium surface was
always cracked. This features would be favorable for corium
cooling by top-flooding.
CONCLUSIONS
Eleven spreading tests have been performed at the VULCANO
facility, including three with prototypic corium containing up to
63%wt of UO2 which are representative of the products of a
corium concrete interaction in the reactor pit. They have shown
with real materials the promises of spreading concepts for
corium management in the hypothetical case of a nuclear reactor
severe accident. Spreading to thin corium thickness has been
obtained even with mixtures containing more than a fifth of
silica, which increases substantially the liquid viscosity (but
simultaneously decreases the liquidus temperature).
Spreading codes and models have been validated to the extent
permitted by the current large uncertainties on corium physical
properties in the solidification range. Improving our physical
properties database is thus one major R&D need.
Important differences have been observed between prototypic
and simulant materials especially for the physico-chemistry with
respect to oxygen exchange processes.
For the complete validation of the spreading concepts, further
experiments are needed to study the effects of spreading on a
concrete substrate – which liberates gases and can be
thermochemically ablated -, spreading under water and to
analyze the long terms effects, for which the slowly diminishing
internal heat generation must be simulated.
ACKNOWLEDGEMENTS
The work and efforts of the whole VULCANO team are greatly
acknowledged.
Some of the described experiments were part of the Corium
Spreading and Coolability project of the European
Commission’s 4th Work Program (Contract FI4S-CT96-0041).
NOMENCLATURE
b
effusivity
J m-2s-1/2 K-1/2
g
acceleration of gravity
m/s²
G
volume flow rate
m3/s
Nu
Nusselt number
dimensionless
Pe
Péclet number
dimensionless
convected heat flow
radiated heat flow
surfacic power (volumic power x height)
time (discharge time or contact time)
temperature
velocity
total spread volume
spread height
capillary limit
enthalpy to remove for immobilization
kinetic viscosity
melt specific mass
W/m²
W/m²
W/m²
s
K
m/s
m3
m
m
J/kg
m²/s
kg/m3
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