MCCI Project Seminar 2007, Cadarache October 10-11 2007
Simulation of CCI-2 test with TOLBIAC-ICB: modelling and sensitivity studies
B. SPINDLER, E. DUFOUR
CEA, Grenoble, France
Abstract
The first simulations of CCI-2 performed with TOLBIAC-ICB were presented in the frame of the
benchmark organized in the frame of the MCCI Project. After the benchmark was closed, new
data concerning the concrete composition were given.
The results of the simulation of CCI-2 test with TOLBIAC-ICB are presented here using the final
experimental data, and the up-to-date version of TOLBIAC-ICB with the NUCLEA07 data base
for GEMINI. Sensitivity studies are also presented, concerning mainly the initial conditions: initial
temperature, initial composition, effects of melt composition and initial transient. These sensitivity
studies are related to the model used in TOLBIAC-ICB: phase segregation and liquidus
temperature of the melt used at the interface between the melt and the crusts. The specific point of
the ablation observed in CCI-2 test above the expected melt level is also discussed in relation with
the simulation results of TOLBIAC-ICB.
1. Introduction
The CCI-1, CCI-2 and CCI-3 tests performed in the frame of the MCCI project give useful data
concerning the corium concrete interaction, for various concrete compositions. The large lateral
ablations observed in CCI-1 and CCI-3 are still not well understood, and the simulation of these
tests is therefore of low interest. On the other hand, the CCI-2 test gives more comprehensive
results, with a long test duration. It is therefore very useful for the code validation.
A short presentation of the model used in TOLBIAC-ICB is first given, then the successive
simulations of CCI-2 test with TOLBIAC-ICB within the MCCI project. Then the results obtained
with the final data proposed for CCI-2 and the up-to-date code version are given. Finally
sensitivity studies are presented, which illustrate the characteristics of the model.
2. MCCI modelling in TOLBIAC-ICB code
The TOLBIAC-ICB code is developed by CEA in the frame of an agreement with EDF. It is based
on the phase segregation model. Due to the high liquidus temperature of oxide melts and despite
the melting of concrete and the presence of gas issued from concrete decomposition, a solid crust
is assumed to form at the concrete wall. The species that encrust are the most refractory species. A
low crust growth, a high liquid diffusivity and a small diffusion boundary layer thickness are also
assumed. With this view, the pool is only composed of liquid and consequently has a low
viscosity. The interface temperature between the liquid pool and the solid crust is equal to the
liquidus temperature corresponding to the current composition of the remaining liquid phase. The
crust thickness is calculated using a steady state assumption. Physico-chemistry (liquidus
temperature, crust composition, chemical reactions) in TOLBIAC-ICB code is calculated with an
intrinsic coupling to the GEMINI2 code, developed by THERMODATA. The code user may
select the heat transfer correlations that are used. In the calculations presented here, the same heat
transfer correlation (BALI correlation) for the different interfaces is used.
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MCCI Project Seminar 2007, Cadarache October 10-11 2007
3. Successive results
3.1 Presentation
Successive simulations of CCI-2 test were performed in the frame of the MCCI project. Table 1
gives the code version, thermodynamical data base and input conditions that were used.
Date
Code version
geometry
and data base
June 2004
TOLBIAC-ICB V2.0
Pre-calculation
NUCLEA-03
November 2004
TOLBIAC-ICB V2.1
Blind calculation
NUCLEA-03
April 2005
TOLBIAC-ICB V2.2
Post-calculation
NUCLEA-03
September 2007
TOLBIAC-ICB V3.0
rectangular
Input
power
Initial
mass
kW
kg
150
400
Concrete
decomposition
9.8 % MgO
28 % gas
cylindrical
120
400
9.8 % MgO
28 % gas
rectangular
120
311
9.8 % MgO
28 % gas
rectangular
120
311
12.2 % MgO
NUCLEA-07
34.6 % gas
Table 1: conditions of the successive simulations of CCI-2 test with TOLBIAC-ICB.
The results are presented on figure 1 (melt temperature versus time), figure 2 (ablation depths
versus time) and figure 3 (final shape of the cavity). The main changes in the results are due to the
modifications of the initial conditions or to the modifications of the model used, but some also are
due to the improvements, corrections or modifications made in the successive versions of the code
and the data base.
2600
temperature, K
june 04
november 04
2400
april 05
september 07
2200
2000
1800
0
60
120
180
240
300
time, min
Figure 1: Melt temperature versus time, successive results with TOLBIAC-ICB.
3.2 Pre-calculation
The pre-calculation was performed using the rectangular geometry corresponding to the test
section, and with the test conditions that were defined before the test. The initial temperature is
lower than the liquidus temperature, and therefore there is a first period during which the ablation
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MCCI Project Seminar 2007, Cadarache October 10-11 2007
does not occur, but the input power is used to increase the melt temperature. After about 10
minutes, the liquidus temperature is reached and the ablation begins.
3.3 Blind calculation
Blind calculation was performed using a cylindrical geometry that was imposed for the
comparison with the other codes results, and the reduced input power which was effectively used
in the test. Compared to the pre-calculation, the difference is low concerning the temperature. It is
large for the ablation and shape of the cavity, due to the power input change on one hand, and on
the scaling factor used for the cylindrical geometry.
0.30
june 04
0.25
november 04
april 05
ablation, m
0.20
september 07
0.15
0.10
0.05
0.00
0
60
120
180
240
300
time, min
Figure 2: Ablation depth versus time, successive results with TOLBIAC-ICB. The same axial and
radial ablation depths are obtained.
3.4 Post-test calculation
The post test calculation is performed with a rectangular geometry. The main change concerns the
initial melt mass, which is reduced because the initial splattering is taken into account. Therefore
the light oxide proportion becomes larger than in the previous calculations, and consequently the
liquidus temperature and melt temperature are significantly reduced. An other modification is that
the ablation by radiation above the melt level is taken into account. The final shape of the cavity is
consequently largely modified.
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MCCI Project Seminar 2007, Cadarache October 10-11 2007
1.0
height, m
0.8
june 04
0.6
november 04
april 05
september 07
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
width, m
Figure 3: Final shape of the cavity, successive results with TOLBIAC-ICB.
3.5 Actual reference case
The main modification in this last calculation is the use of a modified composition of the CCI-2
concrete, with more gas and more magnesia. The consequence is a reduced final temperature.
4. Reference case compared to the experiment
The actual reference case is now compared with the experimental results.
4.1 Melt temperature
Figure 4 shows the comparison between the reference case calculated by TOLBIAC-ICB and the
measurements. The initial temperature is lower than the liquidus temperature, and therefore there
is a first period during which the ablation does not occur, but the input power is used to increase
the melt temperature. After about 10 minutes, the liquidus temperature is reached and the ablation
begins. At about 90 minutes, there is a large temperature decrease, which corresponds to the end
of the oxidation of chromium. Before that, GEMINI found two liquid phases, one metallic and one
oxidic, and the liquidus temperature corresponded to the temperature corresponding to the
miscibility gap. After 90 minutes, the calculated temperature is close to the measured temperature.
At 300 minutes water aspersion occurs and the melt temperature reduces.
It can be seen also that the liquidus temperatures corresponding to the samples compositions that
were analysed after the test (red points on figures 4), also correspond to the measured melt
temperature and the calculated temperature. This phenomenon is in favour of the phase
segregation model used in TOLBIAC-ICB.
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MCCI Project Seminar 2007, Cadarache October 10-11 2007
2600
temperature, K
2400
2200
2000
1800
1600
0
60
120
180
240
300
360
time, min
Figure 4: Melt temperature versus time, reference case with TOLBIAC-ICB, and liquidus
temperatures corresponding to the sampling compositions (red points).
4.2 Ablation depth and shape of the cavity
Figure 5 shows the comparison between the reference case calculated by TOLBIAC-ICB
and the measurements. The same axial and radial heat transfer coefficients are used, and
consequently the radial and axial ablation depths are the same. The ablation rate is rather
good. However it can be seen that the ablation continues after water pouring at 300
minutes, which is not observed in the experiment. The final ablation depths are therefore
overestimated, as can be seen on figure 6. The use of the radiation model which includes
ablation by radiation above the melt level gives an ablation at the top of the cavity that is
still underestimated, even if the results are better (see also the sensitivity studies
hereunder).
0.35
0.30
ablation, m
0.25
0.20
0.15
CCI2 axial
CCI2 radial S
0.10
CCI2 radial N
0.05
TOLBIAC axial and radial
0.00
0
60
120
180
240
300
360
time, min
Figure 5: Ablation depths versus time, reference case with TOLBIAC-ICB.
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MCCI Project Seminar 2007, Cadarache October 10-11 2007
CCI2
north and south
1.0
initial
shape
height, m
0.8
0.6
0.4
TOLBIAC
0.2
0.0
0.0
0.2
0.4
0.6
0.8
width, m
Figure 6: Final shape of the cavity, reference case with TOLBIAC-ICB.
5. Sensitivity studies
4.1 Initial temperature
2600
temperature, K
CCI2
2400
TOLBIAC reference
2200
TOLBIAC mod initial
temperature
2000
1800
1600
0
60
120
180
240
300
360
time, min
Figure 7: Melt temperature versus time, reference case and modified initial temperature.
A calculation is performed changing only the initial temperature. A value of 2600 K is used,
which corresponds to the liquidus temperature of the initial melt composition. The result is
presented on figure 7. In this case, the ablation begins immediately, the end of oxidation occurs
previously, but the melt temperature during the last period is the same as the temperature
calculated for the reference case. Concerning the ablation rate, it is the same, the only difference
consists in the start of ablation: time zero here, and deleted in the reference case.
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MCCI Project Seminar 2007, Cadarache October 10-11 2007
4.2 Initial crust
2600
CCI2
temperature, K
2400
TOLBIAC reference
TOLBIAC initial crust
2200
2000
1800
1600
0
60
120
180
240
300
360
time, min
Figure 8: Melt temperature versus time, reference case and initial crust.
An other calculation is performed in order to get an initial temperature equal to the liquidus
temperature (fig. 8): part of the refractory material is supposed to be encrusted (5 cm crust) at the
initial time, so that the liquidus temperature of the remaining melt corresponds to the initial
measured temperature. The ablation then starts at time zero. After that, the initially encrusted
material progressively melts, until the steady state crust thickness is reached. The consequence is
that the melt temperature remains low, because the remelting of refractory material is
compensated by the concrete ablation, and the liquidus temperature remains low. After about 90
minutes, the melt temperature is close to the reference temperature. The ablation rate is not
modified; the only difference concerning the ablation depth is that the ablation begins at time zero.
4.3 Initial melt without chromium
2600
CCI2
temperature, K
2400
TOLBIAC reference
TOLBIAC all Cr oxidized
2200
2000
1800
1600
0
60
120
180
240
300
360
time, min
Figure 9: Melt temperature versus time, reference case and start with oxidized chromium.
The run time in TOLBIAC-ICB depends on the run time in GEMINI2. This run time in GEMINI2
is high in case of a melt with both metallic and oxidic species. In order to reduce the run time, and
to see the influence of the metallic phase of chromium, a calculation is performed supposing that
all the chromium is oxidized at the initial time. Moreover the initial temperature is taken as the
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MCCI Project Seminar 2007, Cadarache October 10-11 2007
liquidus temperature corresponding to the new initial composition. The result shows (fig. 9) that
the melt temperature decrease is now regular, and the temperature corresponds to the reference
case from time 90 minutes to the end. The ablation rate and shape of the cavity are the same. The
conclusion is that this option (all chromium oxidized) is very useful for sensitivity studies.
4.4 Alternative scenario beginning at time 60 min
An alternative scenario was proposed in the post-test calculations performed in the frame of the
MCCI-OECD benchmarking work. In this scenario, the simulation starts at 60 minutes, a time at
which a steady state ablation regime seams to be established. An estimation of the melt
composition at 60 minutes including addition of some concrete (ablation during the first 60
minutes given by the experimental curves) is used. The ablation depth at 60 minutes is also used
for the ablation depth evolution. The results show that the temperature evolution is close to the
reference temperature after 90 minutes (fig. 10). Concerning the ablation depth, the ablation rate is
the same (slopes of the curves of fig. 11), but the estimated ablation depth used in the new
calculation leads to final ablation depth larger than the reference case.
2600
CCI2
temperature, K
2400
TOLBIAC reference
TOLBIAC modified scenario
2200
2000
1800
1600
0
60
120
180
240
300
360
time, min
Figure 10: Melt temperature versus time, reference case and modified scenario.
CCI2 axial
0.40
CCI2 radial S
0.35
CCI2 radial N
ablation, m
0.30
TOLBIAC reference
0.25
TOLBIAC mod
scenario
0.20
0.15
0.10
0.05
0.00
0
60
120
180
240
300
360
time, min
Figure 11: Ablation depths versus time, reference case and modified scenario.
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MCCI Project Seminar 2007, Cadarache October 10-11 2007
4.5 No ablation above the melt level
The experimental shape of the cavity shows that ablation occurs at a high level of the cavity. In
the blind calculation of the benchmark, a shape of the cavity with such an ablation at a high level
was obtained in the results of TOLBIAC-ICB by EDF, using the option “ablation by radiation
above the melt level”. This option is used in the reference case presented here. A calculation was
performed without this option: radiation but no ablation above the melt level. The result (fig. 12)
shows that the shape of the cavity is very different from what is observed.
CCI2
north and south
1.0
reference
height, m
0.8
0.6
0.4
no ablation by radiation
0.2
0.0
0.0
0.2
0.4
0.6
0.8
width, m
Figure 12: Final shape of the cavity, reference case and no ablation by radiation.
4.6 Void fraction model
An other way to get ablation at a high level of the cavity, without ablation by radiation, is to
modify the void fraction model. The classical Zuber and Findlay model is used in TOLBIAC-ICB,
which gives a void fraction of about 0.3 at the beginning of the simulation and 0.2 at the end. An
other calculation is performed multiplying the void fraction by 3. The result (fig. 13) shows that
the shape of the cavity obtained is equivalent to that corresponding to the calculation supposing
ablation by radiation. It cannot be concluded from these calculations which model is the good one.
However it is doubtful that ablation by radiation is obtained in some experiments (CCI-2) and not
in others (CCI-1, CCI-3). On the other hand, a factor 3 in the void fraction model is very large.
The void fraction model seams to be revised, and a comprehensive study of the different MCCI
tests available is necessary.
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MCCI Project Seminar 2007, Cadarache October 10-11 2007
CCI2
north and south
1.0
reference
height, m
0.8
0.6
mod void fraction
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
width, m
Figure 13: Final shape of the cavity, reference case and modified void fraction.
6. Conclusion
The successive results of the simulation of CCI-2 test with TOLBIAC-ICB, show a decrease of the
melt temperature. This decrease is due to the modification of the test conditions used in the code
data set: first decrease of the initial mass, second modification of the concrete composition. It is an
illustration of the sensitivity of TOLBIAC-ICB code to the melt composition, because the melt
temperature in TOLBIAC-ICB is directly related to the liquidus temperature.
The reference case shows that the melt temperature after 90 minutes (end of chromium oxidation)
is correctly evaluated by TOLBIAC-ICB. By the way the final temperature level corresponds to
the liquidus temperature calculated with the final samples composition, which is consistent with
the phase segregation model of TOLBIAC-ICB.
The sensitivity studies that are presented show a low dependency of TOLBIAC-ICB concerning
the initial temperature level. An other important point concerns the high experimental level of the
ablation in the cavity. It is simulated in TOLBIAC-ICB either using ablation by radiation above
the melt level, or with a multiplying factor of 3 of the void fraction.
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