EXPERIMENTS WITH BORON CONTAINING CORIUM
L. Vasáros, Gy. Jákli, A. Pintér, Z. Hózer
KFKI Atomic Energy Research Institute, Hungary
Abstract
In order to assay the role of B2O3 in the core degradation processes, experimental information,
such as phase diagrams of B2O3 -metal oxide systems, are needed for model calculations. Because in
the open literature phase diagrams of interest are insufficient, the KFKI Atomic Energy Research
Institute had carried out investigations in order to provide experimental phase transition data on the
binary systems of Fe2O3, ZrO2 and UO2 with B2O3. The final results of the tests have been
implemented into a thermodynamic database. The conclusions of the performed work can support the
new series of MASCA project with boron containing corium. In the paper the experimental activities
related to these systems and the main results are summarized.
Introduction
Uranium dioxide, zirconium dioxide and iron oxides are considered as the main corium
components during the late phase of a severe reactor accident. Boron trioxide (produced during the
accident from boric acid or from boron carbide control rods) can be present in the reactor vessel and
due to the large mass of boric acid boron trioxide can create an important part of the corium. Contrary
to the metal oxides, boron trioxide has relatively low melting point (~450 oC), density and viscosity.
Fused B2O3 readily dissolves many metal oxides giving a number of stochiometric compounds and/or
producing different phase relations. Since the concentration of B2O3 in the corium is commensurable
with that of the main metal oxides, its presence may significantly influence the fuel degradation,
melting and relocation processes. In the framework of the ENTHALPY project (EU 5th Framework
Programme, FIKS-CT1999-00001) the KFKI Atomic Energy Research Institute (AEKI) had
carried out investigations in order to provide experimental phase transition data on the binary systems
of Fe2O3, ZrO2 and UO2 with B2O3 [1-3].
Experimental
Materials
High purity powdered Fe2O3, ZrO2, UO2 and amorphous granulated B2O3 were used as starting
materials. Due to the hygroscopic nature, boron trioxide was grinded to powder and dehydrated under
vacuum at 400 oC.
Preparation of test mixtures
The test samples were prepared by mixing the powdered components immediately before the
experiments. In order to eliminate the traces of water and H3BO3, about 10 g of the well-homogenised
mixture was fused in an open nickel crucible by slow heating from 450 oC up to 850 oC as long as
the bubble formation disappeared. The sample was then quenched by submerging the crucible into
cold water and the crashed very hard test material was stored under vacuum.
Temperature history measurements
Temperature history measurements, including linear heating and cooling cycles with the test
mixtures, were carried out in a vertical positioned resistant tube furnace. The temperature regulator
allowed a wide range time-pattern control of the temperature. The arrangement of the ceramic
pipeline, sample holder, together with the supporting ceramic tubes and the thermocouples are
illustrated in Figure 1.
The temperatures of the oven and test sample were measured by Pt/10%PtRh thermocouples,
whose average signals and their differences were recorded at every 30 s, using a measuring card
connected to a personal computer. In order to avoid the oxidation of the molybdenum sample holder
during the heating and cooling cycles, protecting gas, composed of 5 vol % of H2 and 95 vol % N2 was
continuously flowing trough the ceramic pipe-line.
Figure 1. Schematic diagram of the installation
for temperature history measurement
Hydrogen/Nitrogen
(5/95 vol%)
Thermocouple (1)
Silicon septum
Quartz cap
Ceramic pipe-line
Ceramic support tube
Resistant oven
Mo sample holder
Ceramic support tube
Quartz cap
Hydrogen/Nitrogen
(5/95 vol%)
Thermocouple (2)
The sample holder (Figure 2) was machined from molybdenum bar to produce a capsule, having
dimensions of =14 mm and l=80 mm and was positioned in the ceramic pipeline within the nearly
constant temperature zone by means of ceramic support tubes. The open end of the capsule is closed
with a cap having a thermo well to fix the location of the upper thermocouple, measuring the
temperature of the test mixture. The smaller thermo well in the bottom serves for fixing the position of
the lower thermocouple, detecting the oven temperature. The free internal volume of the sample holder
was be ~8 ml. The distance between the two thermocouples was chosen as ~8 mm. At this position
the upper thermocouple is located nearly in the middle of the melted test mixture, producing relatively
high detection sensitivity of the phase transition.
In each experiments, about 5 g of the test mixture was powdered in a closed stainless steel mortar
and loaded into the sample holder.
Figure 2. Diagram of the molybdenum sample holder
The temperature history measurements were carried out with test mixtures, containing different
concentration of the metal oxides. For each composition at least two independent experiments have
been made. In order to obtain reliable phase transition temperatures, the test mixtures are subjected to
at least two simultaneous heating and cooling cycles. Because the dissolution of the metal oxides in
liquid B2O3 is very slow, a rather low linear heating and cooling rate of 20 oC/h was adapted. In the
experiments the top heating temperatures were chosen at least 100 oC above the expected liquidus
temperature (TLiq).
The liquidus temperatures are signalled by a characteristic brake on the temperature difference
(T) curves of the cooling cycles. The values of TLiq are obtained from the sample temperature curves,
corresponding to the location of the brake on the T curves.
Results
Fe2O3-B2O3 system
In order to eliminate the possible effect of the quartz ampoules, used in the first series of the
temperature history measurements reported in [1], and to extend the Fe2O3 concentration range above
10 wt %, additional experiments were carried out using molybdenum sample holder. The mean values
of the liquidus temperatures (Tliq.), determined from the second cooling cycles of the temperature
history experiments are summarised in Table 1. Reproducibility of the data was estimated as ±10 oC.
The phase diagram for the Fe2O3-B2O3 system, shown in Figure 3, is constructed from the
temperature history measurement data. The phase relations bellow liquidus temperatures were taken
from the work Makram et al [4] and are well in line with our Mössbauer results [1]. In Figure 4 the
structure of the binary component mixture is illustrated with a scanning electron microscope (SEM)
picture.
Figure 3. Phase diagram for the F2O3-B2O3 system
1600
1800
1500
Liq. Fe2O3 + Liq. B2O3
1400
1700
1600
1300
1500
1200
1100
1400
Solid Fe2O3 + Liq. B2O3
o
T, C
1200
900
Fe2O3
+
Fe3BO6
700
Solid Fe3BO6
+
Solid FeBO3
Solid Fe2O3
+
Solid FeBO3
600
1100
1000
Solid FeBO3 + Liq. B2O3
FeBO3
Fe3BO6
800
Solid Fe3BO6 +Liq. B2O3
900
500
800
Sintered Fe2O3 + B2O3
700
400
Solid Fe2O3 +Solid B2O3
600
300
0
10
20
30
40
50
60
70
80
B2O3, wt %
Experimental,
Interpolated,
Lit. [4]
90
100
T, K
1300
1000
Table 1. Liquidus temperatures measured in
the binary systems of Fe2O3-B2O3
Fe2O3,
wt %
Tliq.,
o
C
5.0
885
7.5
1010
10.0
1065
15.0
1130
18.0
1165
25.0
1225
37.5
1320
Figure 4. SEM image of Fe2O3-B2O3 binary mixture
ZrO2-B2O3 system
The liquidus temperatures for the ZrO2-B2O3 system were determined by temperature history and
as well as from solubility measurements. In the former experiments test mixtures, containing 2.5 and
5.0 wt % ZrO2 were investigated. The results are given in Table 2.
Table 2. Liquidus temperatures from temperature history experiments
ZrO2,
wt %
Tliq.,
o
C
2.5
1130
5.0
1300
In the solubility experiments the test mixtures, containing ZrO2 in excess as compared with the
saturated solution at a given temperature, were fused in an inductive furnace at constant temperature in
closed tungsten sample holder. The original composition of the test mixture for both temperatures was
chosen as 20 wt% ZrO2. The liquidus temperature on the phase diagram was determined by measuring
the concentration of the components in the quenched solid solution at the heating temperatures of 1800
and 2000 oC.
Table 3. Liquidus temperatures from solubility measurements
Temperature,
o
C
ZrO2,
wt %
1800
6,50,5
2000
13,51,0
UO2-B2O3 system
The liquidus temperatures for the UO2-B2O3 system were determined by temperature history
measurements. As an example, results of the measurements with 5,0 wt% UO2 is shown in Figure 5. In
this experiment the top heating temperatures for the two cycles are chosen to be 1270 and 1295 oC,
respectively.
Figure 5. Heating and cooling cycles of the test mixture with 5,0 wt % of UO2
1400
100
o
Tmax = 1295 C
o
Tmax = 1270 C
1300
90
80
1200
70
1100
40
800
o
o
Ts, C
50
900
 T, C
60
1000
30
700
20
600
10
500
0
10
20
30
40
50
60
70
80
90
100
110
120
Time, h
Figure 6. Variation of temperatures during the
second cooling cycle for sample, containing 7.5 wt% UO2
7,5 wt % of UO2
1500
8
T
Ts
1400
7
o
TLiq=1330 C
1300
6
5
1000
4
900
3
800
700
2
73
74
75
76
77
78
79
Time, h
80
81
82
83
84
85
o
1100
 T, C
o
T, C
1200
The liquidus temperatures (TLiq) are determined from the temperature difference (T) curves of
the cooling cycles. The values of TLiq are obtained from the sample temperature curves, corresponding
to the location of the characteristic brake, recorded on the T curves. Figure 6 shows the variation of
Ts and T during the second cooling cycles in the vicinity of the liquidus temperature for the 7.5 wt %
test mixture. The size and form of the T signal in the vicinity of the liquidus temperature depends on
UO2 concentration and on the temperature gradient in the zone of the thermocouples.
The mean values of the liquidus temperatures, measured in the temperature history experiments
are summarised in Table 4. The TLiq data were determined from the second cooling cycles.
Table 4. Liquidus temperatures measured in the UO2-B2O3 system.
UO2,
wt %
Tliq.,
o
C
2,5
1130
5,0
1275
7,5
1330
10,0
1460
References
[1] L. Vasáros,Gy. Jákli, Anna Pintér, Z. Hózer, High temperature investigation of the ironoxidesB2O3 systems, Progress report SAM-ENTHALPY(01)-D 008, January 2001.
[2] L. Vasáros, Gy. Jákli, P. Windberg, L. Matus, I. Nagy, A. Pintér, Z. Hózer, Phase transitions in
the ZrO2-B2O3 system, Progress report SAM-ENTHALPY(01)-D 009, January 2003.
[3] L. Vasáros, Gy. Jákli, Z. Hózer, Phase transitions in the UO2-B2O3 system, Progress report SAMENTHALPY(01)-D 010, January 2003.
[4] H. Makram, L. Touron, J. Loriers, Phase relations in the system Fe2O3-B2O3, and its
application single crystal growth of Fe2O3, J. Cryst. Growth 13/14 (1972) 585-87