09

FR0200361 ^r.^r IQ 34.

IMPORTANCE OF PROTOTYPIC-CORIUM EXPERIMENTS FOR SEVERE ACCIDENT RESEARCH

P. PILUSO, C. JOURNEAU, G. COGNET, D. MAGALLON*, J.M. SEILER**

CEA Cadarache 13108 St Paul les Durance cedex, France *: detached from the Joint Research Centre of the European Commission ** CEA Grenoble

KEY WORDS: Severe accidents, Nuclear materials, Experiments

Introduction:

In case of a severe accident in a nuclear reactor, very complex physical and chemical phenomena would occur. Parallel to the development of mechanistic and scenario codes, experiments are needed to determine key phenomena and coupling, develop and qualify specific models, validate codes.

Due to lower costs and constraints, experiments using low temperature simulant materials (such as CORINE [1] or Scaled Simulant Spreading Experiments (S3C) [2] for spreading or BALI [3,4] for in-vessel pools) allow the testing of a large number of configurations and the determination of correlations. But some crucial corium phenomena and behaviours such as the importance of radiation heat transfer or the presence of a large liquidus-solidus interval (up to 1000 K) are not reproduced in low temperature experiments. Consequently, it is attempted to simulate real corium with high temperature simulant materials: alumina, thermite, widely used (e.g. KATS [5] or COMET [6] facilities); high temperature salts or mixtures with zirconia and/or hafnia.

However, it is not feasible to simulate all the aspects of corium phenomenology, especially its high temperature behaviour. Therefore, experiments with prototypic material are performed to check the results obtained with simulants and identify possible differences. In this context, CEA has undertaken a large program on severe accidents with prototypic corium [7]. Other programs using prototypic corium have also been engaged in the past; recent experiments are COMAS [8] for corium spreading, FARO-KROTOS [9] for Fuel-Coolant Interaction (FCI), MACE[10] for Molten Corium Concrete Interaction (MCCI) or COTELS for Ex-Vessel Corium behaviour in BWRs [38]. oooe

In this paper, we discuss some specificities of the prototypic corium:: • Spreading: experiments with simulant mixtures and prototypic corium performed in the VULCANO facility showed a behaviour involving gas formation during melt spreading . • Corium pool: the presence of miscibiiity gap in the U-Zr-0 ternary system for liquid phases and the high density of uranium oxides affect solidification paths, stratification and/or macro-segregation. • Corium concrete interaction: the possible reactions between urania and concrete oxides are specific in terms of thermodynamics and kinetics. For instance, the limited solubility of uranium in zircon can lead to the formation of the solid solution called "chernobylite" (Ux,Zri-x)SiO4 which is important for the long term behaviour (fission product release, handling,..) of solidified corium. • Fuel Coolant Interaction: experiments in the KROTOS facility have shown important differences of behaviour between molten alumina and molten 80%wtUO2-20%wt ZrO2, the latter inducing less violent explosions than the former.

Uranium oxide: specificity and properties:

The major component for the molten fuel is uranium oxide. The use of depleted uranium oxide for severe accidents research allows to be as close as possible to the real corium characteristics. In this paragraph, we are going to underline some specific properties of uranium oxide.

Electronic configuration and oxidation states of uranium

Uranium, as chemical element of the periodic table, belongs to the actinide series. It implies that uranium has a specific electronic configuration: 5? 6d 7s2 (Table 1). The existence of electrons in the 5f electronic band makes it possible to have located electrons. In the solid state, these electrons will determine the possible chemical bonds, particularly with oxygen. These chemical bonds control the chemical and physical properties of the uranium oxide. The oxidation states of the uranium are numerous: +3,+4,+5,+6 (Tablei).

Element U Ac Zr Hf Ce Electronic 5f-6d1-7s2 7s2-6d1 4d2-5s2 4f14-5d2-6s2 4f1-5d1-6s2 structure Oxidation 0,+3,+4, +5, 0,+3 0,+1,+4 0,+4 0,+1,+3,+4 state + 6 Ionic radius +3, r= 10.3 + 1,r=10.9 +4, r=7.8 +1,r=12.7 r +4, r = 8.9 +3, r=11.8 +4, r=7.9 +3, r=10.3 (nm) +5, r= 7.6 +4, r= 9.2 Table 1: Electronic structure, oxidation states and ionic radii of uranium and some other elements [11,12]. oooe

The diversity of the uranium electronic structure, oxidation states and ionic radii shows that it is one of the more complex elements of the periodic table, with no equivalent, except for some actinides like for instance plutonium. These specificities imply that uranium can form a great variety of oxides with different physical properties which strongly influence all possible states (e.g., gaseous species observed for example in VULCANO experiment reported in the sub-sequent section).

Uranium-oxygen system

The phase diagram uranium-oxygen is very complex [13-16]. In this paper, we are not going to describe all the phase diagram, but we want only to underline some specific points of this system.

Because of the numerous oxidation states, there are many possibilities for the formation of compounds. - For the solid state, many compounds can exist in the domain of O/U>2,00. The main compounds are [39]: • U02,oo -stoichiometric oxide -crystallographic structure: fluorine (CaF2) -cell parameter: a = 547,14 pm [17]

• U02,oo+x -non-stoechiometric oxide -crystallographic structure: fluorine (CaF2) -insertion of oxygen atom

• U4O9 -crystallographic structure: fluorine (CaF2)

• U3O7 - crystallographic structure: quadratic

• U3O8 -crystallographic structure: orthorhombic

Studies of this system are currently underway for instance at CEA [13,19,20]. All these compounds can be obtained during the course of a severe accident. The existence of all these phases means differences for the properties of the compounds, particularly for the thermo-mechahical properties. This diversity is really specific to the U-0 system.

- For the liquid state, the experimental data are well-known in the UO2-rich part. But for the oxygen solubility in liquid uranium , there are not many data [13,20].

- For the gaseous state, the situation is also very complex. The species which can exist are : UO (g), UO2 (g),UO3 (g) and U (g) [18]. According to the conditions of pressure and temperature, and gas phase composition in a severe accident, it is possible to vaporise a significant amount of one or more of these species. This ability to be volatised is also specific to the uranium oxides. Usually, the high melting point oxides, like for instance ZrO2, are difficult to volatise. This difference is of lesser importance at low temperatures (below 2000°C), where the volatility of all species is lower. OQOO

These examples show the complexity and the specificity of the U-0 system. The consequences on the studies of severe accidents phenomena must be properly assessed through specific experiments (either analytical or integral) as it is not possible to reproduce this complexity by using simulants Simulation of some important properties of uranium oxide

To simulate the uranium oxide, some properties are fundamental for studies of spreading or stratification of corium. We can quote melting temperature, thermal conductivity, heat capacity and density. The best known non-radioactive oxides as potential simulants of uranium oxide are:

• HfO2 • CeO2 • ZrO2 As an example, table 2 presents the density and the melting temperature of these oxides compared to UO2. As shown, the melting temperature and the density of HfO2 are very close to UO2. In general, the thermodynamic properties of these oxides, except ZrO2, are not part of the nuclear materials databases and thermodynamic code such as TDBCR and GEMINI2 [24,25]. The density of the other oxides is much more different.

Oxides Melting point Density (293K) (K) (gem-3) UO2 Tm = 3120+/-30 10.956 [181 M8i CeO2 3073 7.10 T211 [21] HfO2 3031+/-25 9.68 (mono) T231 F231 ZrO2 2973 5.68 (mono) [23] nn

Table 2: Physical properties of oxides in comparison with UO2.

It is possible to simulate one or the other of the properties of UO2, but not all properties together. For instance, HfO2 will be a good simulant of UO2 for severe accident studies in mixtures with metals from the hydrodynamic point of view because of its high density. ZrO2 could be a simulant in oxidic mixtures for some studies of spreading because it simulates the refractory behaviour of UO2 in the freezing process, from the thermodynamic point of view. However, both HfO2 and ZrO2 are very stable oxides and react differently with other materials and with the gas phase.

Reaction between UO2 and other materials

Not only the system U-0 is complex, but also are the systems between U-0 and the main elements existing in a nuclear reactor: Zr (zircaloy cladding of the nuclear fuel); Fe (vessel and elements of structure); Si, Ca and Al (concrete). We are not going to describe all the existing phase diagrams but only discuss two examples. oooe

In case of severe accident, the nature and the fraction of solid and liquid phases must be known as a function of its composition and temperature. A miscibility gap exists in this system [13], but today, it is not yet well known. This miscibility behaviour will also control the possibility of stratification of the U-O-Zr mixture in a corium pool. This stratification will control the heat flux distribution to vessel and thus have an impact on the coolability on the external surface. Another effect of UO2 is the occurrence of a solid solution called "chemobylite" (Zr-i- xllxSiCU) which has been largely observed in the samples from the Chernobyl reactor and has also been observed in some VULCANO corium spreading experiments. Currently, the solubility limit of uranium in zircon (ZrSiQ^) is not well known. Some studies [28,29] including that of "Chernobyl lavas" [30] indicate a solubility limit of 2 to 6% mol. of uranium in zircon. The existence of a solubility limit is specific b the system U-O-Zr-Si; this solubility limit doesn't exist, for example, for hafnium (complete solubility in zircon) [28]. These limits are of some importance because it controls the amount of liquid silica at a given temperature which, in turn, governs Die viscosity [26,37]and the chemical diffusion of the remaining liquid, two major parameters for the convection heat transfer in a solidifying ex-vessel corium pool.

Experiments with prototypic corium

Spreading experiments using UO2 and HfO2

At the first order, corium spreading is controlled by the competition between solidification and flow rates [2].

The volumetric flow rate and the evolution of kinematic viscosity during solidification are the main parameter influencing the spreading hydrodynamics. For the kinematic viscosity (of the solid-liquid mixture) to be equivalent between reactor case and simulant, both solid volume fraction evolution versus temperature and mixture density must be conserved [26,37]. Verifying simultaneously both criteria will only seldom be achievable. Hafnia provides a good simulation of density, but hafnia-zirconia pseudo- binary is quite different from urania-zirconia and thus solid fraction at a given temperature. '

For flows over ceramics, heat transfer in the corium can be greatly affected by the occurrence of large voids. Such a morphology has been observed in VULCANO during two spreading tests with prototypic corium (see e.g. figure 1, VE-U1 test). Such voids have never been observed in any of the VULCANO tests using hafnia even in test VE-07 which had a composition equivalent to that of VE-U1 corium, except that UO2 was replaced by the same molar fraction of HfO2 [27]. oooe

In another VULCANO test (VE-U5), a powder has been found just below the surface of the spread corium. This powder has been analysed by X-ray-diffraction; two main phases were present: LbOs and SiO2. In the bulk of the corium, macro-porosity and bubbles have been observed. It is currently assumed that a volatile species (UO3 ?) with uranium was produced during the test, trapped under the corium skin and condensed during cooling.

Due to the wide range of possible conditions in a severe accident, such phenomena could occur in the reactor case. This porosity, might favour water ingression phenomena into the solidifying melt and also contribute to coolability.

Figure 1: Cross section of the VE-U1 spread showing the great macroporosity. (Screw head diameter: 14 mm)

Corium Pool, Corium-Concrete or Corium-Ceramic interaction experiments

For these experiments, with decreasing sustained heating, the following scales must be conserved between corium and a simulant [33]: • the partition coefficient between liquidus and solidus curves; • the solidification rate (velocity of the displacement of (refractory materials) solid front); • the density ratio between the refractory solids and the remaining liquid ; • the ratio of the solidification range by the liquidus temperature; • the freezing number A=8 V/D (ratio of the product of the solidification rate by the diffusion boundary layer thickness to the diffusion); • the ratio of chemical diffusion and thermal diffusivity; • the ratio of heat transfer characteristic time (crust growth) to the sustained heating decrease characteristic time.

It appears very doubtful to have a simulant material that will satisfy all the scaling relationships. Therefore, it is recommended to study separate effects with simulants and then to conduct a limited number of experiments with prototypic compositions. oooe

Fuel Coolant Interactions

The KROTOS FCI experiments [31] aim at investigating the effect of fuel/coolant initial conditions and mixing on steam explosion energetics. Molten materials (both simulant and prototypic) up to 5 kg were used in a large number of KROTOS tests simulating various initial and boundary conditions.

A number of alumina tests were performed with both sub-cooled and saturated water conditions in a stainless steel test section of diameter 200 mm and for water depth -1 m [31,32]. The sub-cooled tests normally resulted in spontaneous steam explosions. The tests at low sub-cooling were performed to confirm, on one hand, the suppression of spontaneous steam explosions under such conditions and, on the other hand, that such a system can still be triggered by using an external initiator. The other test parameters in these alumina tests included the melt superheat and the initial pressure. All the tests in the investigated superheat range (150 K - 750 K) produced a steam explosion. No evidence of the explosion suppression by the elevated initial pressure (in the limited range of 0.1 - 0.375 MPa) was observed in the alumina tests. The maximum conversion ratio (energy released/initial thermal energy) recorded in these tests was about 2.5 percent.

The corium test series included tests both under sub-cooled and near saturated conditions at ambient pressure and sub-cooled tests at an elevated pressure from 0.2 to 0.37 MPa [9]. Corium melt masses in these tests varied from 2.4 kg to 5.1 kg (~3073 K). None of these tests with corium produced an energetic steam explosion. However, propagating low energy events with a maximum energy conversion ratio of about 0.15 % were observed when an external trigger was applied. Present experimental evidence suggests that the water depletion in the mixing zone suppresses energetic steam explosions with corium melts at ambient pressure and in the present pour geometry. On the other hand, elevated pressure reduces significantly the integral void fraction also with corium allowing better melt/water contact at triggering and generation of mild interactions. However, the coarse mixture so far has not been suitable for the mild interactions to escalate into an energetic steam explosion.

Summarising, the experiments performed at a system pressure of 0.1 MPa and sub- cooled water with small masses of prototypic core melt in the KROTOS facility did not produce an explosion even when an artificial trigger was applied. The experiments performed at higher system pressures in the range of 0.2 to 0.4 MPa showed very weak explosive events when artificially triggered. Conversely, very energetic steam explosions easily occurred when pure alumina melt was used in similar geometrical conditions.

Melt penetration data and preliminary results of visualisation of melt pre-mixing in recent alumina and corium tests suggest differences in melt break-up behaviour between alumina and corium melts (see figure 2). The break-up behaviour determines the premixing configuration and can therefore be an important factor in determining the yield of the interaction. Whether it is possible to produce the same pre-mixing configurations between corium and alumina is not yet known. oooe

A second difference between the two types of experiments can be linked to the production of incondensable gases that make the explosion more difficult to initiate. The two main sources of incondensable gases in FCI experiments are the dissociation of water at high temperatures (which exists above 2500 K and is negligible around the alumina melting point) and the oxidation of UO2 .(which does not exist for alumina or zirconia). For instance, 1 mole UO2 in presence of 1 mole H2O at 1 atmosphere can produce 0,5 mole of gas (mainly hydrogen and UO3) [34]. It must nevertheless be stressed that, if an explosion eventually occurs, these oxidation reactions may increase the explosion yield [35].

Recent analyses based on a simple heat transfer model of a droplet [34] indicate that crust growth is about five times quicker for a drop of uranium dioxide than for alumina or iron oxides. This effect is mainly due to the difference in melting temperature on heat transfer by radiation. A quantitative scale analysis [34,36] of the energy needed to fragment the droplet indicates that pressure waves must thus be one to two orders of magnitude higher to start a steam explosion for urania than for alumina I simulant.

Figure 2: Melt penetration in water in KROTOS tests (left corium, right alumina)

It may be concluded that the material characteristics play an important role in steam explosion triggerability and explosivity, possibly explaining the low yield observed in corium tests. Leaving aside the possible role of chemical reactions during the explosion propagation, hydrogen generation during the premixing phase is believed to make the triggering of an explosion difficult (increase of void fraction and of vapour film stability) and to limit the energetics. The KROTOS results indicate that reliable material properties of high temperature core melt mixtures and experiments involving prototypic core melt in reactor-like configurations are crucial in extrapolating experimental findings on FCI to a real reactor situation. oooo

Conclusion:

Uranium is one of the more complex element of the periodic table. This complexity concerns several aspects of corium behaviour in severe accident research: chemical, physical and thermodynamic properties, different mechanisms (spreading, solidification, interaction, steam explosion, stratification...).There doesn't exist any non-radioactive element sufficiently similar to uranium, even if it is possible to use ZrC>2 or HfC>2 for specific studies. Furthermore, some peculiar behaviour of UO2 is still unknown (miscibility gap, vaporisation...). It's difficult to choose a simulant to reproduce properties which are not well known.

Some experiments on VULCANO or KROTOS programs have already proven the specificity of uranium oxide in severe accident research.

On the other hand, potential simulants, like HfO2 and CeO2, are not incorporated in the thermodynamic databases, presently available for severe accident research. It appears impossible to find a simulant material that will satisfy all the scaling relationships (thermodynamic and chemical behaviour, density, gas release, effects related to high temperatures...).

Simulant experiments are useful for systematic studies with well determined parameters. Due to technological and regulatory constraints , prototypic materials are less easy to use. Nevertheless, the above-mentioned differences between uranium and its simulants require the performance of a limited number of separate effect tests with depleted uranium. Furthermore, the use of prototypic material is required for any overall validation.

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