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Modelling of Zry-4 cladding oxidation by air, under severe accident conditions using the MAAP4 code
Nuclear Engineering and Design 241 (2011) 1217–1224
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Modelling of Zry-4 cladding oxidation by air, under severe accident conditions using the MAAP4 code Emilie Beuzet a,∗ , Jean-Sylvestre Lamy a , Armelle Bretault a , Eric Simoni b a b
EDF R&D, 1 Avenue du Général de Gaulle, F-92140 Clamart, France Institut de Physique Nucléaire, Université Paris Sud XI, F-91406 Orsay, France
a r t i c l e
i n f o
Article history: Received 25 January 2010 Received in revised form 9 April 2010 Accepted 12 April 2010 Available online 15 May 2010
a b s t r a c t In a nuclear power plant, a potential risk in some low probability situations in severe accidents is air ingress into the vessel. Air is a highly oxidizing atmosphere that can lead to an enhanced core oxidation and degradation affecting the release of Fission Products (FP), especially increasing that of ruthenium. This FP is of particular importance because of its high radio-toxicity and its ability to form highly volatile oxides. Oxygen affinity is decreasing between Zircaloy cladding, fuel and ruthenium inclusions in the fuel. It is consequently of great need to understand the phenomena governing cladding oxidation by air as a prerequisite for the source term issues. A review of existing data in the field of Zircaloy-4 oxidation in air-containing atmosphere shows that this phenomenon is quantitatively well understood. The cladding oxidation process can be divided into two kinetic regimes separated by a breakaway transition. Before transition, a protective dense zirconia scale grows following a solid state diffusion-limited regime for which experimental data are well fitted by a parabolic time dependence. For a given thickness, which depends mainly on temperature and the extent of pre-oxidation in steam, the dense scale can potentially breakdown. In case of breakaway combined with oxygen starvation, cladding oxidation can then be much faster because of the combined action of oxygen and nitrogen through a complex self sustaining nitriding-oxidation process. A review of the pre-existing correlations used to simulate zirconia scale growth under air atmospheres shows a high degree of variation from parabolic to accelerated time dependence. Variations also exist in the choice of the breakaway parameter based on zirconia phase change or oxide thickness. Several correlations and breakaway parameters found in the literature were implemented in the MAAP4.07 Severe Accident code. They were assessed by simulation of the QUENCH-10 test, which is a semi-integral test designed to study fuel bundle exposure to steam first and then to air. This paper deals with the main results obtained with MAAP4.07 when simulating QUENCH-10. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Severe accidents in PWRs can lead to the relocation of molten material from the core into the lower head. Heat exchange between the molten debris and the vessel may lead to the rupture of the vessel and air may be drawn from the containment into the vessel. Analysis of the Three Mile Island accident (TMI-2) shows that, in some scenarios, core degradation may not be uniform, the central region being melting and relocating downwards, while the outer region remains largely intact (Powers et al., 1994). Therefore, air entering in the vessel in the late phase of the accident may react with the peripheral, largely intact rods.
∗ Corresponding author. E-mail addresses: [email protected] (E. Beuzet), [email protected] (J.-S. Lamy), [email protected] (A. Bretault), [email protected] (E. Simoni). 0029-5493/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2010.04.024
In such situations, an air atmosphere may affect the core heatup and degradation and the fission product source term to the environment. Indeed, Zircaloy-4 (Zry-4) cladding oxidation by air yields 85% more heat than by steam (Powers et al., 1994). Besides, UO2 can be oxidized to UO2+x , which may lead to a lowering of the fuel melting temperature. Finally, air atmospheres can enhance the release of fission products (FPs), noticeably that of ruthenium which is present in the fuel as metallic inclusions (alloy composed of Mo, Tc, Rh, Pd and Ru (Bramman et al., 1968)). This FP is of particular concern because of its high radio-toxicity due to the combination of both its short and long half-life isotopes (103 Ru, 106 Ru) and its ability to generate volatile gaseous oxides (notably RuO4 ) in very oxidizing conditions such as in air ingress accidents (Iglesias et al., 1999). Considering that oxygen affinity is decreasing between cladding, fuel and ruthenium inclusions in the fuel, it is of great need to understand the phenomena governing cladding oxidation by air
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Fig. 1. Steps of cladding oxidation by air in severe accident conditions (Sarrazin et al., 2000; Huntz and Pieraggi, 2003).
as a prerequisite for the source term issues. The present paper gives an overview of the recent modelling progress made on this topic within the MAAP4.07 code. The first two parts summarize a review of the literature to examine phenomena and to identify reaction rates in air based on separate-effect experiments on Zry-4. In Section 4, we describe the modifications implemented in the MAAP4.07 code for air ingress phenomena. Part 5 focuses on post-test analyses of a 21-rod bundle experiment in the QUENCH facility at FZK (Forschungszentrum Karlsruhe, Germany) considering air oxidation of pre-oxidized electrically-heated Zircaloy-clad rods containing zirconia pellets that simulate the thermophysical effect of UO2 fuel.
with Dim as diffusion coefficient of i in the middle m, Ci as concentration of i in a given point and j as vector of particles current i
density. During this pre-transition regime, cladding oxidation controlled by diffusion can be decomposed in different steps (cf. Figs. 1 and 2; Lacour, 2001; Huntz and Pieraggi, 2003; Panicaud, 2004): • adsorption and dissociation of oxygen at the oxide-oxygen surface, • oxygen diffusion/dissolution in the metal until saturation, which leads to the formation of a zirconia layer, • oxygen diffusion through the oxide layer, • oxide growth at the metal-oxide interface.
2. Overview of phenomena The phenomena related to Zry-4 oxidation in air atmosphere were reviewed in the light of numerous pre-existing separateeffect experiments, which consisted in the oxidation of Zry-4 cladding samples of a few centimeters long at a constant temperature and air flow rate. Those experiments were performed on bare or pre-oxidized Zry-4 specimens over a wide temperature range from 573 K to 1858 K (Powers et al., 1994; Shepherd et al., 2000), (Duriez et al., 2008, 2009; Steinbrück, 2009). Globally, those experiments showed that Zry-4 oxidation by air has similarities with oxidation in steam due to the common reaction partner oxygen, but also important differences. The exothermal energy is around 1.8 times higher, which causes a more pronounced temperature escalation. After initial growth of a protective oxide scale, localized loss of scale protectiveness, also called breakaway, can occur leading to an increase of oxidation rate. The oxidation process is thus divided into two regimes called pre- and post-breakaway regimes. The different steps of the oxidation process are summarized in Fig. 1 and detailed in the following sections. 2.1. Pre-breakaway regime Before breakaway, oxidation by air is controlled by the exothermic reaction: Zr + O2 → ZrO2 with a reaction enthalpy of hair = −1100 kJ/mol(Zr) at 298 K. During the pre-breakaway regime, oxide growth is controlled by oxygen diffusion inside the oxide layer: the oxide layer behaves as a protective barrier against oxidation limiting the oxygen rate through the metal (Duriez et al., 2008). Diffusion leads to an equilibrium of the chemical species concentrations. This phenomenon is characterized by the Fick law which specifies that diffusion rate is proportional to concentration gradient: j = −Dm · ∇ C i i i
Experimentally speaking, the weight gain of the oxide layer is initially fast and decreases with the oxide growth. 2.2. Breakaway The oxide layer loses its protectiveness for a critical thickness, whose value depends on cladding initial state, atmosphere composition and temperature (Steinbrück, 2009). Experimental observations indicate that this change of kinetics is due to the formation of radial surface cracks in the oxide layer and then in the ␣-Zr(O) layer (cf. Fig. 1; Duriez et al., 2008). The most usual explanation given for the formation of these cracks is that they are the result of the combination of accumulation of stresses and zirconia phase change. Density and thermal expansion differences combined with oxide adhesion on the metal lead to compressive stresses in the oxide and tensile stresses in the metal (cf. Fig. 1; Gosmain et al., 2001). In the case of zirconia, the PBR (Pilling–Bedworth ratio) is equal to 1.56 in standard conditions, which means that the oxide has a strong tendenry to expand on the outer surface of metal (Sarrazin et al., 2000). Stresses are re-distributed inside. The differential between the thermal expansion of the oxide and the metal increases stresses (Brunet-Thibault, 2006).
Fig. 2. Diffusion principle.
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Fig. 3. Zirconia phase diagram (Steinbrück, 2009). Fig. 5. Schematic diagram of separate-effect tests.
What is more, the zirconia is not pure because of defects due to the presence of other chemical elements in the alloy and fast transients in accident conditions: temperature and oxidation change quickly. Equilibrium is never achieved. The zirconia formed is thus not pure and metastable phases can be formed. In this way, monoclinic-tetragonal zirconia phase can be found between 1133 and 1473 K and a tetragonal-cubic between 1773 and 2723 K (cf. Fig. 3; Lacour, 2001). It is thus possible to find tetragonal and metastable zirconia phases at a lower temperature. Compressive stresses increase with the oxide growth. According to the increase of stresses with the oxide growth, it is not possible to stabilize this phase, which becomes monoclinic. This phase change creates pores inside the oxide layer, due to volume differences between tetragonal and monoclinic zirconia (Godlewski, 1990). The combined effect of all these phenomena can explain why the oxide layer loses its protective characteristic and cracks.
2.4. Influence parameters Thanks to experiment analysis (Steinbrück, 2009), it has been possible to identify the parameters that influence the course of cladding oxidation in air. The most important are: • the sample initial state: pre-oxidation seems to represent a protective barrier against oxygen and as long as the oxide layer is intact, cladding oxidation is parabolic, • the atmosphere composition: oxygen seems to increase considerably the cladding oxidation kinetics, and the presence of nitrogen enhances cladding degradation between 1273 and 1473 K, • the temperature: strong degradation is observed for temperatures between 1273 and 1473 K and transition is faster when temperature increases.
2.3. Post-breakaway regime
3. Kinetics data from experiments
Experimental observations show that the oxidation rate increases after having reached a minimum at breakaway transition (Duriez et al., 2009). This acceleration can be amplified by the formation of a nitrogen-rich phase (ZrN) in the oxide near the boundary with the ␣-Zr(O) layer. Inside the cracks formed during the breakaway transition, the atmosphere can become rich in nitrogen if the oxygen is locally completely consumed (local oxygen starvation phenomenon). Spots of ZrN are thus formed following the exothermic reaction: Zr + (1/2)N2 → ZrN with a reaction enthalpy hnitrogen = −370 kJ/mol(Zr) at 298 K. When oxygen is again available, there is a fast and exothermic reaction that converts ZrN into ZrO2 . This reaction produces cracks due to density difference between ZrN and ZrO2 and nitrogen releases as bubbles, which contribute to form porosities (cf. Figs. 1 and 4; Duriez et al., 2008). Stresses can even play a role in the porous oxide formation: the oxidizable surface is increased due to crack formation and cladding distortion (and ballooning).
To help understand and quantify cladding oxidation in air and to establish simple correlations, one can interpret separate-effect tests on cladding samples. In this way, it is possible to identify the phenomenon, decoupled from any other. The separate-effect tests studied involve the oxidation of a Zircaloy-4 cladding sample, of a few centimetres long, pre-oxidized or not and under isothermal conditions. A constant air flow rate is injected during a given time (cf. Fig. 5). The oxidation rate is commonly associated with the oxide weight gain per unit surface area as follows: W n = Km (T ) · t
with W, the oxide layer weight gain in kg m−2 , Km (T), the air oxidation rate constant in kgn m−2n s−1 , n = 2 for a parabolic law, 1 for a linear law and 0.5 for an accelerated law and t, the time in s. The pre-breakaway regime, controlled by diffusion, is usually represented by a parabolic weight gain law (n = 2) and the postbreakaway regime, characterized by a porous oxide, by a linear to accelerated law (n ≤ 1). Following standard practice, the temperature dependence is fitted to an Arrhenius-type formalism: Km (T ) = A exp
Fig. 4. Cladding microstructure after exposure to air at about 1000 K (Duriez et al., 2008).
(1)
−E a
RT
(2)
with A, a constant in kgn m−2n s−1 , Ea , the energy of activation in J mol−1 and R, the gas constant, which value is 8.314 J mol−1 K−1 . According to a review done in the OPSA project (Shepherd et al., 2000), there is a high dispersion among air oxidation data of the preexisting separate-effect tests on the bare and steam pre-oxidized
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4. Code modelling
Fig. 6. Correlations for the air oxidation rate constant of Zry-4.
Zry-4 cladding material over a large temperature range between 573 K and 1858 K. Most of them are fitted to parabolic rate correlations (n = 2 in (1)). One of these, called NUREG1, considers two temperature ranges, a first below 1333 K and a second above 1550 K to take into account the zirconia phase change from monoclinic to tetragonal. A second one, called NUREG2, has an higher activation energy and covers the whole temperature range between 1000 K and 1550 K. This correlation is consistent with a third one termed AEKI. The NUREG2 correlation is also recommended to cover the temperature range missing in NUREG1 i.e. between 1333 K and 1550 K. Oxidation on bare and steam pre-oxidized Zry-4 samples in air/steam mixtures has also been addressed at FZK in the 1073–1673 K temperature range representative of air ingress scenarios but the data was not fitted to a correlation (Steinbrück, 2009). More recently, an experimental program, called MOZART, was conducted by IRSN (the French institute of radioprotection in nuclear security) to generate data and identify a new set of correlations for the air oxidation kinetics of Zry-4 cladding material (Duriez et al., 2008, 2009; Coindreau et al., 2008). Correlations were obtained for unexposed samples for temperatures between 873 K and 1473 K. In the pre-breakaway regime, the oxidation rate is fitted to a parabolic law called MOZART-PREB (n = 2 in (1)). After breakaway, the kinetic is modelled by an accelerated law called MOZART-POSTB (n = 0.5 in (1)). The transition occurs when a critical value of weight gain is reached according to a theory related to the tetragonal-monoclinic transformation of zirconia accompanied by crack formation:
m S
b
= 3.9 × 105 ×
Tb Htr × [T − Tb ]
2.27328 (3)
with (m/S)b the critical value of weight gain, Htr , the enthalpy of transformation of tetragonal to monoclinic zirconia and Tb set to 1447 K, a critical temperature above which there is no transition and the kinetics remains parabolic. The above mentioned correlations used to determine air oxidation rate constant dependence on temperature are plotted in Fig. 6.
MAAP (Rahn, 2010) is used to simulate severe accident transients. It was originally developed for the Industry Degraded Core Rulemaking (IDCOR) program in the early 1980s by Fauske and Associates, Inc. (FAI). At the completion of IDCOR, ownership of MAAP was transferred to the Electric Power Research Institute (EPRI), which is now in charge of improving the code and licensing it to utilities, vendors and research organizations. The current version of the code, called MAAP4.07, has no provision for modelling air ingress. Thus, it was modified to make it suitable for the present task. In the standard code, thermal hydraulics are based on the treatment of a mixture of steam, oxygen, nitrogen, hydrogen and non condensable gases such as methane, carbon monoxide and carbon dioxide. The modelling of interaction between atmosphere and cladding material does not treat oxidation and nitriding in air containing atmosphere. It focuses on cladding oxidation by steam using parabolic kinetics and weight gain correlations are expressed in terms of oxide layer growth as follows: xn =
Km (T ) · t n
(4)
with x, oxide layer thickness in m, , zirconium density in kg m−3 , Km (T ), steam oxidation rate constant in kgn m−2n s−1 and n = 2 for parabolic laws. Additions were therefore implemented in the code to model the reaction of Zry-4 with air, essentially the oxygen part. The oxidation rate of Zry-4 in oxygen was modelled using the same approach as for steam (cf. Eq. (4)). One can find in Table 1 the three sets of correlations implemented in the code to calculate the air oxidation rate constant. The sets of correlations can be selected by the user with a simple option choice. If taken into account, the transition from pre- to post-breakaway regime is done with a parameterized temperature (NUREG) or with a critical weight gain (MOZART). Finally, to optimize changes to the code for reactor applications with complex atmospheres, the effect of a steam/oxygen mixture is treated in the code. In MAAP4.07, it is now possible to oxidize with both steam and oxygen: • if there is enough zirconium, cladding oxidation is calculated with both gases, in accordance with their own oxidation kinetics, • if there is little zirconium left, cladding oxidation is calculated in proportion to the gas mass flow rates, • for a ratio of mass flow rates less than 10−6 , oxidation is calculated with the major component only. 5. Code assessment on the QUENCH-10 experiment The QUENCH-10 test was performed at the Forschungszentrum Karlsruhe (FZK) in July 2004. It aimed at studying the cladding oxidation and nitriding formation in case of air ingress (Steinbrück, 2006). It was simulated with MAAP4.07 to determine how well the air oxidation model recently implemented in the code repro-
Table 1 Correlations for the air oxidation rate constant of Zry-4 available in MAAP4.07 (in normal font: parabolic correlation; in bold and italic: accelerated correlation). Correlations
Ai (kgnZr /m2n /s)
Eai /R (K)
Breakaway
15,630 28,485 14,634
T < 1333 K 1333 K ≤ T ≤ 1550 K T > 1550 K
NUREG (Powers et al., 1994)
10.50 25.11 × 104 50.40
AEKI (Shepherd et al., 2000)
21.72 × 104
29,054
For each T
2.27 × 104 261.63
23,442 15,937
m/s < 92.96 exp(7024.3/T) m/s ≥ 92.96 exp(7024.3/T)
MOZART (Coindreau et al., 2008)
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Fig. 9. QUENCH-10 test conduct. Modelling of the QUENCH-10 test facility and test sequence
Fig. 7. QUENCH-10 axial view.
duces the experiment and to indicate areas of weakness that require further investigation. 5.1. Description of the QUENCH-10 test facility and test sequence A axial section and a cross-section of the QUENCH-10 bundle are shown in Figs. 7 and 8. It consists of 20 fuel rod simulators and a central unheated rod with a total length of approximately 2.5 m. The cladding material of fuel rod simulators is standard Zry4 used in PWRs (10.75 mm diameter and 0.725 mm thickness). Fuel
Fig. 8. QUENCH-10 cross-section.
is represented by annular pellets of zirconia, which are internally heated with 1m length tungsten heaters connected to an electric power supply. Thermocouples are implemented at 17 different elevations. Two corner rods made of Zircaloy are used for thermocouple instrumentation. Two others are withdrawn during the sequence in order to study the evolution of axial oxide layer profile. The bundle is surrounded by a shroud with a Zircaloy liner, thermal insulation in porous zirconia and a cooling jacket (Schanz et al., 2006). The QUENCH-10 test sequence is composed of different steps as illustrated in Fig. 9. The first phase consists in a heat stabilization phase at around 873 K under a steam flow rate of 3 g/s, to test the facility. The second one is a pre-oxidation phase in steam at around 1620 K to reproduce the cladding oxidation in steam generated by water boiling at the beginning of core degradation. Then the bundle is cooled to 1183 K and a first corner rod is withdrawn to follow the oxide growth. This cooling is used to prevent the bundle from very rapid escalation temperature due to air (Shepherd et al., 2000). The fourth phase concerns cladding oxidation by air with a flow rate of 1 g/s and a temperature ramp from 1183 K to 2200 K. This phase is the main goal of this test. At the end of air ingress phase, the second corner rod is withdrawn to follow the oxide growth compared with the first one. To finish, the bundle is quenched by a water injection from the bottom of the test section. QUENCH-10 bundle is meshed with 3 radial channels composed of respectively 5, 8 and 8 fuel rods and 58 axial meshes, which 48 represent the heated central zone and the upper and lower plenums are roughly parsed (5 for each). The shroud, the thermoinsulation and the internal wall of cooling circuit are also modelled. The fluid is divided amongst the 3 channels and the 58 axial meshes (cf. Fig. 10). Initial conditions are the average temperature along the shroud and along the fuel rod simulators. Boundary conditions are the
Fig. 10. Mesh of QUENCH-10 bundle in MAAP4.07.
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Fig. 11. Temperature evolution of cladding (Tcl) and shroud (Tsh) during steam pre-oxidation at the bottom (350 mm) and at the top (950 mm) of the heated zone.
injected power history, temperatures on the inner cooling jacket, steam, air and water flow rates and pressure.
5.2. Simulation of the steam pre-oxidation phase This pre-oxidation phase consists of the oxidation of the fuel rod simulators in a steam atmosphere over about 11,600 s. It is a simulation of early phase of core degradation after core uncovery when water that remains boils away. As the Km coefficient is strongly T dependent, the clad temperature has to be as close as possible to the experimental conditions. The computational weight gain is not only sensitive to oxidation model but also to heat exchange. To simulate this phase and to be sure of the initial conditions before air ingress, Cathcart/Pawel and Urbanic/Heidrick correlations are used for the cladding weight gain due to steam oxidation (Cathcart et al., 1977; Urbanic, 1976). One can see in Fig. 11 the cladding and shroud temperature evolutions measured during the experiment and predicted by the code (at 350 and 950 mm high). These curves show that the code predicts quite well the thermal behavior of the bundle: the qualitative and quantitative evolution of the temperatures is quite well reproduced by MAAP4.07 code. The rod withdrawn at the end of the pre-oxidation phase gives us the oxide layer profile. The curve in Fig. 12 puts the good agreement of the calculation with the experiment into relief: the calculated oxide layer profile at the end of the pre-oxidation phase is quite close to the experimental one.
Fig. 12. Oxide layer thickness at the end of the steam pre-oxidation.
Fig. 13. Temperature evolution of cladding (Tcl) and shroud (Tsh) at the bottom of the heated zone (350 mm elevation) during the air ingress phase.
5.3. Simulation of the air oxidation phase The air ingress phase of about 30 min long in QUENCH-10 examines the effect of air on cladding oxidation and degradation. In Figs. 13 and 14, the temperatures evolutions given by the different correlations available in MAAP4.07 code (cf. Table 1) are illustrated for the cladding and shroud for the bottom (350 mm high) and for the top (950 mm high) of the heated zone, which is the hot spot. First, these curves clearly show that the MOZART correlation is not well adapted for QUENCH-10 experiment, although this takes into account breakaway transition. It is something that was expected, as the MOZART correlation is based on experiments made to evaluate the consequences of spent fuel storage pool loss of water inventory accident, that is to say at lower temperatures than in severe accident conditions (Duriez et al., 2008, 2009). Besides, in the QUENCH-10 test, a protective oxide layer was formed during steam pre-oxidation that delays breakaway (Steinbrück, 2009), while the MOZART correlation was obtained using experiments made on samples without pre-oxidation (Coindreau et al., 2008). This may have a strong influence on breakaway transition, as it is the case in Fig. 13. Second, the AEKI correlation gives a better temperature evolution but not as good as that using NUREG correlation. But above all, the definition interval of the AEKI correlation is based on zirconia
Fig. 14. Temperature evolution of cladding (Tcl) and shroud (Tsh) at the top of the heated zone (950 mm elevation) during the air ingress phase.
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and no nitriding treatment in MAAP4.07 (use of parabolic laws, all the simulation long), no oxidation at the hot spot was done. The oxide layer of the experimental hot point was bigger, even if uncertainties exist on it (the rod, which was withdrawn, broke down; Schanz et al., 2006). Moreover, oxide layer thickness is calculated as a homogenous layer with MAAP4.07, which is not at all the case experimentally speaking. 6. Conclusions The separate-effect tests give a good comprehensive basis for the phenomenological understanding of cladding oxidation by air. This phenomenon is indeed well understood: Fig. 15. Oxygen flow rates at different bundle elevations during air ingress.
phase change (1333–1550 K) and not on in the whole temperature interval of QUENCH-10 experiment (Shepherd et al., 2000). The NUREG correlation reproduces well the thermal behavior of the lower part of the bundle but the one of the hot spots is underpredicted. Two reasons can explain this observation. The temperature is slightly underestimated by the code at the end of the pre-oxidation phase at 950 mm elevation. The transition from one regime to another can thus be delayed because the NUREG correlation is based on different temperature intervals. Moreover, the use of a parabolic law after breakaway cannot reproduce perfectly the rapid acceleration of the temperature escalation, which was observed experimentally. From this temperature evolution analysis, the NUREG correlation appears to be the best choice. This conclusion is not code-dependent: a comparison of SCDAP/RELAP5 and MELCOR calculations against QUENCH-10 test comes to similar conclusions to that work (Birchley and Haste, 2004). In Fig. 15, one can see the evolution of oxygen flow rates for different bundle elevations using the NUREG correlation and for the experiment at bundle entrance (‘data-in’) and at bundle exit (‘data-off’). What is important to note is that an O2 starvation (a complete O2 consumption) is first observed at the top of the heated zone, a bit earlier than the experiment, and then propagates to the bottom. This is confirmed in the experiment in which nitriding is observed between 800 mm and 1080 mm elevation which leads to an accelerated degradation, characteristic of the post-breakaway regime (Schanz et al., 2006). This phenomenon is not taken into account in MAAP4.07 code modeling, which is why the cladding oxidation acceleration is not simulated. Fig. 16 represents the oxide layer thickness profile at the end of air ingress, for the experiment and the modelling using the NUREG correlation. As there was no oxygen left at the top of the bundle
Fig. 16. Oxide layer thickness at the end of air ingress for the experiment and for the modelling with the NUREG correlation.
• a pre-breakaway regime, which leads to the formation of a dense oxide layer, with a parabolic weight gain, • a breakaway due to cracks inside the oxide layer (from stresses and zirconia phase change), • a post-breakaway regime, which leads to the formation of a porous oxide layer, with an accelerated weight gain. From this experimental work, weight gain correlations have been established, most of them are parabolic. Breakaway transition is based on zirconia phase change temperature or on a critical weight gain of the oxide layer. The numerical analysis done in this work underlines the need to capture the breakaway effect to model correctly the behavior of Zircaloy oxidation in air. This breakaway has to take into account different parameters such as temperature and oxide layer thickness. Moreover, QUENCH-10 test simulation with the MAAP4.07 code clearly supports the idea that parabolic correlations are not adapted for post-breakaway regime: the temperature acceleration that occurs after breakaway is not captured. Numerous data from separate-effect tests and semi-integral experiments are available and provide an important database for future model development, notably on nitriding treatment. The QUENCH series aim to study reflood behaviour and even if the MAAP4 code calculates this last phase, the analysis is part of other work that will maybe lead to next papers. References Birchley, J., Haste, T., October 2004. Post-test analysis of QUENCH-10 with SCDAP/RELAP5 and MELCOR. In: 10th QUENCH Workshop, Forschungszentrum Karlsruhe, Germany. Bramman, J.I., Sharpe, R.M., Thom, D., Yates, G., 1968. Metallic fission-product inclusions in irradiated oxide fuels. J. Nucl. Mater. 25, 201–215. Brunet-Thibault, E., December 2006. Etude du renoyage par le haut en cas d’accident grave et en particulier oxidation des mélanges (U, Zr, O). Ph.D. Thesis. Institut National Polytechnique de Grenoble. Cathcart, J.V., Pawel, R.E., McKee, R.A., Druschel, R.E., Yurek, G.J., Campbell, J.J., Jury, S.H., 1977. Zirconium Water Oxidation Kinetics. Report, ORNL/NUREG 17. Coindreau, O., et al., September 2008. Modelling of accelerated cladding degradation in air for severe accident codes. In: ERMSAR 2008 Conference, Nesseber, Bulgaria, Paper No. 2-4. Duriez, C., Dupont, T., Schmet, B., Enoch, F., 2008. Zircaloy-4 and M5® high temperature oxidation and nitriding in air. J. Nucl. Mater. 380 (October), 30–45. Duriez, C., Steinbrück, M., Ohai, D., Meleg, T., Birchley, J., Haste, T., 2009. Separateeffect tests on zirconium cladding degradation in air ingress situations. Nucl. Eng. Des. 239, 244–253. Godlewski, J., July 1990. Oxydation d’alliages de zirconium en vapeur d’eau: influence de la zircone tétragonale sur le mécanisme de croissance de l’oxyde. Ph.D. Thesis. Université de Technologie de Compiègne. Gosmain, L., Valot, C., Ciosmak, D., Sicardy, O., 2001. Study of stress effects in the oxidation of Zircaloy-4. Solid States Ionics 141–142, 633–640. Huntz, A.M., Pieraggi, B., April 2003. Oxydation des matériaux métalliques: comportement à haute temperature. Lavoisier, Paris. Iglesias, F.C., Lewis, B.J., Reid, P.J., Elder, P., 1999. Fission product release mechanisms during reactor accident conditions. J. Nucl. Mater. 270, 21–38. Lacour, V., November 2001. Modélisation de la production d’hydrogène lors de la phase de renoyage des cœurs de réacteurs nucléaires en situation d’accidents graves. Ph.D. Thesis. Ecole Nationale Supérieure des Mines de Paris.
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Sarrazin, P., Galerie, A., Fouletier, J., October 2000. Les mécanismes de la corrosion sèche: une approche cinétique, EDP Sciences, Les Ulis. Schanz, G., et al., May 2006. Results of the QUENCH-10 Experiment on Air Ingress. Report FZKA 7087, SAM-LACOMERA-D09. Steinbrück, M., 2006. Experiments on air ingress during severe accidents in LWRs. Nucl. Eng. Des. 236, 1709–1719. Steinbrück, M., 2009. Prototypical experiments relating to air oxidation of zircaloy-4 at high temperatures. J. Nucl. Mater. 392, 531–544. Urbanic, V.F., 1976. Method for estimating the exposure time and temperature for Zircaloy oxidation in steam. J. Nucl. Mater. 59, 90–94.
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Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Modelling of Zry-4 cladding oxidation by air, under severe accident conditions using the MAAP4 code Emilie Beuzet a,∗ , Jean-Sylvestre Lamy a , Armelle Bretault a , Eric Simoni b a b
EDF R&D, 1 Avenue du Général de Gaulle, F-92140 Clamart, France Institut de Physique Nucléaire, Université Paris Sud XI, F-91406 Orsay, France
a r t i c l e
i n f o
Article history: Received 25 January 2010 Received in revised form 9 April 2010 Accepted 12 April 2010 Available online 15 May 2010
a b s t r a c t In a nuclear power plant, a potential risk in some low probability situations in severe accidents is air ingress into the vessel. Air is a highly oxidizing atmosphere that can lead to an enhanced core oxidation and degradation affecting the release of Fission Products (FP), especially increasing that of ruthenium. This FP is of particular importance because of its high radio-toxicity and its ability to form highly volatile oxides. Oxygen affinity is decreasing between Zircaloy cladding, fuel and ruthenium inclusions in the fuel. It is consequently of great need to understand the phenomena governing cladding oxidation by air as a prerequisite for the source term issues. A review of existing data in the field of Zircaloy-4 oxidation in air-containing atmosphere shows that this phenomenon is quantitatively well understood. The cladding oxidation process can be divided into two kinetic regimes separated by a breakaway transition. Before transition, a protective dense zirconia scale grows following a solid state diffusion-limited regime for which experimental data are well fitted by a parabolic time dependence. For a given thickness, which depends mainly on temperature and the extent of pre-oxidation in steam, the dense scale can potentially breakdown. In case of breakaway combined with oxygen starvation, cladding oxidation can then be much faster because of the combined action of oxygen and nitrogen through a complex self sustaining nitriding-oxidation process. A review of the pre-existing correlations used to simulate zirconia scale growth under air atmospheres shows a high degree of variation from parabolic to accelerated time dependence. Variations also exist in the choice of the breakaway parameter based on zirconia phase change or oxide thickness. Several correlations and breakaway parameters found in the literature were implemented in the MAAP4.07 Severe Accident code. They were assessed by simulation of the QUENCH-10 test, which is a semi-integral test designed to study fuel bundle exposure to steam first and then to air. This paper deals with the main results obtained with MAAP4.07 when simulating QUENCH-10. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Severe accidents in PWRs can lead to the relocation of molten material from the core into the lower head. Heat exchange between the molten debris and the vessel may lead to the rupture of the vessel and air may be drawn from the containment into the vessel. Analysis of the Three Mile Island accident (TMI-2) shows that, in some scenarios, core degradation may not be uniform, the central region being melting and relocating downwards, while the outer region remains largely intact (Powers et al., 1994). Therefore, air entering in the vessel in the late phase of the accident may react with the peripheral, largely intact rods.
∗ Corresponding author. E-mail addresses: [email protected] (E. Beuzet), [email protected] (J.-S. Lamy), [email protected] (A. Bretault), [email protected] (E. Simoni). 0029-5493/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2010.04.024
In such situations, an air atmosphere may affect the core heatup and degradation and the fission product source term to the environment. Indeed, Zircaloy-4 (Zry-4) cladding oxidation by air yields 85% more heat than by steam (Powers et al., 1994). Besides, UO2 can be oxidized to UO2+x , which may lead to a lowering of the fuel melting temperature. Finally, air atmospheres can enhance the release of fission products (FPs), noticeably that of ruthenium which is present in the fuel as metallic inclusions (alloy composed of Mo, Tc, Rh, Pd and Ru (Bramman et al., 1968)). This FP is of particular concern because of its high radio-toxicity due to the combination of both its short and long half-life isotopes (103 Ru, 106 Ru) and its ability to generate volatile gaseous oxides (notably RuO4 ) in very oxidizing conditions such as in air ingress accidents (Iglesias et al., 1999). Considering that oxygen affinity is decreasing between cladding, fuel and ruthenium inclusions in the fuel, it is of great need to understand the phenomena governing cladding oxidation by air
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Fig. 1. Steps of cladding oxidation by air in severe accident conditions (Sarrazin et al., 2000; Huntz and Pieraggi, 2003).
as a prerequisite for the source term issues. The present paper gives an overview of the recent modelling progress made on this topic within the MAAP4.07 code. The first two parts summarize a review of the literature to examine phenomena and to identify reaction rates in air based on separate-effect experiments on Zry-4. In Section 4, we describe the modifications implemented in the MAAP4.07 code for air ingress phenomena. Part 5 focuses on post-test analyses of a 21-rod bundle experiment in the QUENCH facility at FZK (Forschungszentrum Karlsruhe, Germany) considering air oxidation of pre-oxidized electrically-heated Zircaloy-clad rods containing zirconia pellets that simulate the thermophysical effect of UO2 fuel.
with Dim as diffusion coefficient of i in the middle m, Ci as concentration of i in a given point and j as vector of particles current i
density. During this pre-transition regime, cladding oxidation controlled by diffusion can be decomposed in different steps (cf. Figs. 1 and 2; Lacour, 2001; Huntz and Pieraggi, 2003; Panicaud, 2004): • adsorption and dissociation of oxygen at the oxide-oxygen surface, • oxygen diffusion/dissolution in the metal until saturation, which leads to the formation of a zirconia layer, • oxygen diffusion through the oxide layer, • oxide growth at the metal-oxide interface.
2. Overview of phenomena The phenomena related to Zry-4 oxidation in air atmosphere were reviewed in the light of numerous pre-existing separateeffect experiments, which consisted in the oxidation of Zry-4 cladding samples of a few centimeters long at a constant temperature and air flow rate. Those experiments were performed on bare or pre-oxidized Zry-4 specimens over a wide temperature range from 573 K to 1858 K (Powers et al., 1994; Shepherd et al., 2000), (Duriez et al., 2008, 2009; Steinbrück, 2009). Globally, those experiments showed that Zry-4 oxidation by air has similarities with oxidation in steam due to the common reaction partner oxygen, but also important differences. The exothermal energy is around 1.8 times higher, which causes a more pronounced temperature escalation. After initial growth of a protective oxide scale, localized loss of scale protectiveness, also called breakaway, can occur leading to an increase of oxidation rate. The oxidation process is thus divided into two regimes called pre- and post-breakaway regimes. The different steps of the oxidation process are summarized in Fig. 1 and detailed in the following sections. 2.1. Pre-breakaway regime Before breakaway, oxidation by air is controlled by the exothermic reaction: Zr + O2 → ZrO2 with a reaction enthalpy of hair = −1100 kJ/mol(Zr) at 298 K. During the pre-breakaway regime, oxide growth is controlled by oxygen diffusion inside the oxide layer: the oxide layer behaves as a protective barrier against oxidation limiting the oxygen rate through the metal (Duriez et al., 2008). Diffusion leads to an equilibrium of the chemical species concentrations. This phenomenon is characterized by the Fick law which specifies that diffusion rate is proportional to concentration gradient: j = −Dm · ∇ C i i i
Experimentally speaking, the weight gain of the oxide layer is initially fast and decreases with the oxide growth. 2.2. Breakaway The oxide layer loses its protectiveness for a critical thickness, whose value depends on cladding initial state, atmosphere composition and temperature (Steinbrück, 2009). Experimental observations indicate that this change of kinetics is due to the formation of radial surface cracks in the oxide layer and then in the ␣-Zr(O) layer (cf. Fig. 1; Duriez et al., 2008). The most usual explanation given for the formation of these cracks is that they are the result of the combination of accumulation of stresses and zirconia phase change. Density and thermal expansion differences combined with oxide adhesion on the metal lead to compressive stresses in the oxide and tensile stresses in the metal (cf. Fig. 1; Gosmain et al., 2001). In the case of zirconia, the PBR (Pilling–Bedworth ratio) is equal to 1.56 in standard conditions, which means that the oxide has a strong tendenry to expand on the outer surface of metal (Sarrazin et al., 2000). Stresses are re-distributed inside. The differential between the thermal expansion of the oxide and the metal increases stresses (Brunet-Thibault, 2006).
Fig. 2. Diffusion principle.
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Fig. 3. Zirconia phase diagram (Steinbrück, 2009). Fig. 5. Schematic diagram of separate-effect tests.
What is more, the zirconia is not pure because of defects due to the presence of other chemical elements in the alloy and fast transients in accident conditions: temperature and oxidation change quickly. Equilibrium is never achieved. The zirconia formed is thus not pure and metastable phases can be formed. In this way, monoclinic-tetragonal zirconia phase can be found between 1133 and 1473 K and a tetragonal-cubic between 1773 and 2723 K (cf. Fig. 3; Lacour, 2001). It is thus possible to find tetragonal and metastable zirconia phases at a lower temperature. Compressive stresses increase with the oxide growth. According to the increase of stresses with the oxide growth, it is not possible to stabilize this phase, which becomes monoclinic. This phase change creates pores inside the oxide layer, due to volume differences between tetragonal and monoclinic zirconia (Godlewski, 1990). The combined effect of all these phenomena can explain why the oxide layer loses its protective characteristic and cracks.
2.4. Influence parameters Thanks to experiment analysis (Steinbrück, 2009), it has been possible to identify the parameters that influence the course of cladding oxidation in air. The most important are: • the sample initial state: pre-oxidation seems to represent a protective barrier against oxygen and as long as the oxide layer is intact, cladding oxidation is parabolic, • the atmosphere composition: oxygen seems to increase considerably the cladding oxidation kinetics, and the presence of nitrogen enhances cladding degradation between 1273 and 1473 K, • the temperature: strong degradation is observed for temperatures between 1273 and 1473 K and transition is faster when temperature increases.
2.3. Post-breakaway regime
3. Kinetics data from experiments
Experimental observations show that the oxidation rate increases after having reached a minimum at breakaway transition (Duriez et al., 2009). This acceleration can be amplified by the formation of a nitrogen-rich phase (ZrN) in the oxide near the boundary with the ␣-Zr(O) layer. Inside the cracks formed during the breakaway transition, the atmosphere can become rich in nitrogen if the oxygen is locally completely consumed (local oxygen starvation phenomenon). Spots of ZrN are thus formed following the exothermic reaction: Zr + (1/2)N2 → ZrN with a reaction enthalpy hnitrogen = −370 kJ/mol(Zr) at 298 K. When oxygen is again available, there is a fast and exothermic reaction that converts ZrN into ZrO2 . This reaction produces cracks due to density difference between ZrN and ZrO2 and nitrogen releases as bubbles, which contribute to form porosities (cf. Figs. 1 and 4; Duriez et al., 2008). Stresses can even play a role in the porous oxide formation: the oxidizable surface is increased due to crack formation and cladding distortion (and ballooning).
To help understand and quantify cladding oxidation in air and to establish simple correlations, one can interpret separate-effect tests on cladding samples. In this way, it is possible to identify the phenomenon, decoupled from any other. The separate-effect tests studied involve the oxidation of a Zircaloy-4 cladding sample, of a few centimetres long, pre-oxidized or not and under isothermal conditions. A constant air flow rate is injected during a given time (cf. Fig. 5). The oxidation rate is commonly associated with the oxide weight gain per unit surface area as follows: W n = Km (T ) · t
with W, the oxide layer weight gain in kg m−2 , Km (T), the air oxidation rate constant in kgn m−2n s−1 , n = 2 for a parabolic law, 1 for a linear law and 0.5 for an accelerated law and t, the time in s. The pre-breakaway regime, controlled by diffusion, is usually represented by a parabolic weight gain law (n = 2) and the postbreakaway regime, characterized by a porous oxide, by a linear to accelerated law (n ≤ 1). Following standard practice, the temperature dependence is fitted to an Arrhenius-type formalism: Km (T ) = A exp
Fig. 4. Cladding microstructure after exposure to air at about 1000 K (Duriez et al., 2008).
(1)
−E a
RT
(2)
with A, a constant in kgn m−2n s−1 , Ea , the energy of activation in J mol−1 and R, the gas constant, which value is 8.314 J mol−1 K−1 . According to a review done in the OPSA project (Shepherd et al., 2000), there is a high dispersion among air oxidation data of the preexisting separate-effect tests on the bare and steam pre-oxidized
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4. Code modelling
Fig. 6. Correlations for the air oxidation rate constant of Zry-4.
Zry-4 cladding material over a large temperature range between 573 K and 1858 K. Most of them are fitted to parabolic rate correlations (n = 2 in (1)). One of these, called NUREG1, considers two temperature ranges, a first below 1333 K and a second above 1550 K to take into account the zirconia phase change from monoclinic to tetragonal. A second one, called NUREG2, has an higher activation energy and covers the whole temperature range between 1000 K and 1550 K. This correlation is consistent with a third one termed AEKI. The NUREG2 correlation is also recommended to cover the temperature range missing in NUREG1 i.e. between 1333 K and 1550 K. Oxidation on bare and steam pre-oxidized Zry-4 samples in air/steam mixtures has also been addressed at FZK in the 1073–1673 K temperature range representative of air ingress scenarios but the data was not fitted to a correlation (Steinbrück, 2009). More recently, an experimental program, called MOZART, was conducted by IRSN (the French institute of radioprotection in nuclear security) to generate data and identify a new set of correlations for the air oxidation kinetics of Zry-4 cladding material (Duriez et al., 2008, 2009; Coindreau et al., 2008). Correlations were obtained for unexposed samples for temperatures between 873 K and 1473 K. In the pre-breakaway regime, the oxidation rate is fitted to a parabolic law called MOZART-PREB (n = 2 in (1)). After breakaway, the kinetic is modelled by an accelerated law called MOZART-POSTB (n = 0.5 in (1)). The transition occurs when a critical value of weight gain is reached according to a theory related to the tetragonal-monoclinic transformation of zirconia accompanied by crack formation:
m S
b
= 3.9 × 105 ×
Tb Htr × [T − Tb ]
2.27328 (3)
with (m/S)b the critical value of weight gain, Htr , the enthalpy of transformation of tetragonal to monoclinic zirconia and Tb set to 1447 K, a critical temperature above which there is no transition and the kinetics remains parabolic. The above mentioned correlations used to determine air oxidation rate constant dependence on temperature are plotted in Fig. 6.
MAAP (Rahn, 2010) is used to simulate severe accident transients. It was originally developed for the Industry Degraded Core Rulemaking (IDCOR) program in the early 1980s by Fauske and Associates, Inc. (FAI). At the completion of IDCOR, ownership of MAAP was transferred to the Electric Power Research Institute (EPRI), which is now in charge of improving the code and licensing it to utilities, vendors and research organizations. The current version of the code, called MAAP4.07, has no provision for modelling air ingress. Thus, it was modified to make it suitable for the present task. In the standard code, thermal hydraulics are based on the treatment of a mixture of steam, oxygen, nitrogen, hydrogen and non condensable gases such as methane, carbon monoxide and carbon dioxide. The modelling of interaction between atmosphere and cladding material does not treat oxidation and nitriding in air containing atmosphere. It focuses on cladding oxidation by steam using parabolic kinetics and weight gain correlations are expressed in terms of oxide layer growth as follows: xn =
Km (T ) · t n
(4)
with x, oxide layer thickness in m, , zirconium density in kg m−3 , Km (T ), steam oxidation rate constant in kgn m−2n s−1 and n = 2 for parabolic laws. Additions were therefore implemented in the code to model the reaction of Zry-4 with air, essentially the oxygen part. The oxidation rate of Zry-4 in oxygen was modelled using the same approach as for steam (cf. Eq. (4)). One can find in Table 1 the three sets of correlations implemented in the code to calculate the air oxidation rate constant. The sets of correlations can be selected by the user with a simple option choice. If taken into account, the transition from pre- to post-breakaway regime is done with a parameterized temperature (NUREG) or with a critical weight gain (MOZART). Finally, to optimize changes to the code for reactor applications with complex atmospheres, the effect of a steam/oxygen mixture is treated in the code. In MAAP4.07, it is now possible to oxidize with both steam and oxygen: • if there is enough zirconium, cladding oxidation is calculated with both gases, in accordance with their own oxidation kinetics, • if there is little zirconium left, cladding oxidation is calculated in proportion to the gas mass flow rates, • for a ratio of mass flow rates less than 10−6 , oxidation is calculated with the major component only. 5. Code assessment on the QUENCH-10 experiment The QUENCH-10 test was performed at the Forschungszentrum Karlsruhe (FZK) in July 2004. It aimed at studying the cladding oxidation and nitriding formation in case of air ingress (Steinbrück, 2006). It was simulated with MAAP4.07 to determine how well the air oxidation model recently implemented in the code repro-
Table 1 Correlations for the air oxidation rate constant of Zry-4 available in MAAP4.07 (in normal font: parabolic correlation; in bold and italic: accelerated correlation). Correlations
Ai (kgnZr /m2n /s)
Eai /R (K)
Breakaway
15,630 28,485 14,634
T < 1333 K 1333 K ≤ T ≤ 1550 K T > 1550 K
NUREG (Powers et al., 1994)
10.50 25.11 × 104 50.40
AEKI (Shepherd et al., 2000)
21.72 × 104
29,054
For each T
2.27 × 104 261.63
23,442 15,937
m/s < 92.96 exp(7024.3/T) m/s ≥ 92.96 exp(7024.3/T)
MOZART (Coindreau et al., 2008)
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Fig. 9. QUENCH-10 test conduct. Modelling of the QUENCH-10 test facility and test sequence
Fig. 7. QUENCH-10 axial view.
duces the experiment and to indicate areas of weakness that require further investigation. 5.1. Description of the QUENCH-10 test facility and test sequence A axial section and a cross-section of the QUENCH-10 bundle are shown in Figs. 7 and 8. It consists of 20 fuel rod simulators and a central unheated rod with a total length of approximately 2.5 m. The cladding material of fuel rod simulators is standard Zry4 used in PWRs (10.75 mm diameter and 0.725 mm thickness). Fuel
Fig. 8. QUENCH-10 cross-section.
is represented by annular pellets of zirconia, which are internally heated with 1m length tungsten heaters connected to an electric power supply. Thermocouples are implemented at 17 different elevations. Two corner rods made of Zircaloy are used for thermocouple instrumentation. Two others are withdrawn during the sequence in order to study the evolution of axial oxide layer profile. The bundle is surrounded by a shroud with a Zircaloy liner, thermal insulation in porous zirconia and a cooling jacket (Schanz et al., 2006). The QUENCH-10 test sequence is composed of different steps as illustrated in Fig. 9. The first phase consists in a heat stabilization phase at around 873 K under a steam flow rate of 3 g/s, to test the facility. The second one is a pre-oxidation phase in steam at around 1620 K to reproduce the cladding oxidation in steam generated by water boiling at the beginning of core degradation. Then the bundle is cooled to 1183 K and a first corner rod is withdrawn to follow the oxide growth. This cooling is used to prevent the bundle from very rapid escalation temperature due to air (Shepherd et al., 2000). The fourth phase concerns cladding oxidation by air with a flow rate of 1 g/s and a temperature ramp from 1183 K to 2200 K. This phase is the main goal of this test. At the end of air ingress phase, the second corner rod is withdrawn to follow the oxide growth compared with the first one. To finish, the bundle is quenched by a water injection from the bottom of the test section. QUENCH-10 bundle is meshed with 3 radial channels composed of respectively 5, 8 and 8 fuel rods and 58 axial meshes, which 48 represent the heated central zone and the upper and lower plenums are roughly parsed (5 for each). The shroud, the thermoinsulation and the internal wall of cooling circuit are also modelled. The fluid is divided amongst the 3 channels and the 58 axial meshes (cf. Fig. 10). Initial conditions are the average temperature along the shroud and along the fuel rod simulators. Boundary conditions are the
Fig. 10. Mesh of QUENCH-10 bundle in MAAP4.07.
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Fig. 11. Temperature evolution of cladding (Tcl) and shroud (Tsh) during steam pre-oxidation at the bottom (350 mm) and at the top (950 mm) of the heated zone.
injected power history, temperatures on the inner cooling jacket, steam, air and water flow rates and pressure.
5.2. Simulation of the steam pre-oxidation phase This pre-oxidation phase consists of the oxidation of the fuel rod simulators in a steam atmosphere over about 11,600 s. It is a simulation of early phase of core degradation after core uncovery when water that remains boils away. As the Km coefficient is strongly T dependent, the clad temperature has to be as close as possible to the experimental conditions. The computational weight gain is not only sensitive to oxidation model but also to heat exchange. To simulate this phase and to be sure of the initial conditions before air ingress, Cathcart/Pawel and Urbanic/Heidrick correlations are used for the cladding weight gain due to steam oxidation (Cathcart et al., 1977; Urbanic, 1976). One can see in Fig. 11 the cladding and shroud temperature evolutions measured during the experiment and predicted by the code (at 350 and 950 mm high). These curves show that the code predicts quite well the thermal behavior of the bundle: the qualitative and quantitative evolution of the temperatures is quite well reproduced by MAAP4.07 code. The rod withdrawn at the end of the pre-oxidation phase gives us the oxide layer profile. The curve in Fig. 12 puts the good agreement of the calculation with the experiment into relief: the calculated oxide layer profile at the end of the pre-oxidation phase is quite close to the experimental one.
Fig. 12. Oxide layer thickness at the end of the steam pre-oxidation.
Fig. 13. Temperature evolution of cladding (Tcl) and shroud (Tsh) at the bottom of the heated zone (350 mm elevation) during the air ingress phase.
5.3. Simulation of the air oxidation phase The air ingress phase of about 30 min long in QUENCH-10 examines the effect of air on cladding oxidation and degradation. In Figs. 13 and 14, the temperatures evolutions given by the different correlations available in MAAP4.07 code (cf. Table 1) are illustrated for the cladding and shroud for the bottom (350 mm high) and for the top (950 mm high) of the heated zone, which is the hot spot. First, these curves clearly show that the MOZART correlation is not well adapted for QUENCH-10 experiment, although this takes into account breakaway transition. It is something that was expected, as the MOZART correlation is based on experiments made to evaluate the consequences of spent fuel storage pool loss of water inventory accident, that is to say at lower temperatures than in severe accident conditions (Duriez et al., 2008, 2009). Besides, in the QUENCH-10 test, a protective oxide layer was formed during steam pre-oxidation that delays breakaway (Steinbrück, 2009), while the MOZART correlation was obtained using experiments made on samples without pre-oxidation (Coindreau et al., 2008). This may have a strong influence on breakaway transition, as it is the case in Fig. 13. Second, the AEKI correlation gives a better temperature evolution but not as good as that using NUREG correlation. But above all, the definition interval of the AEKI correlation is based on zirconia
Fig. 14. Temperature evolution of cladding (Tcl) and shroud (Tsh) at the top of the heated zone (950 mm elevation) during the air ingress phase.
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and no nitriding treatment in MAAP4.07 (use of parabolic laws, all the simulation long), no oxidation at the hot spot was done. The oxide layer of the experimental hot point was bigger, even if uncertainties exist on it (the rod, which was withdrawn, broke down; Schanz et al., 2006). Moreover, oxide layer thickness is calculated as a homogenous layer with MAAP4.07, which is not at all the case experimentally speaking. 6. Conclusions The separate-effect tests give a good comprehensive basis for the phenomenological understanding of cladding oxidation by air. This phenomenon is indeed well understood: Fig. 15. Oxygen flow rates at different bundle elevations during air ingress.
phase change (1333–1550 K) and not on in the whole temperature interval of QUENCH-10 experiment (Shepherd et al., 2000). The NUREG correlation reproduces well the thermal behavior of the lower part of the bundle but the one of the hot spots is underpredicted. Two reasons can explain this observation. The temperature is slightly underestimated by the code at the end of the pre-oxidation phase at 950 mm elevation. The transition from one regime to another can thus be delayed because the NUREG correlation is based on different temperature intervals. Moreover, the use of a parabolic law after breakaway cannot reproduce perfectly the rapid acceleration of the temperature escalation, which was observed experimentally. From this temperature evolution analysis, the NUREG correlation appears to be the best choice. This conclusion is not code-dependent: a comparison of SCDAP/RELAP5 and MELCOR calculations against QUENCH-10 test comes to similar conclusions to that work (Birchley and Haste, 2004). In Fig. 15, one can see the evolution of oxygen flow rates for different bundle elevations using the NUREG correlation and for the experiment at bundle entrance (‘data-in’) and at bundle exit (‘data-off’). What is important to note is that an O2 starvation (a complete O2 consumption) is first observed at the top of the heated zone, a bit earlier than the experiment, and then propagates to the bottom. This is confirmed in the experiment in which nitriding is observed between 800 mm and 1080 mm elevation which leads to an accelerated degradation, characteristic of the post-breakaway regime (Schanz et al., 2006). This phenomenon is not taken into account in MAAP4.07 code modeling, which is why the cladding oxidation acceleration is not simulated. Fig. 16 represents the oxide layer thickness profile at the end of air ingress, for the experiment and the modelling using the NUREG correlation. As there was no oxygen left at the top of the bundle
Fig. 16. Oxide layer thickness at the end of air ingress for the experiment and for the modelling with the NUREG correlation.
• a pre-breakaway regime, which leads to the formation of a dense oxide layer, with a parabolic weight gain, • a breakaway due to cracks inside the oxide layer (from stresses and zirconia phase change), • a post-breakaway regime, which leads to the formation of a porous oxide layer, with an accelerated weight gain. From this experimental work, weight gain correlations have been established, most of them are parabolic. Breakaway transition is based on zirconia phase change temperature or on a critical weight gain of the oxide layer. The numerical analysis done in this work underlines the need to capture the breakaway effect to model correctly the behavior of Zircaloy oxidation in air. This breakaway has to take into account different parameters such as temperature and oxide layer thickness. Moreover, QUENCH-10 test simulation with the MAAP4.07 code clearly supports the idea that parabolic correlations are not adapted for post-breakaway regime: the temperature acceleration that occurs after breakaway is not captured. Numerous data from separate-effect tests and semi-integral experiments are available and provide an important database for future model development, notably on nitriding treatment. The QUENCH series aim to study reflood behaviour and even if the MAAP4 code calculates this last phase, the analysis is part of other work that will maybe lead to next papers. References Birchley, J., Haste, T., October 2004. Post-test analysis of QUENCH-10 with SCDAP/RELAP5 and MELCOR. In: 10th QUENCH Workshop, Forschungszentrum Karlsruhe, Germany. Bramman, J.I., Sharpe, R.M., Thom, D., Yates, G., 1968. Metallic fission-product inclusions in irradiated oxide fuels. J. Nucl. Mater. 25, 201–215. Brunet-Thibault, E., December 2006. Etude du renoyage par le haut en cas d’accident grave et en particulier oxidation des mélanges (U, Zr, O). Ph.D. Thesis. Institut National Polytechnique de Grenoble. Cathcart, J.V., Pawel, R.E., McKee, R.A., Druschel, R.E., Yurek, G.J., Campbell, J.J., Jury, S.H., 1977. Zirconium Water Oxidation Kinetics. Report, ORNL/NUREG 17. Coindreau, O., et al., September 2008. Modelling of accelerated cladding degradation in air for severe accident codes. In: ERMSAR 2008 Conference, Nesseber, Bulgaria, Paper No. 2-4. Duriez, C., Dupont, T., Schmet, B., Enoch, F., 2008. Zircaloy-4 and M5® high temperature oxidation and nitriding in air. J. Nucl. Mater. 380 (October), 30–45. Duriez, C., Steinbrück, M., Ohai, D., Meleg, T., Birchley, J., Haste, T., 2009. Separateeffect tests on zirconium cladding degradation in air ingress situations. Nucl. Eng. Des. 239, 244–253. Godlewski, J., July 1990. Oxydation d’alliages de zirconium en vapeur d’eau: influence de la zircone tétragonale sur le mécanisme de croissance de l’oxyde. Ph.D. Thesis. Université de Technologie de Compiègne. Gosmain, L., Valot, C., Ciosmak, D., Sicardy, O., 2001. Study of stress effects in the oxidation of Zircaloy-4. Solid States Ionics 141–142, 633–640. Huntz, A.M., Pieraggi, B., April 2003. Oxydation des matériaux métalliques: comportement à haute temperature. Lavoisier, Paris. Iglesias, F.C., Lewis, B.J., Reid, P.J., Elder, P., 1999. Fission product release mechanisms during reactor accident conditions. J. Nucl. Mater. 270, 21–38. Lacour, V., November 2001. Modélisation de la production d’hydrogène lors de la phase de renoyage des cœurs de réacteurs nucléaires en situation d’accidents graves. Ph.D. Thesis. Ecole Nationale Supérieure des Mines de Paris.
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