Oxidation behaviour of plutonium rich (U, Pu)C and (U, Pu)O2

Journal of Nuclear Materials 479 (2016) 623e632

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Oxidation behaviour of plutonium rich (U, Pu)C and (U, Pu)O2 S.K. Sali a, *, N.K. Kulkarni a, Rohan Phatak a, Renu Agarwal b a b

Fuel Chemistry Division, India Product Development Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 March 2016 Received in revised form 27 July 2016 Accepted 28 July 2016 Available online 30 July 2016

Oxidation behaviour of (U0.3Pu0.7)C1.06 was investigated in air by heating samples up to 1073 K and 1273 K. Thermogravimetry (TG) of the samples and X-ray powder diffraction (XRD) of the intermediate products were used to understand the phenomenon taking place during this process. Theoretical calculations were carried out to understand the multiple phase changes taking place during oxidation of carbide. Theoretical results were validated by XRD analysis of the products obtained at different stages of oxidation. The final oxidized products were found to be a single FCC phase with O/M ¼ 2.15 (M ¼ U þ Pu). Oxidation kinetic studies of (U0.3Pu0.7)O2 and (U0.47Pu0.53)O2 were carried out in dry air, using thermogravimetry, under non-isothermal conditions. The activation energy of oxidation was found to be 49 and 70 kJ/mol, respectively. Lattice parameter dependence on Pu/M and O/M of plutonium rich mixed oxide (MOX) was established using combined results of XRD and TG analysis of (U0.3Pu0.7)O2þx and (U0.47Pu0.53)O2þx. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Solid solutions of mixed uranium-plutonium monocarbide and mononitride are considered as advanced fast breeder reactor fuels on the basis of their higher breeding ratio, shorter doubling time, better thermal conductivity and excellent compatibility with sodium coolant and stainless steel cladding compared to mixed oxide fuels. In India, plutonium rich mixed carbide, (U0.3Pu0.7)C (MARK-I) and (U0.45Pu0.55)C (MARK-II) have been used as fuel in the fast Breeder Test Reactor (FBTR), at Kalpakkam. The interest for plutonium rich mixed oxide (MOX) fuels is also increasing due to their relevance in IVth generation fast breeder reactors and they are also being considered as potential fuels for plutonium burning reactors [1]. The carbide fuel is highly pyrophoric in nature therefore, understanding the oxidation behaviour of this fuel is of great interest to assess fuel behaviour during fabrication and storage. Managing the oxidation of (U, Pu)C fuel is also one of the main task underlying the safety risk analysis for the backend, which needs to process the scrap and spent fuel (via the Purex process). Although the oxidation behaviour of UCz fuel (z ¼ 1.0, 1.5 and 2.0) has been widely studied [2e7], the data on the oxidation of (U, Pu)C fuel in oxygen

* Corresponding author. E-mail address: [email protected] (S.K. Sali). http://dx.doi.org/10.1016/j.jnucmat.2016.07.062 0022-3115/© 2016 Elsevier B.V. All rights reserved.

containing gases are limited [8e13]. Moreover, there is only one reported work on oxidation of plutonium rich carbide [9]. A gravimetric method for the determination of carbon in (U, Pu)C materials was developed by Kavanaugh et al. [14]. The reliable carbon analysis requires a quantitative ignition of the sample which is assured by a slow heating eoxidizing process. Stoichiometric uranium plutonium mixed oxides have been extensively investigated over wide range of plutonium compositions. An important characteristic of these mixed oxides is the extensive defect structure, which readily occurs within the crystal lattice. The information on hypo- and hyper- stoichiometric domain of U-Pu-O phase diagram is complex and still incomplete. Uranium plutonium mixed oxide (MOX) is stable over a wide range of hyperand hypo-stoichiometric compositions, where the structure is dominated by a number of interstitial oxide ions and their vacancies, respectively. The oxygen to metal ratio (O/M; M ¼ U þ Pu) of (U,Pu)O2±x significantly affects various physical properties such as lattice parameter, thermal conductivity, melting points, diffusion coefficient, vapor pressure, migration of fission products etc. The knowledge of O/M of the mixed oxide fuels, therefore, is very essential. Among the various O/M measurement techniques, thermogravimetry method developed by McNeilly and Chikalla [15] has been commonly used on routine basis. Verma and Roy [16] had suggested that lattice parameter measurement can be conveniently used to measure O/M in single and two- phase hypostoichiometric

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mixed oxides of UO2ePuO2ePu2O3. Markin and Street [17] indicated that lattice parameter of a mixed oxide of given plutonium concentration varies linearly with its oxygen content. The U-Pu-O system exhibits different multiphase equilibria depending on uranium, plutonium and oxygen (O/M) content. The hypo-stoichiometric MOX with high Pu content is known to be biphasic at room temperature [18]. For Pu  35% the co-existing phases of (U, Pu)O2x can be either two FCC phases or a FCC coexisting with cubic or rhombohedral phase, depending on Pu/ (U¼Pu), O/M and temperature of reduction. When two FCC phases coexist, one of the phases is nearly stoichiometric while the other phase is hypostoichiometric with higher lattice parameter [17,19,20]. Among (U, Pu)O2þx phases, the maximum oxygen containing phase is M3O8. The solubility of PuO2 in a-U3O8 phase is ~ 7e9 mol% [21]. For 0  Pu/M  50% a mixture of M3O8 þ MO2þx are obtained and the concentration of M3O8 phase increases with decrease in Pu content. However, for Pu/M  50%, MO2þx (M ¼ U þ Pu) is reported to be only single FCC phase [17]. The air oxidation behaviour of UO2 and (U, Pu)O2 has been paid attention by nuclear chemists for several decades and received considerable interest in recent years from the reprocessing as well as dry storage point of view of spent oxide fuel [22,23]. The present oxidation studies were carried out on both mixed carbide and stoichiometric mixed oxides. The oxidation behaviour of plutonium rich monocarbide (U0.3Pu0.7)C in air were investigated and the products formed at different temperatures were identified by X-ray powder diffraction technique. The oxidation studies of mixed stoichiometric oxides involved heating (U0.3Pu0.7)O2.00 and (U0.47Pu0.53)O2.00 in different oxygen potential atmospheres to get the different O/M values of the final oxidized product. The lattice parameter of single phase fluorite type solid solutions (U0.3Pu0.7) O2þx and (U0.47Pu0.53)O2þx were correlated with the O/M ratio and Pu/M using linear equation. Oxidation kinetics of these plutonium rich MOX was also investigated using thermogravimetry under non-isothermal conditions.

X-ray diffractometer (Diano Corporation, Woburn, MA) with graphite monochromatized Cu Ka (l ¼ 0.154184 nm) radiation. For identification of phases, the samples were scanned over 2q range of 10e100 at a rate of 1 /min. The uncertainty on the lattice parameter was estimated to be less than ±0.0001 nm. 2.2. Reduction and oxidation of oxides, (U0.3Pu0.7)O2 and (U0.47Pu0.53)O2 2.2.1. O/M determination To determine O/M ratio of products formed by oxidation of carbide fuel pellets (Pu/M ¼ 0.7) or oxidation of stoichiometric MOX with Pu/M ¼ 0.53 and 0.7, a method of back calculation was used. This method involves reduction of the samples in moist Ar/8%H2 atmosphere at 1073 K, resulting in an O/M of 2.000. Considering an uncertainty of ±0.01 mg in the measurement of 1 gm of sample, the uncertainty in O/M was estimated to be ±0.005. The moist Ar/8%H2 was obtained by passing dry Ar/8%H2 gas through a water trap maintained at 298 K, giving a pH2O/pH2 z 0.4. This cover gas when passed over the sample held at 1073 K, created an oxygen potential of ~ 400 kJ/mol. As the oxygen potential change in stoichiometric range is very steep, therefore, sample attained stoichiometric composition irrespective of small variation in pH2O/pH2 [24]. Recently, Dawar et al. [25] have reported oxygen potential of (U0.54Pu0.46)O2x for different values of ‘x’ at 1073 K. They have given oxygen potential of the samples with O/M ¼ 1.999 as 458.37 kJ/mol, whereas it varied from 365.90 to 388.28 kJ/ mol for O/M ¼ 2.0. The reduction of weight from MO2þx to MO2.00 (for M ¼ U0.3Pu0.7 or U0.47Pu0.53) was used for calculating O/M of the oxidized product using following expression:

O=M ¼ 2 þ

w1  w2 W  16 w2

(1)

where, w1 ¼ initial weight, w2 ¼ weight after equilibration and W is molecular weight of stoichiometric composition.

2. Experimental 2.1. Oxidation of carbide, (U0.3Pu0.7)C1þz As-fabricated carbide fuel pellets of (U0.3Pu0.7)C1þz were obtained from Radio Metallurgy Division, BARC and the details of oxidation experiments are described here. These pellets were taken from their production stream of MARK-I fuel, fabricated for FBTR reactor, Kalpakkam, India. These were sintered pellets with a density ~95% TD, oxygen and nitrogen impurities ~6000 wtppm, total metallic impurities 3000 wtppm and C/(U þ Pu)z1.06. The carbide pellets of 6e8 mm thickness, 4.18e4.23 mm diameter and 1000e1150 mg weight were used for the present experiments. Thermograms of oxidation of these pellets were recorded using Mettler thermoanalyzer (model: TA-1, Mettler Toledo, Switzerland) in dry air at a flow rate of 100 ml/min, in Al2O3 crucible. The thermoanalyzer has a balance precision of ±0.01 mg and a programmable temperature control giving the temperature within ±2 K of the set value. The samples were heated from room temperature to 1073 K and 1273 K, at a heating rate of 4 K/min. Separate sets of TG and XRD experiments were carried out for the analysis of reaction products formed at intermediate stages of oxidation of mixed carbide fuel pellets. For this purpose, the pellets were heated to different temperatures (698 K, 1073 K and 1273 K), while retaining other experimental conditions similar to TG experiments described above. After the experiment was over, the samples were furnace-cooled to ambient temperature. The products formed after each heat treatment were characterized by X-ray powder diffraction using Bragg- Brentano q2q geometry in Diano

2.2.2. XRD analysis of different O/M samples XRD analysis was used to correlate lattice parameters with O/M of the oxidized MOX samples. For this purpose, another set of TG experiments were carried out in which stoichiometric (U0.3Pu0.7) O2.00 and (U0.47Pu0.53)O2.00 samples were heated up to 1473 K, at the heating rate of 10 K/min and held at that temperature for 1 h, in (i) commercial argon (oxygen impurity 10 ppm) having oxygen potential ~ 140 kJ/mol, at 1473 K, (ii) CO2 with unidentified oxygen potential and (iii) dry air having oxygen potential ~ 20 kJ/mol, at 1473 K. After keeping the samples in these conditions for 1 h, there was no weight change in the sample, indicating that the sample had achieved complete equilibrium with the respective cover gas. The samples were then cooled to room temperature by switching off the furnace, in respective environments. The O/M of the final product was determined from reduction experiment mentioned above. The (U0.47Pu0.53)O2.00 samples used in the present studies were obtained by crushing and powdering sintered microspheres obtained from sol-gel route. These microspheres of average size ~750m were sintered in Ar/8%H2 atmosphere, at 1823 K, for 4 h and had density ~98%TD. The (U0.3Pu0.7)O2.00 samples used for XRD analysis were in powder form and were obtained from oxidation of carbide pellets in our oxidation experiments. 2.2.3. Kinetics of oxidation of stoichiometric MOX The kinetics of oxidation of (U0.3Pu0.7)O2.00 and (U0.47Pu0.53) O2.00 powder samples were studied in dry air, using non-isothermal method. For this purpose thermogravimetry measurements during oxidation processes of mixed-oxides were carried out in duplicate

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using Mettler thermoanalyzer, at a flow rate of 100 ml/min, in Al2O3 crucible, at a heating rate of 10 K/min. After oxidation of the samples, the products were characterized by X-ray powder diffraction. 3. Computation method To elucidate the oxidation mechanism of mixed carbides, theoretical calculations were carried to mimic weight gain by MARK-I fuel by successively adding oxygen at constant temperature. Gueneau et al. [26] have reported optimized thermodynamic interaction parameters for U-Pu-O-C system. The authors have tabulated these thermodynamic parameters in Appendix-A of their publication. The present authors obtained the Gibbs energy file of this thermodynamically assessed system from Dr. Gueneau through private communication. The optimized interaction parameters of this system were used to calculate stable phase fields, their compositions and oxygen potential for varying oxygen content of the fuel, at different temperatures (700 Ke1200 K). All these calculations were carried out using Thermo-Calc software [27]. A starting composition of the mixed carbide for these calculations was taken as (U0.3Pu0.7)C1.06, and then ‘O’ was added slowly in the system under isothermal conditions. The weight change of the sample was calculated by adding weight of ‘O’ and subtracting weights of ‘C’ and ‘O’ content going in the gaseous form. The ‘Pu’ and ‘U’ contents of gaseous species were too low to be considered for weight change calculations. 4. Results and discussion 4.1. Oxidation of carbide: (U0.3Pu0.7)C1þz The oxidation studies of mixed carbides was carried out on (U0.3Pu0.7)C1þz pellets up to 1073 K and 1273 K. These hyperstoichiometric carbides were a mixture of monocarbide (MC) and sesquicarbide (M2C3) phases. Thermogram of oxidation of mixed carbide, recorded in dry air, at a heating rate of 4 K/min, up to 1073 K (exp.1) and 1273 K (exp.2) are shown in Fig. 1. As seen from this figure, that the oxidation of carbide sample started at ~573 K and completed at ~873 K, followed by slow mass loss in the temperature range ~873 Ke1273 K, due to removal of residual carbon. A comparison of exp.(1) and exp.(2) (Fig. 1) shows that the carbon loss is more when the sample was heated up to 1273 K. But, heating to higher temperature at the same heating rate, also meant heating

Fig. 1. Thermogram of (U0.3Pu0.7)C1þz in dry air (a) heated up to 1073 K (exp.1) (b) heated up to 1273 K (exp.2).

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for longer period. So the net weight difference is a combined effect of both higher temperature and longer period of oxidation. When the oxidation was carried out up to 1073 K and the sample was held at 1073 K, under isothermal condition for 20 min, the continuous weight loss was observed till the end, indicating that the removal of free-carbon at 1073 K was not complete. Whereas, when the sample was heated up to 1273 K and held at 1273 K, in isothermal condition, for 20 min, the weight change was negligible. This indicated that complete oxidation of free-carbon was attained when heated at 1273 K. The maximum O/M of the product that was heated to 1273 K was found to be 2.15. This value agrees with similar experiments carried out by Iyer et al., at lower temperature, but longer duration of heating [9]. In the present oxidation studies of carbide pellets, a higher weight gain by the sample heated up to 1073 K (9.1%) than that of sample heated up to 1273 K (8.62%) was used to calculate amount of free-carbon left in the former sample. It was found that ~3500 wtppm of free carbon was left in the oxidized samples heated up to 1073 K, which is ~6% of the initial carbon present in carbide sample. Presence of free carbon left in expt. 1 was also confirmed during reduction experiments with moist Ar/8%H2. It was observed that in both cases the same product, (U0.3Pu0.7)O2.00, was formed after reduction of oxidized carbide pellet in moist Ar/8%H2. However, higher weight loss in samples heated at 1073 K than in 1273 K indicated a loss of CH4(g)þH2O(g) in former as compared to only H2O(g) loss in later. An analysis of the present results with those of Iyer et al. [9] clearly indicate that oxidation of carbide for getting rid of free carbon depends on three factors, (i) temperature (ii) time and (iii) oxygen partial pressure. Heating to higher temperatures and increase in oxygen partial pressure in cover gas, results in completion of oxidation in lesser time. Iyer et al. found that complete removal of carbon from carbide samples could be achieved in 600 min, at 673 K and partial pressure of oxygen of 25.3 kPa. This partial pressure of oxygen is very close to that of air (21.3 kPa) used in the present experiments. However, in the present experiments the samples were heated to 1273 K and the carbon removal was complete in 20 min. When oxygen partial pressure was increased to 60.8 kPa, Iyer et al. observed considerable reduction in oxidation time, 180 min (673 K) and 140 min (723 K). To further investigate the effect of temperature on removal of carbon from carbide samples during oxidation process, some experiments were carried out by us on synthetic mixtures. These synthetic mixtures were prepared by mixing 1.07 and 0.44 wt% free carbon with PuO2 powder. These synthetic powders were heated in dry air in TG-DTA instrument, up to 1190 K, at a heating rate of 10 K/ min. It was observed that the weight loss due to carbon removal from these mixtures started at ~873 K and was complete at 1190 K, with a quantitative weight loss of added ‘C’ (Fig. 2). Similar oxidation experiments were carried out on pure graphite powder. These experiments showed that oxidation of carbon in synthetic mixture started at lower temperature than that of pure graphite powder and heating to 1200 K ensured complete removal of carbon from samples with 0.44 as well 1.07 wt% carbon. Thus under fast heating and non-isothermal conditions, heating the carbide samples to 1200 K can be expected to remove all free carbon in a short period. To study the intermediate products formed during oxidation of carbide, additional experiment was carried out. In this experiment, the pellet was heated under similar experimental conditions, but the sample was removed at an intermediate temperature and analyzed by XRD. During TG experiments (Fig. 1), it was observed that the sample showed increased weight gain kinetics at ~700 K, therefore, the intermediate temperature was set as 698 K Fig. 3, shows the room temperature XRD patterns of (a) starting (U0.3Pu0.7) C1.06 containing MC and M2C3 phases (Fig. 3a), (b) sample heated up

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Fig. 2. Fractional weight loss (a ¼ Dw/wC, Dw is observed weight change and wC is weight of added carbon) of pure graphite and synthetic mixture of PuO2 with 0.44 wt% and 1.07 wt% graphite, when heated in dry air.

Fig. 3. Room temperature XRD patterns of carbide samples after different heat treatments (a) Starting sample: MC þ M2C3 (b) after oxidation in air, up to 698 K (c) after oxidation in air up to 1273 K and (d) after reduction of oxidized sample in moist Ar/8% H2 atmosphere, at 1073 K.

to 698 K, at a heating rate of 4 K/min and cooled in the furnace, in same atmosphere (Fig. 3b), (c) sample completely oxidized after heating in air, up to 1273 K, at 4 K/min, maintained in isothermal condition at 1273 K for 20 min (Fig. 3c), and (d) (U0.3Pu0.7)O2.00 formed by reduction of oxidized sample in moist Ar/8%H2 atmosphere (Fig. 3d). The XRD results of the sample heated to intermediate temperature, 698 K, showed the presence of MO2þMC þ M2C3 phases (Fig. 3b). As compared to XRD pattern of original sample (Fig. 3a), peak corresponding to M2C3 in the sample heated to 698 K was considerably reduced and a new peak for MO2 corresponding to hkl of 111 was appeared. The (111) reflection of MO2 in this pattern is clear, but broad, indicating that the formation of MO2 phase has just initiated. Peaks corresponding to other reflections of MO2 were also present in this pattern (Fig. 3b), but they were of low intensity and broad, therefore, not visible at first glance. The presence or absence of free carbon could not be confirmed in XRD analysis due to its

amorphous nature. As the sample was cooled within the furnace and the furnace took ~10 min to cool below 623 K and, therefore, the oxidation of sample continued even after the furnace was switched off. The final oxidized product formed after heating at 1273 K was identified as face centered cubic, (U0.3Pu0.7)O2.15 phase, with a lattice parameter of 0.5406 nm (Fig. 3c). The lattice parameter of reduced phase (U0.3Pu0.7)O2.00 was found to be 5.415 Å (Fig. 3d), which agree with literature lattice parameter for MOX with this Pu/M and O/M [28]. A comparison of oxidation behaviour of (U0.3Pu0.7)C1þz studied in the present investigation was made with that of Iyer et al. [9]. The oxidation studies of Iyer et al. were limited to lower temperatures, 550e745 K. They have also observed the formation of (U0.3Pu0.7) O2.15 phase on oxidation, and their oxidized product had freecarbon (>1000 ppm) even after long hours of oxidation. They postulated that increasing the temperature may remove all of extra carbon but at the same time may results in the formation of M3O8. In the present experiments, when the samples were heated up to 1273 K, first assumption made by Iyer et al. for the removal of carbon by heating samples in air at high temperature was validated. However, the XRD pattern of the sample did not show peaks corresponding to M3O8 phase, confirming that the only oxidized product for high Pu/M is single (U0.3Pu0.7)O2.15 phase. A close investigation of XRD data reported in literature indicates that completely oxidized product of uranium rich MOX will be FCC þ Orthorhombic-M3O8 and plutonium rich MOX will be single FCC phase, and all the uranium in later case will be in an average oxidation state of þ5. Kumar et al. [29] have reported the presence of single FCC hyper-stoichiometric dioxide phase, when they carried out XRD analysis of (U0.47Pu0.53)O2þx sample, after heating it in air at 1073 K. Brett and Fox [21] have carried out extensive oxidation studies of uranium plutonium oxides (Pu/M 0.01 to 0.789) and analyzed the oxidized product using XRD. They have reported the presence of orthorhombic M3O8 phase along with cubic MO2þx phase in oxidized products of MOX containing Pu/M up to 0.399. However, their samples containing Pu/M ¼ 0.588 and 0.789, did not showed the presence of M3O8 phase. The results of Brett and Fox [21] have shown that the lattice parameter of (U, Pu)O2þx having Pu> 50% fall on a tie-line between U2O5 and PuO2. A similar, O/M (2.15) is reported by Anthonysamy et al. [30] during air oxidation of urania-thoria solid solution with 30 at.% urania where, like plutonium, all the thorium is in þ4 oxidation state and all the uranium is in average oxidation state of þ5 [31]. Benedict [32] carried out extensive investigation of solubility of plutonium in orthorhombic M3O8 phase. He established a relationship between b/a (lattice parameters) ratio with O/M and Pu/M of this phase. Based on these relations, he concluded that Pu/M of this phase did not exceed 6 at.% Pu at 1273 K. Limited solubility of plutonium in M3O8 phase may be the reason for the absence of this phase in oxidized product of plutonium rich MOX. When all the uranium is in an average valency state of þ5 the corresponding molecular formula becomes (UyPu1y)O2þy/2. 4.2. Computational phase changes with increase in oxygen content Based on thermodynamic description of U-Pu-C-O system given by Gueneau et al. [26] phase diagram of (U0.3Pu0.7)-C-O at 1000 K was calculated in the present investigation, using Thermo-Calc software (Fig. 4). According to this phase diagram, the sample passed through the following phase changes with successive addition of oxygen: (i) MCO þ M2C3 (starting mixture) (ii) MCO þ MCO0 þ M2C3 (iii) MCO þ M2C3 þC (iv) MCO þ M2C3 þMO2x þ C (v) MCO þ MO2x þ C (vi) MO2x þ C (vii) MO2þC þ Gas (viii) MO2þx þ gas. These computed phase changes can be interpreted based on the

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(iii)

(iv)

(v) Fig. 4. Calculated phase diagram of (U0.3Pu0.7)-C-O system at 1000 K. The present oxidation calculations were carried out along the dash line (——). Oxygen potential along the composition of the present oxidation calculation, at 1000 K ( ). (M ¼ (U,Pu) and its composition changes in the phase diagram).

previously reported experimental observations. (i) Initial oxidation of mixed carbide causes substitution of ‘C’ by ‘O’ in monocarbide phase, supported by experimental investigations of Potter [33], Anselin et al. [34], Mulford et al. [35] and Brett et al. [36]. This substitution results in increase of carbon potential of the system [37], which in-turn increases phase fraction of M2C3. In the starting sample, (U0.3Pu0.7)C1.06, sesquicarbide had ~ 90e98 at.% Pu2C3, whereas, monocarbide had ~ 66 at.% PuC. The present calculations indicate that increase in oxygen content of carbide system results in successive decrease of Pu/M in sesquicarbide phase. At 1000 K, first carbon precipitation results in MCO þ M2C3þC phase field, and M2C3 phase had 0.42 Pu/M. Further oxidation of carbide gives MCO þ M2C3þC þ MO2 phase field, in which Pu/M in M2C3 is 0.24. According to the present calculations, addition of oxygen in monocarbide phase resulted in increase in plutonium content of monocarbide phase, which was established experimentally by Potter and Roberts [38]. Here, it is important to point out that addition of oxygen and nitrogen in MC1þz has very similar effects, as both get stabilized in monocarbide lattice by substituting carbon and the carbon from monocarbide phase goes in sesquicarbide phase. Hence, addition of ‘O’ or ‘N’ in MC1þz results in increase in carbon potential and increase in content of sesquicarbide phase. Therefore, not only oxygen from dry air will react with carbide fuel, but nitrogen which has higher partial pressure than oxygen in dry air, will also contribute to initial carbide-air interaction. (ii) Theoretically found segregation of monocarbide phase into (a) MCO0 - the plutonium rich monocarbide phase (~99.9% Pu) with most of the oxygen and (b) MCO e the monocarbide phase with negligible oxygen, is not reported in literature. However, absence of this observation in literature may be due to much higher temperatures of experiments reported in literature and very limited research with plutonium rich carbides. It may also be due to limitation of interaction parameters used in the present calculations [26] for high plutonium carbides and low temperatures, as these parameters were calculated based on experimental data reported in literature, which are mostly for uranium rich mixed carbides

(vi)

(vii)

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and at much higher temperatures than the temperatures used in the present investigations. Further addition of oxygen resulted in precipitation of free carbon and increase in MO content in monocarbide phase, depending upon temperature. This observation is supported by many experimental and theoretical investigations reported in literature [33,39,40]. After precipitation of free carbon, further oxidation did not result in formation of CO/CO2. Instead plutonium rich MO2x (x z 0.35) phase was formed at the expense of MCO and M2C3 phases, resulting in C þ MCO þ MO2x þ M2C3 phase field. The solubility of MO in MCO in this phase field was found to decrease with increase in temperature, 47 and 41 mol.% at 700 K and 1200 K, respectively. During their experimental investigations, Jain and Ganguly reported 25 mol.% solubility of MO in MCO, at 1923 K [39]. Adding more oxygen in the system resulted in disappearance of M2C3 phase. As per the present calculations, a small increase in solubility of oxygen in MCO phase was observed after disappearance of M2C3. Jain and Ganguly [39] also reported an increase in solubility of MO in MCO after disappearance of M2C3 from 25 mol.% (MCO þ M2C3þMO2x) to 47e49 mol.% (MCO þ MO2x), at 1923 K. The increase in oxygen solubility in MCO in the present calculations was not as drastic as reported by Jain and Ganguly. Increase in oxygen potential of the system resulted in disappearance of MCO phase. After MCO and M2C3 phases were completely oxidized, further oxidation resulted in decrease of ‘x’ in MO2x phase. The carbon precipitated gets oxidized to form CO(g), only when O/M z 1.99 of MO2x, indicating that the oxygen potential of (U0.3Pu0.7)O1.99 is same as that of CO(g) at these temperatures. This also shows that at these temperatures, Gibbs energy of reaction of M2C3/MCO(s)þ O2(g) / MO2x(s)þC(s) is lower than that of C þ O2(g)/ CO(g). Therefore, first MCO and M2C3 phases get oxidized to form MO2x þ C, then MO2x gets oxidized to MO1.99 and then the oxidation of free carbon starts. Till formation of MO2þC, mass of the system continuously increased due to trapping of oxygen from the gaseous phase. After successive disappearance of M2C3 and MCO, and after attaining near stoichiometric dioxide phase, weight loss started due to removal of free carbon from the system as CO(g). Simultaneously, oxidation of MO2 resulted in formation of MO2þx, giving (U0.3Pu0.7)O2.15 as final product, which will contribute to weight gain. Therefore, net weight change will have a factor of decrease in weight due to CO(g) formation and increase in weight due to oxidation of MO2. With 70 at.% plutonium in mixed oxide, uranium does not form U3O8 or UO3 as it get stabilized in FCC lattice of MO2.15.

4.3. Comparison of calculated results with experimental data The calculated results of oxidation of mixed carbide were compared with available data in literature and our own experimental data to understand the phenomenon. The kinetics of oxidation of mixed carbides is a very complex process due to involvement of multiple phases, dependence of oxidation process on sample composition, morphology, heating process (isothermal or non-isothermal) and environmental conditions. There is very limited information on the phase transitions during oxidation process, as this requires phase and composition analysis at different stages of oxidation, which is not simple, especially for plutonium bearing species. Therefore, in the present research computational

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thermodynamics was used as tool to understand phase changes at different stages of progressive oxidation. The weight changes from the present calculations were compared with experimental weight changes and some of the present XRD analysis results were used as further supporting evidence. These comparisons were made with basic assumption of micro-equilibrium in the small samples being investigated. The effects of changing morphology of the sample on reaction kinetics were not considered during calculations. 4.3.1. Modification of weight change obtained from equilibrium calculations The thermodynamic calculations were carried out with the assumption of equilibrium at each stage. Thermodynamically free carbon should not oxidize till MOX attains a near stoichiometric composition, i.e. phase field MO2.00þC. But kinetic factors allow oxidation of free carbon from the moment it was formed. This is apparent from the difference in the maximum weight gain observed experimentally (9.18 wt%) from the equilibrium calculations (12.7 wt%). To match calculated weight change with experimentally observed value, it was assumed that 30% of the free carbon got oxidized as CO (g), till attaining MO2.00þC phase field. The computed and experimental weight changes of the sample were compared at different O/M values (Fig. 5). The corrected Dw% change obtained from calculated values is given in the figure as (L1). 4.3.2. Calculation of O/M from experimental weight change The change in O/M of the system is not an experimental observation in TG experiments, but it can be calculated from observed weight change. This calculation can be done by assuming that (a) the experimentally observed weight change is only due to addition of oxygen in the solid matrix (b) the observed weight change is a combined effect of addition of oxygen and removal of carbon from the solid matrix. The first assumption is a simplistic approach and the O/M values calculated by this method are given in Fig. 5 as (L2). As discussed in the previous section, the second method is more appropriate to calculate O/M of the system. To estimate the fraction of carbon that got oxidized, an assumption was made that the stoichiometric oxide is formed (MO2.00þC phase field) when weight gain is maximum. The justification for this assumption was that the maximum intake of oxygen takes place during oxidation of carbide and hypostoichiometric oxide. Based on

Fig. 5. A comparison of calculated weight change vs. O/M at different temperatures with experimental weight change and corresponding calculated oxygen potential.

this assumption, the maximum weight gain of 9.18 wt% was set at O/M ¼ 2.00. This meant a loss of 70% of total carbon by the time the system attained maximum weight gain. Using this assumption, the more realistic O/M was calculated from the experimentally observed weight change, given as (L3) in Fig. 5. 4.3.3. XRD results to understand oxidation process The present thermogravimetry results were related with different phase fields, as given in Fig. 5 and the presence of these phase fields were confirmed experimentally by XRD analysis. The temperatures given in Fig. 5 are indicative of the dynamic temperatures of the system during TG experiments. It is apparent that even at a heating rate of 4 K/min, formation of MO2x started at temperatures as low as 710 K and sesquicarbide phase disappeared at ~800 K. A slower heating rate may result in shift of these temperatures to lower values. The same was confirmed by the present XRD analysis (Fig. 3 (b)), which showed a reduced signal of M2C3 in a sample that was cooled after heating to 698 K, at a heating rate of 4 K/min. As the sample was cooled in air, in the furnace itself, so the process of oxidation continued a little longer. XRD pattern showed the presence of MO2, MC and M2C3 phases (Fig. 3b). This conforms to the calculated results showing the presence of MO2x þ MC þ M2C3þC phase-field. As carbon is amorphous, therefore, its presence cannot be confirmed by XRD. The sample was cooled to ambient temperature in air, therefore, hypostoichiometry of dioxide phase was lost during XRD analysis. The replacement of ‘C’ by ‘O’ in MCO, resulting in shift of peaks to lower ‘2q’ values was also confirmed by the XRD results. 4.3.4. Relation of oxidation kinetics with diffusion parameters The literature data of oxygen diffusion in different phases involved during the present oxidation process were used as supporting information to relate thermodynamically calculated phase fields with experimentally observed weight changes. According to Mazaudier et al. [8], temperature does not have a significant affect on kinetic process during air oxidation of mixed carbide samples. Therefore, sudden differences in rate of weight change of the sample observed during TG experiments (Fig. 1) can be only partially assigned to increase in kinetic rates with increase in temperature. The main reason for this change in oxidation rate is attributed to significant difference in oxidation behaviour of different phases appearing at different stages of oxidation. These oxidation reactions are mainly controlled by diffusion of oxygen in solid matrix and adhesiveness/peeling of new phases formed during the process. Kinetics of oxidation is much faster for the MO2x phase and slower for the MCO/M2C3 phases [5]. This can be explained on the basis of higher activation energies of diffusion of non-metallic elements (C/N) in MC and M2C3 phases, ~350e420 kJ/ mol [41]. Similar activation energy values can be assumed for diffusion of similar size atomic oxygen in MC/M2C3. The activation energy of diffusion of oxygen in MO2±x varies in the range 40e90 kJ/ mol [42e44]. The slower diffusion rate coupled with lower temperatures resulted in an initial region of almost constant weight (up to ~600 K), as shown in Fig. 1. This is the stage when oxidation of MCO þ M2C3 resulted in formation of MCO þ M2C3þC (Fig. 5). At next stage of oxidation, on appearance of MO2x phase (Dw % > 1.7%), steeper weight change took place due to much faster diffusion of oxygen in MO2x phase (Figs. 1 and 5). Sari [45] has compared oxygen diffusion coefficients in hypo- and hyperstoichiometric MOX and concluded that the oxygen diffusion rate in MO2x is higher than that in MO2þx. A steep change in oxidation and a corresponding disturbance in the temperature signal observed in the present experiments at ~800 K, may be due to ignition of the sample (Fig. 1). Apparently by this time the MO2 layer formed on MCO grains had attained critical thickness and

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ruptured suddenly, resulting in ignition. The oxidation of most of the free carbon was complete within ~5 min (DT z 20 K), after disappearance of MCO/M2C3. The remaining 30% of the carbon got oxidized slowly, after keeping the sample in air at high temperature. This delayed loss of carbon was assigned to imbedding of some of the free carbon in the lattice of MO2þx. The calculated oxygen potentials for different temperatures are also plotted in Fig. 5, as a function of O/M. It is apparent that oxygen potential is negligibly low in presence of MC and M2C3 phases and increases sharply when all the carbide phases disappear. The oxygen potential does not change much with temperature, but can change significantly with change in phase field. For this four component system, according to phase rule, the oxygen potential remains constant at constant temperature and pressure, in the four phase region, MCO þ M2C3þMO2x þ C. 4.4. Lattice parameter correlation with O/M and Pu/M UO2 and PuO2 both have face centered cubic structure of CaF2 type and form complete range of solid solution (UyPu1y)O2. The following relation was calculated to relate lattice parameter (a) of (U, Pu)O2.00 with Pu/M using experimental lattice parameters for different Pu/M values given by Mufford and Ellinger [35]:

aðnmÞ ¼ 0:54704  0:0078ð1  yÞ for 0  y 
 1 in Uy Pu1y O2:00

Fig. 6. Dependence of lattice parameter on O/M for single phase mixed oxide (U0.3Pu0.7)O2þx and (U0.47Pu0.53)O2þx, the experimental conditions to achieve required O/M are given in Table 1.

of the atmosphere increases the lattice parameter values of the solid solution (U0.3Pu0.7)O2þx and (U0.47Pu0.53)O2þx decreases and follows a linear relation. The two linear equations given in Fig. 6 were obtained by separately fitting lattice parameters of (U0.3Pu0.7)O2þx and (U0.47Pu0.53)O2þx at different O/M values. The uncertainties given for the coefficients of these two equations are fit uncertainties of the experiment data. As seen from Fig. 6, two approximately parallel straight lines were obtained for y ¼ 0.30 and 0.47, with an average slope of 0.007. The slope value, -0.007 was used as coefficient of 0 x0 to modify Equation (1). Thus the equation was rewritten to relate lattice parameter of plutonium rich (UyPu1y)O2þx with O/M (2 þ x) and Pu/M (1y), as given below:

(2)

Where, lattice parameter for UO2.00 (0.54704 nm) was taken from Leinders et al. [46]. It was observed that oxidation of samples in argon and CO2 atmosphere was slower than in air, which was associated with lower oxygen potential of the former two atmospheres, this is also supported by experiments of Iyer et al. [9]. The lattice parameter of FCC solid solution with corresponding values of O/M, O/U obtained in different atmospheres for (U0.3Pu0.7)O2þx and (U0.47Pu0.53)O2þx are given in Table 1. (U, Pu)O2þx shows reduction in lattice parameter with increase in plutonium content or increase in O/M. Former is due to smaller ionic size of plutonium and later is due to increase in oxidation state of uranium, resulted in decreasing the ionic size of uranium ion. Lattice parameter vs O/M relationship of MOX with Pu/M ¼ 0.53 and 0.70 is reported for the first time in the present investigation. The lattice parameters for different Pu/M oxides were plotted against deviation from stoichiometry 0 x0 (O/M ¼ 2 þ x; x > 0) and are shown in Fig. 6. As mentioned earlier, the uncertainty limits in measurement of lattice parameters and O/M analysis were ±0.0001 nm and ±0.005, respectively. The lattice parameter of the specimen containing the same plutonium contents are represented by a straight line. It can be seen from Fig. 6 that as oxygen potential

aðnmÞ ¼ 0:54704  0:0077ð± 0:0025Þð1  yÞ  0:007 ð±0:001Þx for x > 0 and y < 0:5

(3)

This relation relates (i) lattice parameter, (ii) O/M and (iii) Pu/M, of plutonium rich MOX. The first coefficient of this equation, 0.54704 nm, is the lattice parameter of UO2.00 [46]. The second coefficient of this equation was calculated by fitting lattice parameters of (UyPu1y)O2.00 vs (1-y), taken from literature [35]. The fitting was carried out by fixing lattice parameter of UO2.00 at 0.54704 nm. The error given on this coefficient is the fit error of this plot. As mentioned earlier, the third coefficient of Eqn. (3) is an average of the slopes of lattice parameter vs ‘x’ fits for (U0.3Pu0.7)

Table 1 Observed and calculated (eqn. no. (3)) lattice parameters of plutonium rich (UyPu1y)O2þx obtained on heating to 1473 K under different oxygen potential for different Pu/M and O/M values. Pu/M

Cover gas

m (O2) kJ/mol

O/M (2 þ x)

O/U

Lattice parameter (nm) Observed

Calculated

0.7 0.7 0.7 0.7 0.53 0.53 0.53 0.53 0.588 0.789

Moist Ar/8%H2 Ar (10 ppm O2) CO2 Air Moist Ar/8%H2 Ar (10 ppm O2) CO2 Air Air Air

400 96 e 19 400 96 e 19 13 13

2.00 2.04 2.068 2.15 2.00 2.08 2.157 2.20 2.218 2.107

2.00 2.13 2.23 2.50 2.00 2.17 2.33 2.43 2.529 2.507

0.5415(1) 0.5411(1) 0.5410(1) 0.5406(1) 0.5433(1) 0.5425(1) 0.5421(1) 0.5414(1) 0.5413(2)a 0.5401(2)a

0.5416 0.5413 0.5411 0.5405 0.5429 0.5423 0.5418 0.5415 0.5409 0.5401

a

Experimental data taken from Brett and Fox carried out by heating at 1023 K [21].

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O2þx and (U0.47Pu0.53)O2þx, given in Fig. 6. The error of this coefficient was calculated from the fit errors of the two slopes. This equation is useful interrelationship between lattice parameter, Pu/ M and O/M. If any of the two parameters are known, third parameter can be calculated using the above relation. A simultaneous fit of lattice parameter with varying Pu/M and O/M was carried out to get a relation: a(nm) ¼ 0.54704e0.0077 (1y) e 0.0068 x. It can be seen that, the coefficient of Pu/M of this equation and Equation (3) are similar. The second coefficient corresponding to O/M is slightly lower in simultaneous fit equation than the average slope obtained directly from Fig. 6. However, the difference between the two coefficients is within the calculation error limit. We would like to recommend Equation (3) for calculating lattice parameter of Pu rich hyper-stoichiometric oxide as the coefficient of Pu/M in this equation is taken from Mufford and Ellinger [35] which is based on their experimental lattice parameters in the whole composition range for (U, Pu)O2.00. In Table 1, experimental lattice parameters of plutonium rich MOX reported by Brett and Fox [21] are also given. As can be seen from the table, the lattice parameters calculated using Equation (3) is in reasonable agreement with the values reported by them. According to data available in literature [47e51] on lattice parameter for various non-stoichiometric, mixed oxide solidsolutions, containing uranium as one of its constituents and having CaF2 type FCC structure, variation of lattice parameter with non-stoichiometry have some general values. There is a significant difference in these values for hyper-stoichiometric oxide and hypostoichiometric oxide, irrespective of the metal substituent in uranium oxide matrix. The differential of lattice parameter (a) with respect to excess oxygen (x), given as va/vx, have values: (i) for hyper-stoichiometric oxides (x > 0) va/vx ¼ 0.0094 to 0.011 and (ii) for hypo-stoichiometric oxides (x < 0) va/vx ¼ 0.03 to 0.024. The present value of 0.0073 is close to this range of va/vx for hyper-stoichiometric oxides, though the present oxides were richer in plutonium. The coefficient of ‘x’ reported for various solid solutions were 0.0116 for Bay/2Yy/2U1yO2þx [48], 0.0102 for CayU1yO2þx [49], 0.0094 for UO2þx [50], 0.0117 for MgyU1yO2þx [51], 0.0102 for NayU1yO2þx [47], 0.0104 for LiyU1yO2þx [47]. In all these solid solutions reported in literature, uranium was the major constituent. 4.5. Oxidation kinetics of (U0.3Pu0.7)O2.00 and (U0.47Pu0.53)O2.00 in dry air There is limited data available on oxidation kinetics of mixed oxides and that too is limited to uranium rich MOX. To understand the effect of plutonium enrichment of this kinetic process, oxidation kinetics of MOX with Pu/M ¼ 0.7 and 0.53 were studied in duplicate by thermogravimetry under non-isothermal heating conditions, using a heating rate of 4 K/min, up to 1200 K. A typical plot of fractional weight gain (a) against temperature for oxidation of (U0.3Pu0.7)O2.00 and (U0.47Pu0.53)O2.00 in dry air is shown in Fig. 7. The onset temperature of oxidation of (U0.3Pu0.7)O2.00 is lower than that of (U0.47Pu0.53)O2.00. This may be because the later oxide was highly sintered material, whereas, the (U0.3Pu0.7)O2.00 samples were taken after oxidation experiments of mixed carbide. Though both the samples were taken in powder form, but the particle size of the two samples were different due to the difference in the process of their synthesis. The particle size of the sintered (U0.47Pu0.53)O2.00 powder were ~ 8e10 m [52], whereas, that of (U0.3Pu0.7)O2.00 were in the range ~ 0.5e0.8 m [8]. The particle size has direct impact on diffusion rate and hence on the reactivity of material. The procedure for obtaining kinetic parameters and mechanism of oxidation reaction using thermogravimetric data, has been

Fig. 7. Fraction conversion, a, vs temperature plot for oxidation of (U0.3Pu0.7)O2.00 powder obtained from oxidation of carbide fuel and (U0.47Pu0.53)O2.00 powder obtained by crushing sintered microspheres, in (100 ml/min) dry air at a heating rate of 4 K/min.

discussed in detail earlier [47]. Basically it is a method suggested by Zsako [53] and Doyle [54] for non-isothermal analysis and calculations were carried out using a computer program developed by Ravindran [55]. The rate of reaction under non-isothermal conditions was expressed by the following basic equation:

  da ¼ A$f ðaÞ$exp Ea=RT dT

(4)

Where, f(a) is the kinetic conversion function that depends upon the kinetic mechanism, a is fractional conversion (varying from 0 to 1), A is Arrhenius pre-exponential factor, R is gas constant and Ea is activation energy for the oxidation process. The term a (fraction oxidized) was calculated using the following relation

.

a ¼ ðWT  Wo Þ

Wf  Wo



(5)

Where, Wo, W T and Wf are initial weight, weight at temperature T and final weight, respectively. The program first calculates the approximate value of the activation energy by the method of Piloyan and Novikova [56], using TG data. The kinetic parameters were estimated by fitting the experimental data to twenty different reaction mechanisms. The mechanism, activation energies and preexponential factors obtained for the oxidation of (U0.3Pu0.7)O2.00 and (U0.47Pu0.53)O2.00 in dry air are given in Table 2. The sigmoid nature of a versus temperature plot and data given in Table 2 show that oxidation of these mixed oxides follows random nucleation and growth mechanism (unimolecular - Mampel type). Activation energy for oxidation of (U0.3Pu0.7)O2.00 was found to be 49 ± 5 kJ/ mol and activation energy for oxidation of (U0.47Pu0.53)O2.00 was found to be 70 ± 7 kJ/mol. These activation energy values were calculated from the mean of activation energies obtained from two independent sets of experiments carried out for each mixed oxide composition. Tennery and Godfrey [57] have investigated oxidation of MOX with Pu/M: 0.2 and 0.25 in dry air. They have reported two step oxidation of MO2.00, the first step was related with achieving oxygen saturation in FCC lattice and the second step was related to formation of M3O8. The activation energy for the first step was 42 kJ/mol and for the second step it was 167 kJ/mol. Sampath and Chackraburtty [58] have reported activation energy between 71 and 80 kJ/mol for oxidation of (UyPu1y)O2.00 to (UyPu1y)O2þx in

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Table 2 Non-isothermal kinetic data of oxidation of (U0.3Pu0.7)O2 and (U0.47Pu0.53)O2 in dry air. Initial composition

Final composition

a- range

Mechanism

Ea (kJ/mol)

Z (sec1)

(U0.3Pu0.7)O2.00 (U0.47Pu0.53)O2.00

(U0.3Pu0.7)O2.15 (U0.47Pu0.53)O2.20

0.2e0.9 0.2e0.9

MAMP MAMP

49 ± 5 70 ± 7

0.57  101 0.13  102

MAMP: Random nucleation (one nucleation per particle) mechanism, Mampel type g(a) ¼ ln (1a).

air using non-isothermal method for Pu/M ¼ 0.05, 0.30 and 0.75. They have reported that activation energy is not significantly dependent on Pu content for both hypo- and hyper-stoichiometric region. D'Annucci and Sari [59] also have pointed out that activation energies of oxygen diffusion in hyper and hypo-stoichiometry are very much similar, as all these values for oxidation of MOX within FCC lattice are in the same range, 40e90 kJ/mol. The activation energy of oxygen diffusion in MOX are reported to be in the range of 73.4 kJ/mol for (U0.7Pu0.3)O2x by Suzuki et al. [60], 71 kJ/ mol for (U0.8Pu0.2)O2þx by Sari [45]. The activation energy obtained in the present study also agree well with the data obtained for oxidation of (UyTh1y)O2 by Anthonysamy et al. [30], (UyZr1y)O2 by Kulkarni et al. [61], (PuyTh1y)O2 by Sali et al. [62] and UO2þx by Bayoglu and Lorenzelli [63]. The activation energy for oxidation of various mixed actinide oxides are given in Table 3. As the activation energy values of oxidation of plutonium rich MOX, obtained in the present studies, are very close to the activation energy of oxygen diffusion in MOX lattice, therefore, rate determining step in these oxidation process should be diffusion of oxygen in the lattice. The activation energy of oxidation of (U0.47Pu0.53)O2.00 is higher than that of (U0.3Pu0.7)O2.00, which may be mainly because (U0.47Pu0.53) O2.00 samples were sintered at 1823 K for 4 h.

5. Conclusion Oxidation of plutonium rich monocarbide in air gave a single FCC-phase of (U0.3Pu0.7)O2.15, where the mean valency of uranium was þ5. It was observed that the oxidation of carbide is slow in the beginning and became very fast after formation of MO2x phase (T > 720 K). Based on thermodynamic calculation of phase equilibria, this change in oxidation behaviour was assigned to change in phases during the process. The starting material was a mixture of MC and M2C3 phases and the diffusion of oxygen in these phases is relatively slow, thus lowering the rate of oxidation. After some time, appearance of oxide phase greatly enhanced the oxidation kinetics as diffusion of oxygen in MO2x is much higher. By heating the carbide samples to different temperatures, it was concluded that the final oxidized product can be free of carbon if it is heated in air up to 1273 K for approximately 20 min. If heated at lower temperatures, the oxidation process has to be carried out for longer period. A general equation relating lattice parameter of plutonium rich single FCC-phase of MOX with O/M and Pu/M was obtained. Oxidation kinetics of plutonium rich stoichiometric MOX, under

Table 3 Activation energy for oxidation of mixed actinide oxides in air. Actinide oxide

Method

Ea (kJ mol1)

Reference

(U0.3Pu0.7)O2 (U0.47Pu0.53)O2 (U0.95Pu0.05)O2 (U0.7Pu0.3)O2 (U0.25Pu0.75)O2 (U0.8Pu0.2)O2 (U0.3Th0.7)O2 (U0.72Th0.28)O2 UO2

TG-(Non-isothermal) TG-(Non-isothermal) TG-(Non-isothermal) TG-(Non-isothermal) TG-(Non-isothermal) TG-(Isothermal) TG-(Non-isothermal) TG-(Non-isothermal) TG-(Non-isothermal)

49 70 71 80 80 71 49 70 89

Present study Present study [58] [58] [58] [45] [30] [30] [63]

non-isothermal condition, gave activation energy, 49 kJ/mol for (U0.3Pu0.7)O2.00 and 70 kJ/mol for (U0.47Pu0.53)O2.00. As these values are close to the activation energy required for oxygen diffusion in MOX lattice, therefore, it was concluded that the rate controlling step in the oxidation of MOX is diffusion of oxygen. Acknowledgements The authors express their appreciation to Dr. S. Kannan, Head, Fuel Chemistry Division, BARC for his keen interest and support of this investigation. The authors are grateful to Dr. N.D. Dahale for XRD analysis. Thanks are due to colleagues from sol-gel group of Fuel Chemistry Division for providing mixed oxide samples for this neau for sharing Gibbs study. The authors are obliged to Dr. C. Gue energy data file of U-Pu-C-O system and her guidance in the present calculations. References [1] T. Truphemus, R.C. Belin, J.-C. Richaud, J. Rogez, M. Reynaud, M. Martinez, I. Felines, A. Arredondo, A. Miard, T. Dubois, F. Adenot, J. Rogez, J. Nucl. Mater. 432 (2013) 378e387. [2] F. Le Guyadec, C. Rado, S. Joffre, S. Coullomb, C. Chatillon, E. Blanquet, J. Nucl. Mater. 393 (2009) 333e342. [3] R.M. Dell, V.J. Wheeler, J. Nucl. Mater. 21 (1967) 328e336. [4] S.K. Mukerjee, G.A. Rama Rao, J.V. Dehadraya, V.N. Vaidya, V. Venugopal, J. Nucl. Mater. 210 (1994) 97e106. [5] R.G. Sowden, N. Hodge, M.J. Morton-Smith, D.B. White, The behaviour of carbides in hydrogen and oxygen, Proceedings of a symposium held at Harwell, Nov 1963, Carbides Nucl. Energy vol. 1 (1964) 297e314. [6] K.A. Peakall, J.E. Antill, J. Less Comm. Met. 4 (1962) 426e435. [7] C. Berthinier, S. Coullomb, C. Rado, E. Blanquet, R. Boichot, C. Chatillon, Powder Technol. 208 (2011) 312e317. [8] F. Mazaudier, C. Tamant, A. Galerie, Y. Marc, J. Nucl. Mater. 406 (2010) 277e284. [9] V.S. Iyer, S.K. Mukerjee, R.V. Kamat, K.T. pillai, N. Kumar, V.N. Vaidya, D.D. Sood, Nucl. Technol. 91 (1990) 388e393. [10] G.P. Novoselov, V.V. Kushnikov, V.A. Baronov, V.P. Serebryakov, N.M. Stepennova, At. Energiya 53 (2) (1982) 77e80. [11] U. Benedict, K. Richter, J. Tuerlinx, Trans. Am. Nucl. Soc. 31 (1979) 512e514. [12] H.D. Lewis, Tucson, Arizona, USA, in: J. Leary, H. Kittle (Eds.), Proc. of Intl. Meet. on Adv. LMFBR Fuels, 1977, p. 245. US Report ERDA-4455. [13] D.G. Cragg, R.J. Dicker, J.D.L. Harrison, J.W. Isaacs, J.R. Laren, W.T.G. Roberts, ram. 77 (1967) 11e12. Bull. Soc. Française Ce [14] H.J. Kavanaugh, J.W. Dahlby, A.P. Lovell, Gavimetric Determination of Carbon in Uranium-plutonium Carbide Materials Report LA-7981, 1979. [15] C.E. McNeilly, T.D. Chikalla, J. Nucl. Mater. 39 (1) (1971) 77e83. [16] R. Verma, P.R. Roy, Bull. Mater. Sci. 8 (4) (1986) 479e488. [17] T.L. Markin, R.S. Street, J. Inorg. Nucl. Chem. 29 (1967) 2265e2280. [18] Renu Agarwal, B.K. Sen, V. Venugopal, J. Nucl. Mater 385 (2009) 112e116. [19] N.H. Brett, L.E. Russell, Trans. Brit. Ceram. Soc. 62 (1963) 97. [20] N.H. Brett, A.C. Fox, AERE Report R-3937, UKAEA, Harwell, 1963. [21] N.H. Brett, A.C. Fox, J. Inorg. Nucl. Chem. 28 (1966) 1191e1205. [22] R. Vauchy, A.C. Robisson, R.C. Belin, P.M. Martin, A.C. Scheinost, F. Hodaj, J. Nucl. Mater 465 (2015) 349e357. [23] M. Strach, R.C. Belin, J.C. Rishaud, J. Rogez, Inorg. Chem. 54 (2015) 9105e9114. [24] R.E. Woodley, J. Nucl. Mater. 96 (1981) 5e14. [25] R. Dawar, V. Chandramouli, S. Anthonysamy, J. Nucl. Mater 473 (2016) 131e135. neau, N. Dupin, Bo Sundman, C. Martial, J. Dumas, S. Gosse , S. Chatain, [26] C. Gue F. De Bruycker, D. Manara, R.J.M. Konings, J. Nucl. Mater. 419 (2011) 145e167. [27] B. Sundman, B. Jansson, J.O. Andersson, Calphad 9 (1985) 153e190. [28] F. Thuemmler, R. Theisen, Phasenbezihungen, Herstellung und Eigenschaften von unterstoichiometriscem, oxidischem kerbrennstoff (UO2-x und (UPu)O2x), Report KFK-543, 1967. [29] A. Kumar, J. Radhakrishana, N. Kumar, R. Pai, J.V. Dehadrai, A.C. Deb, S.K. Mukerjee, J. Nucl. Mater. 434 (2013) 162e169. [30] S. Anthonysamy, K. Joseph, T. Gnanasekaran, P.R. Vasudeva Rao, J. Nucl. Mater.

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