Calorimetric studies on urania thoria solid solutions

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Journal of Alloys and Compounds 299 (2000) 112 117 L www.elsevier.com/ locate/ jallcom Calorimetric studies on urania thoria solid solutions a b a, * S. Anthonysamy, Jose Joseph, P.R.

1 Journal of Alloys and Compounds 299 (2000) L locate/ jallcom Calorimetric studies on urania thoria solid solutions a b a, * S. Anthonysamy, Jose Joseph, P.R. Vasudeva Rao a Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam , India b Materials Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam , India Received 2 October 1999; accepted 14 November 1999 Abstract he enthalpy increments of (Uyh 12y)O2 solid solutions with y50.1, 0.5 and 0.9 were measured by using a drop calorimeter of isoperibol type. he experimental data cover the temperature range K relative to K. Other thermodynamic functions such as heat capacity, entropy and free energy function of these solid solutions were derived from the measured enthalpy increment values. he results indicate that the enthalpies of (Uyh 12y)O2 solid solutions in the temperature range K obey the Neumann Kopp molar additivity rule Elsevier Science S.A. All rights reserved. Keywords: Urania thoria solid solutions; Enthalpy increments; Drop calorimetry 1. Introduction thermal properties of these solid solutions which in turn will help to understand the U h O system completely. Studies on the oxides and mixed oxides of actinide elements such as thorium, uranium and plutonium are of great interest in nuclear industry since some of the oxides are used as nuclear fuels and some as blanket materials in 2. Experimental various types of reactors [1]. he thermodynamic properties such as enthalpy and heat capacity of these materials 2.1. Preparation of the sample are needed for reactor physics and safety calculations. he thermal properties of urania, thoria, plutonia and urania Urania thoria solid solutions were prepared as follows: plutonia have been studied extensively in the past and a Ammonium diuranate and thorium hydroxide were co- reasonable body of data exists [2,3]. However, there is precipitated from a mixture of uranium and thorium nitrate paucity of data on thoria urania and thoria plutonia mixed solution by the addition of aqueous ammonia. he precipi- oxides. For example, in the case of urania thoria solid tate was dried and calcined at 1073 K in air for 5 h. he solutions, enthalpies and heat capacities are available in calcined powders were then compacted at 500 MPa into the literature only for thoria-rich (#20 mole % of urania) pellets of 10 mm diameter and 2 3 mm thickness by solid solutions over a limited temperature range [4,5]. In employing an electrically operated double action hydraulic addition, it has not been clearly established whether the press. No binder or lubricant was added for the preparation molar additivity rule of Neumann Kopp is obeyed by of the compacts. he compacts thus prepared were sintered these solid solutions. Hence, in the present study, the at 1873 K for 6 h in a flowing argon 8 vol. % hydrogen enthalpy increments of (Uyh 12y)O2 solid solutions with gas mixture. he oxygen to metal ratio of the compact was y50.1, 0.5 and 0.9 were measured in the temperature fixed at by equilibrating the compact with H 2 H2O 21 range K by using a drop calorimeter developed in gas mixture having an oxygen potential of 2510 kj mol. our laboratory, with a view to generate more data on the at 1073 K till equilibrium was established between the gas phase and (U,h)O2 compact [6]. Before carrying out *Corresponding author. el.: ; fax: calorimetric studies, the samples were characterised for 365. their chemical composition (by elemental analyses for address: vasu@igcar.ernet.in (P.R. Vasudeva Rao) uranium and thorium as well as lattice parameter measure / 00/ $ see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S (99)

2 S. Anthonysamy et al. / Journal of Alloys and Compounds 299 (2000) ments), phase composition (by X-ray diffraction studies) and impurity content (by inductively coupled plasma mass spectrometry and optical emission spectrometry). he total metallic impurity of the mixed oxide samples employed in this study was less than 100 ppm (by weight). (U0.1h 0.9)O Calorimeter able 1 Heat capacity and entropy values of (U,h)O2 solid solutions employed in this study for computations O Sample Cp,.15 (J mol K ) S.15 (J mol K ) (U0.5h 0.5)O (U0.9h 0.1)O he calorimeter employed in this study for enthalpy he differentiated form of this equation is the heat measurements was a drop calorimeter of the isoperibol capacity over the given temperature range type. he equipment and the validation of the measurement technique are described elsewhere [7]. Essentially, 22 Cp 5 A 1 2*B* 2 C* the calorimeter consists of a nichrome wire wound furnace, with provisions for heating and holding the sample at the he two boundary conditions used for the fitting were: experimental temperatures, and a calorimeter proper. he temperature of the furnace was controlled within 61 K. O O (i) H 2 H 5 0 he calorimeter proper is essentially a brass receiving at 5.15 and crucible maintained at K. he detector is a novel (ii) Heat capacity of the solid solutions at.15 K (Cp, ) thermopile whose design and construction are described elsewhere [7]. he temperature of the sample was mea- he heat capacity of (U,h)O2 solid solutions at.15 sured using a ype K thermocouple. In order to prevent the K are not available in the literature. Hence, they were oxidation of the oxide samples, argon 8 vol.% hydrogen obtained from the data on the heat capacity of pure urania gas mixture was continuously flown over the sample. and thoria [2] by applying the molar additivity rule. he 2.3. Method of measurement. In a typical experiment, the sample or the reference material was placed in the furnace at the desired temperature for a period of about 30 min. to ensure the temperature of equilibration. he sample size was approximately g. he reference material employed in this study was the calorimetric standard, synthetic sapphire (( g, SRM-720, a-al2o 3), obtained from NIS, USA. During a drop experiment, one sample and two standards at the experimental temperature were dropped alternatively into the working crucible maintained at K. he resultant output of the detector was amplified and monitored 2 (Standard error) continuously by using a high-impedance multimeter and the data acquired using a computer. he output from the computer gives the trace of the temperature change of the working crucible as a function of time. By using the values of the areas of the curves measured in the case of standard calculated heat capacities of (U h )O (U h )O , , and (U0.9h 0.1)O2 solid solutions at K are given in able1. he values of the constants of the fit expressions obtained for the measured enthalpy values are given in able 2. he enthalpy values of (U0.1h 0.9)O 2, (U0.5h 0.5)O2, and (U0.9h 0.1)O2 solid solutions computed from the fit expressions are given in able 3 along with the measured values. he enthalpy values computed based on the molar additivity rule of Neumann Kopp are also shown in this table. he standard error between the experimental result and the smoothed enthalpy values from the fit equation was computed using the following expression: 2 5 (S differences) /(no. of observations no. of coefficients) he computed standard error values are also shown in and sample, and the known enthalpy increment data of the able 3. he estimated error involved in the measurements O O standard, the enthalpy increment, H, of the sample is 62% taking into account the precision of the measurewas calculated. ments as well as the error involved in the measurement of the temperature of the sample. 3. Results and discussion able 2 O O Constants for the fit equations of the enthalpy increments H 2 H he measured enthalpy increments of (U0.1h 0.9)O2, A* 1 B* 1 C* 1 D (U0.5h 0.5)O2, and (U0.9h 0.1)O2 solid solutions were 4 24 Solid solution A B*10 C*10 D fitted to polynomials in temperature of the following form (U0.1h 0.9)O by the least squares method: (U0.5h 0.5)O O O 2 21 (U0.9h 0.1)O H 2 H 5 A* 1 B* 1 C* 1 D

3 114 S. Anthonysamy et al. / Journal of Alloys and Compounds 299 (2000) able 3 Enthalpies of urania thoria solid solutions O O 21 O O 21 O O 21 (K) H (J mol ) for (U0.1h 0.9)O2 H (J mol ) for (U0.5h 0.5)O2 H (J mol ) for (U0.9h 0.1)O2 Measured Calculated Calculated Measured Calculated Calculated Measured Calculated Calculated (from fit (molar (from fit (molar (from fit (molar eqn.) additivity rule) eqn.) additivity rule) eqn.) additivity rule) Standard error: he enthalpy increment values obtained from the fit et al. is not appropriate because of the large difference in equations are shown in Fig. 1 along with the experimental the temperature of the two sets of measurements. Springer values. he literature data on the enthalpy increments of et al. [5] have measured the enthalpy increments of urania and thoria are also shown in Fig. 1 for the purpose (U0.1h 0.9)O2, and (U0.2h 0.8)O2 over the temperature of comparison. Fischer et al. [4] have measured the range K.. However, the data reported by them enthalpy increments of (U h )O (U h )O for (U h )O lie below those of both urania and , , and (U h )O over the temperature range thoria, where as, the enthalpy increment data obtained in K. A comparison between our results and those of Fischer this study lie above that of thoria. Fig. 1. Enthalpy increments of (U,h)O 2 solid solutions.

4 S. Anthonysamy et al. / Journal of Alloys and Compounds 299 (2000) Fig. 2. Variation of heat capacities of (U h )O (U h )O and (U h )O solid solutions with temperature , , he enthalpy increment data of this study were used to ployed in this study for the computations are given in compute other thermodynamic functions such as heat able 1. Fig. 2 shows the variation of heat capacities of capacity, entropy and free energy function of the solid (U0.1h 0.9)O 2 (U0.5h 0.5)O2 and (U0.9h 0.1)O2 solid solusolutions at various temperatures.* he results are shown tions with temperature. he heat capacities of the solid O in able 4. he S values of the solid solutions required solutions lie between the pure components of the solid for the computation of free energy functions were again solutions. obtained from S of pure urania and thoria [2] by Figs. 3 5 compares the measured enthalpy increments applying the molar additivity rule. he S values em- with the values computed by applying the molar additivity Fig. 3. Comparison of measured enthalpy increments of (U h )O solid solution with the values computed by applying the molar additivity rule

5 116 S. Anthonysamy et al. / Journal of Alloys and Compounds 299 (2000) Fig. 4. Comparison of measured enthalpy increments of (U h )O solid solution with the values computed by applying the molar additivity rule rule of Neumann Kopp. he results indicate that the range K and Springer et al. [5] for measured enthalpy increments of (U0.1h 0.9)O 2, (U0.2h 0.8)O2 over the temperature range K (U0.5h 0.5)O2, and (U0.9h 0.1)O2 over the temperature were also found to obey the molar additivity rule. hese range K obey the Neumann Kopp molar ad- results suggest that the stoichiometric urania thoria solid ditivity rule within the experimental uncertainty. he solutions behave ideally. his is in agreement with the experimental results of Fischer et al. [4] for conclusion arrived at based on the oxygen potential (U0.15h 0.85)O2, and (U0.3h 0.7)O2 over the temperature measurements reported in the literature [8 14]. Fig. 5. Comparison of measured enthalpy increments of (U h )O solid solution with the values computed by applying the molar additivity rule

6 S. Anthonysamy et al. / Journal of Alloys and Compounds 299 (2000) able 4 Values for heat capacity, entropy and free energy functions of urania thoria solid solutions (heat capacity, entropy and free energy functions are in J mol 21 K ) (K) (U h )O (U h )O (U h )O O a O O Cp S FEF Cp S FEF Cp S FEF a FEF5free energy function. 21 References [1] C. Ganguly, Development of plutonium-based advanced LMFBR fuels and thoria-based PHWR fuels in India, IAEA-ECDOC-352, 1985, pp [2] J.K. Fink, Int. J. hermophys. 3 (2) (1982) 165. [3] K. Bakker, E.H.P. Cordfunke, R.J.M. Konings, R.P.C. Schram, J. Nucl. Mater. 250 (1997) 1. [4] D.F. Fischer, J.K. Fink, L. Leibowitz, J. Nucl. Mater. 118 (1983) 342. [5] J.R. Springer, J.F. Langedrost, Battelle Memorial Institute Report, BMI-X-10231, [6] S. Anthonysamy, K. Nagarajan, P.R. Vasudeva Rao, J. Nucl. Mater. 247 (1997) 273. [7] M.V. Krishniah, Ph.D hesis, University of Madras, 1999, to be submitted. [8]. Matsui, K. Naito, J. Nucl. Mater. 132 (1985) 212. [9] S. anaka, E. Kimura, A. Yamaguchi, J. Moriyama, J. Jpn Inst. Metals. 36 (1972) 633. [10] M. Ugajin, J. Nucl. Mater. 110 (1982) 140. [11] M. Ugajin,. Shiratori, H. Shiba, J. Nucl. Mater. 116 (1983) 172. [12] J.S Anderson, D.N. Edgington, L.E.J. Roberts, E. Wait, J. Chem. Soc. (1954) [13] L.E.J. Roberts, L.E. Russel, A.G. Adwick, A.J. Walter, M.H. Rand, P/ 26 UN Geneva Conference of Peaceful Uses of Atomic Energy, Vol. 28 (1958) p [14] R.E. Woodley, M.G. Adamson (Eds.), hermodynamics of Nuclear Materials 1979, Vol. 1, IAEA, Vienna, 1980, p. 333.

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