Determination of Oxygen in Some Oxides and Uranium Carbide by Inert Gas Fusion

- '•"- ^ \ <M 00 O CO IGC-102 I™ 0} 6 " 1933 o Determination of Oxygen vn some Oxides and Uranium Carbide by Inert Gas Fusion V. Chandramouli P. R. Vasudeva Rao GOVEWMENT. OF INDIA. DEPARTMENT OF ATOMIC ENEFGY INDIRA GANOHI CENTRE FOR ATOMIC RESEARCH KALPAKKAM IGC- 102 1988 GOVERNMENT OF INDIA DEPARTMENT OF ATOMIC ENERGY DETERMINATION OF OXYGEN IN SOME OXIDES AND URANIUM CARBIDE BY INERT GAS FUSION V.Chandramouli and P.R.Vasudeva Rao Radiochemistry Programme Indira Gandhi Centre for Atomic Research Kalpakkam 603 102 Tamil Nadu, India ABSTRACT Uranium and plutonium carbides contain oxygen as one of the dissolved impurities. This oxygen affects the physico- chemical properties of the carbides. In the present work, oxygen has been determined in some oxides and uranium carbide by the inert gas fusion method, A mixture of thoria and graphite was used to calibrate the determinator for oxygen in percentage levels. Attempts to use tungsten hexacarbonyl gave encouraging results suggesting its use as a calibration standard. KEY WORDS: [Chemical analysis, oxygen, oxides, uranium carbide, inert gas fusion] CONTENTS Page No 1. Introduction 1 2. Principle 2 3. Experimental 3 4 = Procedure 3 5, Results and discussion 4 6, Conclusion 6 7. References 7 8. Tables 8-10 9.. Figures 11-13 10. Appendix 14 DETERMINATION OF OXYGEN IN SOME OXIDES AND URANIUM CARBIDES BY INERT GAS FUSION V. Chandramouli and P.R. Vasudeva Rao Radiochemistry Programme Indira Gandhi Centre for Atomic Research Kalpakkam 603 102 INTRODUCTION The Fast Breeder Test Reactor at Kalpakkam is the first reactor to use plutonium-rich U-Pu mixed carbide (70% PuC-30% UC) as the driver fuel. As the carbides are generally prepared in large scale by the c ar bother mi c reduction of the oxidesd), they contain oxygen as one of the impurities. Presence of oxygen in the fuel enhances the carbon potential(2'3) leading to carburi- sation of the clad. Further the oxygen impurity influences the thermophysical and thermodynamic properties of uranium and plutonium carbides. Similarly, the O/M of uranium dioxide influences the thermophysical properties such as thermal conductivity of the oxides(4). it is therefore necessary to monitor the oxygen contents in carbide and oxide fuel materials before one ventures to study any property,, Various methods are available for the determination of oxygen in metals*5). Among these methods, vacuum fusion and inert gas fusion are simple, fast analytical methods that are used for the determination of oxygen in refractory oxides(6-9) and carbides^O'lD. in the present work, oxygen in some oxides and uranium carbide was determined using the RO-18 oxygen determinator (LECO Corporation, USA) employing the inert gas fusion technique. For the calibration of this instrument the use of carbon monoxide gas has been suggested. But due to its toxic and hazarduous nature, use of alternate working standards is desirable. A mixture of thoria and carbon was found suitable. Since carbonyls decompose at low temperatures, and liberate only carbon monoxide gas, and since this decomposition reaction does not need extra carbon or flux, an attempt was made to use tungsten hexacarbonyl as a calibration standard and the results are reported. PRINCIPLE Oxygen in oxides and carbides is extracted as carbon monoxide by carbothermic reduction in a graphite crucible as per the typical equations: MO2 + 3C f===^ MC + 2 CO (1) M(C,O) + C v===^ MC + CO (2) Since CO is the only non-condensed phase, the equilibrium constant Keg is given by The equilibrium constant is related to the free energy change of the reaction by the expression AG° = - RT In Keq = -nRT In pco It is seen from above that the reduction is favoured at higher temperatures. So, high temperatures are desirable for the carbothermic reduction for the quantitative evolution of oxygen as CO. The CO gas evolved is determined either by oxidising CO to CO2 and measuring the CO2 by a suitable method or determined directly using an infrared detector. EXPERIMENTAL Apparatus used The analyses were carried out using LECO RO-18 dual range oxygen determinator. A schematic of the gas flow in the instrument is given in Fig.l and a description of the characteristics of the instrument is given in the Appendix. The oxygen in samples are reduced to carbon monoxide in the EP-10 furnace (flow diagram in Fig. 2) supplied along with the determinator. Brief description is given in the Appendix. The CO gas is estimated using an IR cell (Fig. 3). The working of the detector is described in the Appendix. Reagents and Chemicals For the purpose of calibration, accurately weighed nuclear grade thoria and graphite powder were mixed for 6 hrs in the grinder 'pulverisette' supplied by M/s Fritch, West Germany. The uranium carbide samples on which estimations for oxygen were carried out, were prepared by the carbothermic reduction of UO2 at 1600°C. The products were identified by X-ray diffraction. The tungsten hexacarbonyl was supplied by M/s Dr Theodor Schuchardt and Co*/West Germany. iron oxide (Fe2C>3) was obtained from M/s Fischer Scientific Co., USA. The rare earth oxides were obtained from Indian Rare Earths Ltd, India. PROCEDURE Accurately weighed (^40mg) samples of the oxides or uranium carbide were taken in a tin capsule. Uranium carbide was sampled inside a high purity inert atmosphere glove box. The tin capsule with the sample was dropped in to a degassed graphite crucible. The crucible was heated to >2000°C in the impulse heating furnace (EF-lO)in an inert atmosphere. The evolved CO was swept in to the oxygen determinator where it was detected and mesured using an IR cell. RESULTS AND DISCUSSION Samples of a mixture of ThO2 and carbon were first analysed repeatedly for oxygen. In the first few analyses/ the oxygen value obtained was adjusted to the value calculated from the stoichiometry. The recovery values for replicate analyses of oxygen in this mixture are given in table 1. It was observed that for the complete recovery of oxygen, an intimate mixture of the oxide and carbon was necessary as was reported earlier(7,10)# The evolution of oxygen in the second and third firings was negligible, which indicated that the recovery was complete in the very first firing. The reproducibility of the recovery values thus suggests that the mixture of ThO2 and C is suitable for the calibration of the determinator. Table 2 gives the recovery of oxygen from some oxides. In these analyses, the mixture of thoria and carbon was used as a standard to calibrate the instrument. In all the oxides except neodymium oxide, the value for the oxygen content agreed well with theoritical value. But in neodymium oxide it was higher. This may be due to absorption of water and CO2 by the rare earth oxide as reported by Alvero et al (12), por the complete recovery of oxygen from these oxides,use of additional carbon and flux was necessary. The flux raises the partial pressure of CO over the oxide, may be by alloying with the metal. This increase in the partial pressure may be caused by the free energy contri- bution of the reaction of the carbide formed with the flux(13).ln this work SS bits were used as flux material and some carbon was added from outside. The agreement in the oxygen content of a number of oxides again suggests that the mixture of thoria and carbon is a suitable calibration standard. In table 3, the oxygen content in tungsten hexacarbonyl is tabulated. Initial calibration in this part of the work was carried out using thoria-graphite mixture. It is seen from the table that the recovery of oxygen from tungsten hexacarbonyl is more than 98% suggesting its use as an instrument calibration standard. Tungsten hexacarbonyl has the following advantages :- (1) known stoichiometry (2) reaction is only decomposition, hence no need for the addition of extra carbon and flux. (3) decomposition temperature is very low ( 150°C), so, complete decomposition is ensured even at the lowest temperature attainable in the EF-10 furnace ( 600°C). (4) the decomposition products are only tungsten and carbon monoxide hence there is no need for any purification train. Further work is planned with other metal carbonyls to assess their suitability for use as calibration standards. Results of the determination of oxygen in the uranium carbides are listed in table 4. The triplicate values in each sample were consistent. In the absence of standards for oxygen in UC, there is no way to find the correctness of the values, but since no oxygen was indicated in the second firing, it was assumed that the reduction was complete. To assure completion of reaction, SS flux was added alongwith some carbon from outside. CONCLUSION Oxygen in the oxides and carbides can be determined using the inert gas fusion method. An intimate mixture of thoria and graphite is suitable for use as a calibration standard. Higher temperatures favour complete recovery of oxygen from the samples. W(CO)g possesses several advantages over conventional standards. Providing one has a pure W(CO)g, the compound can be used as a primary standard. ACKNOWLEDGEMENTS The authors sincerely thank the preparation group, the X-ray group for their valuable services in providing us with uranium carbide and characterising them. Special thanks are due to Shri R.B. Yadav and Dr. D.S. Suryanarayana for their valuable suggestions. We also thank Dr C.K. Mathews, Head, Radiochemistry Programme for the keen interest shown in this work. REFERENCES 1. C Ganguly, P.V, Hegde, G.C. Jain, U. Basak, R.S.

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