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

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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 pluto-

nium 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 con-

tents 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 desi-

rable. 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 hexa-

carbonyl 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. Mehrothra and S. Majumdar, Nucl. Tech , 72, 59 (1986). 2. M. Saibaba, S. Vanavacambhan and CK. Mathews, J, Nucl. Mat., 144,56 (1987).

3. S. Anthonysamy, P.R. Vasudeva Rao and C.K. Mathews, IGC Report No. IGC-82 (1986)..

4. D. White, Guide book on Quality Control of Water Reactor Fuel, Technical Report Series No. 221, IAEA (1983) page 81.

5. W.G. Guldner, Talanta, 8,91 (1961).

6. S.K. Smith and D.K. Krause, Anal.Cham., 40 ,2034 (1968).

7. C.S, MacDoughall, M.E. Smith and G.R. Waterbury, Anal.Chem., 41,372 (1969).

8. H.T.Goodspeed and D.Pettis, ANL-7264 (1967)

9. B.L. Taylor, C Phillips and G..W.C. Milner, Proc. Syrup, on Analytical Methods in Nuclear Fuel Cycle, Vienna (1973), IAEA-SM-149/2 2 (1974).

10. W.R. Laing, L.T., Corbin, IAEA-SM-149/31, ibid,

11. C. Ganguly and G.C. Jain Proc,of Nucl. and Radiochemistry Symp., BHU, Varanasi (1981), p.540.

12. R.Alvero, J.A.Odriozola and J.M. Trillo, j.Chem.. Soco Dalton Transactions, (1984) p,87

13. L. Melnick, L. Lewis, and Bo Holt "Determination of Gaseous Elements in Metals", Wiley, New York (1974). TABLE 1 Recovery of oxygen from ThO2 + C mixture

Wt of the mixture,mg Wt% oxygen obtained

132.44 12.28 161.53 12.08 175.63 12.04 180.08 12.15 184.98 12.31 186.56 12.52 192.32 12.23 195.99 12.15 197.69 12.34 199.48 12.08 200.80 12.30 203.15 12.08 204.50 12.15 267.98 12.30 209.22 12.38 210.81 12.14 214.28 12.08 223.75 12.16 227.68 12.15 240.27: 12.12 245.15 12.43

T = 12.21 ± 0.13 % Theoretical value for ThC>2 = 12.12% TABLE 2 Weight Percent Oxygen Determined in some Oxides

Oxide No. of deter- Theoritical Oxygen Content rsd minations value Determined, wt % (%)

25 18. 59 18.56 + 0.12% 0.65 6 14. 27 19.07 + 0.75 3.9

15 25. 95 26.3 + 0.76 2.87

7 30. 08 30.23 + 0.41 1.34 TABLE 3 Recovery of Oxygen from Tungsten Hexacarbonyl

No. Wt. of W(C0)6/mg Oxygen Content Determined, wt %

1 23.02 26.50

2 39.24 27.27

3 43.83 26.47

4 50.74 27.19

5 55.97 27.69

6 65.28 26.35

7 70.65 27.74

8 72.42 26.51

9 82.53 27.51

10 90.33 27.46

11 95.37 26.27

12 104.94 26.49

T = 26.95 + 0.57% rsd = 2.11% Theoritical value = 27.28 %

TABLE 4 Oxygen determined in uraniua carbide

Oxygen Content, wt %

UC/UC2 mixture 0.48, 0.54, 0.56 UC 0.45, 0.44, 0.41

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IR CELLS. OPTICAL SCHEMATIC

13 APPENDIX 1

CHARACTERISTICS OF RO-18 AND EF-10 UNITS

The LECO RO-18 Oxygen Determinates determines oxygen in steels,

iron, non-ferrous metals and other inorganic materials. It uses

the inert gas fusion method. The sample is fused in a graphite

crucible in the EF-10 Electrode Furnace. A?:gon carrier gas

transfers the released oxygen, as carbon monoxide, to the RO-18

for measurement by infrared detection. The detector output is

amplified, linearized, integrated and displayed directly as

percent oxygen. The instrument contains a gas doser for

theoretical gas dosing for quick and accurate calibration.

RO-18 CHARACTERISTICS

Range: Lo Range 0.0001 % to 0.1000 % (sample weight 0.8 to 1.199 grams) High Range 0.01 % (sample weight 0.4 to 1.0 grams) to 20.0 % (sample weight 0.4 grams maximum)

Accuracy Lo Range ± 0.0002 % or ± 1 % of oxygen content whichever is greater High Range +_ 0.10 % or _+ 1 % of oxygen content whichever is greater

Sensitivity 0.0001 % (1 ppm)

EF-10 CHARACTERISTICS

The EF-10 Electrode Furnace is designed for fusion analysis methods. A graphite crucible, held between two water-cooled metal electrodes, reaches temepratures near 2700°C. The pneumatic closing system of the electrodes offers the best possible crucible contact for high current fusion.

14 METHOD OF DETECTION OF CO

The i.R. cells detect changes in infrared radiation. The

CO detector cell is at the top, the LECO Light Source (Emitter

cell) is at the bottom and the sample cell is in the middle (See

Fig.3.)

The CO detector cell consists of two chambers separated by a thin metal diaphram. This diaphram forms one plate of a parallel plate capacitor. The other plate is a "fixed plug" in the detector housing. The two chambers of the detector cell are filled with carbon monoxide gas and sealed. Each chamber has a window which can accept infrared radiation.

Infrared radiation entering one of the CO detector chambers will cause pressure to increase inside that chamber due to the heating of the CO gas in a fixed volume. If an unequal amount of infrared radiation enters the other chamber of the CO detector, the pressure will not be equal in both chambers and the diaphram will deflect causing a change in capacitance (the capacitor consists of the diaphram and fixed plug). The capacitance change results in an electrical signal.

The sources of infrared radiation consist of heated filaments, one for each of the detector cell chambers. The filaments are heated to a temperature of 900°C, just below the incandescent point.

15 Immediately in front of the infrared sources is a rotating

(chopper) blade which interrupts the infrared radiation. The

blade interrupts both the sources simultaneously and chops the

radiation from maximum to zero. The purpose of the chopper is to

convert the radiation from a continuous radiation to a pulsating

radiation. The pulsating radiation causes pulsating pressure

changes in the CO detector cell resulting in a pulsating (A.C)

signal out of the CO detector cell. Without "chopping" the

detector cell output signal would be D.C. and difficult to

amplify.

A mechanical shutter is immediately in front of the CO detector cell. The shutter is adjustable and is set so that exactly equal amounts of I.R. energy reach each CO detector cell chamber.

Between the chopper blade and the shutter mechanism are two chambers. One is the reference chamber, the other is the measure chamber. During calibration, the chambers are filled with Argon.

Under this condition the amount of chopped radiation reaching each chamber of the CO detector cell is equal, keeping the diaphram stationary.

The CO gas from a fused sample enters the measure chamber of the sample cell. The infrared radiation which is passing through the measure chamber of the sample cell is now partially absorbed by the CO gas causing a decrease in energy reaching the measure chamber of the CO detector cell. Because the energies in the CO detector cell chambers are no longer equal, a difference in

16 pressure results and the diaphram deflects. The magnitude of diaphram deflection and resulting capacitance change are proportional to the concentration of sample CO gas. Because of the chopper, this capacitance is varying at a rate determined by the chopper blade rotation and an A.C. signal is generated. The resulting A.C.signal is amplified, rectified, integrated, weight compensated, and displayed on the DVM.