Fluorination of Uranium Dioxide by Bromine-Fluorine System

Journal of NUCLEAR SCIENCE and TECHNOLOGY, 12[5] pp. 308~313 (May 1975). Fluorination of Uranium Dioxide by Bromine-Fluorine System Tsutomu SAKURAI and Akira TAKAHASHI Fluorine Chemistry Laboratory, Japan Atomic Energy Research Institute* Received November 6, 1974 The addition of bromine to fluorine flow makes it possible to fluorinate UO2 into UF6 even below 200dc, at which temperature fluorination does not proceed with fluorine alone. Presence of bromine corresponding to about 6% of the fluorine concentration is sufficient to induce the fluorination. This effect of bromine is much greater than would be expected from the presence of bromine fluorides in concentrations such as would be produced by direct combination of the fluorine with the added bromine. It appears that the fluorination is enhanced when the mixed gas is held for 3~20 sec before arriving at the sample. The main component of the reactant gas is BrF3. KEYWORDS: fluorination, uranium dioxide, fluorine, bromine, bromine tri- fluoride, bromine pentafluoride, bromine monofluoride, thermobalance, infrared analysis, uranium hexafluoride, reaction rate I. INTRODUCTION after passage through a sodium fluoride column to eliminate hydrogen fluoride. Commer- Uranium compounds are fluorinated into cial-grade bromine with a purity exceeding UF6 by means of elemental fluorine(1) or of a 99.0%, from Wako Pure Chemical Industries, halogen fluoride such as CIF3(2)', BrF3(3)(4) or was previously bubbled with dry nitrogen at BrF5(5). Although the dissociation energy of room temperature to remove traces of chlo- fluorine is smaller than the bond energies rine (<0.3%). Nitrogen used as carrier for of Cl-F and Br-F bond(6), the fluorination bromine and fluorine was of purity higher of uranium compounds by fluorine requires than 99.99%. Uranium dioxide powder of a higher temperature for its initiation than purity above 99.9% was supplied by Mitsu- by a halogen fluoride. But use of the latter bishi Metal Corp. Co. Prior to the runs, this raises another problem in that it requires an material was treated with pure hydrogen at additional device to remove the considerable 1,050dc for 10 hr ; the surface area measured amounts of chlorine or bromine produced as by BET method with krypton was 2.2 m2/g. reaction byproduct. Further, when using 2. Apparatus and Procedure gaseous BrF3, heating of the connecting line The change in weight of the solid sample is necessary to prevent its condensation. brought by the fluorination was traced con- The present authors have found that the tinuously with a thermobalance or by directly direct addition of bromine to fluorine flow weighing the reaction residue at regular time promotes the fluorination of uranium com- intervals. The thermobalance was the same pounds. This improvement is described here as applied to the fluorination of UO2. as used in the previous study(3) ; fluorine and bromine, diluted with nitrogen, were introduced into the reaction tube through separate II. EXPERIMENTAL inlets placed opposite each other. 1. Materials For the runs undertaken to observe the Fluorine, from the Matheson Co., was used * Tokai-mura, Ibaraki-ken. — 50 — Vol. 12, No. 5 (May 1975) 309 dependence of the reaction rate on the time ply, is ascribable to the formation of UO2F2 taken by the reactant gas to reach the solid (an intermediate of the reaction), because sample, a horizontal reaction tube, 100 cm chemical analysis showed that a solution long, was used instead of the thermobalance, prepared from the solid phase contained UO2+2 which was unsuitable for this purpose on and F- in mole ratio [F-]/[UO2+2] ?? 2. The account of its embodying a large space where bromine content in the solid phase was neg- the gas was stagnant, i.e. the space enclosed ligible (less than 10 mgg). within the spring column(3). This horizontal reaction tube consisted of four flanged Monel pipes (2.4 cm I.D.), so that an infrared absorp- tion cell could be inserted between the flanges. The cell (Monel) with AgCl windows was devised so as to minimize the space where the reactant gas was stagnant. Bromine was carried into the reaction tube by nitrogen, which was bubbled at a fixed flow rate through 70 mm deep liquid bromine at 0dc. The concentration of bromine in the carrier gas was monitored by passing it through a cold trap at —78dc and weighing the bromine thus collected. It was verified that the nitrogen was saturated with bromine vapor in its passage through the liquid bro- Fig. 1 Effect of Br2 on fluorination of UO2 mine, if the flow rate did not exceed 8.5 l/hr. The flowmeter for fluorine was equipped with nickel filaments(7) ; for nitrogen a glass- 2. Rate of Reaction rotameter was used. The fluorine-to-bromine In many cases of gas-solid reactions in ratio in the reaction tube was varied by con- which the products are entirely gaseous, the trolling the flow rates of fluorine and the diminishing sphere model has been used for two nitrogen flows. theoretical treatment of the reaction rates(1) The change in composition of the solid (2)(5)(8). The final equation derived from this phase with time was observed in the same model is (1—F)1/3=k•L1—t, where F is the manner as in the previous study(4). fraction of the solid reacted, t the reaction time and k•L a rate constant. The value of k•L can be determined from the slope of the III. RESULTS straight line obtained by plotting (1—F)1/3 1. Effect of Bromine Addition against time. Figure 1 shows the change in weight of Figure 2 shows the plots of (1—F)1/3 and the sample with time observed in the fluori- (1—F) against time derived from the "weight nation at 200dc. In the case of fluorine alone, change vs. time" curve of Fig. 1. It is seen unaccompanied by bromine, no weight de- that, in the present instance, except for the crease is observed, no UF6 being formed in initial and final portions (1—F) vs. time show this case. Upon addition of bromine to the better linearity than the corresponding plots fluorine flow, the weight starts to decrease of (1—F)1/3. This indicates that the thickness immediately. Furthermore, turning off the of the solid phase decreases with time at a bromine flow in the midst of reaction stopped constant rate, as observed in the case of the the fluorination. Thus, it is evident that it UF4-BrF3(3) and U02-BrF3 reaction(4). There- is the addition of bromine that induces the fore, the slope of the straight line approxi- fluorination. The slight increase in weight, mating the middle portion of the "weight seen in Fig. 1 at the outset of fluorine sup- change vs. time" curve was used for deter- - 51 — 310 J. Nucl. Sci. Technol , mining the reaction rate K, in the dimension of mg/min. Fig. 2 Plots of (1- F)1/3 and (1 - F) vs. time for weight-change curve shown in Fig. 1 3. Comparison with UO2-BrF3 Reaction Figure 3 compares the fluorination by fluorine-bromine system with that by BrF3 Fig. 3 Comparison of fluorination by Br2-F3 carried out in the apparatus described in the system with that by BrF3 previous papers(3)(4). The symbols PF2 and rate K increases markedly with PF2 PL, in the figure represent the presumed , : the relation between them can be expressed by partial pressures of fluorine and bromine, re- spectively, which would prevail in the reac- K=5.5 x 10-3(PF2)1_64. On the other hand , tion tube if there were no interaction between when PBr, is varied under constant PF2, the them ; PBrFx means the partial pressure of reaction rate acquires its greatest value at a BrF, BrF3 and/or BrF5, formed in the reaction fluorine-to-bromine ratio around 17. The rate tube by direct combination between F2 and the Br2 added. While PBrFx is indicated in Table 1 Influence of fluorine-to-bromine the figure to be almost equal to BrF3 (BrF3 ratio on reaction rate K partial pressure in the UO2-BrF3 reaction), the bromine-added reaction proceeds much more rapidly than the UO2-BrF3 reaction (reaction rate K=15.8 mg/min for the former and =2.7 mg/min for the latter reaction). The resulting actual effect of bromine hence becomes much greater than would result from PBrFx.• 4. Influence of Differences in Fluorine-to-bromine Ratio Table 1 shows the influence of differences in the fluorine-to-bromine ratio on the reaction rate. Under constant PBr2, the reaction — 52 — Vol. 12, No. 5 (May 1975) 311 declines with either increase or decrease of tinued for a predetermined duration. bromine concentration from this maximum Figure 4 shows the change in weight of point. This would indicate that the addition the sample with time at 200dc. Even in the of bromine corresponding to about 6% of the case of the 3 sec passage, the sample is seen fluorine concentration should suffice to en- to have fluorinated rapidly, and increase of hance the fluorination. the passage time from 3 to 20 sec further 5. Dependence on Temperature raised the reaction rate slightly. Additional increase of the time from 20 to 43 sec this Table 2 shows the temperature depend- time markedly impaired the reaction rate. ence of the reaction rate in the range of Hence, it is evident that the time required to 100-~500dc. In these runs, bromine flow was reach the sample influences the reaction rate. first introduced into the reaction tube, because At 100dc, however, appreciable differences in the reaction of fluorine with UO2 proceeded the reaction rate were no longer observed very rapidly at temperatures above 450dc(4).

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