THE MODELLING OF UF 6 TO UF 4 CONVERSION BY USING THE ATOMIC HYDROGEN GENERATOR

Transcription

1 THE MODELLING OF UF 6 TO UF 4 CONVERSION BY USING THE ATOMIC HYDROGEN GENERATOR B.P. Aleksandrov, V.E. Smirnov, V.N. Beznozdrev (NPO Energomash, Khimki, Russia) Introduction The problem of safe storage and utilization of a by-product of the nuclear fuel manufacture, the depleted on an isotope 35 U uranium hexafluoride UF 6 (DUHF), has reached a critical topicality. The quantity of DUHF that sent on storage in bulbs on the open areas increases by 70 thousand tons annually, over 1 million tons will accumulate by 030 [1]. The most effective and practical way of DUHF utilization is its defluorination, i.e. transformation of UF 6 into more chemically inert substances, for example, into uranium tetrafluoride UF 4, solid nonvolatile nonhygroscopic matter with fusion temperature about 960 C, which is practically insoluble in the water. Following technologies of DUHF defluorination have been developed and realized: two-stage thermal hydrolysis of uranium hexafluoride providing U 3 O 8 and HF at mass concentration of 67%; one-stage thermal hydrolysis of UF 6 in a boiling layer of uranium dioxide in a flow of inert gas; UF 6 reduction to uranium tetrafluoride in a gas phase by organic compounds; plasmachemical defluorination in water plasma resulting to U 3 O 8 and waterless hydrogen fluoride; plasmachemical DUHF reduction in hydrogen plasma with production of UF 4 and waterless HF; DUHF conversion to UF 4 and waterless HF in the fluorine-hydrogen flame. Unfortunately, productivity of these defluorination technologies is insufficient for the decision of problem of DUHF conversion in the foreseeable future. In works [, 3] the way of UF 6 reduction to UF 4 with the use of hydrogen atoms had been offered that is the development of UF 6 reduction in fluorine-hydrogen flame method allowing to raise processing productivity up many times. In this work the numerical study of possibilities of DUHF reduction methods with use of active medium generator of continuous chemical lasers (CCL) is presented. 1. A method of DUHF reduction with the use of the atomic hydrogen generator Molecule UF 6 has rather strong interconnections, and in regard to interaction of UF 6 with molecular hydrogen in a gas phase only reaction UF 6 +1/H UF 5 +HF is exothermic, nevertheless subsequent reactions of UF 5 UF with H are endothermic. The reduction of UF 6 by molecular hydrogen goes in two stages: UF 6 + H UF 4 (gas) + HF 4. KJ/mol (1) UF 4 (solid) + HF + 93 KJ/mol. (1a) The gaseous reaction (1) is thermoneutral nearly, and becomes exothermic after adhesion of uranium tetrafluoride into solid particles has happened only. Besides, this reaction is slow enough, the activation energy is E a ~ KJ/mol. The use of fluorine-hydrogen flame, i.e. the F, H, UF 6 mixture burning, is equivalent to initial reagents temperature increasing and so to the reaction acceleration. However, for thermoneutral chemical reaction initial reagents and products are in comparable amount (specified by a reaction equilibrium constant). Thus, it is impossible to achieve needed depth of UF 6 to UF 4 conversion in gaseous reaction (1) without an additional source of atomic hydrogen. The reduction reaction becomes exothermic after UF 4 condensation. To realize exothermic reduction mode with participation of reaction (1a) one need to satisfy inconsistent requirements: the temperature in a reaction zone should be high enough for intensive gaseous reduction reaction running, and low enough at the same time to realize UF 4 condensation and

2 the solid conglomerates formation providing nonnegative energy balance as a whole. To fulfill these inconsistent requirements is rather problematic. In the case of fluorine-hydrogen flame conversion the catalytic reaction goes on the submicron UF 4 particles formed on reactor walls. The catalysis reduces activation energy, and process goes at rather low temperature near walls when UF 4 is in solid phase, and reduction process is exothermic as a whole and so self-contained. At reactor scaling the role of surface effects will decrease in comparison with the volume ones, and the real productivity will decrease. In papers [, 3] the way of UF 6 to UF 4 reduction has been offered with the use of device that creates a flow of hydrogen atoms, and that in CCL is the active medium generator (AMG). As a matter of fact, this method is development of the fluorine-hydrogen flame conversion, allowing to raise productivity up many times. The schematic diagram of a reactor for UF 6 reduction with use of the atomic hydrogen generator is presented on fig. 1 (the variant of UF 6 feeding into the second zone, see below). The left part of a reactor till supersonic reduction zone is the AMG of CCL adapted for additional feeding of uranium hexafluoride into nozzle unit. In the first zone, i.e. in the combustion chamber, atomic fluorine creation occurs at primary fuel and oxidizer burning, combustion products arrive through supersonic nozzles into the second zone where secondary fuel supplies also through neighbor nozzles. After UF 6 reduction has been done the flow goes through ceramic-metal nickel filters into the storage tank of solid particles UF 4 and into a HF condensation compartment. Fig. 1. The schematic diagram of a reactor for UF 6 reduction with use of the atomic hydrogen generator in a variant with UF 6 feeding into nd zone The idea is the active atomic hydrogen created in the second zone after nozzle unit of GAS should tear off atoms F consistently, forming UF 5, UF 4 etc.: UF 6 + H UF 5 + HF + 87 KJ/mol; UF 5 + H UF 4 + HF KJ/mol. These gaseous reactions are strongly exothermic and fast enough at rather low temperatures, so no need the additional reactor warming owing to a strong self-heating. In the CCL atoms H produced in the second mixing zone of AMG as a by-product of fast pumping reaction of a chemical laser (the upper index reflects the fact of fuel feeding into second zone): F + H () HF * + H. () Atoms F are created in the combustion chamber under thermal dissociation of molecular oxidizer (F, NF 3 ) accompanying its burning with fuel (H, D, C H, C H 4, etc.). When the oxidizer is F, the fuel is H nearly full fluorine dissociation occurs in the first zone at temperatures T>1700K in accordance to reaction

3 [H (1) ] + α [F (1) ] (α 1) [F] + [HF]. (3) With regard to a method of DUHF reduction it is suitable to term the AMG of CCL as the generator of atomic hydrogen. Its important advantage is the possibility to operate at supersonic flows that allows to reach high efficiency of UF 6 reduction. The energy extracted in a reaction zone cannot move upwards, so it is possible to maintain the temperature of nozzle unit low enough, protecting it from destruction and from sedimentation of solid UF 4 on its surface. Should be noted that supersonic CCLs developed in Energomash RPA are capable to operate at productivity from the tenths to tens moles of hydrogen atoms per second that makes it possible to restore UF 6 at productivity up to some tons per hour (tens thousand tons per year) that exceeds the productivity of other DUHF reduction technologies. Some variants of reactor with different way of feeding of uranium hexafluoride into the generator after its gasification in bulbs at heating can be considered. It is necessary to take into account that the temperature of gaseous UF 6 should not fall in feeding path less then ~100C to avoid its condensation. In this work two variants of DUHF feeding are considered reflecting basic features of this conversion method. The variant 1: DUHF in a mix with fluorine and hydrogen goes into combustion chamber, combustion products pass to the second zone through oxidizer nozzles of the generator, and there hydrogen moves also through fuel nozzles. In this method twostage reduction is accomplished thermal conversion in the first zone and additional reduction by atoms H in the second zone. The variant : fluorine-hydrogen mix goes to the combustion chamber, DUHF together with hydrogen moves to the second zone through fuel nozzles, the reduction occurs owing to reaction of UF 6 with atomic hydrogen only.. Numerical model for UF 6 to UF 4 reduction with the use of atomic hydrogen The nozzle unit of flat classical design [4] was chosen for numerical modeling. A ratio of areas of nozzle's exit and throat section for an oxidizer and fuel nozzle has been fitted in compliance with needed exit pressure. The flows in nozzles were calculated by twodimensional model in the narrow channel approximation [5]. The chemical equilibrium state in the combustion chamber is used as initial data at oxidizer nozzle input section. The calculation of parameters in the second zone where the mixing of supersonic reacting flows occurs performed by the model based on full D Navier-Stokes equations [6]. For diffusion fluxes strict Stefan-Maxwell relations was used. For description of chemical and vibrational processes with participation of HF(v), H (v), F, F, H in a mixing zone well studied HF-laser kinetic model [7] was used. It has been added by following reactions with participation of uranium components (rate constants are in sm 3 /mol/c): 1. H + UF 6 UF 5 + HF, K 1 = exp ( 130/T) [8]; (4). H + UF 5 UF 4 + HF, K = exp ( 3140/T) (an estimation on [8]); (5) 3. H + (UF 5 ) UF 4 +UF 5 + HF; (6) 4. UF 4 + UF 6 UF 5 + UF 5, K 4 = exp ( 11380/T) [9]; (7) 5. UF 4 + UF 4 (UF 4 ), K 5 = ; [10] (8) 6. UF 5 + UF 5 (UF 5 ), K 6 = [10]. (9) 7. UF 6 + H UF 5 + HF +H, K 7 = exp ( 17370/T) [11]. (10) Rate constants for these reactions are studied poorly, there are few experimental and theoretical works. For process (5) the estimation was performed under comparison with a rate constant of process (4) taking into account the difference in U-F bond energy in compounds UF 6 and UF 5. The same rate constant for process (6) is accepted.

4 3. A method with UF 6 feeding to combustion chamber (variant 1) The fuel composition can be presented by the schematic formula: [H (1) ] + α [F (1) ] + (α 1) β [UF 6 (1) ] + (α 1) K [H () ]. Here β <1 is factor of UF 6 (1) deficiency with respect to the atomic fluorine in the combustion chamber, K> 1 is excess factor for H (). In this method two-stage reduction is accomplished incomplete thermal conversion in the first zone [1] and additional reduction of UF 6 and UF 5 by atoms H in the second zone. A disadvantage of the this way is necessity of passing a hot mix through supersonic nozzles with undesirable possible UF 4 conversion to the condensed phase at cooling and its sedimentation on nozzle walls. For minimization of this undesirable effect only those fuel compositions were considered that give small amount of uranium tetrafluoride in combustor products and the temperature is grater then the condensation temperature T c ~ K at pressures from 1 to 10 at [1]. a) b) Fig.. The pressure p(x,z) (a) and temperature T(x,z) (b) D distributions in the zone of an oxidizer and fuel mixing. h=14 mm; α=1.69, β = 0.54 The calculation results for the generator with nozzle's structure step of h=14 mm, for the pressure in combustion chamber of p c = 1.5 at and the fuel composition α=1.7, β = 0.54: [H ] (1) [F ] (1) [UF 6 ] (1) are presented on fig., 3 (direction X is along the flow, direction Z is across the flow, the calculation region is between nozzle's axis). The static pressure at nozzle's exit was chosen p 0 = 10 torr. To make the same pressure at the hydrogen nozzle exit plane the hydrogen flow rate corresponds to excess factor K=.. The total hydrogen flow rate across the unit nozzle's area for this variant is m H =0.05 g/sm /s, the fluorine flow rate is m F =0.31 g/sm /s, the uranium hexafluoride flow rate is m UF 6 =1.5g/sm /s.

5 a) b) Fig. 3. The distribution along a flow of component mole flow rates and of averaged temperature T av. A step of mixture of 14 mm (a), 7 mm (b). p 0 =10 torr. The distributions of pressure and temperature are shown on fig. at the distance of 1 cm from nozzle exit plane and on half period of nozzle's structure. One can see that the mixing field with atomic hydrogen formation begins directly at nozzle's exit plane and characterized by raised local pressure and temperature. The field of raised pressure forms the Mach cone caused by perturbation propagation with acoustic speed. The distributions downwards from nozzle unit of component's mole flow rates and of the static temperature averaged with the account of thermal capacities over cross section T av uc TdS p uc ds p are shown on fig. 3a. The uranium pentafluorid UF 5 undergoes practically complete fast dimerization near nozzle unit exit, raising average temperature of a flow on ~130 K. At distance of some millimeters from nozzle unit the reduction uranium fluorides processes are turned on that is visible from concentration of H atoms decreasing. The reduction of concentration UF 6 by a factor of two order and the production of target products UF 4 and (UF 4 ) ended practically completely at the distance of ~0 cm from a nozzle unit. The averaged static pressure grows quickly enough as the temperature: at distance of 1 cm to ~0 torr, at 6 cm to 3 torr, at 30 cm to 83 torr. The reduction length for this kinetics is about of 15 0 cm. Calculation has been executed also at reduced nozzle structure step h=7 mm, results are shown on fig. 3b. At more intensive mixing process the reduction zone, naturally, is shortened, in this case to ~3.5 cm that corresponds to the length of hydrogen diffusion to an oxidizer flow. However local areas with very high heating adjoining directly to nozzle unit are observed in this variant, it will be more difficult technologically to provide nozzle cooling at such step of mixture. The pressure at a nozzle's exit plane had been varied also. The higher pressure the easier pumping out system. The pressure increasing at nozzle exit by four times to p 0 = 40 torr at a step of 7 mm, naturally, slows down the speed of flows mixing and lengthens reduction zone to ~17 cm. Such pressure increasing, and, correspondingly, flow rate increasing leads to the static pressure raising at the end of reduction zone to ~0 torr that gives an ability to carry out an exhaust of products of combustion at atmospheric pressure rather easily. 4. A method with UF 6 feeding to the second zone (variant ) In this method DUHF feeds together with hydrogen through fuel nozzles just in the second zone. The initial fuel composition can be presented by the schematic formula:

6 [H ] (1) + α [F ] (1) + (α 1) K [H ] () + (α 1) β [UF 6 ] () ). Fig. 4. The distribution along a flow of mole fractions of components and of averaged temperature. A mixing step is h=7 mm. p 0 =10 torr The atomic fluorine concentration maximum is reached at α [F ]: [H ] ~3. In this case,5 molecules H and 1,5 molecules F are required for reduction of one UF 6 molecule at least, and 5 molecules HF are thus formed. The results of calculations for nozzle unit step of 7 mm for α=3, K=., β =0.3 are presented on fig. 4. The first flow molecular weight is μ (1) =19.5, of the second is μ ( =43.. A ratio of nozzle's exit areas has been matched as.5:1 as opposed to a variant 1 where an oxidizer nozzle exit area is more then of fuel nozzle one. Calculation was carried out for pressure at nozzle's exit is p 0 = 10 torr. The total hydrogen flow rate across the unit nozzle's area for variant is m H =0.08 g/sm /s, the fluorine flow rate is m F =0.44 g/sm /s, the uranium hexafluoride flow rate is m UF =1.58 g/sm /s. 6 Comparison with fig. 3b shows that there are essential falling of UF 6 reduction rate in comparison with a variant of UF 6 feeding into the combustion chamber. Full hexafluoride reduction and an output of a target product occurs at distance of ~15 cm while in a variant 1 at ~3.5 cm. The distinction is caused by lower averaged temperature of a gas flow at nozzle unit exit and its slower growth downwards on a flow. In addition, the feature of this scheme is the necessity to realize mixing process of supersonic flows, one contains atoms F, and the second H with UF 6 mixture. In this case the flows mixing occurs mainly owing to the diffusion of more light component. At first H diffuse in a flow containing atoms F, and then a product of reaction (), hydrogen atoms, diffuse back in H with UF 6 flow, providing the reactions (4), (5). Such two-stage diffusion mixing limits the rate of full reduction process. Conclusion The fulfilled calculations have shown in principle the possibility of uranium hexafluoride UF 6 reduction to essentially more chemically inert UF 4 with the use of generators atomic hydrogen of supersonic continuous wave chemical lasers at the productivity that exceeds up many times the productivity of other present DUHF reduction technologies. Variants considered in this work of using of atomic hydrogen generators developed by Energomash RPA allow to carry out defluorination of uranium hexafluoride with productivity of order ~5 g UF 6 /sm /s. Theoretically at continuous work of the atomic hydrogen generator of the area of 400 sm is capable to provide reduction with productivity up to ~7 tons of DUHF per hour at maintenance of necessary DUHF flow rate by gasification system. It is shown, that the design of a reactor for UF 6 reduction can be technological enough at a nozzle unit step of an order of 15 mm, thus the full reduction length is nearby 0 cm. The

7 pumping out system can be simple enough with an exhaust to target products selection systems at atmospheric pressure. At long operation the intensive cooling as combustion chamber and supersonic reduction chamber may be required, and possibly by near boundary cold gas screens. References 1. Management of Depleted Uranium. A Joint Report by the OECD Nuclear Energy Agency and IAEA, Gordon E.B., Dubovitsky V.A., Kolesnikov J.A., etc. "The mechanism of uranium hexafluoride reduction by hydrogen atoms". Materials of VII All-Russia (international) scientific conference Physical and chemical processes at atoms and molecules selection, Zvenigorod, 00, p Gordon E.B., etc. The Patent of the Russian Federation No. 0459, Chemical lasers / Ed. by Gross P. and Bott J Aleksandrov B.P., Stepanov A.A., Scheglov V.A. "Model of calculation of a gas flow in small-scale nozzles of cw chemical lasers", Quantum electronics, 1997, v.4, No., p Aleksandrov B.P., Stepanov A.A. "Numerical Simulation of Double-Band cw HF- HBr Chemical Laser", Proc. of SPIE, 007, v.6735, p Cohen N.J., Bott J.F. "Review of Rate Data for Reactions of Interest in HF and DF Lasers", Aerospace Corporation, El Segundo, Calif., Aerospace Rep. TR 0083(3603), Rienacker K. Kinetische. Untersuchungen von Reactionen des UF 6 in der Gasphase. Dissertation, Götingen, Max-PlanckInstitut für Stromung-forschung, Tumanov J.N., Galkin N.P. "About the uranium hexafluoride reduction mechanism by hydrogen", Atomic energy, v. 3, No. 1, 197, p Lewis W.B., Wampler F.B., Huber E.J., Fitzgibbon G.C. J. Photo-chem. 1979, v.11, p Myerson A.L., Chludzinski J.J. J. Phys. Chem v.85, p Beznozdrev V.N., etc. "About possibility of UF 6 to UF 4 reduction in initial H, F and UF 6 mixture at thermodynamic equilibrium". "Trudy NPO Energomash", 005, No.3, p