Icapp2011

Proceedings of ICAPP 2011 Nice, France, May 2-5, 2011 Paper 11114

Development of FBR Fuel Reprocessing by Fluoride Volatility Method

Jan Uhlíř, Martin Mareček, Jan Škarohlíd, František Lisý and Richard Bican Nuclear Research Institute Řež plc CZ-250 68 Husinec – Řež 130, Czech Republic Tel: +420 266 173 548, Email: uhl@ujv.cz Abstract – Fluoride Volatility Method is regarded to be a promising advanced pyrochemical reprocessing technology, which can be used for reprocessing mainly of oxide spent fuels coming from future GEN IV fast reactors. The technology should be chiefly suitable for the reprocessing of advanced oxide fuels with inert matrixes of very high burn-up and short cooling time, which can be hardly reprocessed by hydrometallurgical technologies. Fluoride Volatility Method is based on direct fluorination of powdered spent fuel with fluorine gas in a flame fluorination reactor, where the volatile fluorides (mostly UF6) are separated from the non-volatile ones (trivalent minor actinides and majority of fission products). The subsequent operations necessary for partitioning of volatile fluorides are condensation and evaporation of volatile fluorides, thermal decomposition of PuF6 and finally distillation and sorption used for the purification of uranium product. Fluoride Volatility Method is under technological development in the Nuclear Research Institute Řež plc.

I. INTRODUCTION Fluoride Volatility Method (FVM) is regarded as a promising advanced pyrochemical reprocessing technology, which can be used for reprocessing mainly of oxide spent fuels coming from future Generation IV fast reactors (FR), especially of fast breeders (FBR). The technology should be chiefly suitable for the reprocessing of advanced oxide fuel types e.g. fuels with inert matrixes and/or fuels of very high burn-up, high content of plutonium and very short cooling time, which can be hardly reprocessed by hydrometallurgical technologies due to their high radioactivity.

Fluoride Volatility Method is based on a separation process, which comes out from the specific property of uranium, neptunium and plutonium of forming volatile hexafluorides whereas most of fission products (lanthanides) and higher transplutonium elements present in irradiated fuel form non-volatile trifluorides. This property has led to the development of several technological processes based on fluorination of irradiated fuel either by strong fluorinating agents like BrF3, BrF5, ClF3 or even by pure fluorine gas. Major former activities were carried out in 1950s and 1960s in US at Brookhaven, Argonne and Oak Ridge laboratories, in 1970s in France at Fontenay-aux-Roses and in Belgium at Mol, in 1970s and 1980s in former Soviet Union at Dimitrovgrad and in former Czechoslovakia at Řež.1,2,3,4

Original intention of the development of this dry reprocessing method of spent fuel was motivated in the past by the assumed commercial utilization of fast breeder reactors. Nowadays, the renewed interest in the dry (pyrochemical) reprocessing methods is motivated by requirements on the development of advanced fuel cycle technologies devoted to the Gen IV nuclear reactor systems. Among the pyrochemical technologies under present development, the Fluoride Volatility Method is almost the only one, which is not based on the use of molten salt techniques. Pyrochemical reprocessing based on Fluoride volatility technology is now under development in the Czech Republic.

II. DESCRIPTION OF FLUORIDE VOLATILITY PROCESS The reprocessing technology based on the Fluoride Volatility Method consists of the following main operations: 1. Fluorination of the powdered fuel (the purpose of this operation is the separation of the uranium component from plutonium, minor actinides and most of fission products) 2.

Purification of the products obtained

However before the fluorination of the powdered fuel, the removal of cladding material and subsequent

Proceedings of ICAPP 2011 Nice, France, May 2-5, 2011 Paper 11114

transformation of the fuel into a powdered form of a suitable granulometry has to be done. These two preparatory stages for FVM itself can be realized separately from the FVM. Suitable technology of the cladding material removal is melting in high temperature furnace. The cladding material of oxide fuel is either zircalloy (LWR) or stainless steel (FR) and both can be fully removed. Transformation of the fuel pellets into a powder is possible either mechanically by grinding or by partial oxidation of UO2 into U3O8. This chemical process is known as voloxidation. Either flowing air or oxygen at 575 °C to 650 °C is used as oxidizing agent.5 Flame fluorination reaction of a spent oxide fuel is a basic unit operation of the whole process. The reaction between the fuel powder and pure fluorine gas is spontaneous and highly exothermic. Principal fluorination reactions of main or significant fuel components are following: uranium: UO2 (s) + 3F2 (g) → UF6 (g) + O2 (g) or U3O8 (s) + 9F2 (g) → 3UF6 (g) + 4O2 (g) plutonium: PuO2 (s) + 2F2 (g) → PuF4 (s) + O2 (g) PuO2 (s) + 3F2 (g) → PuF6 (g) + O2 (g) PuF4 (s) + F2 (g)  PuF6 (g), Kp= [PuF6] / [F2] minor actinides: - neptunium: NpO2 (s) + 3F2 (g) → NpF6 (g) + O2 (g) NpO2 (s) + 2F2 (g) → NpF4 (s) + O2 (g) NpF4 (s) + F2 (g)  NpF6 (g), Kp= [NpF6] / [F2] - americium and curium: 2Am2O3 (s) + 6F2 (g) → 4AmF3 (s) + 3O2 (g), 2Cm2O3 (s) + 6F2 (g) → 4CmF3 (s) + 3O2 (g). lanthanides: 2Ln2O3 (s) + 6F2 (g) → 4LnF3 (s) + 3O2 (g), Based on these reactions, the main partitioning of spent fuel is realized directly in the fluorination reactor.

Whereas the volatile products of the fluorination reaction leave the apparatus, the non-volatile fluorides remain catched in the bottom part of fluorinator in the form of ash.

The further separation of most of individual components forming volatile fluorides is generally possible by sorption, condensation or distillation processes. While the fluorination of uranium into volatile hexavalent form is spontaneous, plutonium hexafluoride is thermally unstable and can be obtained only at considerable surplus of fluorine gas. Behavior of neptunium during the process varies between uranium and plutonium, however the

thermal stability of neptunium hexafluoride is substantially higher than of plutonium hexafluoride.

Final purification of uranium hexafluoride from MoF6, TcF6, IF5 and SbF5, which tend to accompany UF6 through the system, could be done by rectification process.

Distillation of UF6 is usually done in temperature range from 75 °C to 90 °C at pressure of about 2 atm in order to keep uranium hexafluoride in liquid form.

III. CURRENT STATUS OF R&D ON FLUORIDE VOLATILITY PROCESS Nuclear Research Institute Řež plc has been developing the FVM in the frame of fuel cycle of devoted to selected Generation IV reactor systems. The current experimental R&D program of the verification pyrochemical reprocessing of current and advanced oxide fuel types was opened in 2004.6 The process flow-sheet of the FVM, which was designed and calculated, is shown in Fig. 1. The process flow-sheet of investigated technology is based on the direct fluorination of powdered spent fuel by fluorine gas and on the subsequent purification of volatile products by condensation, sorption, thermal decomposition and rectification.

For the experimental verification of the technology, the semi-pilot line called FERDA was realized in radiochemical laboratory of NRI Řež plc. The FERDA line is an experimental technology of the batch character with the short-time capacity 1-3 kg of powdered fuel per hour.

The maximum capacity of volatile fluorides (UF6) accumulated in the technology is about 7 kg. Following technological apparatuses were designed and manufactured for the FERDA line:  Scroll feeders for powdered spent fuel with a capacity of 1 - 3 kg fuel/hour  Flame fluorination reactors  Condensers-evaporators for condensation and evaporation of volatile fluorides  Sorption columns  Distillation column for the rectification of UF6 MoF6 - TcF6 - IF5 - SbF5 mixture Current experimental R&D program is focused mainly to the technological verification of individual unit operations, apparatuses and material research and to the process control.7 Today, the main experimental effort is focused to the mastering of the fluorination process and to the elimination of some bottlenecks of the technology.

Proceedings of ICAPP 2011 Nice, France, May 2-5, 2011 Paper 11114

Fig. 1. Process flow-sheet of Fluoride Volatility Method After the first fluorination tests done with the uranium fuel based on U3O8 and UO2, the follow-up program has been aimed to the verification of the main unit operations with the simulated spent oxide fuel constituted from a mixture of uranium oxides and non-radioactive oxides representing selected fission products (lanthanides, Cs, Sr etc.). The objective of the experiments with simulated spent fuel has been the verification of the partitioning of volatile from non-volatile products of fluorination reaction. The simulated fuel consisted from uranium oxides and selected representatives of fission product elements simulated for this case by non-radioactive oxides (Y2O3, Nd2O3, CeO2, Sm2O3, BaO and SrO).

The realized fluorination experiments fully verified the ability of the technology to separate uranium in volatile form of UF6 from non-volatile simulants of fission products elements forming solid fluorides.

Whereas the volatile UF6 passed into the series of condensers, the fission products simulants were collected in the bottom part of the flame fluorinator in the special container. The presence of the fission products simulants, which was tested in the range of 3 – 5 %, has no measurable effect on the flame fluorination reaction.

The subsequent attention in the experimental program will be focused in addition to the further study of fluorination with simulated fuels and inert matrix fuels also to the study of purification of the uranium product (UF6) by rectification process. Because UF6 has not liquid phase at normal atmosphere, the process has to be performed under the pressure over two atmospheres when a liquid phase of UF6 exists.

IV. CONCLUSIONS Fluoride Volatility Method has a good potential to be used within the fuel cycles of several current or advanced reactor types for reprocessing of current or future advanced oxide fuel types. The main attractiveness of the technology can be found in the reprocessing of fast breeder reactors spent fuel owing to the extreme high radiation resistance of the used chemical agents (fluorine gas, inorganic fluorides), which allows to reprocess the spent fuels after the cooling time of about six months only.

Proceedings of ICAPP 2011 Nice, France, May 2-5, 2011 Paper 11114

Also the attractiveness to use FVM as the “Frontend” technology of Molten-Salt Transmutation Reactor is high due to the conversion of original oxide form of spent fuel into fluorides – the chemical form of Molten-Salt Reactors. As the basic principles of Fluoride Volatility Method were already verified the present R&D effort should be focused in the first instance to the engineering development of individual apparatuses and the pilot technology to eliminate technological bottlenecks of the process.

ACKNOWLEDGMENTS Current research and development on the FVM is realized in the Nuclear Research Institute Řež plc thanks to existing financial support of the Ministry of Industry and Trade of the Czech Republic.

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M. Benedict, T. H. Pigford, H. W. Levi, In: Nuclear Chemical Engineering, Second Edition, McGraw-Hill Book Company, New York, 1981, Chapter 10, p. 476

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M. Mareček, F. Lisý and J. Uhlíř, “Technological development of fluoride volatility method for frontend of molten salt transmutation reactor fuel cycle”, Proc. InWor for P&T and ADS 2003, Mol, Belgium, October 6 – 8, 2003

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J. Uhlíř, M. Mareček, “Fluoride volatility method for reprocessing of LWR and FR fuels“, J. of Fluorine Chemistry, 130, 1, pp. 89-93 (2009)