FRO104356

ATALANTE2000P3.13 Improved Fluoride Reprocessing for MOX Fuel Cycle

M. Takahashi, T. Fukasawa, T. Sawa, J. Yamashita, M. Kamoshida, A. Sasahira, F. Kawamura Nuclear Systems Division, Hitachi, Ltd., 3-1-1 Saiwai, Hitachi, Ibaraki, 317-8511 Japan Tel: +81-294-23-5458, Fax: +81-294-23-6626, E-mail: [email protected]

ABSTRACT Several countries had stopped developing fluoride volatility reprocessing method in the 1970's due to difficulties in recovering pure Pu. Although, nuclear societies recently favor dirty Pu (MOX) which has high proliferation resistance and needs remote fuel fabrication technologies. This situation reminded the authors to re-evaluate the fluoride volatility process. Preliminary investigation clarified that conventional fluoride volatility process could be simplified to recover dirty MOX and pure U from spent LWR fuels. Pure U is suitable to transfer it to re- enrichment (LWR cycle again), to storage certain period for future FBRs, and to dispose with relatively simple barrier. The improved process also enables to prepare directly the dirty MOX particles which are suitable for remote fuel fabrication (vibration packing). This paper describes the system of improved fluoride volatility reprocessing, and compatibility of each elemental process such as thermal decladding, two stage fluorination of U and U+Pu, U purification, direct conversion of mixed fluoride into oxide particles and vibration packing fuel fabrication.

INTRODUCTION The future nuclear fuel cycle system requires reduction of recycling cost, reduction of waste generation, minimization of environmental influence, much safety and assurance of Pu non-proliferation (Suzuki, 1999). Various recycle systems, such as advanced aqueous processes and dry pyrochemical methods with molten salt, are under development in Japan to meet these requirements (Asanuma, 1999; Asou, 1997; Inoue, 1999; Ion, 1999; Ueda et al., 1997; Mineo, 1999; Ojima, 1997; Wei, 1999). In these systems, several simplifications are made, for example, the elimination of purification process can decrease the capital and operation costs, and reduce the waste generation and radioactivity release to the environment, and the dirty products (U and Pu) are much more suitable for non-proliferation. But dirty U recovered is difficult to be handled in LWR fuel cycle (ex. re-enrichment) or in interim storage system for FBR fuel cycle or final disposal. This paper describes the concept of the Hybrid Recycle System (HRS), which is developed with improved fluoride volatility reprocessing technology and vibration packing technology for fuel pin production. In this system, is recovered with high purity and non-proliferation is assured since DF of (MOX) products is relatively low. CONVENTIONAL AND IMPROVED FLUORIDE VOLATILITY PROCESS Fluoride volatility reprocessing methods were developed in several countries from 1950s to 1980s as summarized in Table 1. Argonne, Brookhaven and Oak Ridge National Laboratories in United States started the research by using fluoride (BrF5 and C1F3) as fluorination reagents and carried out hot pilot scale tests with the capacity of 2-10kg/batch and 40kg/batch tests were done by using fiuidized bed reactors with actual spent fuels or scaled up capacity . Oak Ridge Gaseous Diffusion Plant designed the 300t/y (lt/d) scale fluoride volatility reprocessing facility. French Fontenay-aux-Roses laboratory also studied the process and constructed lOkg/batch facility for actual spent fuel reprocessing. Belgian status was similar to that in France. Japan Atomic Energy Research Institute had studied the process for 15 years and did tests with 5kg/batch U and bench scale Pu. Russian group, All Russian Institute of Chemical Technology, State Scientific Centre Research Institute of Atomic Reactors, RRC Kurchatov Institute and Nuclear Research Institute Rez pic (Czech) had a unique research program by using flame reactors for fluorination and curried out hot pilot scale tests with the capacity of l-3kg/h and continued the research until 1988. All these institutes except Russian group adopted fiuidized bed reactors, and finished the research late 1960's or early 1970s. The reason of their research termination was that the nuclear society needed not only pure uranium but also pure plutonium at that time and fluoride volatility reprocessing was difficult to get pure plutonium due to the unstable property of PuF6. So nuclear society had chosen the PUREX system for fuel reprocessing. But nowadays, the society requires dirty plutonium for non-proliferation and low cost system is required for future nuclear fuel cycle. Table 1 Development history for fluoride volatility reprocessing

1950 19551 1960 1965, 1970, 1975, 1980, 1985, 1990, BrF , Fluidized lOkg/batch, Hot Tests, Design (lt/d) ANL 5 3ed, I BrF5, Fluidized Bed 2kg/batch BNL I C1K , Fluidized Bed, 40kg/batch, Hot Tests ORNL 3 I Tests, Design (lt/d) ORGDP I I BrF5, CIF3. Fluidized Bed, lOkg/batch, Hot Tests FAR 1 I CIF3, Fluidized Bed, lOkg/batch, Hot Tests M ol 1 1 F2, Fluidized Bed, 5kg/batch, Hot Tests JAERf 1 1 Russian F2, Flame Reactor, 300g/h, l-3kg/h, Hot Tests, Design (70l/y) Group 1 ANL: Argonne National Laboratory, BNL: Brookhaven National Laboratory, ORNL: Oak Ridge National Laboratory, ORGDP: Oak Ridge Gaseous Diffusion Plant, FAR: Fontenay-aux-Roses lab., Mol: Centre de l'Energie Nucleaire, Mol, JAERI: Japan Atomic Energy Research Institute, Russian Group: (RRC Kurchatov Institute, RICT, RIAR, NRI), RICT: All-Russian Research Institute of Chemical Technology, RIAR: State Scientific Centre Research Institute of Atomic Reactors, Russia, NRI: Nuclear Research Institute Rez pic, Czech

A new concept called Hybrid Recycle System (HRS) was then proposed to recycle pure uranium and dirty plutonium (MOX) in the next generation fuel cycle. HRS consists of improved fluoride volatility reprocessing, direct oxide fuel particles production from mixed fluoride gas and vibration packing of the particle fuels. Figure 1 shows the block flow diagrams of the conventional and improved fluoride volatility reprocessing process. In improved process, Pu is not isolated but recovered with U as MOX and the purification step of the mixture of UF6 and PuF6 is eliminated.

U Spent fuel decladding u fluorination U purification UF,

BrF5 or dil. F2 - 1 Pu r Pu Pu fluorination Pu purification PuF,

cone. F..J (a) Conventional process

U Spent fuel j—^- decladding U fluorination U purification UF,

dil. F- J ±U,Pu U,Pu Pu fluorination 6' PuF6

cone. F. t (b) Improved process

Fig. 1 Block flow of conventional and improved fluoride volatility reprocessing process Figure 2 shows the conceptual flow diagram of HRS. Spent fuel is disassembled and fuel pins are chopped into short pieces. The fuel is pulverized and separated from cladding by cyclic oxidation and reduction of UO2 in the thermal decladding reactor. The oxide powder is loaded onto a fluidized bed type fluorination reactor with almost the same quantity of AI2O3.

Fluorine gas is introduced into the fluidized bed and a part of the U is volatilized as UF6. Aluminum oxide acts as the medium for homogeneous contact of fuel powder and F2 gas and for heat removal from the fluorination reaction. Using diluted F2 of about 20% results in the selective fluorination of U, leaving Pu in the bed. The fluorination reagent supply is stopped when the ratio of Pu and residual U is that desired for MOX fuel. Then, concentrated F2 gas is introduced into the bed for the co-fluorination of U and Pu.

Some FPs such as Nb, Mo, Tc and Ru are also fluorinated to form NbF5, MoF6, TcF6 and RuF6 respectively. These species are volatilized along with UF6 and PuFg (Levitz, 1969). The U volatilized in the first fluorination step is purified by chemisorption using fluoride adsorbent such as NaF and MgF2. The DF value can be expected to be about 105-107. In the fluoride volatility reprocessing, U can easily be purified by a simple procedure because of the marked difference in chemical properties of the fluorides of U and the FPs.

Purified UF6 is sent to the re-enrichment process or the de-fluorination process, where UFg is converted to UO2 for storage or disposal. Another product, a mixed fluoride of Pu and U, is converted to the corresponding mixed oxide. For conversion of fluoride to oxide, a pyrohydrolysis process is adopted because of simplification of the conversion and suitability for the vibratory packing in fuel fabrication process. The reaction of (MF6) with H2O and H2 at high temperature yields spheroidal or spherical grains of dioxide (MO2) by the following reaction (Knudsen et al, 1964).

MF6(g) + 2H2O(g) + H2(g) . MO2(s) + 6HF(g) (M = U, Pu) (1) The grains are sintered to increase their density and to remove impurity if necessary. The grain size can be controlled by the reaction conditions such as gas feed rate, reaction temperature and time. The resulting grains are vibratory-packed into fuel pins and the pins are sent to the fuel assembling process. The pyrohydrolytic method for converting fluorides to oxides can also be applied to de-fluorination of excess U.

Volatile FPs To reenrichment Spent Fuel A pure- UF6 or de-fluorination 5 7 process ^Fuel UO2/PuO2 = 10 ^ 10 ) Purification pins

H2 dirty- UO2/PuO2 O, PuF, (DF=1(T 1Q2)

U3O8/PuO2

Thermal decladding reactor (AIROX) H2O,H2 A Fuel Pin Fabrication ( Vibration Packing ) Fluorination reactor Pyrohydrolysis reactor (Alumina fluidaized bed) (Oxide conversion)

Fig.2 Conceptual flow diagram of HRS

FEATURES OF HRS The HRS has the following features. (1) Simple reprocessing and fuel fabrication In the improved fluoride volatility reprocessing, U can be purified easily by the chemisorption methods even if the U quantity is large. Since Pu can be recycled with a low DF in the FBR cycle, its purification step is eliminated. The Pu and U ratio in the MOX fuel is controlled during the fluorination step. Hence, a step for adjusting Pu content is eliminated.

The MOX grains can be obtained directly from gas of UF6 and PuF6. The grains are vibratory packed into fuel pins. Thus, the fuel fabrication process becomes relatively simple. (2) Compact and simple facility Spent fuel is not dissolved, but dispersed in AI2O3 in the improved fluoride volatility process. Since the ratio of

A12O3 and fuel is about 1:1, the process equipment size can be relatively reduced. (3) Low probability of Pu proliferation Plutonium is recycled with a low DF. Eliminating the Pu purification step suppress its loss into the waste stream. These ensure non-proliferation of Pu. (4) Simple waste management and disposal Storage and disposal of a large amount of excess U from LWR are simplified, since the U is highly decontaminated. The quantity of the absorbent for the U purification is small and leads to an insignificant increase of waste volume. The FPs, I29I and 99Tc, are long-lived nuclides and mobile in a deep ground environment. In the improved fluoride volatility reprocessing, I29I is volatilized in the de-cladding step by cyclic thermal treatment and concentrated in absorbent. 99Tc is fluorinated and volatilized with hexafluoride. Thus, the long-lived nuclides can be recovered in the fluoride volatility reprocessing. Therefore, high level waste contains no I and Tc. Nickel is used as material for the equipment in contact with fluoride and F2. Because the temperature of fluorination reaction is as high as 500., only limited corrosion occurs. Hence, the wastes generated with maintenance are expected to be relatively small. (5) Low cost for R&D The techniques for the improved fluoride volatility reprocessing and pyrohydrolytic conversion processsing are similar to those for U enrichment and conversion upstream in the present fuel cycle. The experience from the present fuel cycle can be applied to development of the HRS.

CONCLUSIONS As an advanced nuclear fuel cycle system, the Hybrid Recycle System (HRS) was proposed. The HRS includes the improved fluoride volatility reprocessing and the vibration packing MOX fuel fabrication process. In this system, U is purified to a high DF and Pu is recycled with a low DF. A simple process and compact facility for the nuclear fuel recycle can be realized by adopting the HRS. Waste management is rather simplified because excess U is highly decontaminated. Since Pu is recycled with a low DF, non-proliferation is ensured. The HRS satisfies various requirements of future fuel cycle systems.

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