Claudia Castro

Martínez Cédric

Brandam Felipe
email: cedric.brandam@ensiacet.fr

Ramón Portugal

Pierre Strehaiano

A model based on factorial design to predict the evolution of Brettanomyces sp. population as function of environmental conditions in winemaking

Keywords: Microbial growth, lag phase, Brettanomyces, mathematic model, wine

Contamination of wines by

Brettanomyces is an increasing industrial problem. Here, we propose a model able to predict the evolution of a Brettanomyces population as function of environmental conditions as temperature, ethanol concentration or dioxide sulfur concentration. Polynomial models obtained from factorial design realized on synthetic medium gave the lag phase, the specific growth rate and the maximal biomass concentration as function of these medium factors. A logistic model was then used with the determined growth parameters to represent the evolution of Brettanomyces population. The factorial design showed that temperature has an important effect on maximal biomass concentration and specific growth rate but a negligible effect on lag phase. Conversely, molecular sulfur dioxide had an important effect on lag phase but a little influence on growth parameters. Ethanol concentration played an important role on the three parameters tested.

INTRODUCTION

In wine industry, the market competition is more and more important with notably the arrival of "wines from new world". Today, the wine quality and the productivity are factors more considered than yesterday in winemaking. Winemaking needs micro-organisms to ferment sugars into alcohol, to decrease acidity of wine (malo-lactic fermentation) or to produce aromas and other sensorial properties. With these objectives, selected strains are used in the beginning of the fermentation. Nevertheless, natural microorganisms can contaminate the must because it is a not sterilized process. Among these contaminants, the yeast Brettanomyces/Dekkera is particularly undesirable. It can be found in must but also in wine (Fulgelsang 1997). Typically, these yeasts grow after alcoholic and malolactic fermentation during storage of wine in tanks, barrels, or bottles [START_REF] Chatonnet | The influence of Brettanomyces/ Dekkera sp. Yeasts and lactic acid bacteria on the ethylphenol content of red wines[END_REF]. They are considered as organoleptic contaminants of wine because they produce volatile phenols (4ethylphenol and 4-ethylgaiacol). These components are usually described in sensorial analysis as responsible for "horse sweat" or "mousy taint" flavours [START_REF] Heresztyn | Formation of substituted tetrahydropyridines by species of Brettanomyces and lactovacillus isolated from mousy wines[END_REF][START_REF] Chatonnet | The influence of Brettanomyces/ Dekkera sp. Yeasts and lactic acid bacteria on the ethylphenol content of red wines[END_REF]. For about twenty years, some works have been realized to understand why Brettanomyces appears or not during fermentation and wine storage. However, the influence of the environmental factors has been little studied. Particularly, it is known that growth of yeasts in general is influenced by temperature and ethanol concentration but little and controversial data are available for the genus Brettanomyces. In winemaking, sulfur dioxide is often added to avoid all type of contamination, but its effect of on Brettanomyces growth is not yet really evaluated. In this context, the aim of this work was to develop a model to predict the evolution of Brettanomyces population in wine as function of different factors as ethanol concentration, temperature and sulfur dioxide concentration. A three factorial design was done to evaluate the influence of these three parameters. We obtained polynomial models to determine classical parameters of yeast growth: lag phase, specific growth rate and maximal population. A logistic model was then used to represent the growth phase. Finally, we were able to simulate the evolution of Brettanomyces population for a medium with a known composition in ethanol and sulfur dioxide and for a fixed temperature.

MATERIALS AND METHODS

Yeast strain

The Brettanomyces bruxellensis strain used in this work was isolated in a winemaking plant in France. Its identification was made using PCR by IDAC laboratory (Nantes, France).

Culture media

Cultures were made on synthetic medium close to wine. The composition was in g/L: glucose, 10; fructose, 10; MgSO 4 .7H 2 O, 0.4; KH 2 PO 4 , 5; (NH 4 ) 2 SO 4 , 0.5; yeast extract, 0.5; citric acid, 0.3; malic acid, 3; glycerol, 6. Sterilization was made during 15 min at 120°C. O 2 is:

Culture conditions

Preculture was prepared in two steps of 35 h each. First, in medium without ethanol; then, 3% (v/v) of ethanol was added in culture medium. The experiments were carried out in cotton plugged Erlenmeyer flasks of 500 mL with a 300 mL working volume at 150 rpm and without aeration. After sterilization, the medium was inoculated with 3x10 6 cells/mL. Temperature was controlled. Before inoculation, the alcoholic concentration was set to the wanted level by addition of ethanol. The initial pH value was adjusted and the initial free sulfur dioxide concentration was fixed.

Analytical techniques

Biomass concentration was followed by two techniques. A correlation between the optical density of yeast suspension at 620 nm and the cell dry weight gives weight concentration.

Data treatment

Experimental biomass concentrations were smoothed by a cubic spline function. The lag phase was then predefined as the time needed to double the initial biomass (λ=2Xo). The specific growth rate max and the maximal biomass concentration Xmax were equally obtained from smoothed experimental data.

The statistical software (Statgraphics, Centirion XV) was used to analyze the results of the factorial design. In order to determine the sensibility of the calculated parameters, an analysis of variance (ANOVA) was done.

RESULTS AND DISCUSSION

Role of pH

Preliminary experiments were done to evaluate the effect of the pH on microbial growth and lag phase. The pH values tested were 3.2 and 3.6, with no sulphur dioxide, for two ethanol concentrations (7 and 10% v/v) and two temperatures (18 and 25°C). The results obtained showed that pH had no influence on the lag phase and growth (data not showed). So, pH was not taken into account in the factorial design as individual factor. However, it is well known that pH plays an important role for the action of sulphur dioxide on microbial growth. When SO 2 is added in a medium, a part is combined with other constituents of wine and a part remains free. Depending on pH value, the free SO 2 is partially dissociated and transformed in molecular SO 2 . Only the molecular SO 2 is able to enter the cell and act on micro-organism activity. The amount of available molecular SO 2 is function of pH value and free SO 2 concentration. The equation used to calculate the molecular S

    1 10 81 . 1 2 2    pH

FreeSO O MolecularS

As consequence of this preliminary study, pH value allows adjusting the desired initial molecular SO 2 .

Factorial design

A 2 3 factorial design with four central points was used to determinate the significance of individual and interaction effects of the initial ethanol concentration, initial molecular SO 2 concentration and temperature on the maximal growth (X max ), maximal specific growth rate (µ max ) and lag phase (λ). The value factors are presented in table 1. The results of the adjusted models obtained for maximal biomass concentration, maximal specific growth rate and lag phase as function of the more significant variables were: Where E is the initial ethanol concentration, SO 2 m the initial molecular sulfur dioxide concentration and T the temperature.

X max =
The analysis of variance (ANOVA) showed that µ max and λ presented a high correlation coefficient (>0.95) and X max the highest coefficient (0.99).

The polynomial models could be considered statistically significant according to the T-test with 95% of confidence.

The figures 2 to 4 show the Pareto charts for the three studied responses. They allow visualizing the significance of each factor on studied responses.

Maximal biomass concentration and maximal specific growth rate

Figure 2 concerns the maximal biomass concentration and figure 3 the maximal specific growth rate. The ethanol concentration (E) and the temperature (T) had a significant effect on these two factors whereas the molecular SO 2 (SO 2 m) was not significant. For maximal biomass concentration, no interaction factor was significant whereas for maximal specific growth rate ethanol-temperature and ethanol-sulphur dioxide interactions were little significant. Finally, the most important effects were those of ethanol concentration and temperature for these two growth parameters. Ethanol concentration had a negative effect whereas temperature increased the growth rate and the quantity of produced cells. The inhibitory activity of ethanol can be linked to the cell membrane permeability. Its toxicity is associated with the solubility in membrane lipids [START_REF] Leao | Effects of ethanol and other alkanols on the glucose transport system of Saccharomyces cerevisiae suspensions[END_REF]. Moreover, the ethanol was found to inhibit hexokinase and some amino-acid synthesis enzymes and thus metabolic deficiencies on the cell reproduction [START_REF] Nagodawithana | Study of the feedback effect of ethanol on selected enzymes of the glycolitic pathway[END_REF]. On the other hand, the positive effect of the temperature can be related to the increase of the membrane fluidity. It is equally associated to the increase in the affinity of the protein transport for substrates [START_REF] Nedwell | Effect of low temperature on microbial growth: lowered affinity for substrates limits growth at low temperature[END_REF]). [START_REF] Silva | Studies on the wine spoilage capacity of Brettanomyces /Dekkera sp[END_REF] showed the same behaviour as regards of temperature and ethanol concentration for Brettanomyces and Dekkera in wine.

Lag phase

Figure 4 shows that lag phase was mainly affected by ethanol and molecular SO 2 concentrations. These two factors increased the lag phase. The interaction of the ethanol and molecular SO 2 was also very significant with a synergic effect. A slightly negative effect was observed for the temperature. The other interactions were not really significant. Traditionally, the molecular SO 2 is used as an antimicrobial agent in winemaking [START_REF] Beech | Action antimicrobienne de l'anhydride sulfureux[END_REF]. The addition of this compound in wine decreases the specific ATPase activity and as consequence increases lag phase. In the cell, sulphur dioxide may react with proteins, nucleic acids and some cofactors. It can inhibit enzymes and as consequence cells are unable to replicate [START_REF] Romano | Sulfur dioxide and wine microorganisms[END_REF].

Model for Brettanomyces growth

The factorial design enabled to determine the main influent factors on Brettanomyces growth. It gave three polynomial models to determine λ, max and Xmax as function of temperature and medium composition in ethanol and sulphur dioxide.

To represent the evolution of the Brettanomyces population, we used the logistic model [START_REF] Verhulst | Recherches mathématiques sur la loi d'accroissement de la population[END_REF]). These model is known for its "goodness of in a atch culture. The general form of the logistic equation is:

fit" to describe the dynamic of the biomass b

   max max X dt      1 X X dX  (1)
The integration of equation 1 gives:

) 1 ( ) ( 1 ) ( max max t o t o e X e X t X      (2)
s not allow representing nts nd modeled values for biomass concentrations nce f initial molecular SO 2 on the lag phase but also the absence of effect on the growth rate (5C). max

X

The parameters Xmax and max were calculated for each experiment with the polynomial models. As the logistic model doe the lag phase, it must be combined with the lag phase polynomial model. A good correlation between experimental poi a was obtained (> 0.95) for all the experiments.

Figure 5 shows examples of biomass profiles for 6 experiments. The temperature increase favoured the growth rate and the maximal population but didn't act on the lag phase (5A). Moreover, the increase of ethanol concentration increased the lag phase and decreased the maximal population obtained (5B). We can equally see the importa

CONCLUSIONS

This work showed that high concentrations in ethanol led to decrease the maximal biomass concentration and maximal specific growth rate. Temperature acted equally on growth but by increasing the maximal specific growth rate and the biomass concentration. For the lag phase, the initial ethanol and molecular SO 2 concentrations had a great effect. The interactions between these two factors were equally very important, increasing the lag phase. These compounds are well known as inhibitors of micro-organisms [START_REF] Moulin | Inhibition of alcoholic fermentation[END_REF][START_REF] Du Toit | Microbial spoilage and preservation of wine: using weapons from nature's own arsenal-review[END_REF].

It is interesting to remark that SO 2 delayed the beginning of the growth but didn't act on the growth rate and on the quantity of final biomass. The mechanism of action of this compound remains still little known. Similar results were obtained by [START_REF] Medawar | Yeast growth: lag phase modeling in alcoholic media[END_REF] on another Brettanomyces strain.

In this work, we established a model to predict the evolution of Brettanomyces population in a synthetic wine medium. Industrially, the problem is not really the Brettanomyces presence but some by-products as ethyl-phenol and acetic acid produced by this yeast. The following of this study is to determine the relation between the growth of Brettanomyces and the production of these undesirable compounds.

  1.80 -0.21 E + 0.16 T µ max = 0.039 -0.009 EOH + 0.011 T -0.003 E*SO 2 m -0.004 E*T λ= 104.8 + 78.3 EOH + 72.5 SO 2 m -10.9 T + 73.2 EOH*SO 2 m -6.0 EOH*T -4.2 SO 2 m*T

Figure 2 .Figure 3 .

 23 Figure 2. Pareto chart of all effects on maximal biomass concentration.

Figure 4 .

 4 Figure 4. Pareto chart of all effects on lag phase.

Table 1 .

 1 Coded factor levels and real values for the experimental design

	Factor	Low value	High value
		(-1)	(+1)
	Ethanol (% v/v)	7	10
	(mg/L) Molecular SO 2	0.16	0.39
	Temperature (°C)	18	25


C Albasi

R Van Kaam

M Alliet

D Anne-Archard

HYDRODYNAMIQUE DE L'AERATION ET COLMATAGE DANS UN BIOREACTEUR à MEMBRANE IMMERGEE

Une analyse rhéologique de la boue d'un bioréacteur à membranes immergées a été réalisée afin de relier ses caractéristiques au mode de gestion du procédé : La boue est un fluide viscoplastique. Une étude spécifique a démontré qu'elle semblait pouvoir se restructurer si elle était soumise à une contrainte de cisaillement inférieure à la valeur du seuil d'écoulement. La contrainte de cisaillement dans le réacteur étant directement induite par l'aération, des expériences ont été réalisées en injectant l'air de façon intermittente. Elles ont démontré que ce mode d'aération permet de préserver l'intégrité de la boue.

Introduction :

La gestion du colmatage dans les procédés à membranes est à la base de nombreuses études. Certains auteurs cherchent à comprendre son développement par une analyse des phénomènes intervenant dans la formation du colmatage [1][2]. D'autres s'orientent plus sur l'optimisation dans le choix des moyens de combattre ce colmatage à l'aide d'un mode de filtration adapté [3][4][5]. Dans le cas particulier des BAMI tout le monde s'accorde à dire que l'aération est le paramètre clé afin de prévenir de ce phénomène. Il permet en effet de générer de la turbulence dans le milieu [6] et limite ainsi l'apport de matière à la surface de la membrane. Récemment Gui et al. [7] ont démontré qu'au delà d'un certain débit, l'aération pouvait avoir un effet destructeur. Ce paramètre est également le plus coûteux énergétiquement [8][9]. Une bonne gestion en est donc primordiale.

Pour pouvoir proposer une gestion de ce paramètre on a alors essayer de comprendre, par des études rhéologiques comment réagissait la boue aux contraintes de cisaillement dans un BAMI. De nombreuses études ont déjà abordé sous l'angle rhéologie la liqueur mixte présente dans les procédés de boues activées [10][11] mais peu font mention d'une telle caractérisation dans le cas des BAMI [12] et aucune ne s'en servent réellement pour proposer une gestion du procédé. Cette étude a pour but de caractériser la rhéologie d'une boue activée issue d'un BAMI en vue d'identifier une stratégie dans la gestion du colmatage.

Matériel et méthodes : 2.1 Description du pilote et des conditions opératoires

L'ensemble de l'étude a été réalisé en utilisant un bioréacteur à membranes immergées de conception classique (Hernandez et al, 2005 [13]). Ce réacteur est composé d'une colonne en PVC de 15 L, (1 m de haut), dans laquelle est immergé le module membranaire. Les modules fabriqués par la société Polymem ont une Congrès MEMPRO3 'Intégration des MEMbranes dans les PROcédés, 5-7 avril 2006, Nancy, France surface de 0,3m 2 . Les fibres creuses sont en polysulfone (De = 0,72 mm, Di = 0,28 mm, seuil de coupure 0,1 micron., perméabilité initiale de 200 à 400 L/h.bar.m² ).

Le réacteur est ensemencé par de la boue activée issue de la station d'épuration de Toulouse Ginestous. L'alimentation du réacteur se fait ensuite par le haut. Le perméat est aspiré à travers la membrane par une pompe PCM Moineau série 1 Monobloc et stocké dans un réservoir de 30 L pressurisable. La pression transmembranaire, le débit de perméation, la température, le pH sont enregistrés toutes les secondes à l'aide d'un programme d'acquisition automatique DasyLab. L'apport d'oxygène nécessaire à la bonne épuration est assuré par une diffusion continue de fines bulles d'air en base de colonne à 50 L/h à travers une membrane Flygt. Ce débit permet d'être constamment en saturation d'oxygène dans le milieu. Cette aération conduit de plus à une homogénéisation de la liqueur mixte et à une mise en suspension de la phase solide. La dernière partie de l'étude a été réalisée en utilisant un effluent domestique issu de la collecte des eaux usées de Brax (31) commune de 2000 habitants sans apport d'effluent industriel. La valeur de la DCO oscille entre 300 et 600 mg/L.

L'effluent

Contrôle du colmatage

Toutes les expériences ont été réalisées à un flux constant brut de 10 L/h.m² en accord avec des résultats de filtration de boue activée obtenus au préalable au sein de l'équipe [14]. Le protocole de filtration du pilote est basé sur une séquence de filtration-arrêt avec éventuellement introduction de rétrolavages. Pour contrôler le colmatage il faut prévenir l'apport de matière ou favoriser son relargage en induisant un mouvement des fibres et/ou du fluide proche de la membrane. On a opté pour l'injection de grosses bulles en cinq points à la base de la membrane Une électrovanne permet de contrôler le mode d'injection de ces bulles soit en mode continu soit avec un temps d'arrêt dans l'admission d'air. 1,0E-03

Rhéologie

1,0E-02 1,0E-01 1,0E+00 1,0E+01 1,E-03 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 1,E+04 Contrainte (Pa) 1,00E-03 1,00E-02 1,00E-01 1,00E+00 1,00E+01 1,00E+02 1,00E+03 Viscosité (Pa.s) contrainte viscosité Gradient vitesse (s -1 ) 1,0E-03 1,0E-02 1,0E-01 1,0E+00 1,0E+01 1,E-03 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 1,E+04 Contrainte (Pa) 1,00E-03 1,00E-02 1,00E-01 1,00E+00 1,00E+01 1,00E+02 1,00E+03
Viscosité (Pa.s)

contrainte viscosité

Gradient vitesse (s -1 )

Figure 1 : Rhéogramme en contrainte croissante

Pour valider cette hypothèse, on a réalisé des expériences en oscillation, qui ont confirmé que la boue est un fluide viscoplastique avec un seuil d'écoulement autour de 0,1 Pa.

3.2b Expériences sur effluent domestique

Les expériences précédentes ont été réalisées avec de la boue alimentée par un effluent synthétique. On peut alors se demander si la caractérisation de ce fluide est liée à l'effluent ou si c'est une caractéristique du procédé BAMI. Les analyses rhéologiques d'une boue alimentée par une eau domestique ont alors été réalisées. Dans un premier temps elles ont consisté en une comparaison des propriétés de la boue d'ensemencement et la boue une fois stabilisée. Une dernière manière d'étudier le comportement sous cisaillement de la boue est de travailler en imposant des contraintes en palier. Lorsqu'on applique ou lorsqu'on supprime brutalement un cisaillement, va se manifester un régime transitoire pendant lequel la structure et donc la viscosité de l'échantillon évoluent avant de se stabiliser [START_REF] Tixier | Approches de propriétés rhéologiques de suspensions biologiques floculées[END_REF] [START_REF] Tixier | Approches de propriétés rhéologiques de suspensions biologiques floculées[END_REF]. L'expérience suivante consiste alors en une montée en contraintes par paliers croissants de 120 secondes de 0,05, 0,1, 0,5, 1 et 2 Pa, suivie d'une descente symétrique (figure 3) -La contrainte de 0,1 Pa imposée tend dans un sens à déstructurer, désorganiser la boue. La viscosité chute alors.

-On observe ensuite une réaugmentation de cette viscosité malgré l'application toujours constante de la contrainte, comme si une force opposée à la force tangentielle imposée par la contrainte tendait à restructurer la boue

Utilisation de la rhéologie dans la gestion du procédé :

3.3a simulation d'une aération intermittente avec un rhéomètre. Pour vérifier cette hypothèse une dernière expérience a été réalisée sur un échantillon de boue dont le seuil d'écoulement a été déterminé à 0,01 Pa. L'expérience a consisté à simuler le syncopage de l'aération avec le rhéomètre. On a réalisé une expérience en imposant des contraintes en palier d'une minute à 0,05 Pa suivi d'un palier de 6 minutes à 0,005 Pa et on a suivi l'évolution de la viscosité au cours du temps. Les résultats de cette expérience sont représentés sur la figure 5. Selon cette figure, on s'aperçoit bien que pendant la période où la contrainte est à son maximum (0,05 Pa) la viscosité du fluide est à son minimum (3.10 -3 Pas) ce qui tend à prouver que la boue est dans son état de défloculation le plus avancée. En revanche quand on impose une contrainte plus basse (0,005 Pa) la viscosité a tendance à croître et peut atteindre une valeur de 0,1 Pa.s. Cette expérience semble donc bien démontrer la capacité de restructuration de la boue soumise à des contraintes de cisaillement intermittentes.

3.3b Intérêt de l'aération intermittente dans un BAMI

Pour minimiser la consommation énergétique engendrée par l'injection de ces bulles il faut parvenir à minimiser la dérive de pression tout en utilisant un débit le plus petit possible. Des expériences préliminaires au sein de l'équipe [START_REF] Albasi | Récents progrès en génie des procédés[END_REF] avaient démontré qu'un débit d'air proche de 50 L/h suffisait pour limiter le colmatage sur deux jours d'expériences. On a donc choisi de travailler autour de ce débit mais distribué différemment.

-Dans une première expérience l'air a été injecté à 50 L/h en continu (flux net de 166 L/h.m²), -Dans une deuxième expérience un débit de 200 L/h d'air a été injecté une minute toutes les six minutes (flux net 111 L/h.m²).

Congrès MEMPRO3 'Intégration des

Conclusion

Cette analyse rhéologique a permis de démontrer que la boue activée présente dans le bioréacteur possédait les propriétés d'un fluide viscoplastique. Cela signifie que la boue possède des propriétés de restructuration si la contrainte de cisaillement qu'on lui impose n'est pas continue. Un temps de non application de contrainte est nécessaire. D'un point de vue gestion du colmatage dans le BAMI nous apportons ici une interprétation du bénéfice retiré d'une aération intermittente dans les BAMI : le cisaillement engendré par l'aération non continue permet à la boue de retrouver au moins partiellement sa structure et par la suite une bonne aptitude à la filtration.

3. Résultats : 3 . 2

 32 L'ensemble des études rhéologiques a été réalisé sur un rhéomètre Bohlin C-Vor à 23°C. Ce travail a été effectué selon deux protocoles bien distincts : viscosimétrie et oscillation. Le premier mode aussi appelé de cisaillement simple renseigne essentiellement sur le comportement en écoulement des matériaux alors que le Congrès MEMPRO3 'Intégration des MEMbranes dans les PROcédés, 5-7 avril 2006, Nancy, France deuxième, nommé aussi mode de mesure dynamique, permet de caractériser un comportement viscoélastique des matériaux. (Van Kaam, (2005),[15]) Caractérisation rhéologique de la boue d'un BAMI 3.2a Expériences réalisées sur effluent synthétique. Selon la courbe réalisée en contrainte croissante (figure 1), il semble que la boue suive un modèle de fluide viscoplastique. Les valeurs de viscosités sous 0,1 Pa (valeur approximative du seuil) sont en effet comprises entre 100 et 1000 Pa.s, pour chuter rapidement et se stabiliser autour de 3 10 -3 Pa.s après le seuil.

Figure 2 :

 2 Figure 2 : Comparaison des échantillons en oscillation à fréquence de 0,1 Hz boue STEP : boue d'ensemencement boue de BAMI : boue alimentée par un effluent domestique et stabilisée Pour confirmer la différence de comportement entre l'échantillon d'ensemencement et celui de la boue une fois stabilisée on a opéré des analyses rhéologiques en oscillation. Les résultats de ces analyses présentés sur la figure 2, confirment clairement la différence de comportement. La boue en phase stabilisée est viscoplastique avec un seuil d'écoulement qui se trouve autour de 0,1 Pa alors que la boue d'ensemencement ne montre pas de seuil. Ces résultats confirment donc que la viscoplasticité du fluide est bien une propriété intrinsèque du procédé BAMI et non pas liée à l'effluent.Une dernière manière d'étudier le comportement sous cisaillement de la boue est de travailler en imposant des contraintes en palier. Lorsqu'on applique ou lorsqu'on supprime brutalement un cisaillement, va se manifester un régime transitoire pendant lequel la structure et donc la viscosité de l'échantillon évoluent avant de se stabiliser[START_REF] Tixier | Approches de propriétés rhéologiques de suspensions biologiques floculées[END_REF] [START_REF] Tixier | Approches de propriétés rhéologiques de suspensions biologiques floculées[END_REF]. L'expérience suivante consiste alors en une montée en contraintes par paliers croissants de 120 secondes de 0,05, 0,1, 0,5, 1 et 2 Pa, suivie d'une descente symétrique (figure3)

Figure 3 :

 3 Figure 3 : Rhéogramme par paliers de contraintes.

Figure 4 :

 4 Figure 4 : Rhéogramme par paliers de contraintes croissantes (zoom). Ce résultat est très intéressant d'un point de vue de la gestion du colmatage. En effet il est reconnu que plus la boue est floculée et plus aisée sera la filtration. Il apparaît donc d'après ces derniers résultats qu'il faille minimiser la contrainte de cisaillement au sein du réacteur. Or dans le cadre des BAMI, l'aération génère le plus de cisaillement nécessaire à la limitation du colmatage. Un compromis est donc à trouver afin de satisfaire à ces deux exigences antagonistes. Une aération intermittente pourrait être un bon compromis. En effet on peut émettre l'hypothèse que ce mode d'aération permettrait d'induire un cisaillement suffisant à la limitation du colmatage

Figure 5 :

 5 Figure 5 : contraintes en palier : 0,5Pa / 1 minute et 0,05 Pa / 6 minutes.

  MEMbranes dans les PROcédés, 5-7 avril 2006, Nancy, France -Une troisième expérience a été réalisée avec un débit d'air de 370 L/h d'air mais en espaçant la fréquence d'injection toutes les 12 minutes (flux net 100 L/h.m²). A la fin de chaque expérience la membrane est sortie du réacteur et remplacée par une autre afin de partir du même état initial. En parallèle de ces expériences on a suivi l'indice de décantation de la boue selon ces différents modes d'aération. Cette expérience a été réalisée afin d'étudier l'impact du mode d'aération sur l'état de la boue. Les résultats sont consignés dans le tableau 2. Il est intéressant de noter que l'indice de boue décroît avec une diminution de la fréquence d'aération. Pour une aération continue la boue est non décantable alors que pour une aération syncopée 1/12 l'indice de décantation souligne une boue ayant une bonne floculation. Il semble donc qu'une augmentation du débit d'aération ne perturbe pas l'état de floculation de la boue. En revanche plus l'aération est fréquente (tendance à être continue) et plus l'état de la boue s'en trouve perturbé (augmentation de son indice de boue). Ces expériences confirment les résultats obtenus au paragraphe 3.3a : le fait d'injecter l'air de façon intermittente permet à la boue de se restructurer. Tableau 2 : Evolution de l'indice de boue selon le mode d'aération Mais pour pouvoir conclure quant à la réelle efficacité de l'intermittence de l'aération, il faut évaluer l'état de colmatage de la boue selon ces différents modes d'aération. Les dérives de pression associées à ces différentes expériences sont donc représentées sur la figure 6.

Figure 6 :

 6 Figure 6 : pression transmembranaire selon le mode d'injection du débit d'aération pour une boue stabilisée à un âge de boue de 14 jours.Les dérives de pression diminuent avec la fréquence d'aération. Pour une aération continue on observe une faible dérive de pression pendant les trois premiers jours.

  Deux types d'effluent ont été utilisés lors de cette étude : La majeure partie de l'étude a consisté au traitement d'un effluent synthétique à base de Viandox. (dilué 2 fois et complété en azote par ajout de 4,5 g/L de NH 4 Cl). Les caractéristiques similaires à celles d'une ERU sont rapportées sur le tableau 1.

	Tableau 1 : Composition de l'effluent synthétique
	DCO	130 g/L
	DBO	85 g/L
	NTK	8.75 g/L
	N-NH 4	1.9 g/L
	P-PO 4	1.1 g/L

Remerciements

Ce travail a reçu l'appui financier de la région Midi Pyrénées et de la société POLYMEM. Les auteurs ont bénéficié de l'utilisation à titre gracieux du rhéomètre de l'Institut de Mécanique des Fluides de Toulouse dans le cadre de la fédération de recherche FERMaT.


P Souček

L Cassayre

R Malmbeck

E Mendes

J.-P Glatz

ELECTROREFINING OF U-PU-ZR ALLOY ON SOLID AL ELECTRODES IN LICL-KCL EUTECTIC MELT INITIALLY CONTAINING ZR IONS

Electrorefining process in molten chloride salts using solid aluminium cathodes to recover actinides (An) is developed in ITU. Complete recovery of actinides in electrorefining of An-Zr alloy fuels can not be achieved without co-dissolution of Zr. It has been shown in previous experiments that the presence of Zr ions resulted in a significant deterioration in quality of the deposit. The present work has been focussed on a better understanding of the electrochemical behaviour of Zr ions in LiCl-KCl melt. In addition, electrorefining of UPuZr alloy in a melt containing dissolved Zr at the beginning of the process has been carried out. At first, Zr 4+ content was chemically reduced by addition of UPuZr alloy forming a dispersion of Zr-containing particles in the melt. Electrorefining process in this modified melt yielded compact deposits consisting of UAl 3 , UAl 4 , PuAl 3 and PuAl 4 alloys.

Introduction

Partitioning and transmutation technologies are developed worldwide in order to minimize the long term radiotoxicity of the spent nuclear fuel produced during operation of the conventional nuclear reactors. Management of partitioning processes is also a key point in a development of future reactor systems, where application of an advanced nuclear fuel cycle is considered [START_REF]A Technology Roadmap for Generation IV Nuclear Energy Systems[END_REF][START_REF]Accelerator Driven Systems (ADS) and Fast Reactors (FR) in Advanced Nuclear Fuel Cycles -A Comparative Study[END_REF][START_REF] Magill | [END_REF]. Pyrochemical techniques represent a promising alternative to aqueous reprocessing processes. Electrorefining and liquid -liquid metal reductive extraction are the most developed pyrochemical techniques designed for a group selective separation of actinides (An) from a mixture with lanthanides (Ln) in a molten salt media [START_REF] Conocar | Promising Pyrochemical Actinide/Lanthanide Separation Processes Using Aluminium[END_REF].

Electrorefining process in the molten chloride salts using solid aluminium cathodes is being developed in ITU. In this process, actinides are recovered from a metallic An-Zr alloy fuel in a form of An-Al alloys. In previous experiments, excellent grouped separation of actinides from lanthanides has been demonstrated [START_REF] Serp | Electroseparation of Actinides from Lanthanides on Solid Aluminium Electrode in LiCl-KCl Eutectic Melts[END_REF][START_REF] Cassayre | Investigation of electrorefining of metallic alloy fuel onto solid Al cathodes[END_REF]. A very high capacity of aluminium to load actinides has been also experimentally proven [START_REF] Cassayre | Investigation of electrorefining of metallic alloy fuel onto solid Al cathodes[END_REF][START_REF] Souček | Electrorefining of metallic U-Zr and U-Pu-Zr-alloy fuel onto solid Aluminium cathodes in molten chlorides[END_REF]. However, a complete recovery of actinides during electrorefining of An-Zr alloy fuels can not be achieved without co-dissolution of Zr [START_REF] Ahluwalia | [END_REF]. It has been observed in previous experiments that the presence of Zr ions resulted in a deterioration in quality of the deposit [START_REF] Souček | Electrorefining of metallic U-Zr and U-Pu-Zr-alloy fuel onto solid Aluminium cathodes in molten chlorides[END_REF].

The presented work was focussed on a better understanding of the electrochemical behaviour of Zr ions in LiCl-KCl melt and a determination of Zr distribution during the electrorefining process has been estimated. On basis of the obtained results, electrorefining of U-Pu-Zr alloy (71-19-10 wt. %) in a melt containing Zr ions at the beginning of the process has been demonstrated.

Experimental

All experiments, storage and handling of chemicals were carried out in a glovebox under purified Ar atmosphere (<5 ppm of water and oxygen). The detailed description of the technical background is described elsewhere [START_REF] Souček | Electrorefining of metallic U-Zr and U-Pu-Zr-alloy fuel onto solid Aluminium cathodes in molten chlorides[END_REF]. Cyclic voltammetry measurements were carried out using a three-electrode setup and a PAR 273 potentiostat with EG&G M270 electrochemical software. The working electrode was prepared using a metallic wire (d = 1 mm) and inserted approximately 5 mm into the bath, while a Mo wire bent into the shape of a spiral served as the auxiliary electrode. The active part of the cathode surface area could be determined after each experiment by measuring the immersion depth of the wire.

The reference electrode used was an Ag/LiCl-KCl-AgCl (1 wt %) prepared in a Pyrex glass tube and all potentials stated in this work are related to the corresponding reference potential. During electrorefining experiments, working electrode was replaced by an aluminium plate and a tantalum basket containing oxidised material was inserted instead of the auxiliary electrode. Both anodic and cathodic potentials were monitored during the electrorefining experiments using a specially prepared combination of two multimeters with a laboratory power supply.

The initial bath was prepared by oxidation of Zr metal (ITU stock material) in 40.259 g of LiCl-KCl eutectic melt (Aldrich 99.99%) by reaction with BiCl 3 . Reduced Bi metal was collected in a Bi pool at the bottom of an Al 2 O 3 crucible. The initial concentration of Zr 4+ ions in the melt was 0.55 wt.%. The chemical composition of the salt was monitored by ICP-MS analysis [START_REF] Abousahl | Development of quantitative analytical methods for the control of actinides in a pyrochemical partitioning process[END_REF] of regularly taken salt samples dissolved in 1 M nitric acid.

3.

Results and discussion

Cyclic voltammetry

Cyclic voltammetry (CV) measurements were performed on inert W and reactive Al working electrodes. A comparison of voltammograms measured on W electrode at temperatures 450 and 500°C is shown in Figure 1a. Two electrochemical systems were detected: (i) a pair of peaks II c and II a (E p,c = -1.20 V at both temperatures, E p,a = -0.85 V / -0.67 V at 450 / 500°C, respectively) and (ii) two low intensity waves I c and I a at more positive potentials. The waves I indicate a soluble-soluble transition most likely associated with Zr 4+ /Zr 2+ reduction. The peaks II have a characteristic shape corresponding to a formation of an insoluble product, which is probably Zr metal, while an over-potential is required to facilitate the deposition of the metal. This feature became slightly more significant at 500°C, as the observed difference between potentials of the cathodic and anodic peaks was greater. The electrochemical reduction of Zr 4+ in molten LiCl-KCl can therefore be interpreted as a two-step reaction mechanism on an inert working electrode, which is in a good agreement with the available published data [START_REF] Smolenski | A Potentiometric Study of the Interaction of Zr(IV) and O(II) Ions in the LiCl-KCl Eutectic Molten Salt[END_REF].

A comparison of CVs measured on both electrodes at 450°C is shown in Figure 1b. On the reactive aluminium electrode (bold curve), one intensive cathodic peak was detected at E p,c = -1.26 V, followed only by a negative current slope. As the Zr-Al phase diagram [START_REF] Massalski | Binary Alloy Phase Diagrams[END_REF] contains several different alloys, this peak was expected to be associated with ZrAl 3 alloy formation. However, the reduction peaks starts at the same potential as the one measured on W electrode, which could indicate a metal deposition. A sharp current increase was detected on the anodic side, which disabled distinguishing between re-oxidation of reduced Zr species and dissolution of Al.

Drawdown of Zr dissolved in LiCl-KCl

In order to avoid complications associated with the presence of Zr dissolved in the bath used for the electrorefining process, the possibility of Zr ions removal was examined (i) by chemical reaction with aluminium and (ii) by electrorefining of zirconium metal using solid Al cathode.

The chemical drawdown of Zr was studied in stationary conditions at 450°C using aluminium plate with dimensions 14×18×0.5 mm immersed into the melt composed of LiCl-KCl-ZrCl 4 (0.55 wt.% ∼ 221 mg of Zr). A formation of ZrAl 3 alloy according to the reaction scheme (1) was expected, as a direct reduction yielding Zr metal and AlCl 3 is thermodynamically impossible.

3ZrCl 4 + 13Al → 3ZrAl 3 + 4AlCl 3 (1)
No data concerning Zr-Al alloys are available to perform relevant calculations.

According to the ICP-MS analyses of the salt samples before and after the experiment, the complete Zr content was reduced. At the end of the experiment, the concentration of Zr in the bath dropped below the detection limit. However, only a very small amount of reduced Zr was found in the deposit on the Al plate (19 mg, ICP-MS analysis of the complete deposit scraped from the electrode and dissolved in nitric acid). The aluminium concentration in the melt increased to 0.18 wt. %, which supports the occurrence of reaction (1), however the aluminium mass balance is likely affected by AlCl 3 evaporation.

- The possibility of electrodeposition of Zr on solid Al electrode was investigated using electrorefining of Zr metal in the melt resulting from the chemical drawdown experiment, i.e. LiCl-KCl-AlCl 3 (0.18 wt.%). Two connected runs were carried out using Al plates with the active area app. 5 cm 2 and a charge of 4070 C was passed during the experiment. If the charge consumed for reduction of the aluminium present in the salt at the beginning of the experiment (0.075 g) is removed, then the remaining charge of 3265 C for Zr electrodeposition corresponds to 0.772 g of reduced Zr. The deposits from both runs were scraped from the electrodes, dissolved and analysed by ICP-MS. The total Zr and Al masses in both deposits were determined to be only 0.008 g and 0.011 g, respectively.

However, Zr concentration in the salt remained less than 0. During the Zr electrorefining, 6 salt samples was taken and dissolved in water instead of HNO 3 . However, only 2 samples proved the existence of non dissolved particles in the melt. It can be explained by a high heterogeneity of its distribution in the melt resulting in significant difficulties in sampling. The calculation was made in order to estimate the expected mass of Zr particles in the salt m (Zr particles ) according to the following equation (2):

) ( ) ( ) ( ) ( 0 dissolved particles Zr m F z Q Zr M Zr m Zr m - ⋅ ⋅ + = , ( 2
)
where m(Zr 0 ) is the initial expected mass of Zr particles generated during the previous experimental steps (estimated accordingly) and m(Zr dissolved ) is the mass of Zr analysed by ICP-MS in the salt at the corresponding phase of electrolysis. As shown in Table 1, both analysed values fit within a reasonable range with the expected values calculated according to equation (2).

Zr distribution during electrorefining process

The melt was left equilibrated after the electrorefining runs and the additional treatment was realized to determine the final Zr distribution in the system and to prepare a bath for the UPuZr electrorefining experiment. The following steps were performed:

1. Crucible was broken and the Bi pool with Zr particles absorbed during equilibrating was removed 2. Melt was re-melted in a new crucible and a fresh Bi pool was introduced 3. BiCl 3 was stepwise introduced to oxidise Zr particles remaining in the melt 4. U-Pu-Zr alloy was introduced to reduce Zr ions oxidised in step 3 by reaction with Pu and U (more detailed description of the step 4 see in the following chapter 3.4 due to a thematic overlap)

The mass of the metallic Zr particles presented in the melt was evaluated both from the known mass of added BiCl 3 (0.339g Zr, step 3) and from the mass of oxidised U and Pu obtained from ICP-MS analyses of the salt samples (0.277 g Zr, step 4). The average value, 0.308 g, was taken for the mass balance evaluation. It can be seen that majority of the Zr particles fell down to the Bi pool during the equilibration of the salt, because 0.808 g of Zr particles was analysed to be presented in the melt at the end of the electrorefining step (2.00 wt.%, see Table 1, the last row). Samples from the removed Bi pool were analysed by the ICP-MS to complete the mass balance and an average value of 0.806 g of Zr was detected. The summary of the Zr distribution is shown in Table 2. However the sum of input and output mass is in a reasonable agreement, the evaluated distribution is only approximate due to a high heterogeneity of the sampled material and uncertainties of the mass determination during the applied chemical processes.

Table 2 Mass balance of the Zr distribution

The process was followed by the CV measurements and the results correspond well to the expected composition of the bath. An Al system was detected after the step 2, only peaks associated to a Zr system were observed after the step 3 and finally, a clear spectrum with peaks corresponding only to U and Pu systems was measured after the step 4.

UPuZr electrorefining

Electrorefining of the UPuZr alloy (ITU stock material, 71-19-10 wt.%) was carried out after the above described treatment to evaluate the feasibility of the process starting in a melt initially containing Zr ions (melt after step 3, i. e. LiCl-KCl-ZrCl 4 ). The Zr content was calculated to be 0.75 wt.%, but the exact analysis of the bath is not available at this step. A piece of UPuZr alloy (m 0 = 1.746 g) was introduced to the melt in a tantalum basket to reduce the Zr content by reaction with Pu and U. After the reduction was completed, the basket was connected as an anode and used for the electrorefining experiment. The mass and composition of the starting anodic material was calculated to be m 1 = 0.835g and U:Pu:Zr = 68.5:10.5:21.0 according to the composition of the bath before start of the electrorefining, which was analysed as LiCl-KCl-UCl 3 (1.39 wt.%)-PuCl 3 (0.74 wt.%). An aluminium plate used as the cathode had the dimensions of 18×14×0.5 mm.

Galvanostatic electrolysis was carried out and the maximum applied current was controlled with respect to the cathodic potential, which was limited by a value of -1.3 V to follow the conditions enabling An/Ln separation. Currents in a range between 20 -50 mA were applied and 845 C passed during the experiment. The average value of the cathodic potential was -1.15 V, which corresponded to the actinide-aluminium alloy formation, and it had a slowly decreasing tendency. Up to the passed charge of 600 C, anodic potential remained constant at -1.0 V, which corresponded to U and Pu oxidation only. Then it increased up to -0.8 V, which indicated the start of Zr co-oxidation due to the actinides content decrease.

After termination of the run, a solid, compact and metallic-shiny deposit was found on the electrode surface. The electrode containing the adhered salt was analysed by γ-spectroscopy to determine the total mass of deposited actinides and the mass ratio between uranium and plutonium. After that, the vacuum distillation of the electrode was performed at a temperature of 450°C, but only 0.096 g (22.6 % of the expected value) of the salt was removed. After the distillation, the deposit was scraped out and analysed by XRD. The evaluated results are summarized in Table 3. The aluminium/actinides mass ratio (Al:An) was calculated using data from γ-spectroscopy. XRD analysis proved the presence of UAl 3 , UAl 4 , PuAl 3 and PuAl 4 alloys, but it was not possible to quantify the mass ratio among the presented species. XRD patterns for U-and Pu-Al alloys are at the same positions and in addition, an unidentifiable specie was detected, which is most likely a Zr-Al alloy. However, the Zr content was not positively proved by XRD in this case.

Conclusions

Studies of Zr electrochemical behaviour were carried out in eutectic LiCl-KCl-ZrCl 4 (0.55 wt.%) melt at 450 and 500°C on inert W and reactive Al electrodes. On inert electrode, a two step reduction mechanism was observed. Zr starts to be reduced at a potential of -1.1 V vs. Ag/LiCl-KCl-AgCl (1 wt.%) reference electrode, which is preceded by reduction of Zr 4+ to Zr 2+ . On reactive Al electrode, one reduction step was observed most probably yielding Al-Zr alloys. However, the reduction starts at the same potential as detected on the inert electrode. Aluminium dissolution occurs practically at the same potential as Zr metal re-oxidation detected on W electrode. This potential overlap indicates that a molten chloride salt containing dissolved Zr and solid Al creates a very complicated system, enabling spontaneous chemical reactions between Al and Zr ions.

It has been shown that Zr ions presented in the melt can be completely chemically reduced both by solid Al and by actinides from a fresh UPuZr alloy. Formed product, most likely ZrAl 3 alloy, does not stick on the Al surface during chemical reduction and also during the electrorefining of Zr metal, almost no Zr content was detected in the deposit. Reduction of Zr ions yields a fine dispersion of Zrcontaining solid particles in the bath.

The electrorefining of UPuZr alloy on solid Al electrode was successfully carried out in LiCl-KCl melt, which contained app. 0.75 wt. % of Zr 4+ at the beginning of the process. Initially, Zr ions were completely reduced by the fresh fuel. Electrorefining process was carried out in the formed bath and compact metallic deposit consisting from UAl 3 , UAl 4 , PuAl 3 and PuAl 4 alloys was obtained on Al electrode with a very high current efficiency. Although Zr was co-oxidised during the last third of the process, it did not significantly influence the quality of the deposit.

Fig. 1 a

 1 Fig. 1 a) Comparison of CV measured on W electrode at 450 and 500°C b) Details of CV measured on W and Al electrodes at 450°C

Table 1

 1 11 wt %, i.e. max. 0.045 g of Zr was dissolved (see Table1 below). Reduced Zr thus most likely fell down from the electrode and remained in the salt in a form of fine metallic particles. Concentration profile during the Zr electrorefining experiments

	Run	Total charge [C]	Zr dissolved analysed [wt. %]	Zr non-dissolved calculated [wt. %] analysed[wt. %]	Al dissolved analysed[wt. %]
	E-1	0	0.00	0.13	< dl	0.18
		252	0.00	0.28	< dl	0.28
		932	0.11	0.56	< dl	0.15
		1981	0.03	1.25	1.60	0.03
		2860	0.01	1.78	< dl	0.05
	E-2	4070	0.08	2.43	2.00	0.05

Table 3

 3 Mass balance of the UPuZr electrorefining

		Input	Output	Distribution
	mass	where	mass	where	ratio [%]
	221 mg initial Zr mass in the salt	32 mg Zr dissolved in the melt	2.8
	962 mg	Zr oxidised during electrorefining	308 mg Zr metallic particles in the melt 806 mg Zr in Bi pool 8 mg Zr deposit on the electrodes	26.7 69.8 0.7
	1183 mg sum input	1154 mg sum output		100.0


R Malmbeck
email: rikard.malmbeck@ec.europa.eu

L Cassayre

E Mendes

J Serp

P Sou Ek

J.-P Glatz

RECOVERY OF THE ACTINIDES BY ELECTROCHEMICAL METHODS IN MOLTEN CHLORIDES USING SOLID ALUMINIUM CATHODE

An electrorefining process in molten chloride salts is being developed at ITU to reprocess the spent nuclear fuel. According to the thermochemical properties of thesystem, aluminium is the most promising electrode material for the separation of actinides (An) from lanthanides (Ln). The actinides are selectively reduced from the fission products and stabilized by the formation of solid and compact actinide-aluminium alloys with the reactive cathode material. In this work, the maximum loading of aluminium with actinides was investigated by potentiostatic and galvanostatic electrorefining of U-Pu-Zr alloys. A very high aluminium capacity was achieved, as the average loading was 1.6 g of U and Pu into 1 g of aluminium and the maximum achieved loading was 2.3 g. For recovery of the actinides from aluminium, a process based on chlorination and a subsequent sublimation of AlCl 3 is proposed.

I. INTRODUCTION

Most innovative reactor systems are conceived to have an integrated waste burning and a homogenous recycling of all actinides [START_REF]A Technology Roadmap for Generation IV Nuclear Energy Systems[END_REF]. Therefore, the fuels used in these reactors will be significantly different from the commercial fuels of today. Because of the fuel type and the very high burn-ups reached, traditional hydrochemical reprocessing such as used today will probably not be the preferred method. The main reasons are the limited solubility of the fuel materials in acidic aqueous solutions and the poor radiation stability of the organic solvents used in extraction processes. For this reason different pyrochemical separation techniques are under development, liquid metal-molten salt extraction [START_REF] Conocar | Extraction behavior of actinides and lanthanides in a molten fluoride/liquid aluminum system[END_REF][START_REF] Kinoshita | Separation of Uranium and Transuranic Elements from Rare Earth by Means of Multistage Extraction in LiCl-KCl/Bi System[END_REF] and electrodepostion [START_REF] Iizuka | Behaviour of Plutonium and Americium at Liquid Cadmium Cathode in Molten LiCl-KCl Electrolyte[END_REF][START_REF] Koyama | An Experimental Study of Molten Salt Electrorefining of Uranium Using Solid Iron Cathode and Liquid Cadmium Cathode for Development of Pyromettalurgical Reprocessing[END_REF][START_REF] Laidler | Development of Pyroprocessing Technology[END_REF][START_REF] Nishimura | Development of an Environmentally Bening Reprocessing Technology -Pyrometallurgical Reprocessing Technology[END_REF][START_REF] Sakamura | Separation of Actinides from Rare Earth Elements by Electrorefining in LiCl-KCl Eutectic Melts[END_REF] are the two major processes used. Due to the chemical similarity between trivalent Ln and minor actinides (MA), an efficient separation of Am and Cm from Ln, is the most difficult to achieve.

According to the known activity coefficients of Pu, U and Ce in different reactive metals, aluminium has been identified as the electrode material enabling the highest separation factors of actinides from lanthanides (Ln) [START_REF] Conocar | Promising Pyrochemical Actinide/Lanthanide Separation Processes Using Aluminium[END_REF]. This has led to the development of a process based on the separation of actinides by reductive extraction into a molten Al [START_REF] Conocar | Extraction behavior of actinides and lanthanides in a molten fluoride/liquid aluminum system[END_REF]. In ITU, an electrorefining process in molten chloride salt using solid aluminium cathodes is being developed. [START_REF] Serp | Electroseparation of Actinides on Solid Aluminium in LiCl-KCl Eutectic[END_REF]. In this process, actinides are recovered from a metallic An-Zr alloy fuel in the form of An-Al alloys. A very high selectivity of the process for recovery of An from Ln has been already shown as well as the capability of solid aluminium electrodes to form stable An-Al alloys using Al rods, foils [START_REF] Conocar | Promising Pyrochemical Actinide/Lanthanide Separation Processes Using Aluminium[END_REF] and a foam [START_REF] Serp | Electroseparation of Actinides from Lanthanides on Solid Aluminum Electrode in LiCl-KCl Eutectic Melts[END_REF] as the cathodes. In addition, a large-surface aluminium electrode has been used to study the maximum amount of actinides that can be collected on a single electrode by a constant current electrolysis [START_REF] Cassayre | Investigation of Electrorefining of Metallic Alloy Fuel onto Solid Al Cathodes[END_REF].

In this work this issue has been further investigated by U and Pu electrodeposition applying potentiostatic and galvanostatic electrolytic techniques using Al plates of different dimensions. The experiments were carried out with U-Pu-Zr alloy and a very high capacity of aluminium to alloy An was demonstrated.

The recovery of actinides from the An-Al alloys formed during the electrorefining is a key point in order to complete the actinide separation process. A process based on chlorination of the An-Al alloy was selected and evaluated. A thermochemical study of each step of the chlorination process was performed: (i) distillation of the residual salt incorporated in the deposit, (ii) chlorination of the alloy and (iii) removal of formed AlCl 3 by sublimation. According to the conditions defined by this study, the first experiment was performed showing promising results.

II. EXPERIMENTAL

All experiments, storage and handling of chemicals were carried out in a glovebox under purified Ar atmosphere (<5 ppm of water and oxygen). The detailed description of the technical background is described elsewhere [START_REF] Soucek | Electrorefining of Metallic U-Zr and U-Pu-Zr -Alloy Fuel onto Solid Aluminium Cathodes in Molten Chlorides[END_REF]. A bath consisting of U 3+ , Pu 3+ and Gd 3+ ions dissolved in LiCl-KCl eutectic mixture (Aldrich 99.99%) was prepared by oxidising of U-Pu-Zr metallic alloy (ITU stock material, 71-19-10 wt. % respectively) introduced into the melt in a tantalum basket by adding BiCl 3 according to the reaction (1).

An 0

Bi-ph. + Bi 3+ salt-ph.

An 3+ salt-ph. + Bi 0 (An = U, Pu) (1)

Reduced Bi metal was collected in a Bi pool at the bottom of an Al 2 O 3 crucible. GdCl 3 (Aldrich 99.99%) was used as received from the producer without any additional treatment. The initial concentrations in the bath were analysed to be 3.85 wt. % of U, 1.53 wt. % of Pu and 1.61 wt. % of Gd.

Cyclic voltammetry measurements were carried out using a three-electrode setup and a PAR 273 potentiostat with EG&G M270 electrochemical software. The working electrode was prepared using a wire (d = 1 mm) from the desired material (W, Al) and inserted 5 mm into the bath. A Mo wire bent into the shape of a spiral served as the auxiliary electrode and a reference electrode used was an Ag/LiCl-KCl-AgCl (1 wt %) prepared in a Pyrex glass tube. All potentials stated in this work are related to the corresponding reference potential.

Electrorefining experiments were performed at 450°C and the working electrode was replaced by an aluminium plate. A tantalum basket filled with U-Pu-Zr alloy (71-19-10 wt.%, ITU stock material) was inserted instead of the auxiliary electrode. The experimental set-up is shown in Fig. 1. Both anodic and cathodic potentials were monitored during the electrorefining experiments using a specially prepared combination of two multimeters with a laboratory power supply. The expected electrode reactions were anodic oxidation of actinides from the alloy and their reduction on the cathode forming An-Al surface alloys according to the following reactions (2, 3):

Anodic: M 0 Zr M 3+ + 3e -+ Zr (2) Cathodic: M 3+ + 3e -+ zAl M 0 Al z (M = U, Pu) (3)
Two techniques were used to examine the maximum possible loading of the solid Al electrodes: Galvanostatic electrorefining limited by cut-off potentials in order to ensure a selective An separation, and potentiostatic electrorefining. In the galvanostatic mode, the maximum current was reached after applying a stepwise current increase to keep both cathodic and anodic potentials within the selected limits. The cathodic cut-off potential was set to a value of -1.25 V which ensures conditions for a selective group separation of An from Ln [START_REF] Serp | Electroseparation of Actinides from Lanthanides on Solid Aluminum Electrode in LiCl-KCl Eutectic Melts[END_REF][START_REF] Cassayre | Investigation of Electrorefining of Metallic Alloy Fuel onto Solid Al Cathodes[END_REF]. A potential of -1.00 V was used as the anodic cut-off potential to avoid oxidation of Zr from the U-Pu-Zr alloy. The electrolysis was terminated either when too low current was reached. For all runs realized under potentiostatic conditions, a cathode potential of -1.25 V was applied.

The purity of the bath was regularly checked by cyclic voltammetry (CV) measurements using inert tungsten electrodes. The chemical composition of the salt was determined by ICP-MS analysis of regularly taken salt samples dissolved in 1 M nitric acid. After the electrorefining, adhered salt on the electrodes was removed by vacuum distillation and/or by washing in ethanol. SEM/EDX was used for surface analysis and the deposit composition was investigated by XRD and gamma spectrometry [START_REF] Abousahl | Development of Quantitative Analytical Methods for the Control of Actinides in Pyrochemical Partitioning Process[END_REF].

The chlorination experiments were carried out in a glove box under nitrogen atmosphere, specially designed for work with Cl 2 (g). The box is equipped with a chlorine gas line ended by a system of gas wash bottles containing KOH solutions (4 and 8 M), where non reacted chlorine gas is absorbed. A quartz reactor used for the chlorination was placed in a vertical oven. Chlorine gas was introduced by a quartz tube guided through the lid of the reactor by a gastight connection. The lid was also equipped by sealed ports for a thermocouple and the gas outlet connected to the chlorine off gas treatment. After the reactor was filled with chlorine gas, it was isolated and the gas flow was stopped. Boron nitride crucibles were used as containers for the chlorinated material and positioned on the bottom of the reactor. For the chlorination experiments, a well defined UAl 3 alloy was prepared by arc melting at a temperature >3000°C to represent An-Al alloys from electrorefining experiments. The composition determined by XRD was 98 wt.% of UAl 3 , 1 wt.% of UAl 2 and 1 wt.% of Al.

III. RESULTS AND DISSCUSSION

III.A. Electrorefining experiments

Two galvanostatic runs (labelled 1 and 2 in TABLE I below) were carried out using Al electrodes made from plates 0.5 mm thick with active areas 8 and 9 cm 2 . An excess of U-Pu-Zr alloy was introduced as anodic material to avoid Zr co-dissolution during the electrolyses. The anodic potential varied between -1.30 and -1.00 V, which corresponds to a congruent oxidation of the actinides from U-Pu-Zr alloy. Selective reduction of uranium and plutonium forming a U-Pu-Al surface alloy occurred at the cathode and the potential showed a decreasing trend, starting from -1.05 V and in the end reaching the minimum value of -1.25 V. With the build up of the An-Al surface alloy layer, the cathodic reaction is more and more limited by diffusion of the reduced elements forming the surface alloy layer, i.e. U, Pu and Al. A typical record of potentials and current for the galvanostatic electrolysis is presented in Fig. 2.

Fig. 2 Evolution of cathodic and anodic potentials during the galvanostatic electrolysis

In addition, two potentiostatic runs (labelled 3 and 4 in TABLE I below) were performed using small area electrodes (1.5 and 3.6 cm 2 ) made from plates of 0.5 and 2.0 mm thickness. The cathodic potential was kept constant at -1.25 V in both runs to ensure a selective deposition of only actinides. In this case, the current gradually decreased with increasing thickness of the formed U-Pu-Al surface alloy and the anodic potential dropped from -1.0 V up to -1.3 V. The record of the anodic potential development and the current for a typical potentiostatic electrolysis is shown in Fig. 3.

After the electrodeposition, the electrodes were left for some time just above the bath at 450°C in order to let the adhered salt drop down. For the galvanostatic runs, a vacuum distillation was used to remove the salt remaining on the electrode surface. A quantitative Pu assay of the electrodes with the deposits was performed byspectrometry in far-field counting geometry. Separate measurements with close sample-to-detector distances were carried out for the determination of the Pu/U ratio using gamma rays from 239 Pu and 235 U, knowing the isotopic composition of Pu and U in the used alloy. The results are summarised in TABLE I together with an evaluation of the loading of the electrodes with actinides. The weight percent ratio of An in the Al cathode was calculated according to equation (4):

Al An An m m m Al in wt An .% (4)
The mass of actinides m An resulting from -spectroscopy measurements was used for the calculation and the relevant aluminium mass was derived from the alloy thickness and the measured immersion depths of the electrodes.

III.B. Characterization of the formed alloys

The electrode from galvanostatic run 1 was characterized by SEM-EDX analysis and XRD. SEM micrographs and results of EDX analysis are shown in Fig. 4 and Fig. 5. A thick and homogeneous surface deposit is well visible on the micrograph shown in Fig. 4 and at some places, almost all aluminium has reacted. The Al potential cut-off potential cut-off potential maximum thickness of the electrode with deposit was measured to be 1.2 mm, compared to the 0.5 mm thick Al plate used as the cathode starting material.

Fig. 4 SEM micrograph of the cross-section of the

electrode from run 1 (loading 59.9 wt.% of An in Al)

Fig. 5 SEM-EDX determination of the Al/(U+Pu) molar ratio vs. the distance from the electrode surface for the electrode from run 1

An analysis of the deposit was performed using EDX at 21 spots selected at different thickness of the electrode. Composition profiles from the middle towards the surface of the electrode were compiled for U, Pu, Al, Cl and Gd. The molar ratio Al/(U+Pu) is plotted in Fig. 5 versus the distance from the centre of the electrode. The average Al/(U+Pu) ratio was determined to be 3.01. Thus the deposit is most probably composed of a (U,Pu)Al 3 alloy. The values for the U/Pu molar ratio are scattered, with a U/Pu ratio comprised between 4 and 10. The average value for 21 analyses is U/Pu = 7.0, which is somewhat lower than the ratio determined by gamma spectroscopy (U/Pu = 10.1). The XRD analysis of the deposit was indexed on the base of a cubic (a = 4.2652(1) Å) unit cells corresponding to (U,Pu)Al 3 alloy. This result is in a very good agreement with the EDX determination of the deposit composition.

III.C. Salt concentration profiles

The concentration profile in dependency on the total passed charge is plotted in Fig. 6. The effectiveness in the application of cut-off potentials was verified, as the concentration of gadolinium remained at its initial level and zirconium concentration was below the detection limit. The concentration increase of both actinides at 1500 C was caused by an intentional chemical oxidation of the alloy in order to increase the concentration of the actinides in the bath.

Fig. 6 Concentration profile of U, Pu, Zr and Gd in the bath during the electrorefining

The uranium concentration showed a decreasing trend during the complete run. This is explained by the congruent actinides from the U-Pu-Zr alloy on the anodic side and the predominant deposition of U compared to Pu (U/Pu mass ratio in the deposit is ~ 10). In addition a chemical reaction, i.e. reductive extraction of U 3+ by Al from the electrode, can also occur, which would also lead to a slight U 3+ concentration decrease [START_REF] Cassayre | Uranium Underpotential Deposition onto Solid Aluminium in Molten LiCl-KCl Eutectic[END_REF]. The plutonium concentration gradually increased during the experiment, which supports the above mentioned explanation of the uranium concentration decrease. The chlorination route proposed for the recovery of An from AnAl alloys consists in a three step procedure as presented in Fig. 7. Initially, the salt adhered on the electrode after the electrorefining process is removed by distillation. This salt is composed mainly of LiCl-KCl and some fission products (FP) chlorides. The second step is a chlorination of the An-Al alloy in order to convert the entire alloy to chlorides. The last step consists in the sublimation of AlCl 3 , leaving pure An chlorides, which can be further chemically processed to obtain the desired output product, e.g. An alloy.

III.D. Recovery of An from AnAl alloys

Fig. 7 Principle of the chlorination route (An=actinides, FP=fission products)

In order to assess the thermodynamic possibility of the proposed route a theoretical study of each step was performed using the FACTSAGE software. The main results can be summarized as follows:

-A vacuum distillation (P=10 -5 bar, T = 1000°C) is efficient to completely remove the LiCl-KCl salt and most FP. However, under these conditions, a complete distillation of some chloride compounds (MoCl 2 , SrCl 2 , BaCl 2 ) will be probably impossible.

-According to the existing thermochemical data of the studied compounds, it is possible to perform a full chlorination of the An-Al alloys at suitable working temperatures, i.e avoiding volatilisation of An chlorides.

-The molar ratio between chlorine gas and the alloy has a significant influence on the composition of the final products. With increasing Cl 2 /alloy molar ratio, the temperature for formation of volatile An chlorides decreases. A temperature of 150°C and minimum Cl 2 /alloy molar ratio is recommended for this step in order to avoid the formation of gaseous U chlorides.

-Heating of the resulting mixture under inert Ar atmosphere at 350-400°C is expected to facilitate the selective evaporation of AlCl 3 , leaving pure An chlorides in the reactor.

Although, from a thermodynamical point of view, all steps were proven possible, the applicability of this method depends on the reactions kinetics under working conditions suitable for the An recovery. Therefore, an experimental study is necessary to determine the realistic conditions for the process and to quantify the chlorination efficiency and actinides volatilization under these conditions.

The first experiments were focused mainly on the chlorination step. UAl 3 alloy was selected as input material, as volatile U chlorides are formed at the lowest temperature in comparison with other actinides. Accordingly it is expected that the conditions suitable for recovery of U from U-Al alloy can be applied for a process including all actinides. A fine crushed UAl 3 alloy (300 mg) was chlorinated at a temperature of 150°C using a Cl 2 /alloy molar ratio of 36 which was set by the fixed volume of the reactor. After 21 hours the reactor was refilled and the chlorination continued at the same temperature for another 20 hours. Both the initial material and obtained products after the chlorination were weighted and analysed by XRD to calculate a mass balance and evaluate the process efficiency.

After the chlorination, a pure green powder was obtained, which indicates the presence of UCl 3 and/or UCl 4 as the major constituents of the product. XRD analysis indicated a high efficiency of the chlorination, as only 2.7 % of the initial alloy remained non-chlorinated. The XRD pattern obtained after the chlorination is presented in Fig. 8. Accordingly, the concentration of AlCl 3 has to be calculated from the real mass of the chlorination product weighted after the experiment, which was deduced from the calculated theoretical mass of U chlorides and nonreacted residual alloy. The mass balance and an evaluated composition of the chlorination product are shown in TABLE II. The results indicate that a major part of AlCl 3 was evaporated already during the chlorination process. According to the calculations, a mass increase up to 686 -719 mg was expected after chlorination of the initial 300 mg of the alloy, if all AlCl 3 has remianed in the product.

IV. CONCLUSIONS

The use of solid aluminium electrodes for reprocessing of spent nuclear fuel by electrorefining in molten chlorides seems to be very advantageous. Reduced actinides form solid and dense alloys with aluminium and a very high capacity of aluminium to retain actinides has been proven in this work. If AnAl 3 alloy is formed in the deposit, the maximum possible loading would be 2.94 g of An in 1 g of Al, which corresponds to 74.7 wt. % An in Al. The highest An content in Al during the experiment was achieved for potentiostatic electrolysis, with 2.33 g of An loaded into 1 g of Al ( 94 % of the maximum Al capacity).

The diffusion of An and Al in the formed An-Al alloy layer seems to be the most important factor for this process. Obviously, the diffusion rate increases with a temperature and therefore a formation of more actiniderich alloys could be expected at higher temperatures. More detailed studies are necessary to optimise the condition s for a maximum An loading.

Theoretical studies showed the thermodynamical possibility of An recovery from the obtained An-Al alloys using a chlorination procedure consisting of the three main steps: Distillation of the salt adhered on the electrodes, chlorination using chlorine gas and sublimation of formed AlCl 3 . However, the applicability of this method depends on the kinetics of the reactions under working conditions suitable for the selective An recovery. The first verification chlorination experiment was carried out using UAl 3 alloy, which is the most critical material in the sense of actinide losses during the process by U volatilization. The results indicated that the process can be carried out with a satisfactory chlorination efficiency and without actinides losses caused by volatilization of formed chlorides

Fig. 1

 1 Fig. 1 Picture of anodic Ta basket, Al cathode and Al 2 O 3 crucible used in the experiments (left) and the experimental set-up for electrorefining

Fig. 3

 3 Fig. 3 Evolution of anodic potential and current duringthe potentiostatic electrolysis (run 5)

  -Zr electrorefining, run 1, EDX measured at different levels of the electrode

Fig. 8 4 ,

 84 Fig. 8 XRD pattern of the chlorination product (Symbols = observed points, solid line above = calculated profile, solid line below = difference between observed and calculated profiles, ticks below the peaks = 2 hkl Bragg positions for UCl 4 , UCl 3 and UCl 3 , in descending order)It was impossible to make a quantitative evaluation of AlCl 3 content by XRD due to its high solubility in the epoxy-resin used for the preparation of the sample.

TABLE I

 I

			Evaluation of results for U-Pu-Zr
			electrorefining experiments
	run	Charge [C]	Mass Al [g]	Deposit analysis Pu U/Pu content weight	An wt. %	Al/An weight ratio
				[mg]	ratio
	1	1423	0.61	69.9	10.1	59.9 1:1.49
	2	1527	0.54	47.1	18.1	62.5 1:1.67
	3	773	0.27	51.0	11.4	70.0 1:2.33
	4	766	0.23	67.7	6.4	68.5 1:2.17

TABLE II

 II

		Concentrations of the input material and the
	product obtained after the chlorination
	Specie	Concentration [wt.%] Input material Product
	UCl 3	0.0	17.8
	UCl 4	0.0	72.6
	UAl 3	98.0	2.2
	UAl 2	1.0	0.5
	AlCl 3	0.0	7.0
	Al	1.0	0.0
	mass [mg]	300.0	389.1

Global 2007, Boise, Idaho, September 9-13, 2007

ACKNOWLEDGMENTS

The authors wish to thank H. Ottmar and B. Lynch for -spectroscopy measurements, R. Jardin for XRD analyses, C. Scheppler, S. Birck and S. v. Winckel for ICP-MS analysis as well as M. Ougier and A. Rodrigues for experimental support. This work was carried out with the European Commission financial support in the 6 th framework program, under the contract FI6W-CT-2003-408854, "EUROPART".