tritium recovery from a breeder material: gamma lithium aluminate

Journal of Nuclear Materials 141-143 (1986) 275-281 North-Holland, Amsterdam 275

T R I T I U M R E C O V E R Y F R O M A B R E E D E R M A T E R I A L : G A M M A L I T H I U M A L U M I N A T E

E. R O T H 1, F. B O T T E R 1, M. BRIEC 2, M. R O S T A I N G 2, H. W E R L E 3 and R.G. C L E M M E R 3,.

t Commissariat ~ I'Energie Atomique, Institut de Recherche et de D~veloppement Industriel, Division d'Etudes de S~paration lsotopique et de Chimie Physique, DESICP-C.E.N., Saclay, 91191 Gif-sur-Yvett, France 2 DESICP-C.E.N./G, Chemin des Martyrs, 38000 Grenoble, France

Kernforschungzentrum Karlsruhe GmbH, Postfach 3640 D-7500 Karlsruhe, Fed. Rep. Germany

This paper discusses phenomena that have been observed during tritium extraction from -/-lithium aluminate, specifically: - Increase of rate of extraction when adding hydrogen to the sweep gas, - formation of tritiated water in all cases, - permeation of tritium through gas pipes, - adsorption of tritiated water on gas lines.

To minimize the blanket tritium inventory a flowchart is proposed whose specificity rests in the addition of hydrogen to the gas within the blanket, followed by recovery of the tritium after oxidation of hydrogen to water, electrolysis and reconcentration. This flowchart includes a provision for detritiation of the coolant which is separate from the purge gas.

! . I n t r o d u c t i o n

We have studied the rate of extraction of tritium from -t-LiA102 in out-of pile .experiments following short irradiations ( - 1 5 min) and during in-pile experiments. Experimental procedures are described in refs. [1-3] where interpretation of measurements and mechanisms of tritium release are discussed.

Special attention is given here to the nature of the tritiated species formed and to the influence on inventories and extraction rates of hydrogen additions to the purge gas. Reduction of permeation by such dilutions is

discussed. Whatever the detailed mechanisms may be, the behaviour observed in our experiments should be representative of that in large sections of real blankets with purge gas, as tritium concentration in the gas phase and ceramics would be similar in both cases.' Whereas the larger ratio of structural to blanket material in our experiment probably enhances secondary effects, it also makes it easier to study how to correct perturba- tions due to them. We feel justified to infer, from our observations, the requirements on the extracting system discussed in the third part of this paper.

In summary a flow diagram is proposed for the recovery of tritium from a blanket swept with an inert gas with 0.1% hydrogen added.

2. E x p e r i m e n t a l r e s u l t s f r o m in p i l e e x p e r i m e n t s

In the LILA experiment both tritiated water and hydrogen were observed in the sweep gas. But it was shown that the nature of the tritiated species measured depended on the material of the capsul[ [1]. Stainless steel would reduce at least part of the tritiated water to hydrogen.

To avoid difficulties encountered when tritiated

* From Argonne National Laboratory, presently at Kern- forschungszentrum Karlsruhe.

moisture circulates in unheated tubes, a zinc furnace is placed as close as possible to irradiated samples in the LISA experiment [4] in order to reduce the water to tritiated gas. Thus water can just subsist in the sweep gas between the in-pile sample and the zinc furnace and only in negligible amounts. It was therefore hoped to achieve complete recovery of tritium as gas, whether the purge gas used was pure helium or helium plus hydrogen. Indeed when using pure helium the amount of tritium collected varied according to the location of samples from 0.8 to 0.97 of calculated productions. Interpretations of the difference between these numbers and unity will only be possible when results of measurements from the flux integrators are made available. The addition of 0.1% hydrogen to helium was then made. It should have altered the counting rate temporarily only, as, at equilibrium, extraction rates should be constant and equal to production.

However the tritium counts recording, fig. 1 presents two unexpected features:

- The transient peak, due to a drop of inventory in the sample as could be expected from-out-of pile experiments, appears after several hours only.

- Counts did not fall back to their former value after the peak.

We first rule out that this new apparent equilibrium level could be due to a radioactive nuclide other than tritium. For this purpose we investigated the possiblity, mentioned when discussing a different experiment by Dr. Kwast [5], that some fluorine 18 might be counted when H2 is added, because it would be transported as HF.

lSF is produced by the action of recoiling T atoms on is O. To establish orders of magnitude, Dr. Pinte at Saclay irradiated an aqueous solution of lithium carbonate. A i0 rain exposure of the solution, in a l0 ts n m -2 s - t flux, produced 3000 Bq of tSF per mg of natural lithium. Such high counting rates of 0.511 MeV gammas (higher than those of the T formed!) could

0022 -3115 /86 /$03 .50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publ i sh ing Divis ion)

276 E. Roth et aL / Tritium recovery from y-LiAIO 2

T pCiJ-1 Aluminate 2 nd cycle

I 3000

2000

/ GAS ,1000 .

4- ± -1850pCi1-1

He + 0 , 1 % H 2

18 h ()h 12h oh 12h • 28 Nov 29 Nov 30 Nov t

Fig. 1. Effect of the addition of 0.17o hydrogen in helium sweeping a LiAIO 2 sample during the LISA 1 experiment. More detailed operating conditions are given in refs. [3] and [4]. The amount of tritium extracted during the peak reduces by about one third the

inventory in the sample.

hardly have escaped previous measurements, specially along the tubes in the reactor hall. We assume that any 18F produced forms a stable fluoride with the metal (mostly iron) of tubes or capsules and decays (half life 110 re_in) without entering the counters, also the yield of the 160(T, n)laF reaction may be different in a LiA102 matrix.from that in water.

We have learned that the excess counts in Dr. Kwast's experiment have also another cause than ]SF entrain- ment as by counting the gas enclosed in his ionization chamber no 110 min decay period was found [5].

The counting rate enhancement with hydrogen addition could be due to two other causes. The first one being a higher ionization yield due to the lower ionization potential of H 2 with respect to He. Though a 0.1% addition of H 2 seems hardly sufficient to produce the large difference measured, this possibility is being checked. The second reason could be that when the sweep gas is pure helium, permeation losses of tritium through the walls of the capsules take place to a large extent. In that case isotopic dilution with hydrogen might reduce such losses by a factor that theoretically could be as high as the ratio of the partical pressure of the added H 2 to that of T 2, which in the present cases, illustrated by figs. I and 3, is larger than 1000. Fig. 2 shows that permeation losses through the walls of the LILA capsule would be possible (and the LISA one is not different in that respect) and a space exists where graphite is present and could act as a sink for hydrogen isotopes. Such permeation could also explain the ob-

servation of a delayed tritium peak (figs. 1 and 3), as the decrease in inventory to which it corresponds would only occur after suficient hydrogen had diffused into that space to establish a back diffusion current reex- tracting T from the graphite.

But direct measurements on gases desorbed from the graphite, though showing the presence of tritium in that space, indicate much too small quantities to account for the observed phenomena (some 10 4 Bq are recovered instead of the necessary many 10 z° Bq to support this hypothesis).

Thus no reason for the change in counting levels before and after hydrogen additions is yet given. On the contrary the delay in peak appearances either during a run after hydrogen addition, fig. 1, or at the beginning of a second cycle after a period of sweeping with pure helium during the interruption in irradiation, fig. 3, has been explained by the fact that the sweep gas is purified by a molecular sieve bed kept at liquid nitrogen temperature, and that this bed has to be saturated with hydrogen before this gas can effectively sweep the sample.

In summary previous LILA runs had suggested that, even under pure helium, at least a fraction of tritium is recovered in hydrogen form from the samples because a fairly rapid onset of the counts was observed from the beginning of the sweeps, the following slower growth of peaks beeing due to tritiated water. The LISA runs show that hydrogen addition enables to recover part of the inventory previously accumulated in the sample. When less than 100% recovery is estimated in pure

E. Roth et al. / Tritium recovery from y-LiAlO., 277

helium such additions would help, by a mechanism of isotopic dilution, to minimize the part of this loss due to permeation and, by a mechanism of isotopic exchange,

that due to adsorption of tritiated water or tritium, wherever it can take place.

"LIL/~' RIG

Zr 02 INSULAT ON

O, UARTZ TUBE for fiUing

LIAL02 SPECIMENS

OUTLET LINE

INNER SS TUBE p

O, UARTZ TUBE for filling

STAINLESS STEEL CAPSULE

INNER THERMOCOUPLE

EXTERNAL THERMOCOUPLE ....

IHIIIII!II TUBE GRAPHITE -~. _ CYLINDER

INNER THERMOCOUPLE FLUX

INTEGRATOR

CLUARTZ CAPSULE

HEATER

FURNACE THERMOCOUPLE

Fig. 2. Quartz and stainless steel capsules of the CHOUCA rig in LILA, showing the ouside space accessible to tritium permeation.

278 E. Roth et al. / Tritium recovery from y-LiA/O 2

pCi1-1 Aluminate l"tcycle

2500

2000

15Q0

T = 593°C

INTRODUCTION 0 , 1 % H2 in He

12h Oh 12h ()h 1'2 h 12 NOV 13 N ov 14 Nov

Fig. 3. Behaviour of tritium extraction from y-LiAIO 2 after an irradiation interruption during LISA I. See detailed operating conditions in refs. [31 and [4].

3. Out of pile extraction experiments

These experiments (named "OSI" runs from "Osiris Short Irradiation") enable selection, by short and inex- pensi~,e measurements, of many textures and irradiation conditions to be tested in reactor loops, and to study extraction kinetics [2]. In addition to explain mechanisms, OSI runs demonstrate that:

(a) transfer to the gas phase is faster when hydrogen is added to the sweep gas. This must lead to lower inventories in breeder materials;

(b) The largest part of the tritium formed within the samples themselves, is in the form of tritiated water and

(c) as a natural consequence of (b), hydrogen additions do not provide exactly 100% extraction of the tritium as hydrogen gas.

In OSI runs, 3 to 5 mm spheres of 7-LiA102 are irradiated to a maximum flux of -1017 thermal neu- trons m -2 s -] . After storage times that range from a few weeks to several months, tritium is extracted from those spheres by sweeping them first out of, then in, a furnace with either pure argon or argon plus 0.1% hydrogen, at a flow rate of 0.5 ml per second. Con- centration of tritium in the sweep gas remains well below one vppm. Therefore, though the distance between the furnace and the counter is only 50 cm (one hundred times less than in the LILA-LISA experiments) interactions with residual molecules of oxygen or water, present in the sub vppm range in argon, or on the

walls might still stronly affect the nature of the species observed.

When the sweep gas is argon containing 0.1% hydrogen, a sharp tritium peak appears the rise time of which is about three minutes. This fits quantitatively with the time argon needs to transit through the 90 ml of tubing between the furnace and the counter.

The fact that hydrogen addition to the sweep gas produces an almost immediate appearance of a tritium peak can be explained if hydrogen penetrates rapidly into the lithium aluminate samples, and if isotopic exchange with tritium takes place in situ. Whether it would occur in the bulk or at the surface of grains is not discussed here, nor is the nature of the exchangeable species. This faster extraction, implying shorter resi- dence times, is consistent with the drastic reduction of inventory evaluated during in-pile experiments after hydrogen was added to helium.

However, in the case of argon +0.1% hydrogen, several hours later the maximum of another peak is observed, much wider and the surface area of which - i.e. tritium content - is only one fourth of the previous one. It corresponds to tritiated water because this peak disappears when the U shaped tubing between the furnace and counter is cooled down at - 8 0 ° C . In all the runs in which no hydrogen was added to argon only the large delayed water peak was observed. These two observations prove that tritium combines with oxygen mostly within the sample itself, for two reasons:

E. Roth et al. / Tritium recovery from "t-LiAlO 2 279

Firstly if tritiated water was generated by oxidation of tritium by O 2 in the argon or by isotopic exchange of tritium with existing traces of water in the argon, then the total activity of tritiated water could never be a quarter of that of tritium in the 1000 vppm of hydrogen present, because argon is purified of oxygen and dried at dry-ice temperature such that remaining water or oxygen are far below one vppm. Consequently the presence of the more than 60 vppm of tritiated water that would be necessary to explain the counts observed, at the highest tritium content that it could reach is ruled out. (60 vppm of water would be necessary to extract 30% of the activity from 1000 vppm of H 2 because the maximum specific activity of such tritiated water is limited to, at most, five times that of hydrogen by the isotopic equilibrium that could be established with the hydrogen phase either during oxidation of tritium, or during an exchange with preexisting water.)

Secondly, experiments in pure argon support the conclusion that water is formed within the sample because the appearance of the tritiated water peak de- pends on the nature of the 6eramics investigated as shown by comparisons between LiA102 and LiA15Os: under similar conditions, the release from the latter is slower [3]. This does not prevent successive adsorptions and desorptions on the gas lines from being able to contribute to the delay of the maximum and the widen the peak.

A last result supporting the formation of water within the aluminate is given by the simultaneous extraction of residual helium and tritiated species from samples irradiated to large fluences over a one month period, during which they were only submitted to short periodic outgasing. Under these conditions the residual tritium is five to six times larger than helium. We believe that this is due to tritium being retained by OT bonds within the ceramics, and recall that in metal matrices, where tritium is not oxidized, residual helium is comparatively larger than residual tritium.

4. Implications from the results of experiments on the conception of tritium extraction circuits and proposed flowchart

The function of extraction circuits is to transfer tritium generated in the blanket to the fuel circuit. Losses should be minimal because in a large scale reactor even the release of one millionth of the tritium handled to the environment would not be acceptable, and of course they should never be large enough to affect effective tritium breeding ratios. Inventories must also be very low not only for safety reason but because during the time it takes to build them up equilibrium between tritium generation and extraction is not reach- ed and tritium breeding ratios large enough to supply the final circuit can not be obtained. The most worri- some cause of losses, and may be of large inventories, is

permeation of bred tritium through or into walls of structural materials. To limit such permeation it has been advocated to use an oxidising atmosphere to sweep breeder materials, in order to convert all tritiated species into water that does not permeate trough metals [7]~

Oxygen additions to the sweep gas in TRIO [6] and LISA [4], produced dramatic decreases in tritium extraction rates. They must have caused increases in inventories either in blanket materials or in cold parts of the gas lines.

These observations lead to favor extraction of tritium under the hydrogen form and to achieve this result by addition of hydrogen or deuterium to the sweep gas. This solution reduces permeation losses as mentioned earlier, but one cannot hope to use also diffusion bar- tiers, if they are oxides, to improve the process, as most of them would be reduced under those conditions.

One could alternatively, as in the LISA experiment, reduce all tritiated species inside the blanket. We will not study this process here as it raises others difficulties and still requires further means of limiting permeation.

Isotopic dilution, as mentioned, does not necessarily enable withdrawal of tritium completely from water but, even so, it should limit the risk of building large inventories under that form, as in all locations at mod- erately high temperatures, exchange would take place and provide depletion of the tritium content of water present. But, after the purge gas leaves the blanket, two problems must be solved: recovery of the tritium and reconcentration to the suitable value for injection into the fuel circuit.

It is with respect to this last step that it might be worthwile studying whether the use of deuterium as addition to helium instead of ordinary hydrogen is less costly because reconcentrating to 50% is then sufficient, but this is probably of secondary importance. Using technology available today, reconcentration should be undertaken in the gas phase, most probably by cryogenic distillation, because of the amount of tritium to handle. Tritium compatible materials and equipments are easily available for building such distillation units.

Recovery of tritium and hydrogen from the purge gas by getters might seem promising because getters could release these gases directly to the cryogenic distillation. However to assess the possibility of their complete fixation followed by total recovery at the ppm level from metal getters with low inventories, requires further experimentation, that is planned or underway within the EEC technology program. Also getters would not cope with small remaining amounts of tritiated water.

The first step proposed here for the recovery process is therefore to oxidize hydrogen to water over a catalyst, a reaction which can already be pushed to a great degree of completeness with no or little retention.

The problems of collecting the water formed and coverting it back to hydrogen arise last. The solution

280 E. Roth et al. / Tritium recouety/rom y -LiAlO,

BLANKET

EXTRACTION

EXTRACTION t

-+ To Isotope Separation

Fig. 4. General outline of the detritiation of (a) the sweep gas, (b) the helium coolant of the blanket of a fusion reactor. “OX” are oxidizing units, P,, P;, PC, Pa are partial pressures of tritium at various points. Pa on the extraction circuit of the blanket is less than,Pc over the ceramic. In the absence of extraction in the coolanl circuit, PC would prevail there also. The extraction maintains in this circuit a pressure P, so that the loss by permeation to the V.G., causing PI to decrease to P, is acceptable. V.G. is a vapor

generator. F represents a tritium leak by permeation that migh: reduce P, to Pi.

BLANKET /

ambid. t ---------

c

10 bars

t > tfq

C &

I I I

i I I I I

L-!

I IJ- .

outlined in fig. 4 consists of cryocondensation followed by electrolysis. The latter can provide, with cells being tested now at several places, 100% recovery and can be carried out safely with tritium compatible equipment at the level of dilution designed in the flowchart i.e., several thousand curies per liter.

The cryocondensation unit would require an experimental study, as the condenser should incorporate filters holding back small ice crystals that could be carried by the gas stream and should have a second stage, smaller than the main one, where water, frozen as thin layer on the large surface of the first stage, could be melted and transfered to the electrolyser. The necessity of such a scheme is exemplified by the estimation that 250 ml of water condensed uniformly on a large condenser of the size needed to cool down the extracting gas of the INTOR reactor, would produce only a 1 pm thick layer. A block diagram of the condenser is shown fig. 5. Alternatives to individual steps of this recovery process are being investigated, for instance the use of molecular sieves insiead of the main condenser, etc.

Any solution will require experimental studies before it can be fully evaluated because conditions of extreme dilution that are found in contemplated tritium extraction circuits from fusion reactors have never been in-

Fig. 5. Concept of a condenser unit for recovery of tritiated water. R is a refrigerating unit, OX is an oxidation unit, E,, E, are heat exchangers, (c) is a gas circulator, S.I. stands for isotopic separation. The main features fo this units are (a) the

filters, (b) the two step condensations as outlined in the text.

E. Roth et aL / Tritium recovery from 7-LiAIO 2 281

vestigated, except on a very small laboratory scale. But isotopic dilution enables handling gases or vapors at the 1000 vppm level instead of under the 1 vppm level, and simultaneously reduces losses.

The general flow diagram, fig. 4, shows also that the treatment of helium from the cooling circuit is carried out as well as the treatment of the flushing gas because some tritium is bound to permeate into it, and detritiation is necessary to prevent an undesirable level of permeation through the tubes of the vapor generator.

and by discussions with many of our EA colleagues, specially with those also associated with the work reported in refs. [1-4], whom we cannot mentioh individ- ually here.

The in-pile studies have greatly benefited by the possibility to insert a T-LiA102 sample in the LISA experiment carried out in Grenoble for KfK, reported in ref. [4].

References

5. Conclusions

Results from laboratory and in-pile experiments have led us to consider, contrary to conclusions from other studies, that tritium extraction circuits from breeding materials should be better operated under a reducing atmosphere, using isotopic dilution by hydrogen or deuterium. A flow chart for the global recovery of tritium in a reactor is proposed, using components that can be built with a tritium compatible technology. It does not, however, address the tritium recovery from the coolant of the first wall, a problem that is not yet studied.

This work was performed within the frame of the European Economic Community Technology Program for Fusion. We have benefited of the services com- petently provided by Service des Piles and Department de M&allurgie, at Grenoble, Service des Piles at Saclay,

[1] E. Roth, J.J. Abassin, F. Botter, M. Briec, P. Chenebault, M. Masson, B. Rasneur and N. Roux., J. Nucl. Mater. 133 & 134 (1985) 238-241.

[2] R. Benoit, D. Cherquitte, A. Dubey, H. Boulfroy, B. Rasneur and F. Botter, Fusion technology 1984, in: Proc. 13th SOFT Conf. Vol. II, pp. 1023-1028 (Pergamon Press, New York, 1985).

[3] M. Briec, F. Botter, R. Benoit, J.J. Abassin, P. Chenebault, M. Masson, B. Rasneur, P. Sciers and E. Roth, In and out-of pile tritium extraction from samples of lithium aluminates, these Proc. (ICFRM-2), J. Nucl. Mater. 141-143 (1986).

[4] J.H. Were et al., The LISA I Experiment in-situ tritium release investigations, these Proc. (ICFRM-2), J. Nucl. Mater. 141-143 (1986).

[5] Dr. Kwast, personal communication. [6] R.G. Clemmer, P.A. Finn and al., ANL 8455, Argonne

Nat. Lab. (1984). [7] M. Dalle Donne, U. Fischer and M. Ktichle, Nucl. Tech-

nol. 71 (1985) 15-28 and references therein.

tritium recovery from a breeder material: gamma lithium aluminate

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