294 Journal of Nuclear Materials 141-143 (1986) 294-299

North-Holland, Amsterdam

P O S T - I R R A D I A T I O N T R I T I U M R E C O V E R Y F R O M L I T H I U M C E R A M I C

B R E E D E R M A T E R I A L S *

J . M . M I L L E R , S.R. B O K W A a n d R .A . V E R R A L L

Atomic Energy of Canada Limited - Research Company, Chalk Rioer Nuclear Laboratories, Chalk Rioer, Ontario, Canada KOJ 130

Gamma-LiAIO 2 and Li 2 ° were irradiated in sealed capsules and the tritium release behaviour examined during post-irradi- ation annealing. Sweep gas composition, extraction vessel material and ceramic characteristics were varied to determine their effect on the form of the tritium recovered (oxidized versus reduced) and the release rate. Pure He sweep gas resulted in primarily the oxidized form being recovered from both LiAIO 2 and Li 2 ° with the use of chemically inert extraction vessels such as quartz and Inconel 600. The addition of H 2 to the sweep gas or the use of a stainless steel vessel resulted in primarily the reduced form. Because of the variation in grain size within a sample, and other variations in material characteristics, no conclusive tritium release mechanisms were obtained. The results indicate both diffusion-controlled and surface-controlled release kinetics, depending on the experimental conditions and material characteristics.

1. Introduct ion

Ire the C R E A T E (Chalk River tests to Evaluate Tr i t ium Emission) series of exper iments , l i th ium ceramics are i r radiated in sealed capsules in the N R X reactor and the t r i t ium release examined dur ing post- i r radia t ion anneal ing. The purpose of these exper iments was to examine the effect of sweep gas composi t ion and extract ion vessel mater ia l on the form of the t r i t ium recovered, i'.e. oxidized (HTO, T20 ) or reduced (HT, T:) , and t r i t ium release rate. Both are impor tan t pa ram- eters for the choice and design of solid breeder b lankets

* Work sponsored jointly by the Canadian Fusion Fuels Tech- nology Project (CFFTP) and Atomic Energy of Canada Limited (AECL).

Table 1 Sample characteristics

and t r i t ium recovery systems. F rench [1], US [2] and Japanese [3] in-reactor tests have previously examined individual aspects of extract ion vessel mater ia l and sweep gas composi t ion on the form of t r i t ium recovered from LiAIO 2 and Li20 . Fischer and Johnson [4] ex- pressed the effect of exper imental condi t ions on the form of the released t r i t ium in terms of oxygen activity on the the rmodynamic in ter re la t ionship in a breeder system.

2. E x p e r i m e n t a l

2.1. Sample details

Samples weighing from 50-100 mg were cut f rom sintered pellets of LiAIO 2 and L i 2 0 for i r radiat ion.

Experiment Material Density Enrichment (7o of theoretical) (atTo 6Li)

CREATE-II LiAIO 2 65 7.5

CREATE-Ill LiAIO 2 pellet fabricated from as-received powder 64 b) LiAIO 2 pellet fabricated from powder ground in isopropanol 64 b) LiAIO 2 pellet fabricated from powder ground in methanol 64 b~

CREATE-IV LiAIO 2 75 - 50 Li 2 ° 90 - 90

") From qualitative SEM examination. b) Not measured.

Grain size a~ Pore diameter (#m) (ttm)

- 0.05-0.3 with some clusters b) of - 1 p.m grains

0.2-1 1.29

< 0.1-1 0.29

< 0.1-1 0.27

2-10 b)

4-15 with some larger b)

grains, 30-40 ttm

J.M. Miller et al. / Post-irradiation tritium recovery 295

Table 1 gives details for the various pellets for CREATE-II, III and IV. Each sample was vacuum annealed in a quartz tube, then sealed for irradiation without exposure to air. The LiAIO 2 samples ware annealed at 670 K and 3 × 10 -2 Pa for 1 h, and the Li20 samples at 870 K and 3 × 1 0 -2 Pa for 6 h. Samples were irradiated in NRX reactor for 48 h, flux averaging 7 × 1016 n m -2 s-1, temperature estimated at < 370 K.

2.2. Analysis procedure

Fig. 1 is a schematic showing the apparatus used to recover the tritium. Free tritium (i.e. tritium recovered at room temperature) is measured, as well as the iso- thermal tritium release at the desired temperature. Both tritiated water (T20 /HTO) and reduced tritium (T2/HT) are determined. (For simplicity, only HTO and HT are used to refer to tritium forms.) HTO is removed in the first ethylene glycol bubbler, and the sweep gas then passes through an ionization chamber, which provides on-line monitoring of the HT released. A second measurement of HT is obtained by oxidation in a CuO bed to HTO, then collecting the water in another set of bubblers. Bubbler solutions are analyzed after each test by liquid scintillation counting. The time dependence of only the HT release is obtained from the ionization chamber readings.

Both He and H e - l % H2 sweep gases were used at a flow of 0.5 I/rain. The He was purified by passing it through a hot titanium bed, and the H e - l % H2 by passing it through a Deoxo unit and a molecular sieve drier. The purified gas contained less than I IH/I of oxygen and moisture. Extraction vessels were con-

structed from quartz, stainless steel, Inconel 600 and nickel. All extraction tests were performed at 873 K for 4 h. Tritium remaining in the ceramic after annealing was recovered by dissolving the sample in 6N HC1, neutralizing with 6N NaOH and distilling the resulting solution, followed by liquid scintillation counting.

3. Results and discussion

3.1. General

The total tritium recovered, which includes the free tritium, the tritium released during the 4 h anneal and the residual tritium in the ceramic, ranged from 0.37 to 1.11 GBq per gram of LiA102 for samples with natural 6Li enrichment, and from 7.4 to 16.6 GBq per gram of ceramic for Li20 and LiAIO 2 with high 6Li enrichment. These generally agreed with neutronic predictions. Burnup was approximately 0.05 at% 6Li, calculated from the total tritium yield. The amount of free tritium and residual tritium was generally less than 1% of the total for the LiA102 in CREATE-II and -III. The free tritium increased to 15% for the highly enriched sam- ples, possibly as a result of more 6Li(n, a)3H reactions near the surface.

3.2. Form of tritium recovered

Table 2 summarizes the effect of the sweep gas composition and the extraction vessel material on the form of the tritium recovered. Although HT and HTO proportions vary for samples tested under the same conditions, some general trends emerge. The H T / H T O ratio will also have been influenced by residual moisture

ROTAM~ ~

DRIERI-]Ii F L-] TI BED DEOXO U 1700%) UNIT

i He-l% H z He SWEEP 5AS SWEEP 6AS

~l CAPSULE C>'Q BREAKER

SAMPLE -- RETAINER

EXTRACTION VESSEL

FURNACE~I~ !

IONIZATION __ CHAMBER , • ~-~--~-

kl) DRIER t_ E'I'HYLENE GLYCOL ETHYLENE 6LYCOL

BUBBLER BUBBLERS

TO VENT

Fig. 1. Tritium extraction apparatus flow schematic.

296 J.M. Miller et al. / Post-irradiation tritium recovery

Table 2 Influence of sweep gas composition and extraction vessel material on the form of tritium recovered (the predominant form under each set of conditions is given)

Ceramic a) Extraction Sweep gas vessel He He-l% H 2

Predominant form ~ HTO Predominant form % HT

LiAIO 2 Quartz HTO 67-87 (2) b~ HT 63-81 (2) (CREATE-II) Stainless steel HT 5-20 (2) HT 87 (1)

Nickel HTO 70 (1)

LiAIO 2 (CREATE-III, from as-received powder) Quartz HTO 51 (1) HT 86-88 (3)

(CREATE-III, from powder ground in solvent) ' Quartz cj ~ HT 73-90 (5)

LiA102 Quartz c~ ¢~ HT 47-50 (2) (CR~TE4V)

Li 20 Quartz HTO 58 (1) HT 80 (1) (CREATE-IV) Incone1600 HTO 64 (1) HT 68-70 (2)

, Stainless steel HT 46-53 (2) HT 81 (1)

a) See table 1 for ceramic details. b) Number in brackets is the number of samples tested under the given conditions. c) Material was not tested under these conditions.

in the cera~nic, the sweep gas, and the experimental system. Generally, with either He- l% H 2 sweep gas or a stainless steel extraction vessel, oxygen activity was very low and tritium was recovered primarily as HT, whereas primarily HTO was recovered when oxygen activity was raised by use of pure He sweep gas or relatively inert extraction vessels. Note that with the use of He- l% H 2 sweep gas, the predominant form was HT, independent of the extraction vessel material.

3.3. 7~ritium release rate

Figs. 2, 3 and 4 give results for the time dependence of the HT release for the different ceramic materials and experimental conditions. Time dependence of HTO release was not obtained, as only the total HTO re- covery was determined. As shown in fig. 2, the addition of H 2 to the He sweep gas significantly increased the release rate of HT from LiA102 samples in CREATE-II. This is probably a result of an exchange reaction, such a s

HTOsurra~ + H2t ~ ~ H2Osurta¢ ~ + HTtg~

which enhances the surface desorption kinetics, and tritium release, where surface desorption is rate-control- ling. A similar release rate was not.ed for LiA102 sam- ples in CREATE-III (fig. 3) with He- l% H2; however, the final 10% of HT was released very slowly from samples fabricated from "as-received" powder. Also,

there was no significant difference in release rate be- tween He and He- l% H 2 sweep gas for samples in CREATE-III fabricated from "as-received" powder; no runs were carried out with He sweep gas using samples fabricated from powder ground in methanol or isopro- panol. Fig. 4 shows HT release from LiA102 in CREATE-IV to be much slower than from other LiA102 samples, possibly due to larger grains (2-10 #m) com- pared to other samples (< 0.1-1 #m). There was no significant difference in HT release from the Li20 samples between He and He- l% H2 sweep gas or between the various extraction vessels, except in the very early release stage, as shown in fig. 4. Because addition of H 2 did not enhance tritium release from Li20 or LiAIO2 fabricated from "as-received" powder (CREATE-III), as was observed for LiA102 in CREATE-II, it appears that other factors, such as the microstructure of the ceramic or possible impurities, as well as sweep gas composition are important in control- ling tritium release.

3.4. Tritium release mechanisms

A number of authors [2,3,5-9] have investigated kinetics and release mechanisms of tritium from lithium ceramics ~n post-irradiation and in-situ tritium recovery tests, and the importance of material characteristics and experimental conditions has been emphasized. Both solid state diffusion and desorption from the surface of the

J.M. Miller et aL / Post-irradiation tritium recovery 297

1 - -

o.1 =,

<C

0.01

/ / x G.UARTZ, He-1%H 2 / . / o aUARTZ, .e

0 STAINLESS STEEL, He

I I I I I l I I I I ~ , , , t ,,I t t ~ ~ ~ ~ ,,

10 100 1000

TIHE (rninufes)

Fig. 2. Time dependence of the fractional HT release from LiAlO2; 873 K, sweep gas flow of 0.5 I/min, extraction vessel material and sweep gas composition as indicated. See table 1 for ceramic details.

ceramic have been identified [10] as important to the release of tritium into the sweep gas stream. Although the CREATE experiments were not specifically desig- ned to study tritium release mechanisms, release data were compared to the following models for diffusion and desorption.

(a) Diffusion from spherical grains: Crank [11] gives

0. i 1 10 100

TIME (minutes)

Fig. 3. Time dependence of the fractional HT release from LiAIO2; 873 K, sweep gas flow of 0.5 l/min, extraction vessel material and sweep gas composition as indicated. See table 1

for ceramic details. r

the fractional release ( f ) as:

6 ~ 1 ( D n 2 ~ 2 t ) exp as (1) n--t

for the initial and boundary conditions, which can be applied to tritium release into the sweep gas stream, of C(r ,O)=C o , C ( r , t ) - - C ' , OC/~r=O for r = 0 and C(a, t) = C s, where C is the concentration of diffusing tritium in the grain, r is the distance (0 ~< r ~< a), a is the grain radius, D is the diffusion coefficient and / = (Co - C ) / ( Co - Cs).

Late stage release can be expressed by

6 ( (la) f o , ~ l - ~S exp - - - - ~ - - 1

as only the first term of the series contributes. (b) Desorption kinetics: When desorption from the

surface controls the release behaviour, the kinetics can usually be described by an equation of the form [12]:

dNs = ksN:, (2) dt

where N s is surface concentration of the desorbing species, k s is a parameter assumed to be constant, and n is the desorption order. If the surface concentration N s is equal to the fraction remaining (1 - f ) times the original concentration of tritium, the above equation can be rewritten as

d(1 - f ) = - k ( 1 - f ) " . (2a) dt

If k is assumed to be constant, the first-order and

298 J.M. Miller et a L / Post-irradiation tritium recovery

1.0

p . - =

t ~

o.1

¢Y

, <

0.01

, , , , , , , , = , , , , ,

" / / o,O INCONEL 600, He-1%H z ! / o STAINLESS STEEL, He

,1( x INCONEL 600, He

LiA [Oz(CREATE-~)

. O.UARTZ, He-I°/oHz

i i i i i i i I I i i i I i i I i I I

10 100

TIHE (minutes)

I I I I = J

1000

Fig. 4,Time dependence of the fractional HT release from LiA]O 2 and LizO; 873 K, sweep gas flow of 0.5 I/rain, extraction vessel material and sweep gas composition as indicated. See table 1 for ceramic details.

second-order equations can be integrated to give log(1 - f ) = - kt and 1/(1 - f ) = kt, respectively.

The expressions for diffusion-controlled release (eqs. (1) and (la)~ are only roughly valid for our data because of the large range in grain size in all samples, and preclude an accurate assessment of the diffusion coeffi- cient. I;-Iowever, the release data obtained were fitted to

eqs. (1) and (la) to determine if there was some agree- ment. Late-stage release ( f - 0 . 7 5 - 0 . 9 5 ) from most LiAIO 2 and Li20 samples in CREATE-IV agreed with prediction by eq. (la), as shown in fig. 5 where - l n ( 1 - f ) is plotted versus time. However, early-stage release data did not fit eq. (1), which holds throughout the release. The diffusion coefficient was not de-

A

I

C

I

2.8

2.4

2.0

1.6

1.2 I , , I , , = , 50 100 150 200

J ' I ! • I

LiAI • X

LizO (He-l% Hz. ~ " , ~ " INCONEL 6 0 0 ' 7 / ~ "

/ / ~ LizO (He' quartz) / 1

f / L!AIO z (He-l°/o H z, quartz, CREATE .~) 1

TIME (minutes)

Fig. 5. Application of diffusion-controlled release kinetics to late-stage release; 873 K.

J.M. Miller et a L / Post-irradiation tritium recovery 299

termined for these samples because of uncertainties introduced by low porosity and high 6Li enrichment. Releases from the two samples in CREATE-If that were heated in a quartz vessel with He sweep gas exhibited good agreement with the diffusion-controlled release expression (1) in the region from f - 0.3-0.9. Late-stage release is shown in fig. 5. The calculated diffusion coefficient ranged from 1 X 10-16 to 2 × 10-14 cm 2 s-1 with grain size variation from 0.05 to 1 / t in for LiA102 in CREATE-II.

For some samples, regions were found where release agreed with first or second-order kinetic predictions (eq. (2a)), but the results were not conclusive. The HT release from the two LiAlO2 samples in CREATE-II heated in a quartz vessel with He sweep gas agreed with first-order kinetics from f - 0 . 3 - 0 . 9 . However, release data in this region also agreed with diffusion, as dis- cussed earlier. Release from the CREATE-II LiA102 pellets, fabricated from "as-received" powder, using H e - l % H 2 sweep gas also agreed with first-order re- lease kinetics in the region f - 0 . 3 - 0 . 8 , as did release from some Li20 and LiAIO 2 ~amples in CREATE-IV in the region f - 0 . 6 - 0 . 8 . Only release data in the region f - 0 . 1 - 0 . 4 from the two LiA102 samples in CREATE-II mentioned above agreed with second-order kinetics.

Although some stages of the HT release did agree with diffusion or surface-controlled release expressions, no consistent release mechanisms were determined for the various samples. Material characteristics such as grain size, pellet density and porosity, and variation in 6Li enrichment, as well as the material and experimen- tal system conditions are all factors which may have contributed to observed tritium release kinetics.

4. Conclusions

The effect of the oxygen activity of the experimental system on the form of the tritium recovered from LiAIO~ and Li20 was demonstrated. With H e - l % H 2 or a stainless steel extraction vessel, oxygen activity was low, and tritium was recovered primarily as HT. With pure

He and more chemically inert extraction vessels, tritium was recovered primarily as HTO. There was, however, significant variation in the H T / H T O ratio for" samples tested under similar conditions indicating the impor- tance of possible material and system impurities on the results.

The time dependence of the HT release demon- strated the effect of ceramic material characteristics and experimental conditions on the kinetics of the tritium release. No conclusive tritium release mechanisms were determined.

The authors would like to acknowledge the assist- ance of R.E. Donders, I.J. Hastings, W.J. Holtslander, D.H. Rose and A.G. Tremblay in various aspects of this work. Also, the authors would like to thank B.J.F. Palmer (Chalk River Nuclear Laboratories) and C.E. Johnson (Argonne National Laboratory) for supplying LiAIO 2 and Li2 ° for these experiments.

References

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(1985) 2143. [4] A.K. Fischer and C.E. Johnson, J. Nucl. Mater. 126

(1984) 268. [5] R.H. Wiswall and E. Wirsing, BNL-19766 (1975). [6] D. Guggi, H.R. lhle, D. Brining and U. Kurz, J. Nucl.

Mater. 118 (1983) 100. [7] K. Okuno and H. Kudo, J. Nuci. Mater. 116 (1983) 82. [8] H. Kudo and K. Okuno, J. NucL Mater. 133 & 134 (1985)

192. [9] K.R. O'Kula and W.F. Vogelsang, Fusion Te~hnol. 8

(1985) 2054. [10] C.E. Johnson, J. Nucl. Mater. 103 & 104 (1981) 547. [11] J. Crank, Mathematics of Diffusion (Clarendon, Oxford,

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22, Solid State Physics: Surfaces, Eds. R.L. Park and M.G. Lagally (Academic Press, New York, 1985) p. 430.