Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 35: No. 7, p. 511-514 (July 1998)

SHORT NOTE

Development of an Improved Ceramic Tritium Breeder

-Ceramic Breeder with Catalytic Function

Kenzo MUNAKATA*it, Atsushi BABA', Masabumi NISHIKAWA*

and Ralf Dieter PENZHORN**

* Department of Nuclear Engineering, Kyushu University

** Tritium Laboratory Karlsruhe, Research Center Karlsruhe

(Received March 19, 1998)

KEYWORDS: nuclear f is ion, solid breeder, cat- alyst, platinum, palladium, lithium silicate, ex- change reaction, noble metal, blanket, tritium

All current designs of D-T fusion reactor blankets em- ploying ceramic breeders such as LizO, LiA102, LizZr03 and Li4Si04 allow for the use of a sweep gas consist- ing of helium containing 0.1% of hydrogen to extract tritium via isotopic exchange reactions. However, these exchange reactions-which predominantly take place at the surface-are dominated by the desorption of triti- ated water and are therefore rather slow. For this rea- son, there is still a need for the development of techniques that contribute to the acceleration of the recovery of bred tritium, particularly in the range of moderate tempera- tures.

In previous studies, the isotope exchange reactions and water adsorption at the surface of several ceramic blanket materials were The results revealed that these isotope exchange reactions proceed fast only at the more elevated temperatures. Because of the strong temperature dependency of the exchange reactions, a considerable decrease in the reaction rate takes place as the temperature is lowered. Taking into consideration that there is a broad temperature distri- bution within a blanket module(2), it is anticipated that the tritium bred in regions of lower temperature will pre- sumably be poorly recovered. The result could be an increased overall steady state tritium inventory in the blanket module.

In order to obtain an improved recovery of tritium from a blanket over a broad range of temperatures, the effect of catalytic active metal additives, such as plat- inum and palladium, on the heterogeneous isotope ex-

* Hakozaki, Higashi-ku, Fukuoka 812-8581. ** 76022 Karlsruhe, GERMANY.

Corresponding author, Tel. & Fax. $81-92-642-3784, E-mail: kenzo@nucl.kyushu-u.ac.jp

change reactions at the breeder-sweep gas interface was examined.

1. Liberation Mechanism and Surface Reactions of Tritium in Breeders

The release of tritium bred in breeders is considered to proceed via the following steps:

(1) Diffusion in the crystal grain (2) Desorption from the grain surface (3) Diffusion through the pores of sintered pellets (4) Diffusion through the boundary layer on surface of

In former studies, attention was primarily focused on the mass transfer step (1) and on the quantification of the diffusivity of tritium within the crystal grain. More re- cent studies lead to the conclusion that the desorption step (surface reaction) cannot be ignored for a complete analysis of the liberation mechanism of tritium from the ceramic Since, as already mentioned, hy- drogen is added in small concentration to the blanket sweep gas to promote tritium extraction via a surface exchange reaction, it appears worthwhile to examine sys- tematically the effect of noble metal additives to the ce- ramic breeder on the rate of the exchange reaction.

sintered pellets.

2. Preparation of Ceramic Breeder with Cat-

Platinum and palladium were deposited on ceramic breeder pebbles (Li4SiO4, 0.51-0.94 mm (av. 0.68 mm) in diameter, >98%TD) by the incipient wet impregna- tion method generally used for the fabrication of cat- alysts. For this purpose, Li4SiO4 pebbles placed in a flask were first dried under air a t 150°C for 24h. Then a solution of platinum tetra ammonium nitrate Pt(NH3)4(NOs)z or palladium tetra ammonium nitrate Pd(NH3)4(N03)2 was pored dropwise onto the previ- ously dried Li4SiO4 pebbles. After this treatment the wet LiqSi04 pellets were dried in an oven first at 90°C for 3 h and then at 150°C for another 24h. The ob- tained precursor was calcined in a quartz tube reactor under a He atmosphere containing 10% 0 2 by raising the temperature stepwise up to 400°C. Following this cal- cination, the noble metal impregnated ceramic breeder pebbles were reduced under a hydrogen atmosphere at 400°C for several hours.

The colors of Li4SiO4 impregnated with Pt or Pd were found to be either dark gray or black, indicating that Pt or Pd was successfully deposited on the pebbles. Some pebbles were mechanically broken to find out whether the noble metal is uniformly dispersed throughout the pebble. While both platinum and palladium had been impregnated into the interior of the pebbles, the color of the inner shells was less dark suggesting that the noble metals are preferentially deposited on the outer surface of the pebbles. The electron microscopic observations of these pebbles were also performed, and some small marks (dots), which were not observed on the virgin Li4Si04,

alytic Property

511

512 K. MUNAKATA et al.

were identified at grain boundaries of Li4SiO4. These are thought to be P t or Pd deposited. The exact content of the deposited P t or Pd has not yet been determined, but according to the mass balance in the fabrication process the concentrations can be estimated being within a range of 0.7 to 2wt%; the contents never exceed 2wt%.

3. Experimental A schematic diagram of the experimental appamtus is

shown in Fig. 1. For a run between 2.5 and 20.0g of Pt/Li4Si04, Pd/LidSi04 or LiQSi04 pebbles were placed in reactors made of quartz. The gaseous compositions of inlet and outlet streams of the reactor were analyzed with respect to H2, HD and D2 by gas chromatography. A hy- grometer was used to determine the wat.er vapor content in these streams. The gas flow rate was controlled with conventional mass flow controllers. The gases employed were purified with a cold trap cont,aining 5A molecular sieve that was cooled with an ice/water mixture to re- move residual water vapor. To achieve controlled concen- trations of deuterated water in the carrier gas molecular deuterium was quantitatively oxidized with a CuO bed held at 350°C.

As reported in a previous p a ~ e r ( ~ ) ( ~ ) , water is formed on the surface of breeders when they are heated under an atmosphere containing hydrogen. Although the wa- ter formation cannot completely be terminated if hydro- gen is present,, it is necessary to limit the formation of water by this mechanism during the course of an experi- ment. For this purpose, the sample bed temperature was raised in steps of 50°C under a stream of H2/He gas un- til the concentration of water vapor due to the heteroge- neous formation reaction became less than a few ppms. Once the maximum experimental temperature (700°C for Li4SiO4 and 400°C for Pt/Li4Si04 and Pd/Li4Si04) was established and the water formation was confirmed to be negligibly small, the actual exchange experiment was commenced. During runs, which were carried out under steady state conditions, the reactor temperature was stepwise decreased. The experimental conditions employed are summarized in Table 1. The space ve- locity ( S V ) in Table 1 is defined as

Q sv=-, V

Hygrometer

I Gas chromatograph I

Table 1 Experimental conditions

Amount (g) 30 2.4 2.5 Gas velocity (mllmin) 955 326 320 Bed diameter (mm) 22 16.2 16.2 Space velocity (SV) 2,700 5,500 5,200

0- l ) Temperature (“C) 200-600 25-400 25-400 Inlet gas composition

Hz (PPm) 3,500 3,900 4,300 DzO (PPm) 300 260 260

where Q and V are the volumetric velocity of the gas introduced to the reactor and the bed volume of the breeders, respectively. As seen in the above equation, SV is the reciprocal of the residence time of t,he pro- cess gas. Thus, the higher SV corresponds to the lower residence time. As shown in Table 1, the experiments on Pt8/Li4Si04 and Pd/Li4Si04 were performed under higher SV conditions than that for the virgin material Li4Si04. A blank test was also performed with an empty reactor, and it was confirmed that no exchange reac- tions take place within the experimental condition of this work.

4. Results and Discussion In each experiment a gaseous mixture of helium con-

taining H2 (3,500-4,300 ppm) and D20 (260-300 ppm) was passed through the reactor packed with the breeder pebbles. The extent of t,he exchange reactions taking place on the surface of the breeders was obtained from the composition of the exhaust stream of the reactor measured by gas chromatography.

The ratio of conversion R can be defined as

where CD, in is the molar concentration of D-atoms in D 2 0 in the inlet stream and C D , ~ ~ ~ the molar concen-

60 E

8 ’! 401f 20 Pd/Li,SiO,

O b ” A00 ’ go*& ’ 400 ’ 500 ’ 600 ’

Temperature [“C]

Fig. 1 Scheme of the experimental apparatus Fig. 2 Conversion ratio of DzO into HD or Dz over various

breeders

JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

Development of an Improved Ceramic Tritium Breeder 513

H2 +o(Pt) + H-o(Pt)

D,O+o(Pt) - D-o(Pt)

REACTION PATH (1

H, +OW) + H-o(Pt)

D,O + o(Li,SiO,) -D-o(Li,SiO,)

REACTION PATH (2)

H,O + o(Li4Si04) -H-o(Li4Si0,) o(Li,SiO,)-H

o(Li,SiO,) -D -D20 +o(Li,SiO,) surface

REACTION PATH (3)

Fig. 3 Models for the exchange reaction on the surface of catalytic breeders 1) (r denotes a hypothetic active site on the material surface. 2) Reaction (2) is hypothesized to take place at the interface of a Pt grain and a Li4SiO4

3) The stoichiometry of the reactions is ignored. 4) HD or HDO is not considered in the chemical reactions for simplicity.

grain.

tration of D-atoms in HD and Dz in the outlet stream. Figure 2 shows conversion ratios as function of tem-

perature for the investigated breeders. In the case of Li4Si04 (without noble metals), a conversion ratio of only 33% was attained even at 400°C. On the contrary, on the surface of t.he lithium silicate containing dispersed platinum (Pt/Li4SiO4), conversion ratios of nearly 90% (almost corresponding to the equilibrium condition of the isotope exchange reaction) were obtained at, tempera- tures of only little over 200°C. This higher degree of con- version was attained in spite of employing a much higher space velocity than in the experiment with Li4SiO4 (vir- gin material). Even at 25°C a conversion ratio of 28% was obtained over the Li4Si04 pebbles impregnated with platinum. Similar results were obtained in experiment with Pd/Li4Si04 pebbles, which indicates that palla- dium is also an effective catalyst for the enhancement of the exchange reaction. Thus, it can be concluded that isotope exchange reactions proceed at very fast rates even at temperatures below 400°C when lithium silicate pebbles contain dispersed catalytic active noble metals. The improved ceramic breeders are here designated as “catalytic breeders” .

With respect to the mechanism of the isotope exchange reaction between H2 and D20 two reaction paths (reac- tion paths (1) and (2)) can be considered (see Fig. 3). In previous studies with Pt/alumina, Pt/silica-gel and a Pt/MS5A catalyst, reaction path (2) has been identified as the dominating step(lO)(ll). By analogy it is reason- able to assume that the same mechanism will hold for Li4SiO4 as well as for the other ceramic breeders im- pregnated with noble metals; react.ion path (2) would

dominantly proceeds over the surface of breeders. Pro- vided that this constitutes the main path, the catalytic breeder is regarded as a particularly promising improve- ment in rapid recovery of tritium from the blanket of a fusion reactor over a broad range of temperatures.

Even if reaction path (1) should be instead the rate determining step, the catalytic breeder is still consid- ered to be promising. In this case, as apparent from the experimental results presented in Fig. 2 (the conversion ratios over the catalytic breeders much higher than that over the virgin Li4Si04), it can be assumed that reaction (1) will be very fast. Since several ten ppm of HzO are expected to be present in the blanket sweep gas and it is known from the 1iterature(l2)( 13) that the isotope ex- change reaction between hydroxyl groups on the surface of alumina or silica and gaseous water takes place rapidly (see reaction path (3) in Fig. 3), it can be assumed that the combination of reactions (3) and (1) in series will contribute to an enhanced transport of tritium from the breeder surface to the sweep gas and enable to recover tritium as the chemical form of hydrogen isotopes.

A rough estimation of the mass transfer capacitance of the exchange reaction over the catalytic breeder in- dicates that the mass transfer capacitance over the cat- alytic breeder at 100°C is about several billion times as large as that over conventional breeders. Thus, a consid- erable enhancement effect for the release of tritium from catalytic breeders is expected.

One could consider the use of the insoluble residues from nuclear reprocessing of spent nuclear fuel that is known to contain large amounts of metals like P t , Pd or Rh. While these materials cannot be utilized by con-

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514 K. MUNAKATA et al.

ventional industries, a use for the fabrication of catalytic breeders is thought to become t,heir practical application.

5. Conclusions Catalytic breeders were produced by impregnating

lithium silicate with either platinum or palladium. The isotope exchange reactions between molecular protium and gaseous predeuterated water over these new breed- ers were found to be very fast even at temperatures far below 400°C.

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