The objective of this work was to investigate absorption of iodine by different zeolites with their following ceramization. The technique employed was similar to that described in [1] concerning the Sr-containing zeolite ceramization. Only stable iodine isotope was used in the experiments. Zeolites of two types were used: (1) industrially prepared synthetic NaX-zeolite of the composition Na₂O^.Al₂O₃^.2.5SiO₂^.6H₂O was chosen for its high (63%) sorption capacity to iodine [2] and capability of yielding ceramics with sodalite structure; (2) industrially prepared Cu-zeolite (IONEX) obtained by substitution of Na-ion for Cu-ion in the standard NaX zeolite structure was chosen for its low price, compared with Ag-zeolite, while CuI and AgI are similarly stable (log(K_dissCuI) = -11.96, log(K_dissAgI) = -12.4). The completely saturated Ag-zeolite is known to absorb up to 70% of iodine from the gaseous streams produced in cutting fuel elements. Ag-zeolite can absorb up to 250 mg of iodine per 1 cm³ [2].

The initial zeolites were preliminarily dried at 110^oC and annealed at 300^oC during 24 h for dehydration to attain constant weight. Iodine was introduced in different ways. The dehydrated zeolite samples together with the excess of crystalline iodine were held in evacuated glass capsules at 130-400^oC during 12-24 h. Another method of introduction of iodine into zeolite consisted in the synthesis of ceramics from the dehydrated initial zeolite in the presence of iodine excess and water (1 wt %) in hermetically sealed platinum capsules at 500-800^oC and 100 MPa. Quenched samples appeared as dense cylinders 30 mm long and 3-5 mm in diameter. Iodine was also introduced into NaX zeolite through the ceramization of the zeolite together with dry CuI (20 wt %). In some runs introduction of iodine into the initial Cu-zeolite was preceded by copper reduction. The copper reduction experiments were carried out in sealed quartz tubes divided in two by a detachable nickel gauze. Pure graphite was put on the bottom of the tube, and the granules of dehydrated zeolite were placed on the gauze. The temperatures on the levels of graphite and zeolite were 750-780^oC and 280-500^oC, respectively. Copper reduction was due to the interaction of copper oxides and carbonates with CO produced as a result of heating graphite above 700^oC. Reduced and iodine-saturated zeolites were dried at 100^oC during 2 days and then subjected to ceramization in gasostat at 800^oC and 100 MPa.

¹⁾ - Albite

²⁾ - ceramic matrix (CM) based on Cu-zeolite (mixture of quartz, corundum and copper oxides)

Complex thermal analysis of the initial zeolite samples was performed with Q-1500D derivatograph, and infrared spectra of the samples were obtained using Perkin-Elmer 983 spectrophotometer. Density, d, and open porosity, P, of the samples were determined to evaluate the ceramics quality. The results are given in Table. The X-ray patterns obtained were decoded using ASTM card catalog. The X-ray patterns of products of NaX zeolite ceramization exhibit weak NaI reflexes, the phase of albite type formed by ceramization of the initial zeolite without iodine, and the new phase of composition corresponding to iodosodalite synthesized at 700^oC and 0.1 MPa [3]. The compositions of run products were analyzed by Camebax microprobe with Link detector with accuracy 1-3 rel. % regarding the main elements.

Fig.1. Leaching rate of iodine from ceramic samples based on Cu-exchanged zeolite; in legend indicated numbers of runs. Data - Shidlovsky et al., 1981.

Fig.2. Leaching rate of iodine from ceramic samples based on zeolite NaX; in legend indicated numbers of runs. Data - Shidlovsky et al., 1981

The ceramics quality was evaluated from the element leaching rates in distilled water at 90^oC (test MCC-1). The run duration for each sample was 1, 7, 14, and 28 days. Experimental solutions were acidified with hydrochloric acid and analyzed by ICP and atomic absorption.

Conclusion. Relatively high stability of iodine-containing zeolite ceramics was concluded from the data on leaching. The iodine leaching rate was 0.9 g/m²day after runs under the MCC-1 test conditions during 36 days. It was 4-5 times less than the rate of iodine leaching from iodosodalite obtained by hot pressing of Al and Si oxide powders in the presence of alakline metal iodide or iodate solutions [4]. It correlates with the rates of alkaline and alkaline-earth element leaching from borosilicate glasses [5]. The observed decrease of the leaching rates and concentrations suggests that the leached iodine concentration will not exceed 0.5 in

time, what tolerates the long-time storage of radioactive iodine in ceramics form.

Using two different zeolites (NaX and Cu-zeolite) as initial materials, one can obtain ceramics of different phase composition. In the former case, its composition roughly corresponds to the mixture of feldspar and I-containing sodalite; in the latter case - quartz, corundum, and CuI. The density of such a ceramics is somewhat higher than that of NaX-zeolite ceramics. The ceramics containing CuI proved to be twise as stable as NaX-zeolite ceramics as for iodine leaching (Figs.1 and 2, respectively).

Thus, we developed two methods of iodine fixation. The first (with initial NaX zeolite) offers the following advantages: - the obtained ceramics must be stable when burried in alkaline rocks, where the paragenesis feldspar + sodalite is abundant; - the initial zeolite is low in cost and accessible (it is industrially produced and allows secondary utilization).

The second method (with initial Cu-zeolite) offers the following advantages: - iodine is firmly fixed in CuI form, which is more resistant to leaching, compared with sodalite; - the ceramics has higher density and strength, compared with the former method.

The proposed method of conservation of radioactive iodine in the ceramics form has some advantages: the obtained material possesses high strength and significantly reduces the volume of radioactive waste. It is of principle importance that the ceramics is prepared from the industrial zeolites capable of absorbing radioactive iodine directly from the gaseous streams of nuclear plants. According to the principle of phase and chemical relations in the system matrix-rock [6], this ceramics (especially, that based on the assemblage feldspar + sodalite) is compatible with the rocks of the earth's crust (syenites) and thus excells the iodine-fixing matrixes proposed before: Ag-containing zeolite, based on epoxy glue compositions [7] etc.

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2. Murphy C.P., Staples B.A.& Thomas T.R.(1977). The development of Ag^oZ for bulk ¹²⁹I removal from nuclear fuel Reprocessing Plants and PbX for ¹²⁹I storage. // Allied Chemical Company Report number ICP-1135.
3. Tomisaka. Y. & Eugster H.P.(1968) Synthesis of the sodalite group and subsolidus eguilibria in the sodalite - noselite system.Mineral.J.Japan. 5:249-275.
4. Shidlovsky B., Sabad H. & Strachan D.M.(1981) Method for immobilizing radioactive iodine. U.S.A. Patent N 4229317.
5. Strachan D.M. (1983) Results from long-term use of the MCC-1 static leach test method. // Nucl.Chem.Waste Manage. 4:165-177.
6. Kotelnikov A.R., Zyryanov V.N. & Epelbaum M.B. (1994) Phase and chemical compatibility of matrix materials and wall rocks at disposal of high level wastes. // Experiment in Geosciences. N.3, pp.9-21.
7. Kalinin N.N. & Elizarova A.N.(1988). The study of iodine leaching from composition based on epoxy glue and iodid Pb. // Radiokhimiya. 30:107-111.