A study on sodalite pellets as matrix for spent chloride salts confinement
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A study on sodalite pellets as matrix for spent chloride salts confinement M. Capone 1, C. Fedeli 1, G. De Angelis2, M. Da Ros3, F. Giacobbo3, M. Giola3, E. Macerata3, M. Mariani3 1
ENEA, CR Casaccia, Dipartimento Fusione e Tecnologie per la Sicurezza Nucleare, Via Anguillarese 301, 00123 S. Maria di Galeria, Roma, Italy 2 Retired from ENEA, CR Casaccia, Unità Tecnica Tecnologie e Impianti per la Fissione e la Gestione del Materiale Nucleare, Via Anguillarese 301, 00123 S. Maria di Galeria, Roma, Italy 3 Politecnico di Milano, Dipartimento di Energia, Sezione Ingegneria Nucleare, Piazza L. da Vinci 32, 20133 Milano, Italy ABSTRACT Pellets made of pure sodalite blended with commercial glass frit and pellets made of sodalite, glass frit and a mixture of chloride salts, synthesized through dry pressing and subsequent thermal treatment, were evaluated as a potential matrix for confinement of spent chloride salts coming from pyroprocesses. The sodalite pellets were leached at 23°C and 90°C for 28 days, according to the ASTM C1220-10 procedure. Normalized release rates were estimated for the following elements: Li, Na, Al, Si, K, Rb, Cs, Sr, Ba, La, Nd and compared with literature results. SEM investigations, carried out before and after the leaching tests, show dissolution and re-precipitation phenomena at 90°C. INTRODUCTION The pyrometallurgical processing of spent nuclear fuel, conducted at 500°C in a molten salt medium (LiCl-KCl, 59-41 mol%), generates a chloride salt waste containing alkali-metal, alkaline-earth, and some rare-earth fission products >1,2@. Sodalite, a naturally occurring mineral containing chlorine, was investigated as an immobilization matrix for chloride salt waste due to its potential in retaining chloride >2,3@. Different sodalite synthesis routes were attempted by means of high-temperature methods (700−800°C) [4] and low-temperature (70−200°C) synthesis from kaolinite [5]. In particular, the Argonne National Laboratory developed an immobilization procedure that consists in incorporating the fission products into zeolite by ion exchange and then converting zeolite into sodalite [3,6,7]. The matrix was consolidated by addition of glass frit and through a hot pressing procedure (HIP). The so-obtained pellets were leached for 28 days giving normalized release rates typically below 1 g m-2day-1 [6]. In order to overcome the complexity introduced by the hot pressing step, a simpler method was developed by CEA using nepheline as an intermediate reaction product and reacting it with chloride salt at 800°C [8]. In some cases the powders were pressed under different conditions and then sintered at 800°C. In the present work the authors investigated the leachability of pellets of sodalite added with glass frit and mixed chlorides waste prepared according to the methodology developed by CEA and already tested by some of the present authors >9,10@. The resulting pellets were leached for 28 days and their ability to retain fission products was compared with that of the sodalite-based matrices obtained at ANL
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