Immobilization and Leakage of Krypton Encapsulated in Zeolite or Glass
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IMMOBILIZATION AND LEAKAGE OF KRYPTON ENCAPSULATED IN ZEOLITE OR GLASS A. B. CHRISTENSEN, J. A. DEL DEBBIO, D. A. KNECHT, and J. E. TANNER Exxon Nuclear Idaho Company, Idaho Falls, Idaho, USA ABSTRACT Krypton and other gases are readily adsorbed in zeolite 5A molecular sieve or porous VycorO "thirsty glass" and may be trapped there by sintering 2-4 hours at 7000 or 900 0 C, respectively. Encapsulation in these materials is the basis of a method to "immobilize" krypton-85 for storage. Amounts of gas trapped are approximately proportional to gas density, and are about 50 and 40 cc(standard conditions = STP)/cc of solid at 1000 atm, for zeolite 5A and glass, respectively. Based on tests of a few months duration a leakage of 0.3% of the krypton is predicted for 0 10 years at 400 C for either material. INTRODUCTION EPA regulations now require that no more than about 15% of the krypton-85 produced from fuel placed in a commercial nuclear reactor after January 1, 1 This paper presents 1983 escape to the environment during the fuel cycle. work done at the Idaho Chemical Processing Plant (ICPP) to trap krypton in zeolite 5A molecular sieve or porous glass for storage until most of the radioactivity has decayed. 2 Originally this work was aimed at simple diffusion trapping in sodalite, where at high temperature the diffusional barriers are surmounted, allowing 3 At low temperatures the krypton remains trapped in the pores. adsorption. Penzhorn demonstrated a more effective trapping of krypton in zeolite 5A.415 Tests carried out by Penzhorn and the ICPP confirmed that the krypton is trapped during decomposition of the zeolite into an amorphous (sintered) form, with probable closing off of the pores through which the krypton entered. Tingey et a16 found that krypton could be similarly trapped in borosilicate glass (Vycor') made porous by acid leaching of the boron oxide. This report is devoted to subsequent exploitation of this sinter trapping phenomenon at ICPP and determination of the optimum conditions for trapping krypton. The morphology and leakage rates of the krypton-filled product are characterized. MATERIALS AND METHODS Zeolite 5A. The zeolite 5A source materials are listed in Table I. Most of the data were taken using Grace Davison beads (spheres) in the form received. The pore opening for zeolite 5A is 0.42 nm and the void fraction is 0.4.7 TABLE I Properties of the Zeolite 5A Used Form 2.0 mm Fine 2.5 mm
Binder Sphere Powder Sphere
Few % clay None Unknown
% Ca Exchange
Source
80% 80% 74%
Grace Davison Co. Grace Davison Co. Bayer
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Measurements were also made on pellets pressed uniaxially or isostatically from crushed beads or the powder. Low-melting, borosilicate glass powders were sometimes used as binder. Porous Glass. Corning Vycore "thirsty glass" is a borosilicate glass from which the boron oxide has been leached by acid. The manufacturer gives a porosity of about 28% in the form of interconnected pores of average diameter 4nm. High Pressure Encapsulation. Most of the ga
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