Retention of Halogens in Waste Glass

Abstract

Retention of Halogens in Waste Glass SummaryIn spite of their potential roles as melting rate accelerators and foam breakers, halogens are generally viewed as troublesome components for glass processing. Of five halogens, F, Cl, Br, I, and At, all but At may occur in nuclear waste. A nuclear waste feed may contain up to 10 g of F, 4 g of Cl, and 100 mg of Br and I per kg of glass. The main concern is halogen volatility, producing hazardous fumes and particulates, and the radioactive 129 I isotope of 1.710 7 -year half life. Because F and Cl are soluble in oxide glasses and tend to precipitate on cooling, they can be retained in the waste glass in the form of dissolved constituents or as dispersed crystalline inclusions.This report compiles known halogen-retention data in both high-level waste (HLW) and low-activity waste (LAW) glasses. Because of its radioactivity, the main focus is on I. Available data on F and Cl were compiled for comparison. Though Br is present in nuclear wastes, it is usually ignored; no data on Br retention were found.Reporting on decontamination factors for halogens from HLW processing began in nineteen eighties, starting with the pilot-scale melter PSCM-22 (1986) and continued with several other melter tests, PSCM-23 (1990), LFCM-8 (1994), and SSHTM-3 (1994). The F retention was found relatively stable at 759%, whereas the Cl retention varied from 75% to 85%. Only in PSCM-22 test, retention values for I were determined based on chemical analysis of glass, resulting in a value of 8%, which was lower than the value from the off-gas analysis (23%).Because halides are water soluble, they mostly partition to the LAW portion in waste pretreatment. Therefore, from nineteen nineties onwards crucible studies were performed and melter tests were evaluated with respect to the halogen retention in LAW glasses. Li et al. (1995) melted borosilicate LAW glasses at temperature 1300°C to 1450°C. The excess of F above the solubility limit formed CaF 2 and NaF crystals and the excess Cl formed spherical NaCl inclusions (crystallized NaCl droplets). In 1996, Feng et al. found that LAW glasses heated for 2 h at 1130 to 1380°C with 4-9% CaO retained 100% of F, whereas glasses with 2% CaO retained only 85% and 77% F. The retention of Cl was 0.520.14%. Crichton et al. (1995) observed that F and Cl volatilized from the melt surface, but excess Cl separated into liquid NaCl inclusions that rose to the melt surface by buoyancy, where they made a segregated evaporating layer. Iodine created gaseous I 2 bubbles that rose to the melt surface, where they burst. In addition, iodine evaporates from the feed before it becomes glass.According to by Whyatt et al., who compiled and evaluated various LAW vitrification tests in 1996, about50% of Cl and 20% of I were retained in the glass processed by feeding slurry into a continuous Joule-heated melter. In 2002 and 2003, Matlack et al. and Lee André reported data on the retention of halogens in DuraMelter 100 containing 115120 kg of glass, possessing the melt surface...

Reactions during melting of low-activity waste glasses and their effects on the retention of rhenium as a surrogate for technetium-99

“…The retention (also called "retention factor" or "retention ratio") of the ith component in glass after melting, R i , is in general defined as [36]:…”

Iodine solubility in a low-activity waste borosilicate glass at 1000°C

“…The fraction remaining in the melt relative to the original concentration is defined as the retention ratio, R i , which is determined with Equation (1) where g i,r is the mass fraction measured after melting and g i,0 is the target mass fraction of the i-th species [15].…”

Initial Assessment of the Consolidation of Chalcogels into a Viable Waste Form

“…These melted glasses potentially have high chemical durability. This is in contrast to oxide glasses where halides tend to remain as undissolved salts within the glass network or evolve from the glass surface during melting (Hrma, 2010). The preliminary results reported here fulfill the milestone "M3FT-12PN03030912-Provide an initial assessment of the consolidation of chalcogels into viable waste forms" (FCRD-SWF-2012-000246).…”