Low Temperature Vitrification of Radioiodine Using AgI-Ag₂O-P₂O₅ Glass System

Abstract

A new vitrification process for radioiodine has been developed for safe disposal of the spent iodine adsorbent generated from reprocessing off-gas systems. The proposed process consists of the following two steps: (1) separation of radioiodine as AgI from spent iodine adsorbent and (2) vitrification of the separated AgI with silver phosphate. An AgI-Ag2O-P2O5 glass system can homogeneously contain up to 60mol%AgI, which leads to high volume reduction efficiency (approximately 1/25). It also can vitrify the AgI without volatilization of iodine because of its low melting temperature (below 400°C). The leachabilities of iodine from simulated vitrified waste forms were evaluated by the MCC-1 static leach test in an anaerobically controlled glove box, which was purged by nitrogen gas with 3% hydrogen. The leaching behavior of the Agl-Ag2O-P2O5 glass system was influenced by the composition of the actual glass, type of leachant, and redox conditions. When the leach test was carried out using simulated ground wateroriginating from rainfall, the leach rate of iodine from 3AgI-Ag4P2O7 glass was 6×10−8g/cm2/d in the early period of the leach test at 35°C under the reducing condition, after which it decreased to 3×10−9g/cm2/d. This glass showed a lower leach rate in the simulated ground water originating from sea water or cement saturated water, since a precipitation layer was formed on the surface.

Development and Durability Testing of a Low‐Temperature Sintering Bi–Si–Zn Oxide Glass Composite Material (GCM) 129I Waste Form

“…Near neutral 28‐d, 90°C MCC‐1 degradation rates for a variety of glass compositions are 0.3–6.3 g/m 2 , which corresponds to 0.011 to 0.22 g/m 2 ·d. Fujihara et al . measured Ag 2 O‐P 2 O 5 ‐based low‐temperature glass waste form rates much higher than GCM rates, ~0.08 g/m 2 ·d, for several I loading values despite being measured at a lower temperature.…”

Review of Potential Candidate Stabilization Technologies for Liquid and Solid Secondary Waste Streams

“…Disposal forms that have been considered or proposed for silver sorbents used to control radioiodine emissions from spent nuclear fuel reprocessing plants include direct disposal, immobilization of low solubility iodides and iodates in cement (Atkins and Glasser 1992, Morgan et al 1978, Scheele et al 1984, immobilization in CaI 2 -modified cement (Scheele et al 2002), immobilization by hydrothermal reaction with a metal hydroxide (Yamasaki 1987), sintered metal (Fukumoto and Kanzaki 1998), a silver phosphate glass (Fujihara et al 1998), and vanado-apatite (Uno et al 2004;Uno et al 2001). The disposal challenges for silver sorbents used in a reprocessing plant should be similar to those for Ag 0 Z or unreduced silver mordenite (AgZ) used at the WTP.…”

“…For its use, the iodine would need to be removed from the spent Ag 0 ZI as an iodide, and the alumino-silicate would be prepared. Fujihara et al (1998) and Sakuragi et al (2008) investigated the vitrification of AgI as AgI-Ag 2 O-P 2 O 5 . Fujihara et al (1998) described the process for separating trapped iodine as AgI from silver sorbents by vacuum distillation at 800°C.…”

“…Fujihara et al (1998) and Sakuragi et al (2008) investigated the vitrification of AgI as AgI-Ag 2 O-P 2 O 5 . Fujihara et al (1998) described the process for separating trapped iodine as AgI from silver sorbents by vacuum distillation at 800°C. They prepared their glass by blending AgI with silver metaphosphate, silver pyrophosphate, or silver orthophosphate and melting it in an alumina crucible at 623 K (350°C) for 6 hours.…”

“…Sakuragi et al (2008) estimated a leaching period of 1.3 × 10 6 years for this waste form as a 48-cm-diameter cylinder. Fujihara et al (1998) measured a melting temperature of <573 K (300°C), a glass transition temperature of 327 K (54°C), a compressive strength of 9000 kPa, and a thermal expansion coefficient of 2 × 10 -5 K -1 . This approach would need to be combined with a method for removing the iodine before converting it to a waste form for radioiodine in spent Ag 0 Z from the WTP.…”