Consolidation of iodine-loaded Ag0-functionalized silica aerogel with HUP, HIP and SPS

Technical Report · Fri Jun 12 00:00:00 EDT 2015

DOI:https://doi.org/10.2172/3025224· OSTI ID:3025224

Matyas, Josef; Walter, Anna V.

The U.S. Department of Energy is currently investigating alternative sorbents for the removal and immobilization of radioiodine from the gas streams in a nuclear fuel reprocessing plant. One of these new sorbents, silver-functionalized silica aerogel (Ag0-aerogel), shows promise as a potential replacement for silver mordenite because of its high selectivity and sorption capacity for iodine. Moreover, a feasible consolidation of iodine-loaded Ag0-aerogel to a durable SiO2-based waste form makes this aerogel an attractive choice for sequestering radioiodine. The main purpose of the study reported here was to investigate consolidation of Ag0-aerogel powders with or without iodine and with or without sintering additives with hot uniaxial pressing (HUP), HIP, and SPS. The selected densified samples were analyzed with a helium gas pycnometer for apparent density, with the Archimedes method for open porosity and apparent density, with scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) for the extent of densification, macro- and microstructural changes, and distribution of individual elements, and with X-ray diffraction (XRD) for crystalline phases. To convert the iodine-loaded Ag0-silica aerogel to a waste form, rapid consolidation with HUP, HIP, and SPS at moderate temperatures and pressures yields a final waste form consisting of AgI particles encapsulated in fused silica. HUP-processing (1200°C for 30 min under 29 MPa pressure) of Ag0-aerogel loaded with 20.2 mass% of iodine provided a product with residual open porosity of 16.9%. However, ~93% of sorbed iodine was retained in the sintered material. Densification with HIP at 1200°C for 30 min under 207 MPa pressure resulted in a fully dense silica-based waste form of 3.3 ? 103 kg/m3 bulk density containing ~22 mass% of iodine. Iodine was retained in the form of nano- and micro-inclusions of AgI dispersed in the silica matrix. Additives were not needed to enhance sintering process as depicted from the lack of open porosity and 100% retention of iodine in the consolidated product. The future tests will tell whether additives such as raw silica aerogel or Ag0-aerogel are required for HIPing to produce a fully dense waste form from iodine-loaded Ag0-aerogel containing higher concentrations of captured iodine. SPS-processing of samples with or without iodine revealed a benefit of additive-enhanced sintering. The samples containing additives (raw silica aerogel, colloidal silica, and Ag0-aerogel) shrunk faster and at lower temperatures. In addition, additives significantly improved distribution of silver compounds in the fully densified products, considerably decreasing the number of micron-sized inclusions of AgI, Ag and Ag2S at the grain boundaries.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 3025224
• Report Number(s): PNNL-24365
• Country of Publication: United States
• Language: English

Related Subjects

HIP
HUP
Immobilization
Iodine
SPS
Silver-functionalized silica aerogel