April 1, 2026
Report

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

Abstract

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.

Published: April 1, 2026

Citation

Matyas J., and A.V. Walter. 2015. Consolidation of iodine-loaded Ag0-functionalized silica aerogel with HUP, HIP and SPS Richland, WA: Pacific Northwest National Laboratory.