Fuel Cycle

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, Ag0
-functionalized silica aerogels, shows promise as a potential replacement for Ag-bearing
mordenites because of its high selectivity and sorption capacity for iodine. Moreover, a feasible
consolidation of iodine-loaded Ag0
-functionalized silica aerogels to a durable SiO2-based waste form
makes this aerogel an attractive choice for sequestering radioiodine.
This report provides a preliminary assessment of the methods that can be used to consolidate iodineloaded
Ag0
-functionalized silica aerogels into a final waste form. In particular, it focuses on experimental
investigation of densification of as-prepared Ag0
-functionalized silica aerogel powders, with or without
an organic moiety and with or without a sintering additive (colloidal silica), with three commercially
available techniques: 1) hot uniaxial pressing (HUP), 2) hot isostatic pressing (HIP), and 3) spark plasma
sintering (SPS). Since there are no studies reported in literature on the sintering of aerogels with HIP and
SPS, the preliminary experiments were performed without iodine-loaded aerogels and were focused on
investigating feasibility of these methods to produce a fully dense product. Iodine was excluded from
these initial studies to avoid potential damage to vendor equipment due to the potential release of iodine
during consolidation. The densified products were evaluated with a helium gas pycnometer for apparent
density, with the Archimedes method for apparent density and open porosity, and with high-resolution
scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) for the extent of
densification and distribution of individual elements.
The preliminary investigation of HUP, HIP, and SPS showed that these sintering methods can be used to
effectively consolidate powders of Ag0
-functionalized silica aerogel into products of near-theoretical
density. Also, removal of the organic moiety and adding 5.6 mass% of colloidal silica to Ag0
-
functionalized silica aerogel powders before processing resulted in denser products. Furthermore, the ram
travel data for SPS indicated that rapid consolidation of powders with SPS can be performed at
temperatures below 950 °C.