Radiolytically-induced novel materials and their application to waste processing. Final report

Abstract

SUMMARYIn the present NEER project we investigated two different types of gel materials with respect to potential applications in environmental remediation, including mixed waste generated from the nuclear fuel cycles. The materials under study were: 1) silica-polymer based aerogel composites into which specific metallic cations diffuse into and remain, and 2) polymer gels made of thermo-sensitive polymer networks, whose functional groups can be 'tailored' to have a preferred affinity for specific cations, again diffusing into and remaining in the network under a volumetrically, contractive phase-transition. The molecular, diffusion of specific cations, including those of concern in low-level waste streams, into the gel materials studied here indicates that a scaled, engineered system can be designed so that it is passive; that is, minimal (human) intervention and risk would be involved in encapsulating LLW species. In addition, the gel materials hold potential significance in environmental remediation of and recovery of metallic cations identified in respective domains and physico-chemical processes.In brief, silica gels start as aqueous/liquid solutions of base catalyzed silica hydrogels and metal ions (targeted species), such as silver. The metal ions are reduced radiolytically and migrate through the solution to form clusters. Upon post-irradiation processing, aerogel monoliths, extremely lightweight but mechanically strong, that encapsulate the metals are produced. Interestingly the radiolytic or photonic source can be gamma-rays and/or other rays from 'artificial sources', such as reactors, or 'inherent sources' like those characterizing mixed waste.Polymer gels, in contrast exhibit thermally-induced volumetric contraction at 20°-50ºC by expelling water from the gels' physical state. Further, some functional groups that capture di-or tri-valent cations from aqueous solutions can be incorporated into the polymer networks on synthesis, including by radiolytic means. These polymer gels retain cations after volumetric contraction and have the ability to trap solid particulates in liquid waste streams. With both silica and polymer gels, functional design of a prototypic 'recovery' systems, such as by natural convection in a rectangular cell, appears possible.Additional and specific details are provided in the rest of this Final Report. Student ParticipationThe present NEER project attracted an unexpected number of students who contributed to the success of the project. Table 1 provides a list of students who participated. Many were supported in some form from the grant or via additional funds leveraged with the grant. Theses CompletedThe following Master's theses that were directly related to the project were completed. Conference PresentationsA. Tokuhiro, M. Bertino and T. Tokuhiro, Results to date on radiolytic processing of solid particulae waste using polymer and silica gel materials , ANS Winter Meeting, Washington DC, November, 2005 T. Tokuhiro, A. T. Tokuhiro, S. S. Akella*, J. W. Carey*. Metal bind...

Temperature dependence of density of polymer gels: Effects of ionizable groups in copoly(N-isopropylacrylamide/acrylic acid or sodium acrylate)–water systems

“…Thus, this may in fact be the origin of characteristic behavior of copolymer gels in volumetric contractioneexpansion process. Exact correspondence of characteristic behavior substantiated at both molecular and nano-scale levels were also revealed in other experimental observations: (1) significant change in dynamics of water (HOD) in gels as determined by both self-diffusion coefficient [30] and proton NMR spin-lattice relaxation time (T 1 ) [31]; and (2) characteristic changes in the line-shape of proton NMR spectra for polymer networks [32]. When these results are integrated, a plausible mechanism for the volumetric contractioneexpansion process in gels is suggested at the nano-scale.…”

“…In contrast, our experimental results for the volumetric contractioneexpansion processes occurring at the molecular level were substantiated as follows: (1) by means of proton NMR spectra of polymer networks [38], (2) by proton relaxation times of both polymer and solvent (water) components [31]; and (3) by determination of the self-diffusion coefficient of water [30] in samples consisting of sub-micron gel pieces (100 nm in diameter). These measurements clearly exhibit that gel pieces undergo some sort of the transient state (with rather long lifetime) before attaining new thermal equilibrium in transition from the swollen to shrunken states and vice versa.…”