Survey of matrix materials for solidified radioactive high-level waste

Abstract

Pacific Northwest Laboratory (PNL) has been investigating advanced waste forms, including matrix waste forms, that may provide a very high degree of stability under the most severe repository conditions. The purpose of this study was to recommend practical matrix materials for future development that most enhance the stability of the matrix waste forms. The functions of the matrix were reviewed. Desirable matrix material properties were discussed and listed relative to the matrix functions. Potential matrix materials were discussed and recommendations were made for future matrix development. Th1e matrix mechanically contains waste cores, reduces waste form temperatures, and is capable of providing a high-quality barrier to leach waters. High-quality barrier matrices that separate and individually encapsulate the waste cores are fabricated by powder fabrication methods, such as sintering, hot pressing, and hot isostatic pressing. Viable barrier materials are impermeable, extremely corrosion resistant, and mechanically strong. Three material classes potentially satisfy the requirements for a barrier matrix and are recommended for development: titanium, glass, and graphite. Polymers appear to be marginally adequate, and a more thorough engineering assessment of their potential should be made.

Development and testing of matrices for the encapsulation of glass and ceramic nuclear waste forms.

“…Cumulative doses from ceramic pellets would be less than those presented for glass marbles. Calculations have shown that the alpha dose to the matrix, even in the thin layer just around each waste core, is negligible compared to the gamma dose (Gurwell, 1981).…”

“…The candidate materials selected have been studied previously as engineered barriers (Westerman 1979;Westerman 1980;Charlot and Westerman 1981) or as barrier matrices (Gurwe11 1981). To be considered for use as hot pressed matrices, materials should not only have satisfactory leach resistance, but also should be capable of being hot pressed to high density (high enough to preclude open porosity) at a temperature low enough to minimize loss of radioactive isotopes due to volatilization.…”

“…Titanium -Of all the reasonable metal matrix choices, titanium has high corrosion resistance over the broadest range of environments; and for this reason, titanium offers superior barrier properties (Gurwell 1981). Unalloyed titanium is not susceptible to stress corrosion cracking except in the extreme case of red fuming nitric acid; therefore, unalloyed or low alloy titanium, such as Ticode 12, is preferable for the barrier matrix.…”

“…A schematic of a matrix waste form is shown in Figure 1-1, along with an illustration of it's key features (Gurwell 1981). A continuous "matrix web" can be established by controlling the waste core loading in the matrix, thereby effectively isolating individual waste cores.…”

Graphite matrix materials for nuclear waste isolation

“…Gurwell (10) has made an extensive review of possible matrix materials; specific materials were eliminated because of poor corrosion resistance, high cost, poor availability, or inability to fabricate at temperatures compatible with waste materials. A detailed comparison will not be undertaken here; however, some comparison is in order for this review.…”