PNNL

Abstract

Laboratory testing is often used to assess the chemical durability of a wide range of nuclear waste forms. However, results from these tests and their applicability to disposal of waste forms is not representative of a near-surface disposal site because of the high-temperatures used to accelerate various corrosion mechanisms. Here, we synthesized a set of vitreous samples with equivalent chemical compositions to vitreous archeological samples excavated from a ~1500-year old Swedish hillfort named Broborg. The lab-made hillfort samples were subjected to standard laboratory aging tests, and results were compared with observations made from samples that were excavated directly from the site. Synthesis conditions for these lab-made samples were varied to produce samples that differed in terms of their chemical composition and presence of crystalline phases, as these variables may influence sample alteration. The laboratory aging test methods included the product consistency test (PCT), to distinguish between samples of different durability and homogeneity, and the vapor hydration test (VHT), to study alteration of material upon exposure to water vapor at elevated temperatures. The United States Environmental Protection Agency (EPA) Leaching Environmental Assessment Framework (LEAF) pH dependent leach test (EPA Method 1313) was also performed. The extent of corrosion on the laboratory-made samples was evaluated using the calculated alteration layer thicknesses, and samples were compared in terms of alteration behavior and leaching of alkali elements. The influence of: (i) sample composition; (ii) presence of crystalline phases; (iii) environmental conditions; and (iv) temperature, on the thickness of the alteration layer produced by the three durability tests was determined. With this improved understanding of long-term glass alteration behavior, we recommend that, where possible, the mineral phases present in the alteration layers produced by these lab tests should be identified and compared with alteration phases found on the archeological samples.