ANS IHLRWM 2017 - 16th International High-Level Radioactive Waste Management Conference: Creating a Safe and Secure Energy Future for Generations to Come - Driving Toward Long-Term Storage and Disposal
The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy geological repository for the permanent disposal of defense-related transuranic (TRU) waste. Industrial-grade MgO consisting mainly of the mineral periclase is the only engineered barrier certified by U.S. EPA for emplacement in the WIPP in the U.S. An Mg(OH)2-based engineered barrier consisting mainly of the mineral brucite is to be employed in the Asse repository in Germany. The WIPP is located in a bedded salt formation, and the Asse repository is located in a domal salt formation. Colloids would facilitate transport of contaminants including actinides. The regulator for the WIPP, U.S. Environmental Protection Agency (EPA), expressed its interest that possible formation of mineral colloids by MgO and its hydration and carbonation products under the WIPP-relevant conditions be evaluated. In this presentation, we report a systematic experimental study to address U.S. EPA's interest. We evaluated the possible formation of mineral colloids by using two approaches. In the first approach, as the hydration products, Mg(OH)2 (brucite), and(Mg)3Cl(OH)5′4H2O (phase 5), and the carbonation product, (Mg)5(CO3)4(OH)2•4H2O (hydromagnesite), contain magnesium, should mineral fragment colloids exist, magnesium concentrations in solution samples from MgO hydration and carbonation experiments would show a dependence on ultrafiltration, i.e., a decrease in magnesium concentrations as a function of ultrafiltration with decreasing molecular weight (MW) cut-offs. Therefore, we investigated magnesium concentrations from solutions samples in hydration and carbonation experiments as a function of ultrafiltration. We ultrafiltered solutions with a series of MW cut-off filters at 100 κD, 50 κD, 30 κD and 10 κD. Our results demonstrate that the magnesium concentrations remain constant with decreasing MW cut-offs, implying the absence of mineral fragment colloids. In the second approach, because Cs+ is easily absorbed by colloids, we spiked MgO hydration and carbonation experiments under the WIPP-relevant conditions with Cs+. Then, we ultra-filtered solutions with a series of MW cut-off filters at 100 κD, 50 κD, 30 κD and 10 κD. The concentrations of Cs do not change as a function of MW cut-offs, indicating the absence of colloids from MgO hydration and carbonation products. Therefore, both approaches demonstrate that the absence of mineral fragment colloids from MgO hydration and carbonation products.
Single crystals of Cs2NaGdBr6 with different Ce+3 activator concentrations were grown by a two-zone Bridgman method. This new compound belongs to a large elpasolite halide (A2BLnX6) family. Many of these elpasolite compounds have shown high luminosity, good energy resolution and excellent proportionality in comparison to traditional scintillators such as CsI and NaI; therefore, they are particularly attractive for gamma-ray spectroscopy applications. This study investigated the scintillator properties of Cs2NaGdBr6:Ce+3 crystals as a new material for radiation detection. Special focus has been placed on the effects of activator concentration (0 to 50 mol.%) on the photoluminescence responses. Results of structural refinement, photoluminescence, radioluminescence, lifetime and proportionality measurements for this new compound are reported.