The Accelerated Weathering of a Radioactive Low-Activity Waste Glass under Hydraulically Unsaturated Conditions: Experimental Results from a Pressurized Unsaturated Flow Test

Abstract

To predict the long-term fate of low- and high-level waste forms in the subsurface over geologic timescales, it is important to understand how the formation of an alteration phase or phases will affect radionuclide release from the corroding waste forms under repository-relevant conditions. To generate data to conduct performance assessment calculations for the low-activity waste (LAW) integrated disposal facility at the Hanford Site in southeastern Washington state, accelerated weathering experiments are being conducted with the pressurized unsaturated flow (PUF) test method to evaluate the long-term release of radionuclides from immobilized LAW (ILAW) glasses. The radionuclide release rate is a key parameter affecting the overall performance of the LAW disposal facility.

Currently, there are three other accelerated weathering test methods being used to evaluate the long-term durability of glasses: product consistency test, vapor hydration test, and unsaturated drip test. In contrast to these test methods, PUF tests mimic the hydraulically unsaturated open-flow and transport conditions expected in the near-field vadose zone environment, allow the corroding waste form to achieve its final reaction state, and accelerate the hydrolysis and aging processes by as much as 50 times over conventional static tests run at the same temperature.

In this paper, we discuss the results of an accelerated weathering experiment conducted with the PUF apparatus to evaluate the corrosion rate of an ILAW glass, LAWAN102, made with actual Hanford waste taken from Tank 241-AN-102 (U). Results from this PUF test with LAWAN102 glass showed that after 1.5 yr of testing, the corrosion rate, based on B release, reached a steady-state release of 0.010 ± 0.003 g m^–2 day^–1, which is approximately eight times lower than other glasses previously tested. These results indicate that ⁹⁹Tc is being released from the glass congruently, whereas U is being controlled by the formation of a solubility-limiting phase or phases. These results also highlight the importance of being able to predict, with some level of certainty, the alteration phase or phases that will form and how the formation of these phases may impact the release, retention, and transport of radionuclides from the glass under the hydraulically unsaturated open flow and transport conditions that are expected in the LAW integrated disposal facility.