PNNL

Abstract

The vapor hydration test (VHT) is used to study the reaction between different materials and water with the aim to gain insight into the alteration behavior of the materials in aqueous environments. In nuclear waste immobilization, the test is primarily used as a screening tool to identify durable/non-durable glasses, and as a convenient method to generate and identify alteration products for use in performance modeling. The lack of a standard procedure for conducting this test resulted in a number of test methods, thus decreasing the ability to directly compare test results from different sources. In order to optimize the VHT procedure, a series of tests was conducted on simulated low-activity waste (LAW) glasses at temperatures ranging from 150?C to 300?C, with different volumes of water, specimen holders, specimen preparation techniques, and data-evaluation methods. Reaction progress was monitored by measuring the thicknesses of the remaining glass layer and the alteration layer. The apparent alteration rate at which glass is converted into the alteration products was determined from a linear portion of the alteration curve, plotted as a function of time. The resulting procedure for conducting VHT eliminates problems associated with the measurement of alteration layers and enables direct comparison of apparent alteration rates for different materials. Data obtained according to the developed procedure show a correlation with data found in literature.