Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 147

units of cm 2 /s) for each component during each interval. The test results are generally
reported in terms of a leachability index (LI), where LI = -(Σlog 10 D)/n for each
constituent. Note that two of the most readily solubilized bulk constituents of U.S.
reprocessing waste (sodium and nitrate) evinced diffusivities four orders of magni-
tude lower (better) than the usual U.S. radwaste grout WAC standard (10 -6 cm 2 /sec
or a LI of 6.0). Note also that the concrete's polyvalent cationic components (e.g.,
Zr and Sr) evinced diffusivities several orders of magnitude even lower. A substantial
drop in nitrate mobility (D) was observed between the first and subsequent leach
intervals indicating that the curing process had trapped ~90% of the nitrate in “cage
molecules.” The 10% left in the pore water leached (“diffused”) at a rate (D~10 -8
cm 2 /sec) typical of “good” conventional grout.
6.5.4.2.5 Vapor Hydration Test (VHT)
Currently, the most realistic glass durability test is the Vapor Hydration Test 17 because
of the reasonable assumption used about how water would interact with a “naked”
(breached canister) waste form in a vadose-zone repository such as Yucca Mountain
and the quantitative transformation of glass to “alteration products.” The VHT is
performed by suspending a wafer of sample above the bottom of a stainless steel
vessel with a platinum wire. After addition of sufficient water to saturate the gas
phase at the desired temperature (typically 200ºC), the vessel is hermetically sealed
and put into a preheated oven for a time ranging from several days to several weeks.
The thin film of liquid water that forms on the surface of the specimen is maintained
by the condensation of steam. In practice, the corrosion rate of the glass remains
low until the concentrations of ions leaching into that film reach a level that initiates
the formation/precipitation of stable minerals under such conditions, primarily fel-
spathoids (analcime, sodalite, cancrinites, and so on), zeolites, and silicates. Because
the hydroxide ion tends to remain (accumulate) in the water, the overall glass
corrosion (alteration) rate gradually increases until it becomes limited by the avail-
ability of the rate-limiting constituent of the product-forming reactions (usually
aluminate) in the remaining glass. After the test is complete, the corrosion/alteration
rate is determined by measuring the thickness of the remaining (unaltered) glass
layer with a microscope.
Since VHT conditions are essentially identical to those used to cure HC con-
cretes, a properly made specimen will not change (corrode/alter) when tested. Fur-
thermore, since the application of successively more rigorous hydrothermal curing
conditions tends to improve the leach performance of HCs 5 , any slight changes that
do occur are apt to be for the better. Since HC-type concretes are cured under
conditions generally considered to represent a “worst case” repository scenario
(hydrothermal), it is reasonable to conclude that they would prove to be more rugged
than glasses if those conditions were actually realized. A fundamental difference
between HC and high-level glass formulations is that the latter do not contain
sufficient aluminum to form an intrinsically insoluble mineral assemblage with 100%
of the alkali present. This means that a natural “alteration” process cannot convert
these glasses into low solubility minerals.