Yucca mountain project getter program results (year 1):I-129 and other anions of concern

Although high level nuclear wastes (HLW) contain a daunting array of radioisotopes, only a restricted number are long-lived enough to be problematic, and of these many are either effectively insoluble or are likely to be scavenged from solution by minerals indigenous to all aquifers. Those few constituents likely to travel significant distances through aquifers either form colloids (and travel as particulates) or anions--which are not sorbed onto the predominantly negatively charged mineral surfaces. Iodine (129I) is one such constituent and may travel as either iodide (I-) or iodate (IO3-) depending on whether conditions are mildly reducing or oxidizing. Conventionally, 99Tc (traveling as TcO4-) is regarded as being of greater concern since it is both more abundant and has a shorter half life (e.g., has a higher specific activity). However, it is unclear whether TcO4- will ever actually form in the mildly reducing environments thought likely within degrading HLW canisters. Instead, technetium may remain reduced as highly insoluble Tc(IV), in which case 129I might become a significant risk driver in performance assessment (PA) calculations. In the 2004-2005 time frame the US Department of Energy (DOE)--Office of Civilian Radioactive Waste Management (OCRUM), Office of Science and Technology International (S and T) funded a program to identify ''getters'' for possible placement in the invert beneath HLW packages in the repository being planned by the Yucca Mountain Project (YMP). This document reports on progress made during the first (and only) year of this activity. The problem is not a new one and the project did not proceed in a complete vacuum of information. Potential leads came from past studies directed at developing anion getters for a near surface low-level waste facility at Hanford, which suggested that both copper-containing compounds and hydrotalcite-group minerals might be promising. Later work relating to closing HLW tanks (Hanford and Savannah River) added layered bismuth hydroxides to the list of candidates. In fact, even in the first year the project had considerable success in meeting its objectives (Krumhansl, et al., 2005). ''Batch Kd'' testing was used to screen a wide variety of materials from the above-mentioned groups. Some materials tested were, in fact, archived samples from prior studies but a significant amount of effort was also put into synthesizing new--and novel--phases. A useful rule of thumb in judging getter performance is that the ''Kd'', should exceed a value of roughly 1000 before it's placement can materially decrease the potential dose at a hypothetical (distant) point of compliance (MacNeil, et al., 1999). Materials from each of the groups met these criteria for both iodide and iodate (though, of course, the actual chemistry operating in ''batch Kd'' runs is unknown, which casts a rather long shadow over the meaning of such comparisons). Additionally, as a sideline, a few materials were also tested for TcO4- and occasionally Kd values in excess of 103 were also found for this constituent. It is to be stressed that the ''batch Kd'' test was used as a convenient screening tool but in most cases nothing is known about the chemical processes responsible for removing iodine from the test solutions. It follows that the real meaning of such tests is just as a relative measure of iodine scavenging ability, and they may say nothing about sorption processes (in which case evaluating a Kd is irrelevant). Numerous questions also remain regarding the longevity and functionality of materials in the diverse environments in, and around, the proposed YMP repository. Thus, although we had a highly successful first year, we are still far from being able to either qualify any material for placement in the repository, or quantify a getter's performance for use in PA assessments