Technetium-99 Chemistry in Reducing Groundwaters: Implications for the Performance of a Proposed High-Level Nuclear Wast
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repositories because of its long half-life (2.13 x l0 yr) and its relative abundance inHLW (fission yield 6 percent). A redox-sensitive element, Tc exists in a heptavalent (7+) state under nonreducing conditions as
the pertechnetate (TcO4-) anion (Fig. 1). The TcO 4-anion does not sorb significantly onto mineral surfaces, and compounds of Tc(7+) generally have high solubility so that the concentration of Tc in groundwater is not expected to be limited to low values by solubility constraints. Conversely, under reducing conditions Tc is present in a tetravalent (4+) state. The solubility of Tc(4+) solids, such as TcO 2 .l.6H20 (Fig. 1), is much lower compared to Tc(7+) solids, and Tc(4+) aqueous species sorb more strongly onto mineral surfaces compared to Tc(7+) species. For example, sorption experiments by Lieser and Bauscher' using sediments, predominantly quartz, and groundwaters from Gorleben, Germany, indicate that at pH = 7 and Eh > 0.17 V, in the stability field of TcO 4-, Tc Kd is low and relatively constant at - 0.2 mL/g (Fig. 2). At lower Eh, where the Tc(4+) species TcO(OH) 2(aq) is the dominant aqueous form, Kd is high, with a value of- 1,000 muJg. Although Lieser and Bauscher' indicated that precipitation of low solubility Tc(4+) solids may have contributed to Tc removal in their experiments, the effect of low Eh on sorption is significant. As part of the United States HLW program, the Department of Energy (DOE) and the Nuclear Regulatory Commission (NRC) conducted total-system performance assessment (PA) simulations of the proposed geologic repository at Yucca Mountain, Nevada (YM). 2' 3"4 The DOE and the NRC use PA to systematically analyze what can happen at the repository after permanent closure, and to calculate as a function of time the total dose affecting a hypothetical human receptor group located some distance
downgradient from YM in the event of failure of waste packages (WPs) to isolate the radionuclides. The 231 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
NRC PA activity, part of an ongoing process at the NRC to prepare for the review of a potential DOE license application for a HLW repository at YM, is designed to provide information on the potentially important isolation characteristics and capabilities of the proposed repository system based on the available information regarding the geologic setting and the natural and engineered barrier systems. An important aspect of the NRC PA is quantifying the sensitivity of the results to, and the uncertainty associated with, the input parameters.
1.2F~ 0.8k
-
0
wU
In the DOE and NRC PA analyses, oxidizing conditions were assumed over the entire transport path from YM through both the unsaturated and saturated zones. Under these conditions, there is no pH Figure 1. Eh-pH diagram for aqueous Tc species solubility limiting phase for Tc-99, and the PA source calculated using the NEA thermodynamic data.9 term models for Tc assumed a high solubility limit such that Tc release is essentially constrained by the Also show
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