Repository site

Miscellaneous. Iridium dioxide, like RUO2, is useful as an electrode material for dimensionally stable anodes (DSA) (189). SoHd-state pH sensors employing Ir02 electrode material are considered promising for measuring pH of geochemical fluids in nuclear waste repository sites (190). Thin films (qv) ofIr02 ate stable electrochromic materials (191). 

The wide variation between different ground waters in both sets of experiments emphasizes the necessity of using actual ground waters in all laboratory studies, since the observed plutonium behavior is "ground-water-specific". Moveover, these results reinforce the suggestion made elsewhere (12) that ground-water characterization should be included as a viable repository site-selection criterion. 

Selection criterion for radioactive nuclear waste repository site and... 

Waste, repository site-selection criterion, ground-water leaching. 335-36... 

For nuclear waste disposal, in a site such as Yucca Mountain, if the maximally exposed individual receives the proposed annual limit of 0.15 mSv, present estimates (based on the linearity hypothesis) suggest a 0.00 1 % risk of an eventual fatal cancer. The maximum dose is reached only if the wastes are dissolved in a small volume of water, and therefore only a limited number of people would receive this dose. If this number were as high as 1000, the implied toll for Yucca Mountain neighbors would be one cancer fatality per century per repository site.19 This toll would not start for many centuries, when the waste canisters begin to fail, and it not unreasonable to expect that cancer prevention and treatment will be much improved by then. Ignoring this prospect, and assuming many repositories and some doses above the prescribed limit, it still appears that the expected toll would be well under a thousand deaths per century. 

Most repository sites under consideration for commercial spent fuel disposal are in reducing environments, such as in Boom clay formations of the Mol site, Belgium, where U02 is thermodynamically stable. In oxygen-free conditions, Spahiu et al. (2002) have shown that fuel in an... 

Such studies are sometimes criticized because they appear to be based on the concept of a global analogy between the specific example studied and a waste repository site (e.g., Miller et al. 1994). Clearly no geological site fully resembles a radioactive waste repository site. Confusion also seems to arise from the fact that analogy has sometimes been considered as a particular... 

Finally, a study is underway to determine the suitability of the Nevada Test Site in southern Nevada which has been used in the past for both surface and underground testing of nuclear weapons, to see if it may possibly be suitable as a potential permanent radioactive waste repository site. 

If the waste is isolated in a geologic repository, the iodine form should be stable to at least 100°C and possibly at 250°C depending on the repository site. If the waste form satisfies the thermal stability requirement, the most likely release mechanism then becomes leaching in the event that groundwater contacts the immobilization form. Allard et al. (11) report log Kd values for silicate minerals ranging from -0.5 to -3.5. Fried et al. (12) found little retention of iodine (as iodide or iodate) by Los Alamos Tuff. Thus, once the Iodine has been removed by leaching, it will potentially move at the same velocity as the groundwater. 

Choosing the waste repository sites is an especially sensitive issue. Many states have resisted the plan, but Congress has the power to override a state s disapproval. In fact, Congress amended the Nuclear Waste Policy Act in 1987 to make Yucca Mountain in Nevada the primary potential site. Studies are now being carried out to evaluate the feasibility of this site as a safety repository for nuclear waste. 

Ogard A. E. and Kerrisk J. F. (1984) Groundwater chemistry along the flow path between a proposed repository site and the accessible environment. Los Alamos National Laboratory. 

Tien P.-L., Siegel M. D., Updegraff C. D., Wahi K. K., and Guzowski R. V. (1985) Repository Site Data Report for Unsaturated Tuff, Yucca Mountain, Nevada. US Nuclear Regulatory Commission. 

The perfect high-level waste repository (or toxic-waste site) is one in which individual waste components are at thermodynamic equilibrium with the host water-rock system. For such conditions there is no tendency for the waste components to dissolve and be transported from the site to the accessible environment. It was shown earlier that low-Eh crystalline-rock groundwaters are often near saturation with respect to UO2 (Section 13.2.3). Spent fuel UO2 in such a system should have little or no tendency to dissolve and release other radionuclides to the groundwater. An appropriate first task in characterizing a potential repository site should, therefore, be to obtain accurate groundwater analyses to determine if the groundwater is saturated with respect to UO2. 

Use of geologic media for waste isolation involves an action phase and a prediction phase. The action phase is concerned with site selection, waste repository design, waste form selection, etc. i.e., all activities related to selecting a repository site and putting it into use. 

Application of the geosphere transport models is highly site-specific i.e., data for the proposed repository site should be used. This requirement imposes a need to assess the viability of, and methods for obtaining, laboratory and field data. Capability to produce representative information is needed. Data used to date can best be characterized as generic for many nuclides, numerical values used to represent holdup phenomena are extrapolations of limited data for similar nuclides. The validity of such extrapolations should be evaluated. 

Models of biosphere transport have been developed. 5,6) As is the case for geosphere transport models, the data base for parameters in the biosphere models is limited. The scope of available data is being expanded, (7-9) but careful assessment is needed of the validity of the data, the validity of the models, and the need for the data and models to be site-specific. One site-specific factor to consider is the possibility that geosphere migration could extend for considerable distances from the repository radioactivity might therefore enter the biosphere in an environment different from that of the repository site. 

The prediction phase of safety assessment is also coupled to activities concerned with waste treatment, selection of the final form of the waste, and selection of repository sites and designs. Results of the predictions can impact these activities. LITERATURE CITED... 

Mercer, J.W. Thomas, S.D., Ross, B. "Parameters and Variables Appearing in Repository Siting Models" NUREG/CR-3066, Nuclear Regulatory Commission Washington, D.C., 1982, p. 244. 

Figure 1.1. Yucca Mountain, in southern Nevada, is the only proposed geological repository site for high-level nuclear waste in the USA. Photograph courtesy of Neil Coleman of US Nuclear Regulatory Commission.

The updated model focuses on a volume which includes the tunnel spiral and will be contained within a cube having a 1 km side length, extending from the surface to 1000 m depth. This volume is to be linked to the earlier model (Rhen et al. 1997). Additionally, the development and refinement of the methodology and tools for geoscientific model construction will support the geoscientific characterization of a future repository site. 

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