Near-Field Performance Assessment for the Saltstone Disposal Facility

Abstract

A near-field performance assessment (PA) was conducted for the Saltstone Disposal Facility (SDF) at the Savannah River Site in South Carolina. The analysis was conducted in four parts: general screening calculations, degradation calculations, and flow and transport through the fractured and nonfractured facility. Modeling approaches and example sensitivity analysis results from the simulations of the fractured facility are discussed. Design considerations that may not be apparent without addressing flow and transport through fractures and lessons learned during the process are also presented.

Dissimilar Behavior of Technetium and Rhenium in Borosilicate Waste Glass as Determined by X-ray Absorption Spectroscopy

“…NH 4 ReO 4 was initially used as the Re(VII) standard, but its Re L 2 XANES spectrum is somewhat different from that of Re(VII) in glass in that the large peak at the absorption edge (white line) of Re(VII) in glass is broader than that of NH 4 ReO 4 . The EXAFS spectra of Re(VII) in glasses prepared in both crucibles and melters are shown in Figure 1, and the fit parameters are given in Table 4.…”

“…42 A number of assumptions were necessary to calculate the energy of reaction 5. The heat capacities of NaTcO 4 and TcO 2 were assumed to be the same as those of NaReO 4 and ReO 2 , and the heat capacity of Re was assumed to be the same as W since the heat capacity of Re at high temperature was not available. However, the main assumption is that reaction 5 is a good approximation of the behavior of these species in the glass melt.…”

Research Program to Investigate the Fundamental Chemistry of Technetium

“…(Oblath 1984;Oblath 1989;Serne, Lokken et al 1992;Council 2001) The longterm effectiveness of these measures to prevent the migration of radionuclides is described by performance assessments that depend on the leach rates of the radionuclides. (Oblath 1984;Serne, Lokken et al 1992;Seitz, Walton et al 1993;Kaplan and Serne 1998) 99 Tc is one of the radionuclides of greatest concern for leaching from CWFs because of the high mobility and lack of sorption of pertechnetate, TcO 4 -, the most stable form of technetium under aerobic conditions. (Gilliam, Spence et al 1990;Kaplan and Serne 1998) For soluble contaminants such as TcO 4 -or NO 3 -, leach rates from CWFs can be modeled using an effective diffusion coefficient, D eff = N m D m where D m is the molar diffusion coefficient of the contaminant in water and N m is the MacMullin number, a characteristic of the porous solid that is identical for solutes such as gases or anions that are highly soluble and not adsorbed by the matrix.…”

“…(Smith and Walton 1993) The thickness of the oxidized region, X, can be modeled using equation 4 where C 02 is the concentration of , the thickness of the oxidized region would be 17 cm after one 99 Tc half-life (213,000 yr), and after ten half-lives, the oxidized region would be 53 cm thick. For comparison, the dimensions of a Savannah River Saltstone cell are 30.5 m × 30.5 m × 7.5 m. (Seitz, Walton et al 1993) Therefore, approximately 4% of the technetium in the waste form would be oxidized after one 99 Tc half-life, and approximately 14% would be oxidized after ten half-lives based on the assumption that oxidation occurs at the top and bottom of the Saltstone cell. Cracking and flow of surface water through the CWF could greatly increase these numbers and the leaching of TcO 4 -by effectively decreasing the size of the Saltstone cell to the intercrack spacing, (Seitz, Walton et al 1993;Kaplan and Serne 1998) ).…”

“…For comparison, the dimensions of a Savannah River Saltstone cell are 30.5 m × 30.5 m × 7.5 m. (Seitz, Walton et al 1993) Therefore, approximately 4% of the technetium in the waste form would be oxidized after one 99 Tc half-life, and approximately 14% would be oxidized after ten half-lives based on the assumption that oxidation occurs at the top and bottom of the Saltstone cell. Cracking and flow of surface water through the CWF could greatly increase these numbers and the leaching of TcO 4 -by effectively decreasing the size of the Saltstone cell to the intercrack spacing, (Seitz, Walton et al 1993;Kaplan and Serne 1998) ). (Colton 1965) UV-visible spectra were obtained using an Ocean-Optics ST2000 spectrometer.…”