Estimating Model Parameter Values for Total System Performance Assessment
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ABSTRACT The intrinsic dissolution rates of nine borosilicate waste glasses were extracted from the results of MCC- I tests conducted for durations long enough that the solution pH reached a nearly constant value but short enough that the buildup of dissolved species did not affect the dissolution rate. The effects of the pH and temperature on the measured rates were deconvoluted to determine the sensitivity of the rate to the glass composition. The intrinsic dissolution rates were similar for all of these glasses and were not correlated with the glass composition. The mean and standard deviation of the intrinsic dissolution rates of these glasses are log {ko/[g/(mZed)]} = 8.2 ± 0.2. INTRODUCTION The results of total system performance assessment (TSPA) calculations will play an important role in the design and licensing of the federal repository for high-level radioactive waste disposal. These calculations will be used to evaluate the long-term containment of radionuclides by the waste forms and the engineered barrier systems and to ensure that regulatory requirements will be met throughout the service life of the disposal system. A mechanistic rate expression has been developed for borosilicate waste glasses and was included in the TSPA-Viability Assessment report [1]. That expression contains terms for the forward dissolution rate (kf), which depends on the glass composition, pH, and temperature, and for the reaction affinity term, which quantifies the feedback effect of solute species on the glass dissolution. The rate expression can be written as: rate = S 9 kf 9 (1 - Q/K)
(1)
where S is the surface area. The value of the affinity term, which is the term in parentheses, depends on the saturation index of the solution, Q/K, where K is a quasi-thermodynamic constant. The forward rate can be written as: kf = ko * 10
h*pH e e(-Ea/RT)
(2)
where k0 , h, and Ea are parameters for the effects of the glass composition, pH, and temperature on the rate. These parameters values must be measured experimentally. Previous tests have shown the values of h and Ea can be assumed to be independent of the glass composition [2-5]. In this paper, we present the results of tests conducted to determine the values of the intrinsic dissolution rate (k0) for glass compositions that are representative of likely high-level waste glasses.
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Mat. Res. Soc. Symp. Proc. Vol. 608 0 2000 Materials Research Society
EXPERIMENTAL METHOD The glasses used in these tests include reference compositions for the Defense Waste Processing Facility (DWPF), the West Valley Demonstration Project (WVDP), and Hanford tank wastes. The glass compositions are given in Table 1. The SRL 51S glass is a nonradioactive homologue of glass that was made at DWPF with sludge from Tank 51 [6]. The SRL 202U, SRL 165U, and SRL 131U glasses represent possible DWPF waste glasses. The WV6 glass is a nonradioactive homologue of the glasses made at the WVDP. A glass formulated to represent a potential waste glass made with Hanford tank wastes [7] is also being tested;
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