Prediction of Waste Package Life for High-Level Radioactive Waste Disposal at Yucca Mountain
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selected a basecase(defined as a particular set of models and model parameters describing the YM repository system) to perform sensitivity and uncertainty analyses and to study the general behavior of the system as simulated by the TPA code.' The DOE has recently released a VA for the proposed HLW repository at YM [2]. A major objective of the DOE YM repository program is to show the feasibility of near-complete containment of radionuclides within the WPs for several thousand years [3]. The proposed standard for the repository requires the evaluation of the predicted -- over 10,000 yr- performance of the system. Of interest to this paper are the Total System Performance Assessment studies [2, Vol. 3], particularly the analysis of the corrosion phenomena leading to WP failure (i.e., number of failed WPs and the time of failure). In this paper, the NRC basecaseis combined with the parameters characterizing the corrosion phenomena reported in the VA (i.e., the NRC TPA code is used to simulate the corrosion processes as described in the VA). The objectives are to exercise and strengthen the NRC review capabilities in preparation for receipt of the DOE license application for the proposed HLW repository. PROBLEM DESCRIPTION The WP design proposed in the VA includes two metallic barriers: an inner corrosion resistant material composed of Alloy 22 (2 cm thick) and a corrosion allowance material (CAM) composed of ASTM
'The DOE has an analogous definition to the NRC basecase, referred to in this paper as the DOEbase case. 147 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
A516 (10 cm thick) [2, Vol. 2]. The carbon steel (ASTM A516) is meant to provide structural strength, protection against rock fall, and reduction in the intensity of alpha and gamma radiation (thus decreasing the probability of the presence of radiolytic products such as hydrogen peroxide, which are detrimental to the corrosion resistance of the WP materials) [2, Vol. 2]. The NRC models the CAM behavior by considering several corrosion modes, including dry air oxidation and corrosion under humid air and aqueous environments. The physical condition determining the corrosion mode is the relative humidity (RH). Thus, when the RH exceeds a lower threshold, the corrosion mode changes from dry oxidation to humid air corrosion, and when the RH lies above an upper threshold, aqueous corrosion is initiated (i.e., at high RH it is assumed the formation of a water film on the WP surface). Current NRC modeling does not consider the presence of dripping (i.e., the direct impingement of water on the WPs) [I]. Localized corrosion in aqueous environments (an accelerated corrosion process) is assumed to occur when the corrosion potential is above a critical potential, defined by the temperature and chloride concentration. Experimental data indicate that Alloy 22 is a material highly resistant to localized corrosion with a passive dissolution rate independent of environmental factors such as pH and chloride concentration and only slightly dependent
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