Monte Carlo Simulations of the Degradation of the Engineered Barriers System in the Yucca Mountain Repository using the
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0985-NN03-06
Monte Carlo Simulations of the Degradation of the Engineered Barriers System in the Yucca Mountain Repository using the EBSPA Code Z. Qin and D. W. Shoesmith The University of Western Ontario, London, Ontario, N6A 5B7, Canada ABSTRACT Based on a probabilistic model previously proposed, a Monte Carlo simulation code (EBSPA) has been developed to predict the lifetime of the engineered barriers system within the Yucca Mountain nuclear waste repository. The degradation modes considered in the EBSPA are general passive corrosion and hydrogen-induced cracking for the drip shield; and general passive corrosion, crevice corrosion and stress corrosion cracking for the waste package. Two scenarios have been simulated using the EBSPA code: (a) a conservative scenario for the conditions thought likely to prevail in the repository, and (b) an aggressive scenario in which the impact of the degradation processes is overstated.
INTRODUCTION The Yucca Mountain (Nevada, USA) is the proposed site for a geologic repository for the disposal of nuclear waste [1]. The spent fuel and high-level radioactive waste will be stored in an engineered barriers system (EBS) that combines a waste package (WP), the primary barrier to radionuclide release, and a drip shield (DS) which will divert potential seepage drips and rockfall from the WP [2]. This combination, coupled with the defence provided by the wasteform cladding and wasteform itself, will complement the natural barrier provided by the mountain. The EBS is the only absolute barrier within the sequence of barriers, and its performance is primarily controlled by the corrosion performance of the titanium Grade 7 (Ti-7) DS and the Alloy 22 (C-22) WP. A probabilistic model to predict the lifetimes of the EBS has been proposed earlier [3-5]. In this model, corrosion commences once aqueous conditions are established on the surfaces of the DS and the WP. It assumes that the DS will fail by hydrogen-induced cracking (HIC), and the WP by a combination of general passive corrosion (GC), crevice corrosion (CC), and stress corrosion cracking (SCC). The assessment of the EBS performance requires long time predictions based upon experimental databases measured over short time periods. The resulting uncertainties are generally accounted for by using probabilistic distributions instead of determinate parameter values. The use of such distributions in long time predictions makes Monte Carlo simulation, which simulates processes depending upon random variables, an ideal tool for predicting EBS lifetimes. A Monte Carlo simulation code (EBSPA) has been developed to predict the lifetime of the EBS based on the proposed model. The EBSPA, currently in its third version, can calculate the cumulative probability of failures (CPF) of the DS, the WP, and the EBS, the responsibility of an individual component failure for total EBS failure (expressed as the responsibility ratio), and the contributions of different degradation modes to failure.
FAILURE MODEL The temperature of the DS/WP will first rise
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