A Probabilistic Model for Degradation of the Engineered Barriers System in the Yucca Mountain Repository
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A Probabilistic Model for Degradation of the Engineered Barriers System in the Yucca Mountain Repository Z. Qin and D.W. Shoesmith Department of Chemistry The University of Western Ontario London, Ontario, Canada, N6A 5B7
ABSTRACT A probabilistic model to predict the lifetimes of the engineered barrier system proposed for the Yucca Mountain repository is described. The model assumes that the titanium Grade-7 drip shield will fail by hydrogen-induced cracking and the Alloy-22 waste package by a combination of passive and crevice corrosion. The model predicts that crevice corrosion of the waste package can be completely avoided if the drip shield deflects seepage drips for between 2000 (realistic behaviour) and 6000 years (conservative behaviour). Sensitivity calculations on the crevice corrosion model suggest that early waste package failure is extremely unlikely providing the drip shield performs its function for a minimum of ~ 300 years.
INTRODUCTION The engineered barrier system proposed for the high level radioactive waste repository at Yucca Mountain is a combination of a mail-box shaped drip shield (DS), fabricated from titanium Grade-7 (Ti-7), and a corrosion-resistant waste package (WP) consisting of an Alloy-22 outer corrosion barrier and a stainless steel inner cylinder to add structural rigidity [1]. This combination, coupled with the defense provided by the wasteform cladding, is expected to compliment the natural barrier provided by the mountain itself. This sequence of barriers must ensure waste containment so that the annual radiation dose to any exposed individual remains below the regulatory limit for ≥ 104 years [1]. While the WP is the primary barrier to radionuclide release, the DS will divert potential seepage water and rock-fall from the WP. The primary function of the DS is the protection of the WP from seepage drips which, via wetting-evaporative cycles, could lead to the formation of corrosive, high ionic strength aqueous solutions at high temperatures [2]. On its own, the failure of the DS does not constitute a breach of the engineered barrier system, but defines the period of protection from seepage drips supplied to the WP. In this paper, we outline failure models for the DS and WP, build on deterministic mechanisms and based on anticipated repository environments and available corrosion data on relevant Ti alloys [3] and the C-series of Ni-Cr-Mo alloys [4]. Monte Carlo simulations are employed to predict DS and WP lifetimes. The primary emphasis is on the dual barrier nature of the DS/WP and defining the DS performance required to avoid localized corrosion damage on the WP itself.
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FAILURE MODELS On emplacement of WPs in the repository a period of ventilation will guarantee low relative humidities (RH) and temperatures (T). On closing the repository, the T of the DS/WP will first rise and the RH will be further suppressed [1]. As T eventually decreases the RH will rise and aqueous corrosion will become possible. Depending on the nature of the deliquescent solids forme
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