Creep Analyses of Titanium Drip Shield Subjected to Rockfall Static Loads in the Proposed Geologic Repository at Yucca M
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Creep Analyses of Titanium Drip Shield Subjected to Rockfall Static Loads in the Proposed Geologic Repository at Yucca Mountain Brett W. Neuberger,1 Charles A. Greene,1 and G. Douglas Gute2 1 Division of Waste Management, U.S. Nuclear Regulatory Commission, Mail Stop T-7C6, Washington DC 20555-0001, USA 2 Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Rd, San Antonio, TX 78238, USA ABSTRACT The U.S. Department of Energy (DOE) has included a drip shield (DS) as a principle component of the engineered barrier system (EBS) for the proposed high-level nuclear waste geologic repository at Yucca Mountain, Nevada. The current DS design consists of titanium grade 7 (Ti Gr 7) plates and Ti Gr 24 support beams and bulkheads. The intended functions of the DS are to divert dripping water around and prevent rockfall damage to the waste package (WP). Sustained static loading of the DS may occur as a result of rockfall or drift collapse. These static loads may cause residual stress that approaches the yield stresses of the different DS materials. This level of residual stress would enable various creep mechanisms to transpire. A preliminary assessment of the potential for DS creep after a dynamic rock block impact is presented in this paper by expressing the DS residual Von Mises stress levels as fractions of the Ti alloy yield stress (YS). It was determined, using creep data from similar alloys that the residual stress levels within a DS after a 2-tonne rock block impact per DS segment length could cause creep in both the Ti Gr 7 plates and Ti Gr 24 bulkheads and support beams. The results of this study will assist the U.S. Nuclear Regulatory Commission (NRC) in evaluating the risk significance of the expected DS performance characteristics under actual repository conditions. INTRODUCTION The DS plates will be fabricated using Ti Gr 7 (0.12–0.25 Pd wt%), which is an alpha Ti alloy (see Figure 1 [1]). Alpha Ti alloys offer good corrosion resistance and weldability. Ti Gr 24 (similar to Ti 6Al-4V with 0.04–0.08 wt% Pd addition) is characterized as an alpha-beta alloy, which differs from alpha alloys primarily by exhibiting greater strength. As a result, the DS bulkheads and lateral support beam structural stiffeners will be manufactured using Ti Gr 24. Ti alloys typically have a high strength-to-weight ratio and are highly corrosion resistant in a wide range of environments [2]. The low-temperature, alpha, phase is a hexagonal close-packed (hcp) structure, while the high-temperature, beta, phase has a body centered cubic (bcc) structure and is metastable at room temperature. Ti undergoes an allotropic transformation from the hcp phase to the bcc phase as its temperature is raised through 1,156 K (883 °C). Beta phase Ti is then stable to the melting point, 1,941 K (1,668 °C). Alloying elements that cause the allotropic transition temperature to remain unchanged or increase are known as alpha stabilizers. Conversely, alloying elements that cause the allotropic transition temperature to lower
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