Creep Analyses of Titanium Drip Shield Subjected to Rockfall Static Loads in the Proposed Geologic Repository at Yucca Mountain

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.