Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. This presentation describes uncertainty and sensitivity analysis results for the early waste package failure scenario class and the early drip shield failure scenario class obtained in the 2008 YM PA. The following topics are addressed: (i) engineered barrier system conditions, (ii) release results for the engineered barrier system, unsaturated zone, and saturated zone, (iii) dose to the reasonably maximally exposed individual (RMEI) specified in the NRC regulations for the YM repository, and (iv) expected dose to the RMEI. The present article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA.
Uncertainty and sensitivity analysis are fundamental components of the 2008 performance assessment (PA) conducted by the U.S. Department of Energy (DOE) for a proposed high-level radioactive waste (HLW) repository at Yucca Mountain (YM), Nevada [1] and [2]. The following presentation describes uncertainty and sensitivity analysis results obtained for the early failure scenario classes [3] in the 2008 YM PA. Additional presentations describe uncertainty and sensitivity analysis results obtained in the 2008 YM PA for the nominal scenario class [4] and [5], igneous scenario classes [6] and [7], seismic scenario classes [8] and [9], and all scenario classes collectively [10].
Analyses for the early failure scenario classes determine the contribution to expected dose to the reasonably maximally exposed individual (RMEI) that results from waste packages (WPs) or drip shields (DSs) with manufacturing defects or undetected damage that occurred during emplacement [3]. These scenario classes involve only WPs or DSs which experience early failure. With two exceptions, the models used to estimate these contributions are the same as those used to determine the expected dose to the RMEI that results from nominal processes (i.e., in the absence of early failures, seismic events and igneous events; see [11], Section 4 and Fig. 2). In the early WP failure scenario class, the models for corrosion of the WP outer barrier are replaced by the assumption that, for early-failed WPs, the entire WP outer barrier is failed at time 0 and does not impede flow of water or transport of radionuclides ([11], Section 4; [1], Section 6.4). In the early DS failure scenario class, the models for corrosion of the DS are replaced by the assumption that, for early-failed DSs, the entire DS surface is failed at time 0, and does not impede flow of water onto the underlying WP ([11], Section 4; [1], Section 6.4.1.3). Further, (i) the entire outer barrier of a WP underlying an early-failed DS is assumed to fail if seepage occurs at the location of that DS and (ii) no WP failure is assumed to occur for a WP under an early-failed DS if seepage does not occur at the location of that DS. Analyses for early failures involving both commercial spent nuclear fuel (CSNF) WPs and codisposed spent nuclear fuel (CDSP) WPs are presented.
The uncertainty and sensitivity techniques in use are described in Section 2 of Ref. [5] and involve the use of Latin hypercube sampling, partial rank correlation coefficients (PRCCs) and stepwise rank regression. The presented uncertainty and sensitivity analysis results are obtained with the first of the three replicated Latin hypercube samples (LHSs) described in Sections 11 and 12 of Ref. [2]. This is the same LHS used in the generation of the expected dose results for the early failure scenario classes [3] and also in the generation of results for the other scenario classes under consideration [4], [5], [6], [7], [8] and [9]. Descriptions of the epistemically uncertain analysis inputs under consideration and extensive references to additional sources of information on these variables are given in App. B of Ref. [2]. Further, additional information on the uncertainty and sensitivity techniques in use is available in several reviews [12], [13], [14] and [15].
The following topics are considered in this presentation: engineered barrier system (EBS) conditions (Section 2), radionuclide movement from the EBS (Section 3), radionuclide movement from the unsaturated zone (UZ) (Section 4), radionuclide movement from the saturated zone (SZ) (Section 5), dose to the RMEI (Section 6), and expected dose to the RMEI (Section 7). The presentation then ends with a summary discussion (Section 8).
The primary focus of this presentation is on uncertainty and sensitivity analysis results obtained for the early WP and early DS failure scenario classes. Summary descriptions of the models that underlie these results are given in Ref. [11] and in Section 6 of Ref. [1], and more detailed descriptions are available in the reports cited in Refs. [1] and [11] and in App. B of Ref. [2]. Further, an extensive description of the development process that led to these models is given in Refs. [16], [17], [18], [19], [20], [21], [22] and [23].
