System-Level Repository Sensitivity Analyses, Using TPA Version 3.2 Code (NUREG-1746)

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Publication Information

Manuscript Completed: June 2001
Date Published: August 2001

Prepared by:
R. B. Codell, M. R. Byrne, T. J. McCartin^a
S. Mohanty, J. Weldy, M. Jarzemba, G. W. Wittmeyer^b
Y. Lu^c
R. W. Rice^d

^aDivision of Waste Management
Office of Nuclear Material Safety and Safeguards
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

^bCenter for Nuclear Waste Regulatory Analyses
6220 Culebra Road
San Antonio, TX 78238-5166

^cOffice of Nuclear Waste Safety
Swedish Nuclear Power Inspectorate
(Statens Kidrnkraftinspektion)
S-106 58 Stockholm, Sweden

^dIndependent Consultant
307 Quincy
El Paso, TX 79922

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Abstract

To review and quantitatively evaluate the safety case in a potential license application by the U.S. Department of Energy (DOE) for the proposed Yucca Mountain (YM) repository, the U.S. Nuclear Regulatory Commission (NRC), with technical assistance from the Center for Nuclear Waste Regulatory Analyses (CNWRA), developed a Total-system Performance Assessment (TPA) code. The most recent versions of the TPA code used in evaluation and calculation of YM performance are 3.2 and 3.2.3. This report describes a series of computations performed using these codes for determining the confidence in the estimation of future repository performance in light of the uncertainty in conceptual models and parameters of those models. This report primarily presents: (i) the system-level and process-level results (e.g., intermediate results) to demonstrate trends and variabilities in outputs; (ii) the results of system-level sensitivity and uncertainty analyses using a variety of analysis techniques to determine the paiameters that have the most influence on repository performance; and (iii) the relative importance of the integrated subissues in reviewing the DOE total-system performance assessment. An influential parameter is one that either drives uncertainty in performance, or one to which performance is sensitive. The sensitivity and uncertainty analyses were conducted using numerous TPA code runs (several thousand realizations) for each sensitivity analysis technique. Results of system-level analyses are based on peak dose and peak expected dose to a receptor group 20 km (12.4 mi) from the repository at two time periods of interest (TPIs): 10,000 yr (the likely compliance period in the draft regulation) and either 50,000 or 100,000 yr [a longer period for investigating any significant effects that may not be evident because of the calculated long waste package (WP) life].

Using the basecase, which included the seismic disruptive event scenario, peak expected doses of 0.003 mrem/yr and 4 mrem/yr were obtained for the 10,000- and 100,000-yr TPIs, respectively. The faulting scenario changed the peak expected dose negligibly. The igneous activity scenario increased the peak expected dose to 0.6 mrem/yr. For both TPIs, it was found that the most influential parameters were: (i) the fraction of the repository wetted by infiltrating water; (ii) the fraction of water entering the WP; (iii) the well pumping rate at the 20-km receptor group location; (iv) alluvium retardation factors for radionuclides (specifically, ⁹⁹Tc and ¹²⁹I), and (v) the present-day infiltration. The most influential parameters for the 10,000-yr TPI, but not for the 50,000-yr TPI, were the initially defective fraction of WPs and the factor that focuses flow onto the WP. The most influential parameters for the 50,000-yr TPI, but not for the 10,000-yr TPI, were the alluvium retardation factors for radionuclides (specifically, ²³⁴U and ²³⁷Np). The influential parameters were then compared to the current integrated subissues, which are used by the NRC to focus work on items important to repository performance. Nine out of 14 of the integrated subissues reflected at least one influential parameter.

The analyses and results are limited by the use of simplifying assumptions, models, and sparse data in certain areas. As a consequence, these results are preliminary. However, the estimates resulting from this study allowed the staff to focus attention on what are likely to be the most important phenomena relative to repository performance and point out deficiencies in the current state of knowledge. The manner in which these analyses were conducted or the assumptions and approaches used should not be construed to express the views, preferences, or positions ofthe NRC staffregarding the nature of site-specific regulations for YM.