Temporal Damping Effect of the Yucca Mountain Fractured Unsaturated Rock - on Transient Infiltration Pulses

Sep 10, 2026

Uploaded by Patrick

Performance assessment of the Yucca Mountain unsaturated zone (UZ) for an underground repository of high-level radioactive waste relies on the assumption that water percolation can be approximated as steady state. Temporal damping effects from geological units within the unsaturated tuff formation support this assumption, especially the highly porous nonwelded tuff of the Painbrush Group (PTn). Using 3-D and 1-D models, the study simulates transient pulse infiltrations and examines percolation-flux changes and tracer transport under episodic boundary conditions.

LanguageEnglish

CategoryResearch & Report

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Pages36

Reading Time1 h 31 min

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Abstract

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1. Introduction

Temporal Damping Effect of the Yucca Mountain Fractured Unsaturated Rock on Transient Infiltration Pulses Keni Zhang, Yu-Shu Wu, and Lehua Pan Earth Sciences Division, Lawrence Berkeley National Laboratory Abstract Performance assessment of the Yucca Mountain unsaturated zone (UZ) as the site for an underground repository of high-level radioactive waste relies on the crucial assumption that water percolation processes in the unsaturated zone can be approximated as a steadystate condition. Justification of such an assumption is based on temporal damping effects of several geological units within the unsaturated tuff formation. In particular, thenonwelded tuff of the Painbrush Group (PTn unit) at Yucca Mountain, because of its highly porous physical properties, has been conceptualized to have a significant capacity for temporally damping transient percolation fluxes. The objective of this study is to investigate these damping effects, using a three-dimensional (3-D) mountain-scale model as well as several one-dimensional (1-D) models. The 3-D model incorporates a wide variety of the updated field data for the highly heterogeneous unsaturated formation at Yucca Mountain. The model is first run to steady state and calibrated using fieldmeasured data and then transient pulse infiltrations are applied to the model top boundary. Subsequent changes in percolation fluxes at the bottom of and within the PTn unit are examined under episodic infiltration boundary conditions. The 1-D model is used to examine the long-term response of the flow system to higher infiltration pulses, while the damping effect is also investigated through modeling tracer transport in the UZ under episodic infiltration condition. Simulation results show the existence of damping effects within the PTn unit and also indicate that the assumption of steady-state flow conditions below the PTn unit is reasonable. However, the study also finds that some fast flow paths along faults exist, causing vertical-flux quick responses at the PTn bottom to the episodic infiltration at the top boundary. Keywords: Unsaturated Zone; Damping Effect; Yucca Mountain; Episodic Infiltration; Model. 1. Introduction Technical considerations for selection of the Yucca Mountain site as the U. S. national waste disposal site include arid climate, thick unsaturated zone, and remote location. In addition, the strategy of building the permanent subsurface repository is based on the capability of the thick UZ as a natural barrier, slowing rapid water percolation, limiting the availability of water for contacting waste and transporting radionuclides from the unsaturated zone to the saturated zone. This natural barrier idea is also supported by the presence at Yucca Mountain of several geological layers with large pore spaces, strong capillary barriers and damping effects. One of these geological layers identified during site characteriation is the layered, nonwelded PTn unit tuffs that exist between the ground surface and the repository horizon. Infiltrating water descending from the land surface may be effectively damped spatially and temporally by these layers, and thus percolation could be approximated as a steady-state condition once passing them (Montazer and Wilson, 1984; Wu et al., 2000; 2002a; 2002b) . The performance of the proposed repository for long-term storage of high-level radioactive wastes is critically dependent on the rate of UZ percolation. This is because percolation flux through the UZ is one of the most important factors in underground repository performance. The quantity as well as spatial and temporal variations in percolation flux directly affect: (a) the amount of water flowing into waste emplacement drifts; (b) moisture conditions and the corrosion environment of canisters within the drifts;(c) waste mobilization from the potential repository; (d) thermo-hydrologic behavior of the potential repository; and (e) radionuclide migration from the UZ to th

FAQs

Why is steady-state percolation assumed for the Yucca Mountain unsaturated zone?

Long-term performance assessment depends on approximating UZ water percolation as steady state. This assumption is supported by temporal damping effects of geological units within the unsaturated tuff formation.

What role does the PTn unit play in damping transient infiltration?

The nonwelded tuff of the Painbrush Group (PTn) has highly porous physical properties and is conceptualized as having significant capacity to temporally damp transient percolation fluxes.

How do the simulations test the damping behavior under episodic infiltration?

A 3-D mountain-scale model is run to steady state, calibrated with field-measured data, and then subjected to transient pulse infiltrations at the top boundary. Percolation flux changes within and at the PTn unit are examined under episodic infiltration boundary conditions, and 1-D models evaluate long-term responses and tracer transport.

Sep 10, 2026

Uploaded by Patrick