Total System Performance Assessment-License Application Design Selection (LADS) Phase 1 Analysis of Surface Modification Consisting of Addition of Alluvium (Feature 23a)

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The objective of this report is to document the analysis that was conducted to evaluate the effect of a potential change to the TSPA-VA base case design that could improve long-term repository performance. The design feature evaluated in this report is a modification of the topographic surface of Yucca Mountain. The modification consists of covering the land surface immediately above the repository foot-print with a thick layer of unconsolidated material utilizing rip-rap and plants to mitigate erosion. This surface modification is designated as Feature 23a or simply abbreviated as F23a. The fundamental aim of F23a is to reduce the net ... continued below

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Erb, N. June 11, 1999.

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Description

The objective of this report is to document the analysis that was conducted to evaluate the effect of a potential change to the TSPA-VA base case design that could improve long-term repository performance. The design feature evaluated in this report is a modification of the topographic surface of Yucca Mountain. The modification consists of covering the land surface immediately above the repository foot-print with a thick layer of unconsolidated material utilizing rip-rap and plants to mitigate erosion. This surface modification is designated as Feature 23a or simply abbreviated as F23a. The fundamental aim of F23a is to reduce the net infiltration into the unsaturated zone by enhancing the potential for evapotranspiratiration at the surface; such a change would, in turn, reduce the seepage flux and the rate of radionuclide releases from the repository. Field and modeling studies of water movement in the unsaturated zone have indicated that shallow infiltration at the surface is almost negligible in locations where the bedrock is covered by a sufficiently thick soil layer. In addition to providing storage for meteoric water, a thick soil layer would slow the downward movement of soil moisture to such an extent that evaporation and transpiration could easily transfer most of the soil-water back to the atmosphere. Generic requirements for the effectiveness of this design feature are two-fold. First, the soil layer above the repository foot-print must be thick enough to provide sufficient storage of meteoric water (from episodic precipitation events) and accommodate plant roots. Second, the added soil layer must be engineered so as to mitigate thinning by erosional processes and have sufficient thickness to accommodate the roots of common desert plants. Under these two conditions, it is reasonable to expect that modification would be effective for a significant time period and the net infiltration and deep percolation flux would be reduced by orders of magnitude lower than the present levels. Conceptually, the topographic surface above the repository foot-print would be re-contoured to make it more suitable for placement of unconsolidated materials (e.g., alluvium). Figure 1 shows the region of the surface modification in relation to the location of the repository foot-print. The surface contours in this region after modification are shown in the plot presented in Figure 2. Basically, the surface modification would be accomplished by applying cuts to the ridges slopes on the east flank of Yucca Mountain to produce a relatively uniform slope of about 10%. The alluvium would be covered with rock fragments (to imitate the desert pavement) to reduce erosion. This report documents the modeling assumptions and performance analysis conducted to estimate the long-term performance for Feature 23a. The performance measure for this evaluation is dose-rate. Results are presented that compare the dose-rate time histories for the new design feature to those of the TSPA-VA base case calculation (CRWMS M&O 1998a).

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  • Report No.: B00000000-01717-0210-00053, Rev. 00
  • Grant Number: NA
  • DOI: 10.2172/894033 | External Link
  • Office of Scientific & Technical Information Report Number: 894033
  • Archival Resource Key: ark:/67531/metadc890444

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  • June 11, 1999

Added to The UNT Digital Library

  • Sept. 22, 2016, 2:13 a.m.

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  • Dec. 8, 2016, 11:59 p.m.

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Erb, N. Total System Performance Assessment-License Application Design Selection (LADS) Phase 1 Analysis of Surface Modification Consisting of Addition of Alluvium (Feature 23a), report, June 11, 1999; Las Vegas, Nevada. (digital.library.unt.edu/ark:/67531/metadc890444/: accessed September 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.