Three-dimensional calculations that explicitly represent a realistic mixture
of waste packages (WPs) are used to analyze decay-heat-driven
thermal-hydrological behavior around emplacement drifts in a potential
high-level waste facility at Yucca Mountain. Calculations, using the NUFT
code, compare two fundamentally different ways that WPs can be arranged in
the repository, with a focus on temperature, relative humidity, and
liquid-phase flux on WPs. These quantities strongly affect WP integrity and
the mobilization and release of radionuclides from WPs. Point-load spacing,
which places the WPs roughly equidistant from each other, thermally isolates
WPs from each other, causing large variability in temperature, relative
humidity, and liquid-phase flux along the drifts. Line-load spacing, which
places WPs nearly end to end in widely spaced drifts, results in more
locally intensive and uniform heating along the drifts, causing hotter,
drier, and more uniform conditions. A larger and more persistent reduction
in relative humidity on WPs occurs if the drifts are backfilled with a
low-thermal-conductivity granular material with hydrologie properties that
minimize moisture wicking.