Post by 1dave on Nov 29, 2019 23:32:25 GMT -5
The Yucca Mountain (Nevada) Nuclear Waste Repository was designated by the Nuclear Waste Policy Act amendments of 1987.
By 2008, Yucca Mountain was one of the most studied pieces of geology in the world.
www.nrc.gov/docs/ML0907/ML090720221.pdf
pubs.usgs.gov/of/2013/1245/pdf/of2013-1245.pdf
Lithophysal Units
The Topopah Spring Tuff Formation underlies Yucca Mountain and is the host horizon for the underground repository. It consists of ~300 m of silica-rich, densely welded, pyroclastic flow units laid down 12.8 million years ago and contains the distinctive upper (Tptpul) and lower (Tptpll) lithophysal units.
Lithophysae (Thundereggs) are cavities that form in pyroclastic flows and volcanic tuffs as a result of the volcanic gases exsolved during the cooling process, forming bubbles within the cooling rock mass. These cavities constitute approximately 10–30 percent of the rock mass volume.
Tptpul l ithophysae tend to be roughly spherical, uniform in size and distribution, and small in dimension (diameters of 1 to 10 cm); the matrix material is largely unfractured.
Tptpll lithophysae are more irregular in shape, size, and distribution (fig. 2). They range in size from about 1 cm to nearly 2 m in dimension and have spacing that ranges from about 10 to 50 cm, but may be more closely spaced in local regions.
The shapes range from elliptical or spherical to irregular, cuspate and merged cross sections, or elongate along fractures. The matrix between lithophysae often has a fabric of short length (<1m) and discontinuous cooling fractures that have a primary vertical orientation. These fractures typically are not interconnected and do not intersect lithophysal cavities.Approximately 85 percent of the emplacement drifts of the repository are to be excavated within the lower lithophysal unit (Tptpll), 250 to 300 m below the ground surface. The overall porosity of Tptpll due to the lithophysae is ~20 percent, weakening this unit considerably with respect to the passage of seismic waves. Very few of these lithophysae are cut by cooling fractures or joints, and none show appreciable effects of damage, offsets, or collapse that can be attributable to the passage ofseismic waves.
Concerns about possible problems from all the lithophysae in the area created some interesting studies. Here is some data from one of them in 2004.
www.researchgate.net/publication/223607896_Porosity_dependence_of_the_elastic_modulus_of_lithophysae-rich_tuff_Numerical_and_experimental_investigations
1.0 INTRODUCTION
The current Site Recommendation study for the proposed high level nuclear waste
repository at Yucca Mountain locates the repository emplacement drifts approximately
81% within the lower lithophysal unit of the Topopah Springs Formation (Tptpll), 4%
within the upper lithophysal unit of the Topopah Springs Formation (Tptpul), and
roughly 15% within the middle, non-lithophysal unit (Tptpmn) of the same formation. A
major geomechanical issue facing the Yucca Mountain Project is to understand the
thermomechanical behavior of lithophysal tuff, which comprises roughly 85% of the
repository host rock.
The mechanical response is complex due to the presence of voids of varying
shape, size, and distribution within a hard, brittle rock matrix. During past years,
significant testing and numerical modeling investigation has been performed to assist in
understanding the mechanical behavior of lithophysal rock. A series of large-scale
laboratory and in-situ scale field tests have been performed to provide data on both
lithophysal porosity and size effects on rock mass strength and deformability. Even with
this testing program, the database of mechanical properties for this rather complex
material is small. Since it is very difficult to core the lithophysal rock and since it is
impossible to perform a large number of in-situ compression experiments, an alternative
approach to understanding variability and uncertainty of rock mass properties needs to be
developed.
Page 6:
C. General Specimen Geometry
Specimen Shape and Size: The basic geometry of all specimens will remain constant.
All specimens will be cubes with nominal dimensions of 6” x 6” x 6” (Length x
Width x Height).
Presence of Holes: Specimens will contain holes of various shapes and sizes; however,
all holes will extend completely through the specimen. This uniformity will allow
3-dimensional specimens to be evaluated as 2-dimensional specimens.
Lithophysal Porosity: Lithophysal porosity, is the ratio of the volume of the lithophysae
to the volume of the entire specimen. For this Task, a specimen’s porosity will
refer to the lithophysal porosity of a specimen. As the lithophysae are uniform
holes extended through the length of the specimen, the lithophysal porosity may
be expressed by the following equation:
Lithophysal
total
holes A
Porosity(n) = A
D. Internal Specimen Geometry
The internal geometry of specimens will vary with respect to the holes that are
found within the specimen.
Page 7:
Location of Holes: The location of individual holes will be provided using x- and y coordinates.
Both the x- and y-axis pass directly through the center of the specimen; thus, the center of the specimen will be the origin.
Bridge Distance: Bridge distance is the distance (B.D.) between two adjacent holes.
The bridge distance of two circular holes is shown in Figure 3.2.1. This length will vary depending on hole location, shape, and size. In general, specimens will be generated by using three different bridge distances: B.D. ≥ 0.25"; B.D. ≥ 0.10"; and B.D. ≥ 0.50" .
E. Holes Properties
Hole Shape: This property refers to the geometry and relative rotation of the hole
within the actual specimen. Three different shapes of holes will be used.
(1) Circle: These holes will be true circles - plane curves everywhere
equidistant from given fixed points, centers. The rotation of this
shape is not relevant as circles are symmetrical about any axis.
(2) Square: Square holes are true squares – plane areas with sides of equal
distance that intersect at 90-degree angles. The sides of this shape
are parallel to the adjacent specimen walls.
(3) Diamond: Diamond holes are true squares – plane areas with sides of equal
distance that intersect at 90-degree angles. The sides of this shape
will form 45-degree angles with the specimen walls.
Hole Size: This property reflects the relative characteristic dimensions of each shape.
The characteristic dimensions of the holes are diameter for circle (d1) and length
for both square (d2) and diamond (d3). Sizes are found by transcribing square and
diamond holes within respectively sized circular holes. This process is displayed
in Figure 3.2.2 and was used to compute the sizes found in Table 3.2.1
The current Site Recommendation study for the proposed high level nuclear waste
repository at Yucca Mountain locates the repository emplacement drifts approximately
81% within the lower lithophysal unit of the Topopah Springs Formation (Tptpll), 4%
within the upper lithophysal unit of the Topopah Springs Formation (Tptpul), and
roughly 15% within the middle, non-lithophysal unit (Tptpmn) of the same formation. A
major geomechanical issue facing the Yucca Mountain Project is to understand the
thermomechanical behavior of lithophysal tuff, which comprises roughly 85% of the
repository host rock.
The mechanical response is complex due to the presence of voids of varying
shape, size, and distribution within a hard, brittle rock matrix. During past years,
significant testing and numerical modeling investigation has been performed to assist in
understanding the mechanical behavior of lithophysal rock. A series of large-scale
laboratory and in-situ scale field tests have been performed to provide data on both
lithophysal porosity and size effects on rock mass strength and deformability. Even with
this testing program, the database of mechanical properties for this rather complex
material is small. Since it is very difficult to core the lithophysal rock and since it is
impossible to perform a large number of in-situ compression experiments, an alternative
approach to understanding variability and uncertainty of rock mass properties needs to be
developed.
Page 6:
C. General Specimen Geometry
Specimen Shape and Size: The basic geometry of all specimens will remain constant.
All specimens will be cubes with nominal dimensions of 6” x 6” x 6” (Length x
Width x Height).
Presence of Holes: Specimens will contain holes of various shapes and sizes; however,
all holes will extend completely through the specimen. This uniformity will allow
3-dimensional specimens to be evaluated as 2-dimensional specimens.
Lithophysal Porosity: Lithophysal porosity, is the ratio of the volume of the lithophysae
to the volume of the entire specimen. For this Task, a specimen’s porosity will
refer to the lithophysal porosity of a specimen. As the lithophysae are uniform
holes extended through the length of the specimen, the lithophysal porosity may
be expressed by the following equation:
Lithophysal
total
holes A
Porosity(n) = A
D. Internal Specimen Geometry
The internal geometry of specimens will vary with respect to the holes that are
found within the specimen.
Page 7:
Location of Holes: The location of individual holes will be provided using x- and y coordinates.
Both the x- and y-axis pass directly through the center of the specimen; thus, the center of the specimen will be the origin.
Bridge Distance: Bridge distance is the distance (B.D.) between two adjacent holes.
The bridge distance of two circular holes is shown in Figure 3.2.1. This length will vary depending on hole location, shape, and size. In general, specimens will be generated by using three different bridge distances: B.D. ≥ 0.25"; B.D. ≥ 0.10"; and B.D. ≥ 0.50" .
E. Holes Properties
Hole Shape: This property refers to the geometry and relative rotation of the hole
within the actual specimen. Three different shapes of holes will be used.
(1) Circle: These holes will be true circles - plane curves everywhere
equidistant from given fixed points, centers. The rotation of this
shape is not relevant as circles are symmetrical about any axis.
(2) Square: Square holes are true squares – plane areas with sides of equal
distance that intersect at 90-degree angles. The sides of this shape
are parallel to the adjacent specimen walls.
(3) Diamond: Diamond holes are true squares – plane areas with sides of equal
distance that intersect at 90-degree angles. The sides of this shape
will form 45-degree angles with the specimen walls.
Hole Size: This property reflects the relative characteristic dimensions of each shape.
The characteristic dimensions of the holes are diameter for circle (d1) and length
for both square (d2) and diamond (d3). Sizes are found by transcribing square and
diamond holes within respectively sized circular holes. This process is displayed
in Figure 3.2.2 and was used to compute the sizes found in Table 3.2.1
I found this one most interesting!
"Box openings" (square) are common, "Bi-conoidal" (Diamond) are less common, Mexican Coconuts are usually spheres (circles) because of the high calcium concentration. Some correlation to their formation I'd love to know.
But it was just a preliminary guess and the study was apparently never carried through.
Oh, They worried that the lithopysae weakened the rock, but decided as the 3rd most earthquake active state, and 13,000 years hadn't harmed them, they must be OK.
