52 © The Containment of Underground Nuclear Explosions
distance of vertical drill hole shots ('/2 depth of
burial) for tests of the same yield (compare figures
3-2 and 3-3).
Consequently, neither material
« J*
a ves he es
strength, burial depth, nor separation distance
would makeleakageto the surface morelikely for
a tunnel test on Rainier Mesa than for a vertical
drill hole tests on Yucca Flat.
we
Photo credit: Department of Energy
Fracture on Rainier Mesa.
been detonated in alluvial deposits, which are
essentially big piles of sediment with nearly no
internal strength in an unconfined state. Despite the
weakness and lack of cohesiveness of the material,
such explosions remain well contained.
Compared to vertical drill hole tests. tunnel tests
are overburied and conservatively spaced. The
tunnel system in Rainier Mesais at a depth of 1,300
feet. By the standards for vertical drill hole tests
(using the scaled depth formula‘), this is deep
enoughtotest at yields of up to 34 kilotons; and yet
all tunnel tests are less than 20 kilotons.?’ Conse-
quently, all tunnel tests in Rainier Mesa are buried
at depths comparatively greater than vertical drill
hole tests on Yucca Flat. Furthermore, the minimum
separation distance of tunnel shots (twice the com-
bined cavity radii plus 100 feet) results in a greater
separation distance than the minimum separation
Despite the relative lack of importance of strength
in preventing possible leakage to the surface, the
volume of material weakened or fractured by an
explosion is of interest because it could affect the
performance of the tunnel closures and possible
leakage of cavity gas to the tunnel complex. Dispute
overthe amountof rock fractured by an underground
nuclear explosion stems from the following two,
seemingly contradictory, but in fact consistent
observations:
1. Post-shot measurements of rock samples taken
from the tunnel complex generally show no change
in the propertiesof the rock at a distance greater than
3 cavity radii from the point of the explosion. This
observation implies that rock strength is measurably
decreased only within the small volumeof radius =
165 (yield)?,38 where the radius is measuredin feet
from the point of the explosion and the yield is
measured in kilotons (figure 3-10).
2. Seismic recordings of underground explosions
at Rainier Mesa includesignals that indicate the loss
of strength in a volume of rock whose radius 1s
slightly larger than the scaled depth of burial. This
observation implies that the rock strength is decreased throughoutthe large volume ofradius = 500
(yield) ‘5 ”, where the radius is measuredin feet from
the point of the explosion and the yield is measured
in kilotons (figure 3-11). The loss of strength in a
large volume seems to be further supported by
cracks in the groundat the top of Rainier Mesa that
were created by nuclear tests.
Thefirst observation is based on tests of samples
obtained from drilling back into the rock surrounding the tunnel! complex after a test explosion. The
core samples contain microfractures out to a distance
from the shot point equal to two cavity radii.
Although microfractures are not seen past two cavity
radii, measurements of seismic shear velocities
36Depth(ft) = 400 (yield(kt))'4
37** Announced United States Nuclear Tests, July 1945 through December 1987,"' United States Department of Energy, NVO-209(Rev.8). April, 1988.
38If the radius of a cavity produced by an explosion is equal to 55 (yield), a distance ofthree cavity radii would be equal to three umes this, or 165
(yield)'4.