15
to about 400 cps as the counter descended through the falli-back z
and rose again to about 3400 cps at the true bottom of the crater
high count at the surface of the fall-back zone is probably due tq@
The
the
fact that small particles, which absorb more activity per unit of
weight than large particles, fell back more slowly.
The high permeability of the coral rock ensures that the radi
active material within the fall-back zone has been continuously lgached
by sea water since 1958.
Nevertheless, substantial quantities o
radioactive material were present beneath the apparent bottom o7
e
crater before any of the soil and debris from the islands was plabed
into it.
It igs also possible that a part of Cactus Crater was formed
quit of
a man-made extension of the island on the lagoon side of the ree
(Defense Nuclear Agency 1981, p. 409); at least there is no
appreciable beachrock present on the lagoon side of the crater.
3.9
Filling of the Crater
The contaminated soil was transported by barge to Runit Islagd,
where it was mixed with cement and attapulgite to form a mixture
designed for use in the tremie method of underwater concrete plagement.
Using this method, water is added to the cement-soil mixture to form a
slurry that is pumped through a pipe to the underwater location;{ the
end of the pipe is kept below the surface of the ejected slurry
prevent segregation of the cement and soil.
The crater was filled to the low-tide water level using the
method.
The key-wall then was sunk to a depth of 1 foot where
beachrock was solid and to a depth of 8 feet where the beachrock]
fractured or absent.
The key-wall apparently was placed by d
through water that inevitably entered the forms because of the
permeability of the formations on which the key-wall was placed
Above the water level, a common soil~cement placement me
used in which a layer of contaminated sci]. was spread and bags
fo
Fremie
e
was
ition
hgigh
was
cement were placed at designated intervais and punctured.
The dement
was blended into the soil with a disc and the layer was compactdd.
Using this procedure a dome-shaped mound was form:4 over the crgter.
Radioactive debris (i.e., metallic debris, contaminated concretd@, and
other large pieces of material) too large to pass through the tfemie
pipe later was placed in an area, (called the "donut hole”) res@rved
for it in the center of the structure and was “choked” in placefwith
slurry.
Before the filling of the crater was completed, constructioh
of the
concrete cap or dome was started.
It consists of 358 panels injil
rings, and the panels vary in size from 20 by 21 feet at the
side
to 6 by 7.5 feet near the center.
The panels were made in plach
in
forms and rested on polyethylene sheet.
The design thickness
the
24 inches, with a mean of 17.3 inches
(Ristvet 1980).
The out
“Segregation, however, was observed in the core samples onjy at
the bottom of the key-wall.