Mile Island: The Inside Story
Topographic Maps and Model of the Cavity
8.1. Topographic map of the bottom of the cavity
in the core of the TMI-2 reactor.
Back at INEEL, the numerical data collected with the core topography
system were reduced in a three-step process. On a mainframe computer
(CDC-176 “Cyber”), the positions in space of all the
first-surface echoes were calculated. A “first-surface echo”
is that from the first surface encountered by the spatially and
temporally narrowly delimited pulse of ultrasound emitted by a transducer.
It is typically followed, and may be partially overlaid, by “ghosts”
arising from secondary reflections and from fringes of the beam
reflecting off nearer or more distant objects. Discrimination between
such ghost reflections and the first-surface echoes was aided by
recording reflections received above set intensity thresholds.
The results of those calculations were presented as four sets of
tomographs, as we would call them today: three sets were taken perpendicular
to each of the three coordinate directions, while the fourth set
comprised stepwise rotations of a vertical plane passing through
the axis of the core. Each of the tomographs in the first three
sets comprised all “hits” lying within a 2-inch (51
mm) thick slice through the core, while those in the fourth set
comprised hits lying within 2° wedges.
In the second step, these four sets of tomographic presentations
of the data were compared and correlated manually—largely
by Larry Beller and Joseph Holm (Holm, phone conversation, 2004
May 15)—in order to eliminate artifacts due to ghost reflections
and to other effects. Shadowing (e.g., by the APSRs hanging in the
cavity) necessitated interpolation to fill blanks in the contour,
and such features are distinguished in the model by a yellow tinge.
The third step was to input these massaged tomographs to a computer-aided
drafting (CAD) system by tablet entry, tracing over the contours
with a track ball. The CAD system was then used to print the final
set of horizontal contours, and to plot contour (topographic) maps
of the bottom, sides, and top of the cavity, and of the top looking
up from the bottom. These individual contours and hand-labeled topographic
maps at 3/16 scale, on E-size drafting paper, 44 inches (1.1 m)
wide, were the primary and most authoritative products of the core
topography project. On them the radial positions of the sides of
the cavity were located within an accuracy of 15 mm and the lateral
positions of wall features within an accuracy of 40 mm.
8.2. Topographic model of the
cavity in the core of the TMI-2 reactor.
A secondary, but yet important, product of the project was a three-dimensional
Lucite topographical model of the cavity in the core. This model
was fabricated by Scale Models, a small firm in the small town of
Yoncalla, Oregon, in four or five essentially identical copies.
It is at 3/32 scale, half the scale of the primary contours and
the topographic maps.
The model is formed of a stack of 3/16-inch (5 mm) thick Lucite
sheets, each representing a 2-inch (51 mm) contour interval in the
core. The stack of Lucite sheets, with each layer numbered, is drilled
in two places and held together by two vertical rods capped by acorn
nuts. M. R. Martin recalls (conversation, 2004 April 20) that the
sheets were cut by a numerically controlled milling machine, for
which the input was not numerical data from the CAD system, but,
again, tablet entry through tracing of the E-size contour drawings.
8.3. The topographic model is constructed in
two halves, divided by the north-south plane
through the axis of the core. This is the west
Two and a half copies of the model are known still to exist: one
at INEEL; one at this Museum, a gift of INEEL and the Department
of Energy. This Museum also has one of the two halves of the model
held for some years by the Department of Energy at its headquarters
in Washington, then discarded and retrieved from the dumpster by
DoE employee Ivan Fergus, who donated it. Another copy is known
to have been in the possession of General Public Utilities, the
company operating the TMI power plants. It has disappeared along
with most of the vast amount of documentation of the accident, the
crisis, and the cleanup that the now-defunct firm had accumulated
at the TMI site.