Smithsonian - National Museum of American History, Behring Center
Three Mile Island
Unit 2 nuclear power plant
First looks inside the reactor

Three Mile Island: The Inside Story

Topographic Maps and Model of the Cavity

 
Topographic map of the bottom of the cavity in the core of the TMI-2 reactor.

Click to enlarge imageFigure 8.1. Topographic map of the bottom of the cavity in the core of the TMI-2 reactor.

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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.

Topographic model of the cavity in the core of the TMI-2 reactor.

Click to enlarge imageFigure 8.2. Topographic model of the cavity in the core of the TMI-2 reactor.

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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.

The topographic model is constructed in two halves, divided by the north-south plane through the axis of the core. This is the west half.

Click to enlarge imageFigure 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 half.

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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 Ivon E. Fergus Jr., 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.


 


Close-up of the west half of the model.

Click to enlarge imageFigure 8.4. Close-up of the west half of the model.

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Close-up of the east half of the model.

Click to enlarge imageFigure 8.5. Close-up of the east half of the model.

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The topographic maps and model were the principal but not the sole aids for visualizing the TMI-2 core topography that were constructed from the sonar data.

Click to enlarge imageFigure 8.6. The topographic maps and model were the principal but not the sole aids for visualizing the TMI-2 core topography that were constructed from the sonar data.

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