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Three
Mile Island: The Inside Story
Collecting Ultrasonic Echoes at TMI-2
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Figure
7.1. Radiation levels in the TMI-2 reactor building
in 1980, when it was first entered after the
accident, and in 1983, when the core topography
survey was done.
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The core topography team arrived at TMI in August 1983. After having
choreographed their moves at the mockup in Idaho, they practiced
them again on a mockup in the TMI-2 turbine building. August 31
was the first entry into the containment building to set up their
equipment. Installation of the apparatus on the work platform took
just 45 minutes, after which all personnel left the reactor building.
First a reconnaissance survey was done to establish quickly the
shape and dimensions of the cavity and to select optimal operating
parameters for the piezoelectric transducers. That first survey
provided plenty of cause for worry about the success of the project:
the six 10-MHz transducers produced no useful data because the sound
waves they emitted proved to be very strongly absorbed by the water
in the reactor vessel, loaded as it was with dissolved and suspended
matter. And two of the other six transducers, operating at 2.25
MHz, also produced no useful data. Mysteriously, those two transducers
were found to be working the next day at the start of the second
survey, which provided the principal data for subsequent reductions
and displays.
Figure
7.2. Typical on-the-spot sonar-like
range-bearing plot of the data from the horizontal
transducer.
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Surveys were done from the bottom up. The 44-foot (13 m) probe-tipped
boom was allowed to descend slowly under its own weight to within
about six inches (15 cm) of the floor of the cavity—as determined
by the downward-looking transducer. The boom was then stepping-motor-driven
upward in 1-inch (2.5 cm) steps. This ensured a more controlled
vertical motion in case there were any binding in the control-rod
guide tube, whose inside diameter was only 1/8 inch (3 mm) greater
than the diameter of the probe. After rising an inch, the probe
was rotated through a bit more than 360° in 0.9° steps.
At each vertical step a plot was printed of the range-bearing data
acquired with the horizontal-pointing transducer, this being the
one transducer for which range-bearing data were self-interpreting.
A complete survey, with over 50 vertical steps, took somewhat less
than four hours.
Figure
7.3. A similar plot of horizontal
transducer data with the outline of the core
former (sides), to the same scale, superimposed.
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Only two surveys were conducted. On a third day the reactor building
was reentered to remove the apparatus. “Management policy
throughout the cleanup was that research work could not significantly
interfere with cleanup work” (ref. 12, ch.5, p.1). And that,
despite the fact that much of what the Department of Energy paid
for as “research” was the acquiring of data that proved
essential for the planning and execution of the cleanup work.
The sonar survey established that the cavity in the reactor’s
core was substantially larger than had previously been supposed.
This was ascertained immediately with the first preliminary survey.
It was presented in dramatic visual form to TMI
personnel in the following way: the range-bearing plots of the data
from the horizontal transducer were photocopied onto mylar sheets
(overhead projector transparencies). These were then stacked, successive
plots separated by disks of Lucite, and the whole illuminated from
below. (The disks had been prepared in advance for this purpose.)
The effect achieved with this improvised 3-D representation of the
core void was suggested in the Museum exhibit with a similar construction
making use of the original Mylar transparencies.
Figure
7.4. The August 31, 1983, improvised 3-D representation
of the core void as reproduced in the spring
2004 Museum of American History exhibit.
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Standing out very clearly in the range-bearing plots are 3-foot
(0.9 m) lengths of the axial power shaping rods (APSR) hanging down
into the cavity. These are the portions of the APSR that were above
the core at the time of the accident, and that in the tests of their
operability in June, 1982, were driven down into the void. They
are all that remained of the APSR after the accident: the 9 feet
(2.7 m) that were within the core at the time were melted away.
Among the detailed features revealed by the core topography survey—and
to whose revelation particular attention had been given in the design
of the survey instruments—was what remained adhering to the
upper grid structure, hanging down from it into the cavity.
Figure
7.5. The walls of the cavity are not smooth
and continuous as they appear in maps and model,
but are formed largely of broken, hanging fuel
rods.
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Such adhering core debris would encumber and complicate the removal
of the upper grid, the first step in getting access to the mess
in the core. With this in mind, the INEEL team had incorporated
upward-looking transducers into the sonar probe. And in working
up the data, they again gave particular attention to the presentation
of the upward-looking data in visualizable form.
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