Mile Island: The Inside Story
The first looks inside the reactor
5.1. First entry into the reactor containment
building, summer 1980.
In the months following the accident, “the
actual condition of the core was the subject of intense, often heated,
speculation.” And yet, as this General Public Utilities engineer
in charge of much of the cleanup acknowledged as the job was nearing
completion, “the most severe prediction was short of the mark”
(ref.23). The prevailing view among the reactor engineers was that
“most of the core damage was to the cladding”—i.e.,
to the zirconium alloy tubes holding the uranium oxide fuel pellets—and
“damage to the fuel pellets . . . was minimal at Three Mile
Island” (ref.13, p. 33). Although the independent inquiry
instituted by the Nuclear Regulatory Commission surmised that some
fuel had melted and run down the channels between the fuel rods,
it concluded, “Despite this amount of damage, a core meltdown,
as normally considered, did not occur” (ref. 29, v.2, pp.
To some few experts, one piece of evidence strongly suggested that
a considerable part of the fuel in fact lay in the bottom of the
reactor vessel, where none should be. A few hours into the accident
the neutron detectors toward the bottom of the inner wall of the
cylindrical concrete shield surrounding the reactor vessel (solid
black in Figure 2.6) began reporting anomalously large numbers of
neutrons. As only the uranium fuel itself, not any of its highly
radioactive fission products, could be a significant source of neutrons,
the onset and continuance of such high readings from detectors in
the vicinity of the bottom of the reactor vessel pointed to a “relocation”
of a considerable part of the fuel. But as no one could imagine
that a considerable part of the fuel had melted, there was no plausible
explanation for how such a “relocation” could have taken
The “relocation” hypothesis was so much at variance
with the tendency of nuclear power plant engineers and operators
to take at every stage an overly optimistic view of the seriousness
of the damage to the core, that relocation was not accepted until
finally it could be seen by video cameras some six years after it
had occurred. And even when seen and accepted as fact, the conclusion
that it had poured down molten was not drawn.
Nothing but speculation was possible without inspection of the
interior of the reactor pressure vessel. The first step in this
direction, entrance into the containment building, was not possible
until—more than a year after the accident and amid great but
unwarranted public outcries—the radioactive krypton gas, which
had been released from the ruptured fuel rods and had accumulated
in the sealed containment building, was exhausted into the atmosphere.
The first results on the condition of the reactor’s core
came from analyzing debris accumulated on filters in the reactor’s
cooling-water system. The fact that “only” 6 percent
of this material was uranium, while most was zirconium (the main
component of the tubes surrounding the uranium fuel) along with
the relatively low melting point elements from which the control
rods were alloyed, was construed optimistically as indicating some
melting of control rods but not of fuel rods (ref. 19).
The first active intervention to test the conditions within the
reactor’s core—the operation of the so-called axial
power shaping rod (APSR) assemblies—took place in June 1982,
two years after first entrance into the containment building. The
APSRs are, in effect, a special sort of control rod. Although they
play no role in starting up and shutting down the reactor as control
rods do, the APSR contain, like control rods, not fuel, but neutron-absorbing
elements. Those elements are so chosen and so distributed along
the 12-foot (3.5 m) length of an APSR that power generation along
the axis of the reactor’s core—i.e., from top to bottom—can
be made more uniform by appropriate degrees of insertion of the
APSR assemblies. (Because the concentration of neutrons is greater
at the center of the core, fuel in the center “burns”
more rapidly and generates more heat. Hence the need for a corrective.)
5.2. An axial power shaping
rod (APSR) assembly comprises 16 rods, held
by a “spider” attached to a leadscrew.
At the time of the accident, TMI-2 was operating with all eight
of its APSR assemblies so positioned that 75 percent of the total
length of the rods was inserted into the core, while 25 percent
remained in the space in the reactor vessel above the core, the
so-called upper plenum. No steps were taken to raise or lower the
APSR at any time during the accident or in the months afterward.
In initial planning, looking inside the reactor vessel seemed difficult
to accomplish. But it was thought that the operation of the APSR,
if carefully prepared and extensively instrumented with accelerometers
and vibration analyzers, could provide reliable information about
the condition of the core. Such instrumentation was tested in an
engineering laboratory and then in TMI-1. In June 1982, the TMI-2
APSRs were activated and driven down into the core as far as they
would go. The result of doing so is represented by the thin black
rods hanging down into the cavity in the Lucite topographic model
pictured on the home page and in Section 8.
5.3. Axial power shaping rod dynamic test. There
were eight APSR assemblies in TMI-2, roughly
equally spaced around a circle about two-thirds
of the distance out from the axis of the core.
Although four of the eight assemblies were practically immovable,
four did move freely inward to the full extension of their drive
mechanisms. Again this was, at first, interpreted optimistically
“as proof that the core was not badly damaged” (ref.
12, ch.8, p.12; similarly ref. 20 as optimistic). When, however,
the written reports on this test were prepared five to ten months
afterward, the interpretation was reversed: the ability to move
four of the APSR drives freely “probably results from the
nearly total breakup of the fuel assemblies and the APSR assemblies
at these locations, such that only the APSR leadscrews, the spider
and perhaps the uppermost portion of the APSR clusters were actually
moving” (ref. 10, p. 38).
What brought about this 180-degree reversal of the conclusions
initially drawn from the APSR test? It was a look inside the reactor
vessel. This took place in July, 1982, just a month after the APSR
tests—much sooner than had initially been thought feasible.
The concept of “lowering a video camera through a Control
Rod Drive Nozzle and manipulating it around the plenum and down
to the top of the reactor core” had emerged early on as a
way to get beyond mere “analysis and speculation as to the
condition of the reactor core.” But these quotations from
a 1981 planning document (ref. 11, abstract) do not envision getting
a camera down into the core.
Contemplated at that early date was the removal of an entire 2¾
inch (6.8 cm) diameter control rod drive mechanism, an operation
that would require the removal of the heavy “missile shields”
above the reactor, and that, in turn, would require operation of
the heavy-duty polar crane at the top of the reactor building. But
after a year of being exposed to the high levels of radiation that
existed in the building before the purging of the accumulated krypton
gas, and to the 100 percent humidity at higher than tropical ambient
temperatures—not to mention the near explosion of burning
hydrogen that had occurred several hours into the accident—the
overhead crane needed to be tested carefully before doing any heavy
lifting. Thus video inspection seemed some while away.
The Technical Assessment Advisory Group, experts who had been assembled
by General Public Utilities to advise it on problems of cleaning
up after the accident, pointed out that lifting of the overhead
missile shields would not be necessary if, rather than removing
an entire control rod drive mechanism, only the control rod leadscrew
were removed—removed by cutting it into short lengths as it
was screwed out of the reactor vessel. This would create a 1½
inch (3.8 cm) diameter access hole in the top of the reactor vessel.
5.4. The lowering of a video camera down a control
rod guide tube near the center of the reactor.
There were limitations to the “Quick Look” concept.
While removing an entire control rod drive mechanism would have
allowed the camera an open view of the plenum, removing only the
leadscrew meant that the (much smaller) camera would be inside the
leadscrew’s guide tube down to almost the top of the core.
And this was all the greater a limitation as hardly anyone expected
that the camera would be able to get any farther down than the top
of the core. Still the undertaking seemed worthwhile even if all
that one could see was the tops of fuel assemblies in the immediate
vicinity of the control rod guide tube. A test of the concept in
TMI-1 showed that this much was feasible (ref. 9).
The first “Quick Look” video examination, July 21,
1982, is recalled as a dramatic event by those present, and by many
who only heard others tell of it. “There were many exclamations
of surprise, disbelief, and confirmation,” as the video camera
was lowered below the level of the top of the core without anything
at all coming into view (ref. 12, ch. 5, p.13). The camera was able
to see only 3 inches (8 cm) ahead, due to its small size, incorporating
lens and light in one unit, and the turbidity of the water in the
reactor vessel. Not until five more feet (1.5 m) of cable had been
paid out did the operator call out, “Got something!”—rubble
lying at the bottom of the cavity in the core (ref. 18, p.40). A
steel probe inserted down through the same access port showed that
the layer of loose rubble was about a foot (0.3 m) thick (ref. 12,
ch. 5, p.13).
With two further entries in the following weeks, a total of six
hours of videotape were obtained from two control rod positions,
one at the center and one somewhat off center. These videos then
were reviewed by a panel of experts. And here once again the tendency
was to draw optimistic conclusions where the video did not clearly
show the contrary. What the camera could not see was supposed to
be still OK. The conclusion drawn was that the damage was confined
to “the top center” of the core (ref. 1, abstract).
Likewise, the fact that the “Quick Look” operatives
had failed to get a camera into the core through a third channel
created by removing a control rod lead screw that was still farther
from the center of the core “led to the belief that the only
damage was in the center of the core and that all peripherals were
intact” (ref. 12, ch. 5, p. 13).
This hopeful conclusion was not, however, the only one drawn from
“Quick Look.” The authors of ref. 10 inferred, as quoted
above, the nearly total breakup of the fuel assemblies in the vicinity
of the APSR even though the camera could not see as far out from
the center of the core as the location of the APSR.