Mile Island: The Inside Story
Digging Deeper into the Wreckage
10.1. This drawing of the interior of the TMI-2
reactor vessel represents the state of knowledge
attained by 1989.
By April 1984, five years after the accident, the fact of the total
destruction of the upper 30-40 percent of the core of TMI-2 was
accurately known and fully accepted as a result of the sonar and
video inspections. Yet almost nothing was known about the condition
of the lower 60 percent. This appears graphically in Figure 9.1,
published in April 1984, which gives a drastic depiction of the
upper part of the core, while representing the lower part of the
core as entirely undamaged. In fact, the actual condition of that
lower 60 percent, while entirely different, was in important respects
far worse than the mere void that obtained above it.
It would require another three years for a fairly complete picture
to emerge of the condition of the lower core—and with it a
good understanding of the course of the accident. The conclusion
that a substantial part of the material then resting in the lower
half of the core was once molten was resisted until the evidence
was beyond argument. This was also the case with the further and
still more alarming conclusion that in the course of the accident
some twenty tons of that molten material had spilled into the bottom
of the reactor vessel.
Rather, the long preferred supposition was that the production
of the void in the upper part of the core involved no melting of
fuel rods—or, anyway, no melting of the uranium in them—or,
at least, very little melting of uranium. The views varied, but
all excluded substantial melting. Instead, the preferred explanation
of the void was that the portion of the fuel rods standing above
the water level in the reactor vessel, being extremely hot, shattered
when flooded with cold water at 7:20 a.m. And this view, which was
partially, but only partially true, made it easy to continue to
picture the fuel assemblies as essentially intact underneath the
rubble of their shattered upper portions.
In September and October 1983, shortly after the sonar survey, and
again in March 1984, long-handled tools—also designed at INEEL—were
inserted through the same control rod channel and retrieved samples
of the material lying on the floor of the cavity, and lying as far
beneath the cavity as these probes could be thrust into the bed
of rubble (ref. 16, p. 852).
10.2. Retrieving samples of the debris on and
below the floor of the cavity, autumn 1983.
Not immediately, but eventually the study of those samples led
INEEL metallurgists early in 1985 to the conclusion that a substantial
amount of fuel had melted. Yet, to Dennis E. Owen, who led this
effort, it seemed in retrospect that the acceptance of substantial
melting came not from the chemical and metallographic analysis of
the material retrieved, but from the in-vessel video images showing
rounded edges on the wreckage and other obvious visual signs of
melting having occurred (telephone conversation, April 29, 2004).
In this persuasiveness of the pictures there is a peculiar irony,
for those obvious signs were there to be seen from the earliest
video inspections. Yet without a willingness to believe the conclusion,
even the pictures were not persuasive.
After the overhead crane was declared operable, the reactor vessel’s
head was unbolted and lifted away in July 1984. Then a much wider
range of investigations became possible, as we have seen in Section
9. In December, when the upper grid structure was jacked up, a video
camera was snaked down between the core former (the side wall of
the core) and the inner wall of the reactor vessel. The camera could
see large quantities of debris lying at the bottom of the vessel.
By the spring of 1985, INEEL researchers were describing that debris
as “slag-like.” And in July 1985, another of their cleverly
designed long-handled tools grabbed a bit of material lying in the
bottom of the vessel (ref. 16, p. 853). But the conclusion that
the debris had gotten there in molten form—by pouring—was
not yet drawn.
In July 1986, core-boring equipment adapted from the petroleum
industry brought up 2½-inch (6.4 cm) diameter cores from
the reactor’s core (ref. 16, p. 854; ref. 12, ch. 5, p. 18).
The custom made bits—tungsten carbide faced with artificial
diamond—were good for only one core, so hard and abrasive
was the solidified slurry of grains of uranium dioxide ceramic imbedded
in an alloy of steel, zirconium, uranium, and miscellaneous other
metals. (M. R. Martin, conversation April 20, 2004.) Three months
later, after unsuccessful efforts to dig out this consolidated material,
the boring machinery was set up again and provided with solid-faced
bits for a “Swiss cheesing operation” that reduced the
mass to removable rubble (ref. 12, appx A, p. 19).
10.3. Core boring produced unequivocal evidence
of a mass of once-molten material in the center
of the core.
By the end of 1986 it was indubitable that much of the core had
melted, but it would be two years and more before it was clear what
a serious danger that had occasioned. After “Swiss cheesing,”
the mass of solidified material in the lower part of the core was
laboriously cut and chipped apart, and then fished and sucked up
out of the water-filled reactor vessel. Then the structures supporting
the core had to be cut out before access was gained to what lay
in the bottom of the vessel. Only then, early in 1989, was it discovered
just what that “slag-like” material lying in the bottom
of the reactor vessel was—and thus, also, that there had been
one really horrific moment in the course of the accident. That was
the moment portrayed in Figure 3.7 when twenty tons of molten fuel
broke out of the center of the core and cascaded down the side of
the reactor vessel into its bottom.
10.4. Overall view of deployment of plasma arc
(MDM = “metal disintegration machining”)
apparatus for cutting prismatic samples from
inside of bottom of TMI-2 reactor vessel.
Thus the last question to be investigated, after all that rubble
had been excavated, was the condition in which the bottom of the
reactor vessel had been left by that thermal assault—why it
had withstood so unexpectedly well the softening that megawatts
of heat at so high a temperature were expected to effect. Special
plasma arc cutting equipment—designed, as always, to be operated
from above the reactor vessel and under water—carved prismatic
samples out of the inside of the 5-inch (13 cm) thick vessel wall.
With the utility pushing, as always, to move as quickly as possible
to complete the cleanup and put an end to that expensive operation,
15 samples were extracted in 30 days early in 1990, working 24 hours
a day, 7 days a week (ref. 7). These specimens were then distributed
to laboratories around the world for analysis, in a research program
that was concluded only in 1994 (ref. 25). In all, TMI showed that—contrary
to common belief—a disaster inside a nuclear reactor does
not necessarily lead to a disaster outside the reactor.