OF THE ACCIDENT
Just how serious was
the accident? Based on our investigation of the health effects of the
accident, we conclude that in spite of serious damage to the plant, most
of the radiation was contained and the actual release will have a
negligible effect on the physical health of individuals. The major health
effect of the accident was found to be mental stress.
The amount of
radiation received by any one individual outside the plant was very low.
However, even low levels of radiation may result in the later development
of cancer, genetic defects, or birth defects among children who are
exposed in the womb. Since there is no direct way of measuring the danger
of low-level radiation to health, the degree of danger must be estimated
indirectly. Different scientists make different assumptions about how this
estimate should be made and, therefore, estimates vary. Fortunately, in
this case the radiation doses were so low that we conclude that the
overall health effects will be minimal. There will either be no case of
cancer or the number of cases will be so small that it will never be
possible to detect them. The same conclusion applies to the other possible
health effects. The reasons for these conclusions are as follows.
An example of a
projection derived for the total number of radiation-induced cancers among
the population affected by the accident at TMI was 0.7. This number is an
estimate of an average, such as the one that appears in the statement:
"The average American family has 2.3 children."
In the case of TMI,
what it really means is that each of some 2 million individuals living
within 50 miles has a miniscule additional chance of dying of cancer, and
when all of these minute probabilities are added up, they total 0.7. In
such a situation, a mathematical law known as a Poisson distribution
(named after a famous French mathematician) applies. If the estimated
average is 0.7, then the actual probabilities for cancer deaths due to the
accident work out as follows: There is a roughly 50 percent chance that
there will be no additional cancer deaths, a 35 percent chance that one
individual will die of cancer, a 12 percent chance that two people will
die of cancer, and it is practically certain that there will not be as
many as five cancer deaths.
can be calculated for our various estimates. All of them have in common
the following: It is entirely possible that not a single extra cancer
death will result. And for all our estimates, it is practically certain
that the additional number of cancer deaths will be less than 10.
Since a cancer caused
by nuclear radiation is no different from any other cancer, additional
cancers can only be determined statistically. We know from statistics on
cancer deaths that among the more than 2 million people living within 50
miles of TMI, eventually some 325,000 people will die of cancer, for
reasons having nothing to do with the nuclear power plant. Again, this
number is only an estimate, and the actual figure could be as much as
1,000 higher or 1,000 lower Therefore there is no conceivable statistical
method by which fewer than 10 additional deaths would ever be detected.
Therefore the accident may result in no additional cancer deaths or, if
there were any, they would be so few that they could not be detected.
We found that the
mental stress to which those living within the vicinity of Three Mile Island
were subjected was quite severe. There were several factors that
contributed to this stress. Throughout the first week of the accident,
there was extensive speculation on just how serious the accident might
turn out to be. At various times, senior officials of the NRC and the
state government were considering the possibility of a major evacuation.
There were a number of advisories recommending steps short of a full
evacuation. Some significant traction of the population in the immediate
vicinity voluntarily left the region. NRC officials contributed to the
raising of anxiety in the period from Friday to Sunday (March 30-April 1)-
On Friday a mistaken interpretation of the release of a burst of radiation
led some NRC officials to recommend immediate evacuation. And on Friday
Governor Thornburgh advised pregnant women and preschool aged children
within 5 miles of TMI to leave the area. On Saturday and Sunday, other NRC
officials mistakenly believed that there was an imminent danger of an
explosion of a hydrogen bubble within the reactor vessel, and evacuation
was again a major subject of discussion.
We conclude that the
most serious health effect of the accident was severe mental stress, which
was short-lived. The highest levels of distress were found among those
living within 5 miles of TMI and in families with preschool children.
There was very
extensive damage to the plant. While the reactor itself has been brought
to a "cold shutdown," there are vast amounts of radioactive material
trapped within the containment and auxiliary buildings. The utility is
therefore faced with a massive cleanup process that carries its own
potential dangers to public health The ongoing cleanup operation at TMI
demonstrates that the plant was inadequately designed to cope with the
cleanup of a damaged plant The direct financial cost of the accident is
enormous. Our best estimate puts it in a range of $1 to $2 billion, even
if TMI-2 can be put back into operation. (The largest portion of this is
for replacement power estimated for the next few years.) And since it may
not be possible to put it back into operation, the cost could even be much
The accident raised
concerns all over the world and led to a lowering of public confidence in
the nuclear industry and in the NRC.
From the beginning, we
felt it important to determine not only how serious the actual impact of
the accident was on public health, but whether we came close to a
catastrophic accident in which a large number of people would have died.
Issues that had to be examined were whether a chemical (hydrogen) or steam
explosion could have ruptured the reactor vessel and containment building,
and whether extremely hot molten fuel could have caused severe damage to
the containment. The danger was never -- and could not have been -- that
of a nuclear explosion (bomb).
We have made a
conscientious effort to get an answer to this difficult question. Since
the accident was due to a complex combination of minor equipment failures
and major inappropriate human actions, we have asked the question: "What
if one more thing had gone wrong?"
We explored each of
several different scenarios representing a change in the sequence of
events that actually took place. The greatest concern during the accident
was that significant amounts of radioactive material (especially
radioactive iodine) trapped within the plant might be released. Therefore,
in each case, we asked whether the amount released would have been smaller
or greater, and whether large amounts could have been released.
Some of these
scenarios lead to a more favorable outcome than what actually happened.
Several other scenarios lead to increases in the amount of radioactive
iodine released, but still at levels that would not have presented a
danger to public health. But we have also explored two or three scenarios
whose precise consequences are much more difficult to calculate. They lead
to more severe damage to the core, with additional melting of fuel in the
hottest regions. These consequences are, surprisingly, independent of the
age of the fuel.
Because of the
uncertain physical condition of the fuel, cladding, and core, we have
explored certain special and severe conditions that would, unequivocally,
lead to a fuel-melting accident. In this sequence of events fuel melts,
falls to the bottom of the vessel, melts through the steel reactor vessel,
and finally, some fuel reaches the floor of the containment building below
the reactor vessel where there is enough water to cover the molten fuel
and remove some of the decay heat. To contain such an accident, it is
necessary to continue removing decay heat for a period of many months.
At this stage we
approach the limits of our engineering knowledge of the interactions of
molten fuel, concrete, steel, and water, and even the best available
calculations have a degree of uncertainty associated with them. Our
calculations show that even if a roeltdown occurred, there is a high
probability that the containment building and the hard rock on which the
TMI-2 containment building is built would have been able to prevent the
escape of a large amount of radioactivity. These results derive from very
careful calculations, which hold only insofar as our assumptions are
valid. We cannot be absolutely certain of these results.
Some of the limits of
this investigation were: (1) We have not examined possible consequences of
operator error during or after the fuel melting process which might
compromise the effectiveness of containment; (2) We have not examined the
vulnerability of the various electrical and plumbing penetrations through
the walls or the doorways for people and equipment; (3) The analysis was
specific to the TMI-2 design and location (for example, the bedrock under
the plant); (4) We recognize that we have only explored a limited number
of alternatives to the question "What if . . .?" and, others may come up
with a plausible scenario whose results would have been even more serious.
We strongly urge that
research be carried out promptly to identify and analyze the possible
consequences of accidents leading to severe core damage. Such knowledge is
essential for coping with results of future accidents. It may also
indicate weaknesses in present designs, whose correction would be
important for the prevention of serious accidents.
have not prevented us from reaching an over- whelming consensus on
corrective measures. Our reasoning is as follows: Whether in this
particular case we came close to a catastrophic accident or not, this
accident was too serious. Accidents as serious as TMI should not be
allowed to occur in the future.
The accident got
sufficiently out of hand so that those attempting to control it were
operating somewhat in the dark. While today the causes are well
understood, 6 months after the accident it is still difficult to know the
precise state of the core and what the conditions are inside the reactor
building. Once an accident reaches this stage, one that goes beyond
well-understood principles, and puts those controlling the accident into
an experimental mode (this happened during the first day), the uncertainty
of whether an accident could result in major releases of radioactivity is
too high. Adding to this the enormous damage to the plant, the expensive
and potentially dangerous cleanup process that remains, and the great cost
of the accident, we must conclude that -- whatever worse could have
happened -- the accident had already gone too far to make it tolerable.
While throughout this
entire document we emphasize that fundamental changes are necessary to
prevent accidents as serious as TMI, we must not assume that an accident
of this or greater seriousness cannot happen again, even if the changes we
recommend are made. Therefore, in addition to doing everything to prevent
such accidents, we must be fully prepared to minimize the potential impact
of such an accident on public health and safety, should one occur in the