15
Nuclear Reactor Accidents
15.1 Historical Overview of Reactor Accidents . . . . . . . . . . . . . . . . 411
15.2 The Three Mile Island Accident . . . . . . . . . . . . . . . . . . . . . . . 414
15.3 The Chernobyl Accident . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436
15.1 Historical Overview of Reactor Accidents
Despite the largely successful precautions taken to avoid nuclear reactor accidents,
the record is not perfect. We list here the more important known reactor
accidents, excluding accidents in submarine reactors and possible accidents in
the former USSR and Soviet Bloc countries, other than the Chernobyl acci-
dent.1
The decision as to which accidents qualify as “major” accidents is somewhat
arbitrary. In particular, ordinary industrial non-nuclear accidents are
omitted. For example, in 1972 two workers at the Surry Power Station were
fatally scalded by steam escaping from a faulty valve. This did not involve
the nuclear components of the power station and therefore is not pertinent to
the broader issue of nuclear reactor safety. The major past accidents are as
follows:
N Chalk River, Canada (1952). There was a partial meltdown in a 30MWt
experimental reactor. The reactor was cooled by light water and
moderated by heavy water. The accident was initiated by operator errors
and a failure of the control rod system. This led to an elevated power
1
There are reports of reactor accidents in these countries prior to the much larger
Chernobyl accident (see Refs. [1] and [2]), although accounts of their course and
magnitude are in dispute. In addition, there was a major release of radioactive
material in an accident in 1957 at the Kyshtym nuclear complex in the Urals. The
accident was a non-nuclear explosion in tanks of reprocessed radioactive wastes,
not a reactor accident. It led to the reported evacuation of 10,730 people and
caused a collective eﬀective dose of about 2500 person-Sv [3, p. 116].
411
412 15 Nuclear Reactor Accidents
output and some boiling and loss of cooling water. In a typical LWR,
the accident would have been mitigated by a negative void coeﬃcient (see
Section 14.2.1), but in this case the feedback was positive. The reactor was
eventually shut down by draining the heavy water moderator. There were
no known injuries or deaths, but the reactor core was damaged and there
was escape of an unspeciﬁed amount of radioactivity [4, p. 101].
N National Reactor Testing Laboratory, Idaho (1955). The 1.4-MWt
experimental breeder reactor, EBR-I, suﬀered a 40% to 50% core meltdown
during a test in which the power level of the reactor was intentionally
raised but, due to operator error, was not reduced promptly. There was
little contamination of the building, no injuries occurred, and the release
of radioactive material was “trivial” [4, p. 103].
N Windscale, England (1957). Overheating and ﬁre occurred in a graphite
moderated reactor used for plutonium production. The accident began in
the course of heating the fuel above normal operating temperatures to
release energy stored in the graphite crystal lattice.2
This energy is a consequence
of radiation damage to the graphite, a problem that arises in
graphite-moderated reactors if they operate below the temperature necessary
for annealing radiation damage. In this case, the heating and the
energy release, although intentional, were too rapid. The reactor was shut
down with control rods, but the heating had been suﬃcient to cause a ﬁre
in the uranium fuel and, eventually, in the graphite. The ﬁre smoldered for
about 5 days, until extinguished by ﬂooding with water [6]. The most serious
consequence was the release of about 20,000 Ci of 131
I (T = 8.02 days),
which was carried by winds over much of central and southern England.
The estimated consequences for England and continental Europe are 260
thyroid cancers and 13 thyroid cancer fatalities over a period of 40 years,
plus 7 additional fatalities or hereditary eﬀects [7, p. 24].
N National Reactor Testing Laboratory, Idaho (1961). Three army
technicians were killed when one of them apparently rapidly removed
(manually) a control rod from a 3-MWt test reactor, known as SL-1, on
which they were working. Reactors of this type were intended for heating
and electricity production at remote sites, and they were so primitive that
the control rods could be moved by an operator standing on top of the
reactor. There was a rapid increase in reactor output, followed by a steam
explosion, leading to lethal levels of radiation within the reactor building.
Most, but not all, of the activity was contained within the building [4,
p. 109].
N Fermi Reactor, Detroit (1966). There was a partial meltdown in a
200-MWt (61-MWe) commercial breeder reactor, which was a one-of-akind
prototype. The cause was a blockage in the ﬂow path of the sodium
coolant. There were no injuries or signiﬁcant release of radioactivity, and
2
The storage and release of energy in a graphite moderator is the so-called Wigner
eﬀect (see, e.g., Ref. [5]).
15.1 Historical Overview of Reactor Accidents 413
the reactor was brieﬂy put back into operation before its ﬁnal shutdown
in 1973 [4, p. 32].3
N Lucens, Switzerland (1969). There was partial fuel melting in a 30MWt
experimental reactor due to loss of CO2 cooling. There was severe
damage to the reactor but no radiation release beyond permitted levels [4,
p. 121].
N Browns Ferry 1, Alabama (1975). A ﬁre in the electrical wiring did
extensive damage to the control systems and threatened the reactor, but
the reactor was turned oﬀ and cooling maintained with no radiation release
and no injuries other than one individual suﬀering a minor burn from the
ﬁre. Despite the absence of damage to the reactor itself, this accident was
of importance because it was the ﬁrst major accident in a commercial
LWR and demonstrated a serious vulnerability in the control systems of
that period due to inadequate redundancy.
N Three Mile Island, Pennsylvania (1979). This accident is discussed
in more detail in Section 15.2.
N Chernobyl, USSR (1986). This accident is discussed in more detail in
Section 15.3.
Some aspects of these accidents are summarized in Table 15.1.
The only reactor accidents that caused a clearly identiﬁable loss of life were
at Idaho Falls, where three workers died from the eﬀects of the explosion and
radiation, and at Chernobyl, where 31 operating and ﬁreﬁghting personnel
died within about 2 months, primarily from high radiation doses. In addition,
there possibly will be a small number of eventual, or “delayed,” cancer faTable
15.1. Major nuclear reactor accidents.
Environmental Consequences
Capacity Radioactivity Prompt Delayed
Year Reactor Purpose (MW) Release Deaths Cancersa
1952 Chalk River Experimental 30 (t) Some 0 0
1957 Windscale Pu production Large 0 ∼ 13–20
1961 Idaho Falls Test (army) 3 (t) Small 3 0
1966 Fermi I Demo breeder 61 (e) Very little 0 0
1969 Lucens Experimental 30 (t) Very little 0 0
1975 Browns Ferry 1 Power 1065 (e) None 0 0
1979 TMI-2 Power 906 (e) Small 0 ∼ 0–2
1986 Chernobyl Power 1000 (e) Very large 31b
∼ 30,000b
a
Indicated cancers are possible cancer fatalities, calculated on the basis of the linear
hypothesis (see Section 4.3).
b
See Section 15.3.4 for further discussion of Chernobyl fatalities.
3
For two very diﬀerent assessments of the signiﬁcance of this accident and the level
of hazard it created, see Refs. [8] and [9].
414 15 Nuclear Reactor Accidents
talities from the Windscale radiation release and a large number of delayed
fatalities from Chernobyl.4
It may be noted that none of these three reactors
were commercial LWRs, and except for Chernobyl, the accidents took place
more than 35 years ago. Their history therefore has only limited pertinence
to the present safety of commercial LWRs or of other non-Soviet commercial
reactors.
Beyond reactors, the most serious nuclear accident since Chernobyl occurred
at Tokaimura, Japan on September 30, 1999, at a facility for preparing
reactor fuel.5
Although this was not a reactor accident, we mention it here because
a signiﬁcant accident at any nuclear facility reﬂects unfavorably on the
nuclear industry in general. The accident occurred in the course of preparing
fuel for an experimental fast reactor which used uranium enriched to 18.8%
in 235
U. In one stage of the process, in violation of authorized procedures,
workers poured buckets of enriched uranium solution into a tank, apparently
unaware that given the size and shape of the tank (45 cm in diameter and
61 cm high) they could create a critical mass. When they ﬁlled the tank
with about 40 L of the solution, criticality was reached and there was an
intense burst of gamma rays and neutrons, setting oﬀ radiation alarms. The
three workers involved left the building, but all were heavily exposed and two
eventually died. The only “signiﬁcant” health consequences cited in an IAEA
report on the accident were to these workers, although other workers were exposed
to some extent, including some involved in measures taken to terminate
the chain reaction [10, p. 30].6
15.2 The Three Mile Island Accident
15.2.1 The Early History of the TMI Accident
The Three Mile Island (TMI) accident occurred in one of two similar reactors
at the Three Mile Island site in Pennsylvania.7
The accident was in the second
4
The number of fatalities is in question, given the uncertainties surrounding the
eﬀects of radiation at low doses and dose rates, but in the discussion of these
accidents, we quote numbers based on the adoption of the linearity hypothesis
(see Section 4.3).
5
This summary is based largely on an IAEA report prepared shortly after the
accident [10].
6
There was no explosion, but criticality continued with a low power output for
about 20 h, stabilized by thermal expansion of the ﬂuid and the formation of
bubbles. The chain reaction was terminated by draining water from a cooling
jacket surrounding the tank, which reduced reﬂection of neutrons back into the
tank, and as a precaution by injecting a boric acid solution into the tank.
7
Extensive studies were carried out after the accident. One, referred to later as
the “Kemeny report,” was by a commission appointed by President Carter and
15.2 The Three Mile Island Accident 415
Fig. 15.1. Schematic of the TMI-2 facility, including reactor building and turbine
building. Piping goes, from right to left in the diagram, through the containment
building wall to the auxiliary building (not shown); piping also goes, from left to
right, through the turbine building wall to the condensate storage tank and cooling
tower (not shown). (From Ref. [11, pp. 86–87].)
unit, known as TMI-2. It was a 906-MWe pressurized water reactor built by
Babcock and Wilcox, the smallest (in terms of number of units completed) of
three U.S. manufacturers of PWRs. It had ﬁrst received a license to operate
at low power in February 1978 and was in routine operation at full power by
the end of 1978. A schematic of the TMI-2 facility is shown in Figure 15.1 [11,
pp. 86–87].
The accident started with a failure of the cooling system of TMI-2 in the
early morning of March 28, 1979. The initial problem was an interruption in
the ﬂow of water to the secondary side of the steam generator. This water
is the so-called feedwater. In the secondary loop, feedwater enters the steam
chaired by John Kemeny, the president of Dartmouth College [11]. The second,
the Rogovin Report, was by a special inquiry group instituted by the Nuclear
Regulatory Commission and chaired by Mitchell Rogovin, a partner in an independent
Washington law ﬁrm. The description here is drawn largely from the
Kemeny Report [11] and Part 2 of Volume II of the Rogovin Report [12], as well
as a further review article [13].
416 15 Nuclear Reactor Accidents
generator, and steam emerges to drive the turbine. The steam is condensed in
a second heat exchanger (the condenser), and water is returned to the steam
generator after passing through a “polisher,” in which dissolved impurities
are removed. The ﬂow of water between the condenser and steam generator
is maintained by the condensate pump and the main feedwater pump (see
Figure 15.1).
The chain of events that led to the accident appears to have been initiated
by work done to clean the polishers. In a sequence that has not been conclusively
established, this operation may have caused one or more of the valves in
the condensate polisher system to close, automatically shutting oﬀ (tripping)
one of the condensate pumps. The tripping of the condensate pump, whatever
the cause, in turn, tripped the main feedwater pumps.8
This failure caused
the emergency feedwater pumps to start automatically, in order to maintain
the ﬂow of water to the steam generator. Maintenance of feedwater ﬂow is
essential to cool the water from the reactor that ﬂows through the primary
side of the steam generator.
Up to this point, everything was “normal,” in the sense that reactors are
designed to handle occasional equipment failures; protection then comes from
backup systems. However, the block valves in the emergency feedwater lines
(there were two) were closed; according to proper operating procedures, they
were supposed to be open. Indicator lights in the control room showed the
closed status, but the operators at ﬁrst did not notice this. Thus, no water
was being fed to the secondary side of the steam generator because the pumps
for the main supply were oﬀ and valves in the emergency line were closed. With
no ﬂow of water, the pressure in the steam generator rose and in response, the
pilot-operated relief valve (PORV) on the “hot” side of the steam generator
heat exchanger opened. The pressure excursion also caused the reactor to
trip, with automatic insertion of the control rods. With the reactor turned
oﬀ and the PORV open, the pressure dropped. The PORV should then have
automatically closed.
At this point, there were additional equipment and design failures. The
PORV did not close properly, but the control panel indicator light displayed
the status of the control power to the valve (namely that it was supposedly
closed), not the actual status of the valve (namely that it was open). Thus, the
operators had to cope with unusual conditions in the cooling system without
knowing the actual status of the valves in it. In particular, the PORV remained
open for almost 2.5 h, causing a very large loss of needed cooling water.
Within 2 min after the start of the accident, the steam generators boiled
dry because they had no feedwater source and there was a substantial heat
output from the reactor core due to radioactive decay. Overall, the conditions
of the cooling system were both unusual and confused, with the operators
8
Figure 15.1 does not show the redundancy in the system. There were two main
feedwater pumps and three emergency feedwater pumps.
15.2 The Three Mile Island Accident 417
not having correct information or suﬃcient training to recognize the nature
of the evolving anomalies and cope with them. They did recognize that there
were serious problems, and by 4:45 am supervisory personnel began to arrive
at TMI, only three-quarters of an hour after the start of the accident. By
6:22 am, the PORV was closed, but the problems were not over. At 7:00 am
a “site emergency” was declared because there had been some release of ra-
dioactivity.
15.2.2 Evolution of the TMI Accident
Over the next few days, the accident continued to unfold, with continued
diﬃculty in establishing proper cooling conditions. There were some small
releases of radioactivity outside the plant, as well as one misinterpreted report
of radiation levels that led to the incorrect belief that there had been a large
release. There was a great sense of emergency both at the site and in the
surrounding area, as no one was willing to give unequivocal assurances that
matters were under control. This led to a recommended evacuation of pregnant
women and preschool children from the immediate vicinity and a large selfinitiated
evacuation by individuals.
Concern reached a peak on Saturday, March 31, over the possibility of a
hydrogen explosion inside the pressure vessel. As described in the subsequent
Kemeny Report:
The great concern about a potential hydrogen explosion inside the
TMI-2 reactor came with the weekend. That it was a groundless fear,
an unfortunate error, never penetrated the public consciousness afterward,
partly because the NRC made no eﬀort to inform the public it
had erred. [11, p. 126]
Hydrogen is produced by the reaction of steam with the zircaloy cladding
at high temperatures. Oxygen is formed by the breakup of water under radiation,
so-called radiolysis. Together, hydrogen and oxygen can form an explosive
mixture. There was fear that such an explosion could occur within the
pressure vessel. Within a day or so, some NRC experts came to the conclusion
that a hydrogen explosion was impossible, but this conclusion was not
immediately accepted by all of the authorities. In the meantime, the hydrogen
bubble had become a matter of great public concern, a concern not unambiguously
dismissed by the NRC. However, by 6:00 pm on April 1, the hydrogen
was removed from the bubble by “letdown, leakage, and venting” [12, p. 535].
It never had been the threat that had been believed.
The reason that the problem was not a real one was an insuﬃcient accumulation
of oxygen. In a PWR, it is normal to have some hydrogen dissolved
in the water and to have continued recombination of oxygen and hydrogen.
This recombination prevented the amount of oxygen from rising suﬃciently
418 15 Nuclear Reactor Accidents
to create a danger of explosion. This point is brought out in the Rogovin
Report:
Little or no oxygen was present in the bubble and a very low probability
of explosion existed. The incorrect perception of an explosion
hazard stemmed from contradiction among supposed experts. This
perception was known or should have been known to be false by the
afternoon of April 1. [12, p. 535]
It may be noted that a Babcock and Wilcox scientist had given assurances
from the ﬁrst that there was no problem from oxygen production [12, p. 534],
but apparently this assurance did not receive much attention.
President Carter visited the site on Sunday, April 1; the hydrogen bubble
itself dissipated (although not the perception of a near miss with hydrogen)
and the worst of the crisis was over. However, it took more than another week
for the advisory evacuation of pregnant women and preschool children to be
withdrawn by the governor of Pennsylvania.
15.2.3 Eﬀects of the TMI Accident
Core Damage and Radionuclide Releases
In retrospect, several major aspects of the Three Mile Island accident were
not fully appreciated at the time and might seem to be in conﬂict:
N There was very little release of radioactivity and very little exposure of the
general population. According to the Kemeny Commission, “the maximum
estimated radiation dose received by any one individual in the oﬀ-site
general population (excluding the plant workers) during the accident was
70 millirems. . . . three TMI workers received radiation doses of about 3 to 4
rems; these levels exceeded the NRC maximum permissible quarterly dose
of 3 rems” [11, p. 34]. In essential agreement, the Rogovin Report found
that “the maximum oﬀ-site individual dose was less than 100 mrem” [12,
p. 400].
N The total collective dose to the 2 million people living within 50 miles of
TMI was approximately 2000 person-rem (20 person-Sv).9
From this, the
Kemeny Commission estimated a 50% chance of no fatal cancers from the
accident, a 35% chance of one fatal cancer, and a 15% chance of more
than one [11, p. 12]. These results correspond to an average expectation
of 0.7 cancer fatalities. If the 1993 NCRP risk estimate of 0.05 per sievert
is adopted, then one fatal cancer is calculated for the collective dose of 20
9
See Ref. [11, p. 34] and Ref. [12, p. 399].
15.2 The Three Mile Island Accident 419
person-Sv (see Section 4.3.4). Among these 2 million people, it is expected
that 325,000 will die of cancers unrelated to TMI, so the TMI impact, if
any, will be far below any detectable level.
N The core damage was very great. As cleanup and dismantling of the TMI-
2 reactor proceeded, it was found that the core damage was greater than
originally thought, and some observers have expressed surprise that the
reactor vessel itself withstood the molten fuel at its bottom.
Thus, those who thought that the accident was causing, or was about to
cause, large releases of radioactive material were proven to be wrong. Those
who thought that matters were being exaggerated and that there was relatively
little actual damage to the reactor were also wrong. The biggest surprise,
however, was having these two outcomes together—great core damage
and almost negligible external radionuclide releases. Only about 15 Ci of 131
I
were released to the environment outside the containment [12, p. 358], despite
an initial core inventory more than 1 million times greater. It had been commonly
assumed that with core damage of this magnitude, a large fraction of
the iodine would escape. Thus, the containment system, including the system
to spray water into the containment to remove radionuclides, performed unexpectedly
well. In the aftermath of TMI, understanding this performance—now
part of what is known as the source term question—became a major issue in
reactor safety studies (see Section 14.1.4).
Studies of Health Eﬀects of TMI
The release of radioactivity from the Three Mile Island plant and the resulting
radiation exposures were too small to have produced any observable
eﬀects, if one accepts oﬃcial accounts of the magnitude of the releases and
standard dose–response relationships. One or even 10 cancer deaths would
be lost among a total of over 300,000 “natural” cancer deaths. Nonetheless,
there have been persistent claims of health problems from TMI. In response
to some of the early concerns, the Pennsylvania Health Secretary stated in a
news release: “After careful study of all available information, we continue to
ﬁnd no evidence to date that radiation from the nuclear power plant resulted
in an increased number of fetal, neonatal, and infant deaths. That simply isn’t
the case” [14]. This was based on an examination of death rates near TMI
and in Pennsylvania as a whole, before and after the accident.
The Pennsylvania Department of Health carried out a later study of spontaneous
abortions, in the face of a rather widespread belief among residents
of the TMI area that there had been an increase in stillbirths and miscarriages
[15]. The study identiﬁed 479 women living within 5 miles of the plant
who were pregnant at the time of the accident. For this group, there were 436
live births, 28 spontaneous abortions or stillbirths, and 15 other abortions.
The rate of spontaneous abortions and stillbirths was compared to the rate
420 15 Nuclear Reactor Accidents
expected from earlier studies of nonexposed populations and it was found that
there was no excess. In the author’s words, the TMI incidence rates “compared
favorably with the four baseline studies.”
In a broader study of cancer rates near Three Mile Island, investigators
found a statistically signiﬁcant increase in cancer incidence, compared to rates
at greater distances, during 1982 and 1983 [16].10
However, this excess did not
persist, and in 1985, the cancer incidence rate was slightly lower for the nearTMI
group than for the more distant group. Further, the increase was seen
only in cancer incidence, not in cancer fatalities. This lack of increase was
commented on particularly for lung cancer, which progresses rapidly from
incidence to fatality, as pointing to possible “screening bias.” The authors
concluded:
We observed a modest postaccident increase in cancer near TMI that is
unlikely to be explained by radiation emissions. The increase resulted
from a small wave of excess cancers in 1982, three years after the 1979
accident. Such a pattern might reﬂect the impact of accident stress
on cancer progression. Our study lacked a direct, individual measure
of stress, however. The most plausible alternative explanation is that
improved surveillance of cancer near the TMI plant led to the observed
increase.
These results are consistent with the belief that there is virtually no possibility
that there have been or will be observable health eﬀects from radioactivity
released in the TMI accident, given the low exposure levels. However,
the post-TMI history illustrates the extent of skepticism about oﬃcial reassurances
in situations of possible radiation hazard. This skepticism is fed by
anomalies in the data (such as the increase in observed cancer incidence in
1982). Anomalies often cannot be explained in any conclusive fashion, and the
ruling out of radiation exposure as the cause may hinge on somewhat indirect
arguments, such as comparisons to standard models of the time intervals
between radiation exposures, cancer incidence, and cancer fatalities. The families
and friends of the “victims” of the anomalies may have little incentive to
accept these arguments.
These diﬃculties may be of only marginal interest in the case of TMI,
where the weight of evidence and scientiﬁc opinion is strong, but they could
assume much greater importance in evaluating the Chernobyl accident, where
the exposures were very much greater and the conditions for systematic epidemiological
studies are poorer. It is probable that there will be large health
consequences from Chernobyl, and it is possible that some will be observable,
but it may prove diﬃcult to assess the validity of individual reports and to
resolve the disagreements that will arise.
10
The authors were from the School of Public Health at Columbia University, with
the exception of one from the Audubon Society.
15.3 The Chernobyl Accident 421
15.3 The Chernobyl Accident
15.3.1 The Chernobyl Reactors
Among reactor accidents, the Chernobyl accident in 1986 stands alone in
terms of magnitude.11
The design features of “Chernobyl-type” reactors are
unique, and it is the standard assumption of nuclear analysts that a similar
accident could not occur in any of the other types of reactors operating today.
However, Chernobyl demonstrated the seriousness of a near “worst-case”
accident. It intensiﬁed preexisting public concern about reactors of all sorts
and strengthened the position of those who oppose nuclear power on safety
grounds.
The Chernobyl reactor was one of the Soviet RBMK-1000 reactor series,
designed to operate at a (gross) capacity of 1000 MWe. These are graphite
moderated and water cooled, of a type originally used in the USSR (and with
important diﬀerences in the United States) for the production of plutonium.
Such reactors were also used for the generation of electricity in the USSR,
dating back to a 5-MWe water-cooled, graphite-moderated reactor at Obninsk,
put into operation in 1954 [18, p. 9].
At the beginning of 1986, there were four RBMK-1000 reactors at the
Chernobyl site in the Ukraine, located about 130 km north of the major city
of Kiev. The most recently installed reactors, completed in 1983, were Units
3 and 4, housed in a single building. At the time of the accident, all four
reactors were operating and two more were under construction. The accident
itself occurred in Unit 4 on April 26, 1986 at 1:24 am. Unit 3 was turned oﬀ
by the operators after almost 5 h and the nearby Units 1 and 2 were turned
oﬀ after about 24 h. Units 1 and 2 were returned to operation in late 1986
and Unit 3 in December 1987. Construction was suspended and then canceled
on the two reactors being built at Chernobyl [19].
Unit 2 was permanently closed, following a ﬁre on October 1, 1991. The ﬁre
was in a non-nuclear part of the plant, and there was no release of radioactivity,
but there was damage to the engine room [20]. Units 1 and 3 continued in
operation beyond 1991, with the Ukrainian authorities balancing the need
for their electrical output against concerns about their safety, but both were
eventually shut down permanently—in 1996 and 2000, respectively. Outside
Ukraine, there are 11 RBMK-1000 reactors operating in Russia (4 at Kursk, 4
near St. Petersburg, and 3 at Smolensk) and two larger RBMK reactors (1380
MWe) at Ignalina in what is now Lithuania [21].12
In an eﬀort to reduce
the chance of another accident, steps have been taken since 1986 to improve
operator training, and signiﬁcant modiﬁcations have been made in the RBMK
reactors themselves.
11
The account of the accident progression given here is based largely on parts of
Refs. [17], [18], and [24]–[27].
12
Lithuania has agreed to shut down these plants, one in 2005 and the other in
2009, as a condition for joining the European Union [22].
422 15 Nuclear Reactor Accidents
Although the USSR in the past exported reactors to its neighbors, these
were all PWRs, not RBMKs. Outside the USSR, the only reactor bearing
some similarity to the Chernobyl reactor was the not so very similar N reactor
at Hanford, which stopped operations in 1988. Future Russian plans are
based primarily on LWRs, although the Chernobyl accident did not stop the
completion of all RBMK reactors: One of the RBMK-1000 units now operating
at Smolensk was put on-line in 1990 and a ﬁfth unit at Kursk, for which
construction began in 1985, remains under construction [23].
15.3.2 History of the Chernobyl Accident
Deﬁciencies in Attention to Safety
There was no crisp single cause of the Chernobyl accident. Even 6 years later,
a review by a major international technical body, the International Nuclear
Safety Advisory Group (INSAG), reported: “It is not known for certain what
started the power excursion that destroyed the Chernobyl reactor” [24, p. 23].
The quotation may suggest more ignorance than is the case. A great deal
is known about the conditions before and during the accident, even if the
development of a deﬁnitive, detailed scenario has been made diﬃcult by the
complexity of the reactor conditions, the speed of unfolding of the accident,
and the damage itself.
Overall, the accident was the result of a combination of design deﬁciencies,
operator errors, and an unusual set of prior circumstances, all of which put
the reactor and the operators to a test that they failed. In the INSAG view,
“the accident can be said to have ﬂowed from deﬁcient safety culture, not
only at the Chernobyl plant, but throughout the Soviet design, operating
and regulatory organizations.” The INSAG report listed a whole gamut of
weaknesses in institutions and attitudes [24, p. 24]. In this view, with greater
vigilance in both design and operations, there would have been no accident.
Design Weaknesses
Two aspects of the design have been particular targets of criticism: a positive
void coeﬃcient of reactivity and an improperly conﬁgured control rod system.
Both contributed positive feedbacks, which turned an initial excursion
in reactor performance into the Chernobyl disaster.
The void coeﬃcient in a water-cooled reactor would be negative if the water
acted only as a coolant and moderator. However, the water also acts as
a poison, due to capture of neutrons in hydrogen. In an LWR, when water is
lost or steam bubbles develop, the dominant eﬀect is a decrease in reactivity,
corresponding to the negative void feedback discussed in Section 14.2.1. However,
in a Chernobyl-type reactor, most of the moderation is provided by the
graphite, and the water acts mainly as a poison. When some of the cooling
water is replaced by steam, there is a diﬀerent balance than in an LWR be-
15.3 The Chernobyl Accident 423
tween the competing feedbacks: increased resonance absorption in 238
U (negative
void coeﬃcient), increased neutron leakage from the reactor (negative
void coeﬃcient), and decreased absorption of thermal neutrons in H2O (positive
void coeﬃcient). The net outcome depends on the relative amounts and
arrangement of the uranium, carbon, and water in the reactor. For the Chernobyl
reactor, the overall void coeﬃcient was positive, whereas a graphitemoderated,
water-cooled reactor in the United States, the Hanford-N reactor,
had a negative void coeﬃcient.
The second major defect, involving the control rods, is also related to
the role of water. At the onset of the accident, most of the control rods had
been fully withdrawn from the reactor to compensate for an excursion in
xenon poisoning to above the normal level (see next subsection). Remarkably,
the ﬁrst eﬀect of the insertion of the control rods from the full-out position
was to increase the reactivity. This was due to a peculiarity of the RBMK-
1000 control rod system, which has since been corrected. The control rods
move vertically through the reactor core and are withdrawn by being lifted
upward. To prevent the control rod from being replaced by water, which acts
as a poison and lessens the eﬀect of withdrawing the rod, a long graphite
“displacer” was attached to the bottom of the rod. When the rod was fully
withdrawn, most of the channel in the core was occupied by this graphite,
and the control material was above the core. However, the graphite displacer
did not completely ﬁll the channel; instead, there was still a 125-cm column
of water below the graphite displacer [24, p. 5]. The ﬁrst eﬀect of inserting
the control rod, before the absorbing part of the control rod reached the core,
was for the graphite to drive out the water column, increasing the reactivity
by reducing the poison. The full motion of the control rods was slow, and by
the time the control rod proper entered the core region, it was too late.
Since Chernobyl, there have been a number of changes to correct these
defects in the RBMK reactors. These include increasing the enrichment of 235
U
in the fuel, changing the control rod geometry so that the displacer will not
displace water when inserted, and speeding up the control rod insertion [25].
Reactor Operations Prior to Accident
The accident evolved from a test that disturbed normal operating conditions.
Ironically, the test was undertaken to demonstrate a safety feature of the
reactor. Power for the pumps and other plant facilities normally comes from
the plant’s own turbogenerator units or from the oﬀ-site power grid. Should
there be an oﬀ-site power outage and a shutdown of the reactor, standby
diesel generators at the plant come on-line to supply power. There could be an
interval of several seconds between the loss of normal power and the start-up
of the diesel generators. The test was to demonstrate that the inertial coasting
of the turbogenerator would provide suﬃcient power to operate pumps during
this interval.
424 15 Nuclear Reactor Accidents
The ﬁrst step taken to perform this test was a reduction of reactor power
to one-half of its normal 3200 MWt, beginning at about 1:00 am on April
25. One of the turbogenerators was switched oﬀ at 1:06 am, and the power
reached 1600 MWt at 3:47 am [24, p. 53]. The remainder of the test, involving
a further reduction of power, was to start at about 2:00 pm. As part of the
test, the emergency core-cooling system was disconnected at 2:00 pm.
However, the reactor’s power was required for the electricity grid fed by
Chernobyl and instructions were given to postpone the further power reduction.
The test did not resume until 11:10 pm on April 25. It was then intended
to reduce the power to about 700–1000 MWt, but there was an overshoot in
the shutdown and the power level dropped to 30 MWt. By about 1:00 am on
April 26, it had been brought back to 200 MWt, but the period of operation
at low power caused a buildup of xenon poisoning (see Section 7.5.3), which
was compensated for by removal of a large number of the control rods, more
than proper under operating guidelines.13
At this point, there were at least two unusual circumstances: The power
level for the test was lower than planned and the margin of safety, in terms
of the ability to shut down the reactor with the control rods, was less than
the normal operating limit. In addition, a number of safety systems had been
turned oﬀ to facilitate the planned test. In hindsight, it is clear that the test
should have been terminated at this point, but it was continued.
Initiation and Progress of the Accident
As the test proceeded at low power, water ﬂow conditions were not normal,
there was some decrease in steam, and the reduced reactivity caused automatic
control rods to withdraw to restore the reactivity. This was a manifestation of
the fact that the Chernobyl reactor operated with a positive void coeﬃcient.14
This action was in itself harmless, but it raised the control rods to unusually
high positions out of the core. At 1:23:04 am, despite warning indications
of the dangerous control rod conﬁguration, the operators initiated the turbine
test by shutting a valve and reducing steam ﬂow to the turbine. The resulting
changes in steam pressure and in water ﬂow from the cooling water pumps
led to a decreased water ﬂow through the core and some boiling in the core.
The displacement of water by steam caused the reactivity to rise.
In response, at 1:23:40 am, an emergency shutdown (scram) was attempted.
However, the control rods had been withdrawn too far to take im-
13
After power is reduced, the decay of 135
I (T = 6.57 h) to 135
Xe (T = 9.14 h)
continues, but the destruction of 135
Xe by neutron capture is decreased. Therefore,
the amount of 135
Xe increases for several hours.
14
According to Richard Wilson [26], Russian experts explained to him that this
design was adopted for cost savings in establishing the graphite conﬁguration. For
power levels of 20% of normal or higher, the negative fuel temperature coeﬃcient
(the Doppler coeﬃcient) was supposed to provide an adequate margin of safety,
and at lower power levels, there were to be stringent operating regulations.
15.3 The Chernobyl Accident 425
mediate eﬀect and, due to the graphite displacers at their ends, their ﬁrst
eﬀect was to increase rather than decrease the reactivity. Within 3 s there
was a sharp increase in the neutron ﬂux and power output, as the reactor
went superprompt critical. Within not more than 20 s, there were two large
explosions—one apparently a steam explosion that exposed the reactor fuel
to the air and the other an explosion due to exothermic reactions, including
the interaction of liberated hydrogen and carbon monoxide with the air.
These explosions breached the reactor building (there was no true containment)
and sent burning fragments into the air, which started ﬁres on the roof
of the reactor. Fireﬁghters from the nearby towns of Chernobyl and Pripyat
arrived shortly after the accident and put out the building ﬁres by 5:00 am
There had been a threat that the ﬁres would spread to the other units at
Chernobyl (Units 1, 2, and 3), but this was prevented by ﬁremen working
under extreme conditions of heat and radiation exposure. Remarkably, Unit
3 was not turned oﬀ until about 6:00 am.
Although the exterior ﬁres had been extinguished, the problem of heat
generation in the reactor continued, due to (chemical) burning of the fuel
and graphite in the reactor and to radioactive decay heat. These interior ﬁres
were not extinguished until May 6, following a series of attempts to quench
them by dropping massive amounts of boron carbide (intended to prevent
recriticality), limestone, lead, sand, and clay. In total, Unit 4 was entombed
under about 5000 tons of material, and the acute phase of the accident was
then over.
15.3.3 Release of Radioactivity from Chernobyl
The initial explosions and subsequent reactor ﬁres caused large amounts of
radioactive materials to be released from the reactor. The release was not all
immediate, with about 24% the ﬁrst day, 28% over the next 5 days, and 48%
over the following 4 days [27, p. 3.9]. The release was mainly of the volatile
nuclides, including the noble gases, iodine, and cesium. Much less of the nonvolatile
nuclides, such as strontium, escaped. Estimated release fractions and
total releases are given in Table 15.2 for some of the important radionuclides.
The cloud of radioactivity from the accident during the ﬁrst 2 days spread
generally to the north and west and, thus, did not severely impact Kiev.
In later days, when releases were lower, the winds shifted to form plumes
in other directions, including to the south [28, p. 459]. The accident was not
immediately made public, and the ﬁrst awareness outside the USSR came from
radiation measurements in Sweden and Finland. The cloud reached Sweden
at about 2 pm on April 27 and was ﬁrst detected about 18 h later by monitors
at the Forsmark nuclear power station [18, p. 13]. This was approximately 2
days after the start of the accident itself.
Eventually, the radioactive cloud spread over most of the northern hemisphere,
depositing radionuclides widely. The amounts deposited decreased
with increasing distance, although the correlation with distance was not pre-
426 15 Nuclear Reactor Accidents
Table 15.2. Radionuclide releases from the Chernobyl
accident, for selected radionuclides.
Core Releasea
Fraction
Isotope T1/2 (MCi) (MCi) Released
85
Kr 10.8 years 0.89 0.89 1.0
133
Xe 5.24 days 176 176 1.0
131
I 8.02 days 86 48 0.6
134
Cs 2.07 years 4.1 1.5 0.36
137
Cs 30.1 years 7.0 2.3 0.33
90
Sr 28.8 years 5.9 0.27 0.05
a
The actual release was less for short-lived isotopes because
of decay before release; these numbers are “corrected” back
to the activity at the time of the accident.
Source: Ref. [28, pp. 518–519].
cise due to wind patterns and rainfall. In general, it could be said that the
fallout was substantial near Chernobyl, moderate in some other parts of Europe,
and negligible in North America.
15.3.4 Observations of Health Eﬀects of Chernobyl Accident
Overall Summary up to 2000
Subsequent to the Chernobyl accident, there have been many studies of its
health impacts, and studies are likely to continue for many decades. A succinct
summary of current knowledge was presented in the 2000 Report of the United
Nations Scientiﬁc Committee on the Eﬀects of Atomic Radiation (UNSCEAR)
in an Overview section on “The Radiological Consequences of the Chernobyl
Accident”:
The accident at the Chernobyl nuclear power plant was the most
serious accident involving radiation exposure. It caused the deaths,
within a few days or weeks, of 30 workers and radiation injuries to over
a hundred others. It also brought about the immediate evacuation, in
1986, of about 116,000 people from the areas surrounding the reactor
and the permanent relocation, after 1986, of about 220,000 people
from Belarus, the Russian Federation and Ukraine. It caused serious
social and psychological disruption in the lives of those aﬀected and
vast economic losses over the entire region. Large areas of the three
countries were contaminated, and deposition of released radionuclides
was measurable in all countries of the northern hemisphere.
There have been about 1,800 cases of thyroid cancer in children
who were exposed at the time of the accident, and if the current trend
15.3 The Chernobyl Accident 427
continues, there may be more cases during the next decades. Apart
from this increase, there is no evidence of a major public health impact
attributable to radiation exposure 14 years after the accident. There
is no scientiﬁc evidence of increases in overall cancer incidence or mortality
or in non-malignant disorders that could be related to radiation
exposure. The risk of leukemia, one of the main concerns owing to its
short latency time, does not appear to be elevated, not even among the
the recovery operation workers. Although those most highly exposed
individuals are at an increased risk of radiation-associated eﬀects, the
great majority of the population are not likely to experience serious
health consequences as a result of radiation from the Chernobyl accident.
[29, p. 4]
Thus, two groups have been unambiguously harmed by radiation from
Chernobyl: emergency workers at the site of the accident and children in
a wide surrounding region who have developed thyroid cancers. No other
radiation damage had been observed by the time of the UNSCEAR report
in 2000. Further details on the aﬀected groups are presented in the next two
subsections.
Eﬀects on Plant Workers and Firemen
Severe medical eﬀects of the accident were suﬀered by workers at the plant
and the ﬁremen who responded to the accident. By 8:00 am of the morning of
the accident, these totaled about 600, including 69 ﬁremen [28, p. 522]. In all,
31 died within several months of the accident: 28 from acute radiation syndrome
(ARS), 2 from nonradiation injuries, and 1 apparently from coronary
thrombosis [30, p. 6].15
Most of those who died from radiation sickness also
received severe skin burns from beta-particle radiation. These early deaths
were exclusively among plant personnel and ﬁremen. The latter appear to
have performed in an exceedingly dedicated and self-sacriﬁcing manner.
A total of 237 people were suspected of having ARS and this diagnosis
was conﬁrmed for 134, including the 28 who died. The deaths among the ARS
group were strongly correlated with the magnitude of the radiation exposures.
The fractional death rates at diﬀerent exposure levels were: 0 out of 41 up to
2.1 Sv, 1 out of 50 from 2.2 to 4.1 Sv, 7 out of 22 from 4.2 to 6.4 Sv, and 20
out of 21 above 6.4 Sv [28, p. 523]. In the decade following the accident, from
1987 to 1996, an additional 14 of the original 237 patients died, but these
deaths do not appear to be primarily attributable to radiation exposure [31,
p. 187].
15
ARS was deﬁned by “at least minimal bone-marrow suppression as indicated by
depletion of blood lymphocytes” [28, p. 488]. In many accounts of Chernobyl,
the number of prompt fatalities is given as 30, apparently omitting the coronary
thrombosis victim.
428 15 Nuclear Reactor Accidents
Childhood Thyroid Cancer
The accident led to high thyroid exposures, primarily due to 131
I in the cloud
of radionuclides from the reactor. The relatively short half-life of 131
I (8.02
days) means that the dose was received within several weeks of the accident.
The main pathways for this dose were through inhalation and consumption of
milk or locally grown produce.16
High thyroid cancer rates began to be seen
among children in the early 1990s. Sixty-two cases were observed in 1990 and
the number of cases grew steadily until about 1994, when the rate leveled
oﬀ at roughly 250 per year [28, p. 545]. At ﬁrst, it was suspected that the
high rate of observed thyroid cancers was due to the careful search for them.
However, the rate of cancer incidence is seen to be much higher for young
children (based on age at the time of the accident) than for older ones, and
there is no increased rate for children born after the accident [28, p. 501].
Further, the cancer rate correlates with inferred dose. Thus, the excess is well
established.
Through 1998, a total of 1791 thyroid cancer cases were reported in children
up to the age of 17 [28, p. 545]. A 2002 UN report states that “some two
thousand cases of thyroid cancer have been diagnosed” and anticipates that
“the ﬁgure is likely to rise to 8–10,000 in coming years” [32, p. 7]. Thyroid
cancer is rarely fatal, but requires continued medical treatment.
15.3.5 Radiation Exposures at Chernobyl and Vicinity
Eﬀects on Cleanup Personnel
A prolonged cleanup was carried out in the aftermath of the accident by military
servicemen and civilians who were brought in to work for short periods.
These were the so-called liquidators. About 600,000 people have been so designated
for the years 1986 to 1989, including about 200,000 for 1986 and 1987
when the radiation levels were highest [28, p. 469]. Individual doses often
reached several hundred millisieverts [28, p. 525], which is well above the U.S.
occupational limit of 50 mSv in 1 year.
There have been reports of increased fatalities and sickness among the
liquidators, but in the absence of comparisons to similar populations of nonexposed
individuals, it is not clear that the results are meaningful [28, p. 516].
The 2000 UNSCEAR document reported no ﬁndings of increased rates of
leukemia or other forms of cancer among the liquidators [28]. However, this
remains an important group for continued study over the next several decades,
because most radiation-associated cancers appear more than 10 years after the
exposure. As summarized in the UNSCEAR report:
16
Consumption of milk from cows that grazed in contaminated soil provides a quick
path for transfer of 131
I from the ground to the human body. Use of potassium
iodide tablets reduces uptake of other iodine to the thyroid, and many of the
children in the Chernobyl vicinity received these tablets.
15.3 The Chernobyl Accident 429
Apart from the radiation-associated thyroid cancers among those exposed
in childhood, the only group that received doses high enough to
possibly incur statistically detectable increased risks is the recovery
operation workers. Studies of these populations have the potential to
contribute to the scientiﬁc knowledge of the late eﬀects of ionizing
radiation. Many of these individuals receive annual medical examinations,
providing a sound basis for future studies of the cohort. It is,
however, notable that no increased risk of leukemia, an entity known
to appear within 2–3 years after exposure, has been identiﬁed more
than 10 years after the accident. [28, p. 517]
In short, by the year 2000, no statistically signiﬁcant radiation-caused harm
was seen among the liquidators, but that does not establish that there has
been no harm or that no statistically signiﬁcant eﬀects will ever be identiﬁed.
Exposure of Population in the “Aﬀected Region”
The largest exposures of people near Chernobyl initially came from 131
I
and other short-lived radionuclides, in part through inhalation. After several
weeks, the iodine had decayed suﬃciently to be a lesser contributor, and
over the longer term most of the dose came from 134
Cs and 137
Cs and, later,
just 137
Cs.
After passage of the radioactive cloud, the most important pathways for
exposures were from ingested radionuclides and from gamma rays and beta
particles emitted by radionuclides deposited on the ground. Ingestion is the
more important in the ﬁrst year, but as deposition from the atmosphere ends
and the radionuclides are washed oﬀ vegetation by rain, the ground exposure
becomes the more important. Overall, in terms of the long-term dose commitment
(including the dose from ingested radionuclides that remain in the
body), the external surface dose and the ingestion dose are roughly equal,
with the former somewhat predominating.
On the day after the accident, almost 50,000 people were evacuated from
Pripyat, 3 km from the reactor site [28, p. 527]. Later, the evacuation was
extended to cover an “exclusion zone” around Chernobyl, which included the
region within 30 km of Chernobyl plus a few outlying areas [28, pp. 472–
473]. In all, about 116,000 people were evacuated in 1986 and another 220,000
people were relocated after 1986 [28, p. 453].
A series of zones have been deﬁned around Chernobyl, in part in terms of
the deposition of 137
Cs:
N Exclusion zone. This is the 30-km zone (cited above) from which 116,000
people were evacuated in 1986.17
17
There has been some ambiguity about this total. Earlier estimates put it at
135,000, as reﬂected in Table 15.3 [28, p. 473].
430 15 Nuclear Reactor Accidents
N Strict control zone (SCZ). Region where the 137
Cs density on the ground
exceeds 15 Ci/km2
is the region of strict control [28, p. 475]. The initial
population of this region was about 270,000 [28, p. 475]. At this level,
resettlement is obligatory in Ukraine, but not necessarily mandatory in
Belarus and Russia [32, p. 36]. In all three countries, people have a right
to resettle from regions where the density exceeds 5 Ci/km2
.
N Other contaminated areas. These are regions where the 137
Cs ground density
is between 1 and 15 Ci/km2
. Most of the people in these regions were
in areas at the low end of the range (i.e., 1–5 Ci/km2
).
The future health impacts of Chernobyl on these groups can be estimated
from a study of the radiation doses. A summary of average and collective
doses for the groups considered above is given in a review by Elizabeth Cardis
and colleagues, prepared as a background paper for the One Decade After
Chernobyl conference that was held in 1996 with the joint sponsorship of the
IAEA and the World Health Organization (WHO) [30]. It also included data
for the subgroup of liquidators who worked in 1986–1987, when the radiation
levels were highest. In this paper, cancer mortality was estimated separately
for solid cancers and leukemia [33]. The results are presented in Table 15.3.
The doses for the populations other than the liquidators are doses received
from 1986 to 1995.18
This represents about 60% of the total estimated longterm
dose (1986–2056) [28, Table 33], suggesting that the doses in Table 15.3
underestimate the total impact of the accident. On the other hand, as the
authors pointed out, no dose and dose rate eﬀectiveness factor (DDREF)
was applied in estimating the cancer fatalities (see Section 4.3.4). Thus, the
calculated number of fatalities is not an underestimate if a DDREF of 2 is the
“correct” factor to apply. Overall, however, it is to be remembered that there
are large uncertainties in both the doses and the dose–response relation. In
particular, the number of fatal cancers may be greatly overestimated, because
the calculation is based on the linearity hypothesis. Thus, the 9,000 excess
fatalities of Table 15.3 are the predictions of a speciﬁc model, not an assured
outcome, and even in the context of the model, the results are approximate.
It is seen from Table 15.3 that the calculated number of excess fatalities
is, in most cases, small compared to the normal natural rates. Even when in
principle the excess is statistically signiﬁcant, uncertainties in the “normal”
rate may make it diﬃcult to conﬁrm the result by observations. The best opportunity
for doing this may be for leukemia incidence among the liquidators,
where a 25% excess is predicted.
The average doses for most of the people in these groups are well below
the 10-year background level of 20–30 mSv (roughly 2.4 mSv/year), but the
18
In such calculations, the calculated dose includes the contributions from all radionuclides,
with the 137
Cs density taken as an indicator of the concentrations
of other radionuclides. After the ﬁrst year, most of the dose is due to 134
Cs and
137
Cs; after the ﬁrst decade, most of the 134
Cs has decayed, leaving 137
Cs as the
dominant contributor.
Table15.3.Cumulativeradiationdoses(1986–1995)andcalculateddeathsovera95-yearperiodamong
populationsinthevicinityofChernobylandamongliquidatorsworkinginthe1986–1987period(seetext).
SolidCancerDeathsLeukemiaDeaths
PopulationAverageNormalCalc.NormalCalc.
Groupsizedose(mSv)RateExcessa
RateExcessa
Liquidators(1986–1987)200,00010041,5002,000800200
Evacuees(30-kmzone)135,0001021,50015050010
Residents,SCZ270,0005043,5001,5001,000100
Residents,otherareas6,800,0007800,000b
4,60024,000370
a
Thiscalculatednumberofexcessfatalitiesisbasedonthelinearityhypothesis,withoutaDDREF.
b
Thisnumber,giveninRef.[33]forthenormalrate,appearsinconsistentwiththe16%normalcancerincidence
rateindicatedforallgroupsotherthantheliquidators.
Source:Ref.[33,p.255].
431
432 15 Nuclear Reactor Accidents
liquidators and the SCZ residents received higher doses. By 2003, with most
of the long-term dose already received, the annual dose from Chernobyl radionuclides
is below natural background radiation levels, even in the SCZ.
Controversies over Health Eﬀects from Chernobyl
It is too soon for the health eﬀects of the Chernobyl accident to have been fully
manifested, for either the workers who participated in the Chernobyl cleanup
eﬀorts or the surrounding population. Most cancers, other than leukemia, have
a latent period of 10 years or longer. Further, it is impossible to attribute
a given observed cancer to a particular cause. The ambiguities of statistical
evidence, the dramatic nature of anecdotal evidence, and the strong incentives
to reach one conclusion or another almost guarantee that there will be very
diﬀerent assessments of the consequences of Chernobyl.
Even at Three Mile Island, there has been some controversy over the consequences
(see Section 15.2.3), and the situation is likely to be far more difﬁcult
in the Chernobyl case, especially in view of the political and economic
problems in the area. These may make it diﬃcult to obtain satisfactorily comprehensive
and reliable records. It is important that vigorous eﬀorts be made
to carry out detailed health surveys and data analyses, in order to improve
our understanding of the eﬀects of prolonged exposures to radiation at low
and intermediate levels.
The disagreements about Chernobyl could be reduced if there are careful
epidemiological studies by a group whose legitimacy is widely accepted. There
is a precedent for this in the Radiation Eﬀects Research Foundation,19
which
has carried out continuing studies of the aftermath of Hiroshima and Nagasaki.
In this vein, the 2002 United Nations report proposed the establishment of
an International Chernobyl Foundation to “channel resources into health and
ecological research relating to the eﬀects of the Chernobyl accident” [32, p. 17].
15.3.6 Worldwide Radiation Exposures from Chernobyl
One of the important set of results reported at the One Decade After Chernobyl
conference was an UNSCEAR assessment of total global population
doses. These results, as presented in a paper by R.G. Bennett, are summarized
in Table 15.4.
The total collective dose commitment in the northern hemisphere is estimated
to be 600,000 person-Sv, where the dose commitment is calculated
until 2056 (70 years after the accident). For populations that were not near
Chernobyl, the accident added relatively little to the natural background. In
19
This was formerly known as the Atomic Bomb Casualty Commission. It is a joint
research activity of the United States and Japan.
15.3 The Chernobyl Accident 433
Table 15.4. Distribution of radiation doses
from Chernobyl accident (total lifetime collective
dose commitment = 600,000 person-Sv).
Category Percent
Geographical distribution
Former Soviet Union (FSU) 36
Other Europe 53
Other, northern hemisphere 11
Time distribution
First year ∼ 33
Later years ∼ 67
Contributing radionuclides
137
Cs 70
134
Cs 20
131
I 6
Othera
4
Mode of exposure
External radiation 60
Internal radiation 40
a
Ascribed to “short-lived radionuclides deposited
immediately after the accident.”
Source: Ref. [34, p. 125].
the ﬁrst year after Chernobyl, when the impact of the accident was greatest,
the collective dose in Europe (outside the FSU) was about 100,000 person-Sv,
corresponding to an average individual dose of 0.2 mSv for a population of
about 500 million.20
Thus, on average, Chernobyl in the ﬁrst year added about
8% to the average natural radiation dose of 2.4 mSv/yr. The total per capita
doses in Europe (outside the FSU) summed over the 70 years following the
accident are expected to average about 0.7 mSv from Chernobyl and 170 mSv
from natural radiation sources [35, p. 369]. For North America, the average
dose is estimated to be 0.001 mSv in the ﬁrst year and 0.004 mSv summed
over all years.
If one uses a risk factor of 0.05 deaths per person-Sv, the worldwide collective
dose of 600,000 person-Sv would imply a total toll from Chernobyl of
30,000 deaths spread over more than 70 years (mostly in Europe, including
the FSU).21
For the most part, these 30,000 deaths would be impossible to
identify or verify. For example, about 16,000 of these deaths would be in Europe
(outside the FSU). In the same time period, the European population is
20
The country outside the FSU with the highest ﬁrst-year dose was Bulgaria, with
an average ﬁrst-year dose of slightly under 0.8 mSv [34, p. 122].
21
It should again be noted that considerable controversy surrounds the linearity
hypothesis, especially when applied to the low individual doses considered here.
434 15 Nuclear Reactor Accidents
expected to suﬀer over 100,000,000 “natural” cancers.22
The calculated 0.02%
increase—if it occurs—would be undetectable.
Diﬀerent perspectives on Chernobyl may be stated as follows: (a) The
accident may lead to about 30,000 cancer deaths; (b) the number of cancer
deaths attributable to Chernobyl is a very small fraction of those occurring
“naturally”; or (c) it is inappropriate to calculate expected deaths from the
collective dose, when most of the collective dose is made of individual doses
that are well under 10% of the natural background. Depending on which of
these formulations is taken to be the more appropriate, Chernobyl may be
considered to have been either a major global disaster or no more than a
serious accident, with tragic local consequences.
15.3.7 General Eﬀects of the Chernobyl Accident
Human Consequences for People in the Aﬀected Region
One of the major consequences of the Chernobyl accident was the fear engendered
in nearby populations. An IAEA study of the consequences of the
Chernobyl accident was requested in 1989 by the USSR government, apparently
prompted by concerns among people living in the vicinity of Chernobyl
but not close enough to have been among those originally evacuated from the
exclusion zone. The regions considered were places where the ground surface
concentrations of 137
Cs exceeded 5 Ci/km2
. It embraced an area of about
25,000 km2
, with a population of about 825,000 [37, p. 3].
This study, known as the International Chernobyl Project (ICP), was undertaken
by an international committee under the sponsorship of the IAEA,
with the assistance of the WHO, UNSCEAR, and other international organizations.
An overview of these results was published in Spring 1991 [37]. In
the populations studied, the International Chernobyl Project found no indications
of adverse medical consequences from the radiation. In its conclusions
it stated
. . . .[there were] no health disorders that could be attributed directly
to radiation exposure. The accident had substantial negative psychological
consequences in terms of anxiety and stress. . .[37, p. 32]
This referred to health eﬀects as of 1991.
A similar stress on psychological factors appears in a report on the Human
Consequences of the Chernobyl Nuclear Accident that was published in 2002.
This report was commissioned by several United Nations agencies and the
WHO, with the goal of studying the “current conditions in which people
aﬀected by the Chernobyl accident are living” and to make recommendations
for addressing their needs [32, p. ii]. It lays particular stress on the disruption
22
The “over 100,000,000” ﬁgure is a crude extrapolation from Ref. [36], where an
estimate of 88,000,000 deaths in Europe is given for the next 50 years.
15.3 The Chernobyl Accident 435
of the lives of the people who were evacuated and the fears experienced by
them and people still living in contaminated areas. Economic conditions have
been poor in the countries of the former Soviet Union, and the Chernobyl
accident has made matters worse due to the costs of remedial measures and
the inhibitions on growing crops in contaminated areas.
While disclaiming an intent to “minimize the seriousness of the situation
for health and well-being or the role played by the exposure to ionizing radiation,”
the report suggests giving priority to improving “basic primary health
care, diet, and living conditions” [32, p. 8]. Among the adverse consequences of
the Chernobyl accident, the report describes the social demoralization which
has accompanied the dislocation of populations and their living with a poorly
understood hazard. It suggests that “determined eﬀorts need to be made at
national and local level to promote a balanced understanding of the health
eﬀects of radiation among the public, many of whom at present suﬀer distress
as a result of ill-founded fears” [32, p. 10].
One of the diﬃculties in assessing the health impacts of the radiation
exposures is the generous compensation aﬀorded to people who can establish
injury and who, therefore, have an incentive to ﬁnd health damage. Children in
areas with a 137
Cs concentration of over 5 Ci/km2
are entitled to 2 months of
health holidays. With accompanying adults, in the year 2000 almost 300,000
people took holidays in Belarus and about 372,000 in Ukraine. Overall, as
described in the UN report:
[s]carce resources are allocated not primarily on the basis of medical
need but rather on an individual’s ability to register as a victim.
The system has promoted an exaggerated awareness of ill-health and a
sense of dependency, which has prevented those concerned from taking
part in normal economic and social life. The pattern of behavior was
described by the Kiev Conference on the Health Eﬀects of the Chernobyl
Accident. . .as the “Chernobyl accident victim syndrome.”[32,
p. 32]
Extended Eﬀects of the Chernobyl Accident
The above-cited report suggests that Chernobyl produced a demoralization
among the neighboring population, beyond that which could be directly attributed
to the health eﬀects of radiation. Chernobyl may also have had profound
eﬀects on the Soviet Union as a whole. The technological failure at
Chernobyl and the attempted coverup of the accident is cited as one of the
reasons for the collapse of conﬁdence in the Soviet system. This possibility
is reﬂected, for example, in an op-ed piece entitled “Will SARS be China’s
Chernobyl?” Before drawing the parallel with SARS, the author writes:23
23
This article, by James Goldgeier, Director of the Institute for European, Russian,
and Eurasian Studies at George Washington University appeared in a number of
newspapers in April 2003, e.g., the Los Angeles Times on April 23, 2003, [38].
436 15 Nuclear Reactor Accidents
. . .a major event in the Soviet Union’s downward spiral was the accident
at the Chernobyl nuclear power station in April 1986. The Soviet
Union initially tried to keep a tight lid on what had occurred, but radioactive
fallout was not conﬁned to Soviet territory, and European
governments were quick to announce that initial reports from Moscow
underplayed the danger. . . .
Chernobyl alone would not have brought down the Soviet Union. However,
the government’s clumsy reaction and the growing demands for
the truth on this and other issues helped stir the caldron of resentment
that culminated in Soviet collapse.
The eﬀects of Chernobyl on the stability of the Soviet Union may have been
magniﬁed by the fact that, by chance, the radiation exposures and negative
impacts of the accident were greatest in two Soviet Republics that already
had separatist tendencies. As described by Martin Malia, “Chernobyl, in particular,
accelerated the development of Ukrainian and Belorussian separatist
sentiments; and the local apparatus easily found it in their interests to espouse
this sentiment against Moscow” [39, p. 440].
The accident also had a great eﬀect on nuclear power. Like the Soviet
Union, nuclear power was facing problems of its own, prior to Chernobyl. The
shock of the Three Mile Island accident, 7 years earlier, had not fully dissipated
and the post-TMI improvements in nuclear power plants had not yet paid oﬀ
(e.g., in the higher capacity factors that began in the United States in the
1990s). Chernobyl came at a time when nuclear power was already facing
economic and political diﬃculties, and it reinforced the existing public fears
that contributed to these diﬃculties. There is little justiﬁcation for extending
the analogy and assuming that nuclear power will go the way of the Soviet
Union. However, even if not a fatal setback, the Chernobyl accident was a
major blow to its progress.
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