Paper
INCREASED OCCUPATIONAL RADIATION DOSES: NUCLEAR FUEL CYCLE
André Bouville* and Victor Kryuchkov†
AbstractVThe increased occupational doses resulting from the
Chernobyl nuclear reactor accident that occurred in Ukraine
in April 1986, the reactor accident of Fukushima that took place
in Japan in March 2011, and the early operations of the Mayak
Production Association in Russia in the 1940s and 1950s are
presented and discussed. For comparison purposes, the occupational doses due to the other two major reactor accidents
(Windscale in the United Kingdom in 1957 and Three Mile Island
in the United States in 1979) and to the main plutonium-producing
facility in the United States (Hanford Works) are also covered but
in less detail. Both for the Chernobyl nuclear reactor accident and
the routine operations at Mayak, the considerable efforts made to
reconstruct individual doses from external irradiation to a large
number of workers revealed that the recorded doses had been
overestimated by a factor of about two.
Introduction of Increased Occupational Exposures: Nuclear
Industry Workers. (Video 1:32, http://links.lww.com/HP/A21)
Health Phys. 106(2):259Y271; 2014
Key words: National Council on Radiation Protection and
Measurements; accidents, nuclear; Chernobyl; occupational safety
INTRODUCTION
HUNDREDS IF not thousands of radiation accidents have
occurred in nuclear facilities, as well as in industrial,
medical, research, and academic facilities, that make use of
sealed radioactive sources or of machine-generated radiation. A detailed survey of accidents ‘‘whereby exposure to
radioactive material affected workers or members of the
public in a fashion that results in acute (i.e., deterministic)
health effects’’ was conducted by the United Nations
Scientific Committee on the Effects of Atomic Radiation
(UNSCEAR 2011a), and the medical management of such
*National Cancer Institute (retired), 9609 Medical Drive, Room
7E590, MSC 9778, Rockville, MD 20850; †Federal Medical Biological Agency, Burnasyan Federal Medical Biophysical Center, 46
Zhivopisnaya Street, 123182, Moscow, Russia.
The authors declare no conflicts of interest.
For correspondence contact: André Bouville, 9609 Medical
Drive, Room 7E590, MSC 9778, Rockville, MD 20850, or email at
andre.bouville@nih.gov.
Supplemental Digital Content is available in the HTML and PDF
versions of this article on the journal’s Web site (www.health-physics.com).
(Manuscript accepted 5 November 2013)
0017-9078/14/0
Copyright * 2014 Health Physics Society
DOI: 10.1097/HP.0000000000000066
types of radiation accidents was described extensively by
Gusev et al. (2001). This paper will present and discuss
the increased occupational doses that were received as a
consequence of severe reactor accidents, defined as those
that resulted in irreparable damages to the plant (Windscale
in the United Kingdom in 1957; Three Mile Island in the
United States in 1979; Chernobyl in the former Soviet
Union in 1986; Fukushima in Japan in 2011). The facilities
of the ‘‘military’’ fuel cycle Ei.e., the reactors used for the
production of plutonium and the plants involved in the
processing of plutonium (Hanford in the United States and
Mayak in the former Soviet Union)^ have been considered
as well, even though the increased doses that were received
in the early years of operation in the late 1940s and early
1950s at Mayak were not due to accidents but to the
conduct of routine operations.
External doses to workers are usually derived from
the reading or processing of personnel dosimeters. Over
the course of the years, the external doses have been
reported in terms of exposure, absorbed dose, or dose
equivalent. For regulatory practices, the operating principle is to estimate the dose conservatively in order to protect the worker. However, when the doses are to be used
in the framework of epidemiologic studies, care must be
taken to ensure that unbiased estimates of individual dose
are obtained. In that respect, considerable efforts have been
made to evaluate the validity of the reported doses resulting from the Chernobyl nuclear reactor accident and from
the early operations of the Mayak Production Association
(MPA). In this paper, the doses recorded for regulatory
practice are presented in terms of dose equivalent (sievert or
millisievert); when the doses are used for epidemiologic
purposes, they are presented in terms of absorbed dose
(gray or milligray).
Substantial internal doses were also received by some
of the workers at Mayak, as well as those involved in
emergency and cleanup activities related to the Chernobyl
and Fukushima accidents. The internal occupational doses
are estimated retrospectively and are derived from the
analysis of the available environmental and bioassay
measurements. Based on the experience of Mayak and
Chernobyl, the assessment of the internal doses improves
gradually with time and takes a number of years to be
259
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260
Health Physics
implemented satisfactorily. In this paper, the internal doses
are expressed in terms of absorbed dose (gray or milligray),
except for Fukushima, where the use of effective doses
seemed to be more appropriate.
Following a brief summary of the average annual
occupational doses that have been reported worldwide for
routine operations in facilities of the nuclear fuel cycle,
a detailed presentation of the increased exposures among
the workers involved in early operations of plutoniumproducing facilities and in the emergency and mitigating
activities related to the four major nuclear reactor accidents will be made, and the efforts made to improve the
dose estimates for the purposes of epidemiologic studies
will be described.
AVERAGE ANNUAL DOSES TO WORKERS
DUE TO ROUTINE OPERATION OF THE
NUCLEAR FUEL CYCLE
Doses to workers in the whole range of facilities and
activities involving radiation exposure have been compiled and presented in a number of reports, notably by the
National Council on Radiation Protection and Measurements (NCRP 1989) and UNSCEAR (1993, 2010). For
epidemiologic purposes, occupational doses covering a
large number of reactor workers were collected and analyzed also (Cardis et al. 2007; Thierry-Chef et al. 2007).
In an ongoing study, Boice (2012) is collecting and
analyzing the dosimetric information for about one million
U.S. workers who were exposed to radiation in facilities of
various types or a as a result of work in contaminated areas.
With regard to the nuclear fuel cycle, which includes
the facilities that are covered in this paper, UNSCEAR
has reported information periodically on the number of
workers involved worldwide in each component of the
nuclear fuel cycle and on the related average annual effective doses. The results for two time periods (1975Y1979
and 2000Y2002) are presented in Table 1 (UNSCEAR 2010).
The number of workers in uranium mining and milling decreased from one time period to the other, while
Table 1. Variation with time of the numbers of workers worldwide
in the components of the nuclear fuel cycle and of the average annual
effective doses (UNSCEAR 2010).
Monitored workers
(thousands)
Average annual
effective dose (mSv)
1975Y1979 2000Y2002 1975Y1979 2000Y2002
Uranium mining
Uranium milling
Uranium enrichment
Fuel fabrication
Reactor operation
Fuel reprocessing
240
12
11
20
150
78
12
3
18
20
437
76
5.5
10
0.5
1.8
4.1
7.1
1.9
1.1
0.1
1.6
1.0
0.9
February 2014, Volume 106, Number 2
the number of workers involved in reactor operations increased and the workers in other components of the nuclear fuel cycle were relatively constant. The average
annual effective doses during the 1975Y1979 time period
were G5 mSv in uranium enrichment, fuel fabrication,
and reactor operation, and between 5Y10 mSv in uranium
mining, uranium milling, and fuel reprocessing; during
the 2000Y2002 time period, average annual effective
doses had decreased to G2 mSv in all components of the
nuclear fuel cycle.
INCREASED EXPOSURES DURING ROUTINE
OPERATION: EARLY YEARS
At the beginning of the nuclear era in the 1940s,
nuclear energy was developed for military purposes. It
was deemed interesting to present the occupational doses
at two sites with very similar activities: Hanford in the
United States and Mayak in the former Soviet Union.
The Hanford site, located in south-central Washington
State, was selected in 1943 for the production of plutonium and other nuclear materials in support of the World
War II effort. Reactor operation started in 1944. Over several years, nine nuclear reactors were constructed for the
production of plutonium (Shipler et al. 1996), which was
used first for the Trinity nuclear weapons test of July 1945.
A few thousand workers were monitored for radiation at
the Hanford site during the mid- and late-1940s. The personnel dosimetry program at Hanford has been described
and evaluated in detail by Wilson et al. (1990). The predominant source of exposure, which was external irradiation from high-energy photons (9100 keV), was judged to
have been measured adequately for all years of operation.
From 1944Y1957, a film badge dosimeter was used. In 1957,
a multi-element film badge dosimeter was introduced.
This led to a significant improvement in measurement of
low-energy photons, the dose from which had previously
been underestimated. In 1972, the film badges were replaced with thermo-luminescent dosimeters, thus allowing for the estimation of the neutron doses, which had
also been underestimated until that time.
Following the detonation of nuclear weapons by the
United States in 1945, large efforts were undertaken by
the former Soviet Union to develop their own nuclear
weapons technology. The MPA, which is located in the
Southern Urals, was the first industrial complex in the
former Soviet Union that was built for the production of
plutonium. The initial industrial complex included nuclear
reactors, a radiochemical plant for separation of plutonium
from irradiated fuel, and a plutonium production plant
(Vasilenko et al. 2007). The construction of the first reactor began in 1945 and became operational in 1948. The
construction of the chemical processing plant began in
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Increased occupational doses c A. BOUVILLE
1946, and plutonium ready for use in nuclear weapons
was available in 1949. Approximately 20,000 workers were
exposed to radiation in the early years of operation. Occupational doses were due to external irradiation, mainly
from photons, and internal irradiation arising from intakes
of plutonium. External exposures were monitored by means
of film badges, which were without filters from 1948Y1953
and not suitable for appropriately measuring photons with
energies below 400 keV. A compensating filter (0.75 mm
of lead) was added to the film badges to measure external
doses from 1954Y1960. In 1961, a 0.5-mm aluminum filter
was added to the lead filter (Romanov et al. 2002).
Estimates of annual effective doses from external
irradiation to workers at the Hanford Works (Buschbom
and Gilbert 1993) and at MPA (Romanov et al. 2002;
Vasilenko et al. 2007) are shown in Table 2 and in Fig. 1.
For comparison purposes, it was assumed that the reported
doses, which were expressed as whole-body penetrating
doses (Hanford) and tissue-equivalent dose in free air
(MPA), are approximately equal to the effective doses.
The differences in the magnitude of the average annual
effective doses from external irradiation at the Hanford
Works and at MPA are striking. Annual doses at the Hanford
Works were generally G5 mSv, and there were only three
workers with annual doses 950 mSv during the early years:
one in 1947 with an annual effective dose of 60 mSv,
another one in 1951 with a dose of 55 mSv, and the third
one in 1954 with an annual dose of 144 mSv. By contrast,
at MPA, the average annual effective dose was È1,000
mSv or so around 1950 and did not decrease to 10 mSv
Table 2. Variation with time of the average annual effective doses
(millisievert) from external irradiation to workers of the Hanford
Works (Buschbom and Gilbert 1993) and of MPA [based on Romanov
et al. (2002) and on Vasilenko et al. (2007)].
MPA
a
Year
Hanford
Works
Reactor
facilitya
Radiochemical
planta
Plutonium
facilityb
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1960
1965
1970
0.59
2.25
1.39
0.79
0.48
0.65
0.90
0.95
1.41
2.19
1.88
2.12
3.18
6.99
3.20
200
950
300
190
150
200
90
95
29
40
14
400
950
1020
650
300
190
210
170
20
16
10
170
230
130
90
40
From Fig. 3 in Romanov et al. (2002).
From Fig. 1 in Vasilenko et al. (2007).
b
AND
V. KRYUCHKOV
261
Fig. 1. Variation with time of the average annual effective
doses from external irradiation to workers of the Hanford Works
(Buschbom and Gilbert 1993) and of MPA Ebased on Romanov et al.
(2002) and on Vasilenko et al. (2007)^.
until 1970. Reasons for these high doses from external
irradiation at MPA include:
1. the fact that reactor, chemical processing, and plutonium chemical-metallurgical facility technologies were
emerging rapidly;
2. there were limitations in MPA resources and capabilities to protect workers; and
3. there was poor understanding of the consequences of
relatively high occupational radiation doses.
It is worth noting that important efforts have been
made during the last 20 y to reconstruct the doses to the
MPA workers, resulting in a database called ‘‘Doses-2005’’
(Vasilenko et al. 2007). It is now established that the
initial doses shown in Table 2 and Fig. 1 were mostly overestimated when the film badges without filtration were
used during 1948Y1953, in some areas by a factor of 2.8.
With regard to the Hanford Works, it seems that the recorded doses are biased to some extent but that they are
reasonably adequate for the purposes of epidemiologic
studies (Gilbert and Fix 1995).
Another important feature of the working situation
at MPA is the high internal doses that were received as a
result of plutonium intakes. Cumulative organ doses to
workers were calculated using the Doses-2005 internal
dosimetry model (Vasilenko et al. 2007). As expected,
the greatest cumulative doses were estimated for the lung
(mean of 205 mGy, median of 23.5 mGy, 53 workers with
cumulative doses 9500 mGy), liver (mean of 284 mGy,
median of 42.2 mGy, 77 workers with cumulative doses
9500 mGy), and bone surface (mean of 1040 mGy, median of 156 mGy, 243 workers with cumulative doses
9500 mGy).
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REACTOR ACCIDENTS
Four major nuclear reactor accidents, which resulted
in irreparable damages to the plant, occurred in four different countries: the United Kingdom in 1957 (Windscale),
the United States in 1979 (Three Mile Island), the former
Soviet Union in 1986 (Chernobyl), and Japan in 2011
(Fukushima). In each case, the type of reactor was different, and the main cause of the accident was, to some
extent, different.
Windscale accident
The Windscale reactors, also called ‘‘Piles,’’ used
uranium metal as fuel, were moderated by graphite, and
were air-cooled. Their main purpose was the production
of plutonium for the U.K. atomic weapons program. The
heat produced by nuclear fission was not used to generate
electricity. Pile No. 1, on which the accident happened,
was operational in October 1950 (Wakeford 2007). Since
the potential energy stored in the graphite (the Wigner
energy) needed to be released in a controlled manner,
anneals were organized periodically. Unfortunately, during the ninth anneal, a fire broke out in the reactor core
on 10 October 1957 and resulted in a partial core meltdown (Arnold 2007).
Within the framework of an epidemiologic study,
McGheoghegan and Binks (2000) collected the recorded
external doses relative to the 471 workers who were involved in fire activities. For October 1957, the median of
the recorded dose was 4.5 mSv, while the 95th percentile
was 16 mSv, and the maximum dose was 44 mSv. Over
a 3-mo period encompassing the time of the accident,
14 workers received a dose 930 mSv, the highest dose
being 47 mSv. No information could be found in the
open literature on the occupational exposures related
specifically to the cleanup activities.
Three Mile Island accident
Unit 2 of the Three Mile Island reactor was a
pressurized-water reactor with an installed capacity of
906 MW of electricity. It had started operating in December
1978. An accident occurred on 28 March 1979, with failures in the non-nuclear secondary system, followed by a
stuck-open relief valve in the primary system, which allowed loss of coolant to occur and resulted in melting at
least 45% of the reactor core.
Detailed information on the occupational doses related to the accident and to the cleanup of the damaged
reactor could not be found in the open literature. Occupational external doses, which were reported together for
the workers of Units 1 and 2 until 1985, show for 1979 an
average of 3.5 mSv among the 3,975 workers with measurable doses and a maximum dose of 45 mSv (Table 3).
Because the containment of the reactor had held up,
there was no urgency to clean up the reactor. After 6 y
February 2014, Volume 106, Number 2
Table 3. Variation with time of the average annual doses from
external irradiation to workers exposed as a result of the Three Mile
Island accident [based on NRC (1984) and on Radiation Exposure
Information and Reporting System data.]a
Yearb
Number of workers
with measurable dose
Highest dose
(mSv)
Average dose
(mSv)
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
3,975
2,328
2,103
2,123
1,592
1,079
1,890
1,497
1,378
1,247
1,014
484
153
315
167
45
21
21
42
27
3.5
1.7
1.8
4.7
7.3
6.4
4.5
6.1
7.1
7.4
6.3
2.8
2.4
5.0
2.0
a
Data provided by D. Hagemeyer, Oak Ridge, TN: Oak Ridge Associated
Universities.
Units 1 and 2 of the Three Mile Island reactor reported together until 1985
and separately from 1986 onwards.
b
of preparation, defueling began in October 1985, and
decontamination activities took place until December
1993. About 1,000 workers were involved in those operations. The reported average annual external doses for
the workers with measurable doses over the 1979Y1993
time period are presented in Table 3; they were a few
millisievert each year during that time period.
Chernobyl nuclear reactor accident
The reactor was a graphite-moderated, light watercooled system known as RBMK-1000. With an installed
electrical generating capacity of 1,000 MW, it was used to
produce electricity for commercial purposes; it had started
operating in December 1983. The accident occurred on 26
April 1986 during a low-power engineering test of Unit 4.
Improper, unstable operation of the reactor, which had
design flaws, allowed an uncontrollable power surge to
occur, resulting in successive steam explosions, which
destroyed the reactor and part of the building in which
the reactor core was housed (UNSCEAR 1988, 2000,
2011b). It is the most severe accident that has ever occurred in the nuclear power industry.
With regard to occupational exposure, a distinction is
made between the emergency workers, including the
persons who were on the site during the day of the accident,
and the recovery operation workers, who performed a
variety of tasks at the site and in the 30-km zone surrounding the site from 1986Y1990. Among the emergency
workers, who included 374 reactor staff, 69 firemen, 113
guards, and 10 medical staff, two workers died in the
immediate aftermath and 134 reactor staff and firemen
suffered from acute radiation sickness (ARS) (Mettler et al.
2007; UNSCEAR 2011b). The distribution of the external
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Increased occupational doses c A. BOUVILLE
doses received by the workers with ARS and their outcomes are shown in Table 4. The external whole-body
doses that are presented in Table 4 are based on biological measurements and clinical symptoms, as the dosimeters worn by the reactor personnel were all overexposed,
and the firemen were not equipped with dosimeters. The
external whole-body doses ranged from 0.8Y16 Gy; most
of the 28 ARS victims who died within a few months
after the accident had received doses between 6.5 and
16 Gy. The skin doses resulting from beta exposures may
have been much higher than the external whole-body
doses; an evaluation of the skin doses for eight patients
showed that the skin doses ranged from 10Y30 times the
external whole-body doses (Barabanova and Osanov 1990).
On the other hand, the internal doses, based on whole-body
counting and bioassay measurements performed while the
patients were under treatment, were found generally to be
much smaller than the external whole-body doses (Mettler
et al. 2007; UNSCEAR 2000).
Following the acute emergency phase of the accident, È530,000 recovery operation workers were called
from 1986 to 1990 to carry out a variety of tasks, including the decontamination of the reactor block and of the
reactor site, and the construction of the entombment of
the reactor (known as object Shelter, sarcophagus, and
Ukrytie) (UNSCEAR 2011b). The enormous scale of
the problems that had to be faced necessitated a massive
engagement of several ministries of the former Soviet
Union, most notably the Ministry of Defense, the Ministry of Atomic Energy, and the Ministry of Medium
Machinery. The numbers of recovery operation workers
decreased from year to year, from È300,000 in 1986 to
È6,000 in 1990. Altogether, È200,000 recovery operation
workers were from Ukraine; È200,000 also from Russia;
È100,000 from Belarus; and È5,000 from each of the Baltic
countries (Estonia, Latvia, and Lithuania). The time spent
by the workers on the site was extremely variable but was
generally less than a year. The dosimetry of the recovery
operation workers, also called cleanup workers or liquidators, proved to be very challenging, in part because of the
very large number of workers originating from a variety
of organizations. The ‘‘official’’ doses presented in Table 5
are based on doses that were recorded for about half of
Table 4. Emergency workers of the Chernobyl nuclear reactor
accident with ARS (UNSCEAR 2010).
Degree of
severity
Mild
Moderate
Severe
Very severe
Dose range
(Gy)
Number of
workers
Number of
deaths
0.8Y2.1
2.2Y4.1
4.2Y6.4
6.5Y16
Total
41
50
22
21
134
0
1
7
20
28
AND
V. KRYUCHKOV
263
the recovery operation workers, using the assumption that
the mean doses obtained for the workers with recorded
doses apply to the entire population of workers. The
recorded doses, which represent external irradiation from
photons only, were usually obtained by means of one of
three methods:
1. reading of a personal dosimeter;
2. group dosimetry: assignment of the same dose to a
group of workers performing a given task, based on
the reading of a personal dosimeter worn by a member
of the group (in some cases, no member of the group,
in which case the dose was assigned on the basis of
previous experience); or
3. group estimation: crude time-and-motion analysis
(measurements of gamma-radiation levels were made
at various points of the reactor site, and the dose was
estimated as a function of the locations where work
was to be done and of the time spent at those locations)
(Pitkevitch et al. 1997; UNSCEAR 2000).
As shown in Table 5, the annual averages of the officially recorded external whole-body doses decreased
from 146 mGy in 1986 to 96 mGy in 1987 and 43 mGy in
1988 and remained approximately stable in 1989 (41 mGy)
and 1990 (47 mGy); the average annual official external
whole-body dose over the 1986Y1990 time period was
117 mGy. Neither the external doses to the skin nor to
the lens of the eye that were due to beta exposure nor the
internal doses due to intakes of radionuclides were recorded.
The extent to which the officially recorded wholebody doses are valid has been discussed in many publications (e.g., Ilyin et al. 1995; Pitkevitch et al. 1997;
Auvinen et al. 1998; UNSCEAR 2000; Chumak 2007;
Kryuchkov et al. 2012), mainly in relation to their usefulness in epidemiologic studies. The quality of the dosimetry depends to a large extent on the worker category
and on the way in which his or her dose was estimated.
As a result of the broad variety of the tasks related to
Chernobyl activities, the population of the recovery operation workers was very heterogeneous. From the point
of view of exposure conditions and dosimetric monitoring, the population of recovery operation workers can be
divided into the following categories (Chumak 2007;
Kryuchkov et al. 2012), presented in Fig. 2 in terms of
percentages of the total number of workers. It should be
noted that the numbers of workers given below for each
category are taken from Kryuchkov et al. (2012) and may
not be in complete agreement with those found in other
publications (e.g., UNSCEAR 2011b); they give, however,
a good idea of the population size of each category of
recovery operation workers:
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Health Physics
February 2014, Volume 106, Number 2
Table 5. Numbers of recovery operation workers and external dosesa (millisievert) to these workers as officially recorded in
national registries (based on UNSCEAR 2011b).
1986
1987
1988
1989
1990
Average dose
Number of workers
Ukraine
Russia
Belarus
Lithuania
Latvia
Estonia
186
127
57
49
51
151
229,219
149
89
35
34
39
107
188,174
60
28
20
20
V
51
91,000
144
108
43
50
28
109
6,960
146
106
31
45
55
117
6,065
109
111
32
45
V
44
4,832
All countries
146
96
43
41
47
117
526,250b
a
The external dose is expressed in millisievert for reasons of convenience. In fact, the quantity measured was, in many cases, exposure. It is
assumed that these external doses are representative of the whole-body doses.
b
The total includes 1,074 recovery operation workers with unknown year of exposure and five 1991 workers.
& Witnesses and victims of the accident (È2,000): the personnel of the Chernobyl Nuclear Power Plant (ChNPP)
and of other organizations, who were at the site at
the time of the accident or arrived at the site before 30 April 1986. This category includes the emergency workers previously discussed. Doses during the
period of exposure were not recorded for those workers.
The official doses, when available, are based on clinical
and biological monitoring performed while the workers
were hospitalized;
& Early liquidators (È21,600): civilian and military workers, who were used to decontaminate the site and the
30 km zone between 27 April and 31 May 1986. A
consequence of the chaotic situation at the ChNPP
site during the few weeks following the accident was
that all information related to reading personal dosimeters until mid-May 1986 was either inadequate or lost.
The official doses, when available, were based on a
conservatively applied time-and-motion analysis;
Fig. 2. Categories of recovery operation workers (percent).
& ChNPP personnel (2,358 in 1986; 4,498 in 1987):
professional atomic workers, charged with the control
of operations in Unit 4 and the operation of Units 1, 2,
and 3. Good quality dosimetric monitoring was established in mid-May 1986. However, all dose records
related to MayYJune 1986, presumably the period of
highest radiation doses, were lost and were never recovered. Personnel who started their work after June
1986 have adequate personal dosimetry;
& Sent to ChNPP (È2,000 in 1986; 3,458 in 1987): personnel temporally assigned to ChNPP from other nuclear power plants. The dosimetry system was identical
to that for the ChNPP personnel;
& Sent to the 30 km zone (31,000 in 1986; 32,000 in
1987): the most diverse category of workers, who were
involved in Chernobyl cleanup activities on a taskoriented basis and who visited the 30 km zone only
for the duration of their mission; to this category belong, for instance, drivers who delivered equipment
and supplies, specialists who were engaged to solve
specific technological problems, as well as all kinds of
inspectors and representatives of authorities, research
institutes, etc. The dosimetric monitoring system depended
on the task to be performed and was very uneven;
& AC-605 (21,500 in 1986; 5,376 in 1987): the personnel of
Administration of Construction (AC) No.605 Especialized
enterprise of the Ministry of Medium Machinery, which
was established for the purpose of construction of the
sarcophagus (‘‘Object Ukrytie’’)^. Except for the period
before mid-June 1986, the dosimetric management of
that category of workers was performed adequately by
means of personal dosimeters;
& Ukrytie personnel (several hundreds in 1990): personnel monitoring the sarcophagus. Dosimetric monitoring
was controlled by the ChNPP dosimetry service (personal dosimeters);
& IAE personnel (3,521 in 1988): personnel of the
Kurchatov Institute of Atomic Energy (IAE) and of
other organizations, involved in various activities inside the sarcophagus. The dosimetric monitoring of
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Increased occupational doses c A. BOUVILLE
that category of workers is similar to that of the AC-605
personnel, and, therefore, of high quality;
& Military workers (61,762 in 1986; 63,751 in 1987): the
most numerous category of recovery operation workers,
involved in most of the decontamination activities
(including manual removal of reactor debris from the
roofs of ChNPP in SeptemberYOctober 1986 and
JanuaryYFebruary 1987), as well as the demolition of
abandoned villages, transportation of contaminated
materials, etc. The military had its own dosimetric monitoring system, where each unit of battalion level had its
own radiation protection officer who kept the dose records and conducted liaison with the dosimetrists on
the spot. Due to lack of adequate personal dosimeters
(wartime dosimeters had a sensitivity threshold of
È100 mSv) and because of numerous cases of misuse
of the dosimeters, the group dosimetry and the groupestimation methods were used for dose assignment to
all team members, often numbering several dozen.
Both methods were applied in a conservative manner
(e.g., overestimated the doses);
& ‘‘Combinat’’ personnel (6,281 in 1987): civilian staff
permanently employed to perform and supervise all
offsite activities within the 30 km zone (e.g., activities
not related to the recovery and operation of the ChNPP
itself ), including mainly radiation monitoring, handling of radioactive waste, decontamination, and life
supporting infrastructure within the 30 km zone. Because of organizational problems, the dosimetric monitoring of that category of workers was largely not
conducted in 1986 and part of 1987. The quality and
completeness of the dosimetric information regarding the ‘‘Combinat’’ personnel and the visitors to the
30 km zone became adequate in mid-1987; and
& Belarusian workers (È24,000 in 1986; È28,000 in
1987): civilian personnel who worked in the Belarusian
part of the 30 km zone. Only 9% of the Belarusian
workers have official recorded doses.
In summary, the population of recovery operation
workers was extremely heterogeneous, with durations of
exposure that could vary from hours to years, locations
of work with very low to very high radiation levels, and
activities varying from manual removal of reactor debris
to working as a cook or in an office. Also, many types of
dosimeters (Fig. 3) were used, because the workers came
from a number of organizations with their own dosimetric
monitoring systems with little coordination. The quality
of the official recorded doses was relatively low in 1986
because the authorities were not prepared to monitor a
very large number of workers, but it improved with time
and was adequate after mid-1987. Unfortunately, the
AND
V. KRYUCHKOV
265
Fig. 3. Primary dosimeters used by the recovery operation workers.
highest doses were received during the first year following the accident.
A number of risk projections and epidemiologic
studies have been devoted to the risk of leukemia among
recovery operation workers (e.g., Ivanov 2007; Kesminiene
et al. 2008; Romanenko et al. 2008). The case control
studies sponsored by the International Agency For Research on Cancer (Kesminiene et al. 2008) and by the
National Cancer Institute (Romanenko et al. 2008) required the estimation of individual bone marrow doses
for all study subjects. Because external doses are not
available for about half of the recovery operation workers,
and because of doubts regarding the quality of the external doses for some categories of workers, it was decided
not to rely on the official recorded doses and to develop
a method of dose estimation that could be applied to all
study subjects, whether dead or alive, irrespective of their
radiation exposure. The method of dose estimation that
was selected for the case control studies sponsored by the
International Agency For Research on Cancer and by the
National Cancer Institute is a sophisticated time-andmotion analysis called RADRUE, which is an acronym
for Realistic Analytical Dose Reconstruction with Uncertainty Estimates (Kryuchkov et al. 2009). Briefly,
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266
Health Physics
results of exposure rate and nuclide deposition measurements were embedded in RADRUE and were used to derive
exposure rates at places where liquidators lived and worked
and thus to calculate external dose to bone marrow according to the liquidators’ itineraries. Liquidators’ routes were
reconstructed by dosimetry experts familiar with the organization and conditions of work in the 30-km zone based
on information obtained through a study questionnaire
(Kryuchkov et al. 2009).
The individual bone marrow dose estimates, along
with their uncertainties, that were obtained for the 572
subjects of Phase I of the study of leukemia among the
Ukrainian recovery operation workers (Romanenko et al.
2008) are presented in Table 6. In that table, the workers
have been classified according to the worker categories
given above. The individual bone marrow dose estimates
vary in a large range, from G0.01 to 3,260 mGy (even in
the same category of workers; for example, the military
worker category or sent to the 30-km zone category). The
variability of the doses can be important in comparing
differing results. The overall average bone-marrow dose
obtained for the Ukrainian study subjects is 87 mGy,
which is somewhat lower than the estimated average wholebody dose of 117 mGy for all recovery operation workers
(UNSCEAR 2011b). Kryuchkov et al. (2012) compared
the official doses and the doses calculated by RADRUE
for the 1986 recovery operation workers from Ukraine,
Russia, and the Baltic countries, and concluded that:
& the RADRUE-calculated dose distributions for the
red marrow are wider than the distribution of the
official doses;
Table 6. Individual mean bone-marrow dose estimates (milligray)
for the recovery operation workers included in the leukemia study of
Ukrainian recovery operation workers (Chumak et al. 2008).
Category
Witnesses of
the accident
Victims of
the accident
Early liquidators
ChNPP
personnel
Sent to ChNPP
Sent to the
30 km zone
AC-605
IAE personnel
Military workers
Combinat
personnel
Mixedb
All
a
Number of Average Minimum Maximum Average
workers
dose
dose
dose
GSDa
3
160
38
377
2.3
2
2,880
2,580
3,170
3.4
66
9
97
234
0.5
23
1,010
966
2.0
1.7
1
181
44
30
44
G0.01
44
694
1.9
2.0
5
2
220
4
110
129
71
16
1
15
0.01
3
295
242
554
45
2.0
2.1
2.1
1.7
79
572
164
87
0.40
G0.01
3,260
3,260
1.7
2.0
GSD = geometric standard deviation.
Belonged to more than one category of workers.
b
February 2014, Volume 106, Number 2
& the mean values of the RADRUE-calculated doses
for the 1986 workers from Ukraine, Russia, and the
Baltic countries are lower than the mean value of the
official doses for the 1986 workers, the average ratio
being 0.6; and
& a larger percentage of doses exceeding 250 mGy is
predicted by the RADRUE calculations.
Similar conclusions had been reached previously by
Ilyin et al. (1995). Also, Littlefield et al. (1998) performed
a cytogenetic analysis of lymphocyte cultures from 118
Estonian workers (mean recorded dose of 103 mGy and
maximum recorded dose of 250 mGy) and concluded
that it is likely that recorded doses for these workers
overestimate their average bone marrow doses, perhaps
substantially. Finally, Pitkevitch et al. (1997) performed a
statistical analysis of the doses registered for the Russian
workers and did not find a major evidence of unreliability,
but they recommended the use of a time-and-motion
analysis to verify the individual values.
In addition to whole-body doses from external gamma
irradiation, the workers received doses to the skin and to
the lens of the eye from external beta irradiation. In a
cohort study of cataracts among Ukrainian Chernobyl
recovery operation workers (Worgul et al. 2007), individual doses from beta particles had to be added to the
external doses from photons for the 8,607 cohort members.
The dose reconstruction process was very complex
(Chumak et al. 2007). In summary and in a simplified way,
it involved:
& the recalibration of the official doses from photons,
which was obtained by means of a comparison of a
high-quality set of EPR measurements of workers’ teeth
against their official dose records;
& the assessment of the beta-to-gamma dose ratios for a
variety of exposure scenarios, which was obtained using
Monte Carlo calculations; and
& the distribution of the official doses according to a
range of work locations with similar beta-to-gamma
dose ratios, which was derived from a questionnaire
addressed to the study subjects.
It was found that:
& the official external doses were biased high by a factor
of È2.2;
& the beta-to-gamma dose ratios varied as a function of
time after the accident but were similar for all work
locations; and
& the beta-to-gamma dose ratios varied substantially from
one worker to another, with È32% of the study subjects
with beta doses as large as or larger than the gamma
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Increased occupational doses c A. BOUVILLE
Table 7. Distribution of the lens of the eye dose (milligray) among
the Ukrainian Chernobyl recovery operation workers enrolled in the
cataract study (Worgul et al. 2007).
Dose range (mGy)
0 Y 49
50 Y 99
100 Y 199
200 Y 399
400 Y 699
700+
All
Number of subjects
1,300
1,550
3,776
1,431
364
186
8,607
doses and È56% with beta doses less than half as large
as the gamma doses (Chumak et al. 2007).
The overall distribution of the total doses (beta +
gamma) is presented in Table 7. The median dose to the
lens of the eye was estimated to be in the range from
100Y200 mGy, while È200 subjects had estimated doses
exceeding 700 mGy.
Because of the abundance of 131I and of shorter-lived
radioiodines in the environment of the reactor during the
accident, the workers who were on the site during the first
few weeks after the accident may have received substantial
thyroid doses from internal irradiation. Kesminiene et al.
(2012) performed a case-control study among 530 workers
from Belarus, Russia, Estonia, Latvia, and Lithuania with
the objective of evaluating more precisely the relationship between the dose to the adult thyroid from both external irradiation and internally incorporated 131I and risk
of thyroid cancer. For each study subject, individual doses
to the thyroid were reconstructed by considering the following pathways of exposure: (1) external irradiation from
gamma-emitting radionuclides; and (2) internal irradiation arising from the intake of 131I via inhalation of contaminated air or ingestion of contaminated foodstuffs.
To estimate the external doses received by the workers
during their cleanup missions, the RADRUE method, which
had been selected for the leukemia studies (Kesminiene
et al. 2008; Romanenko et al. 2008), was also used. The
main difficulty resided in the estimation of the thyroid
doses resulting from intakes of 131I. Internal doses due to
inhalation intake of 131I during the period of work as a
liquidator were calculated for six study subjects who
worked on the ChNPP site during the first few weeks after the accident. The approach was based on the data on
concentration of 131I in air in settlements in the 30-km
zone (Prohl et al. 2000) and was validated by measurements of the dose rate near the neck taken in 30 AprilY5 May
1986 in a group of 624 early liquidators who were not
study subjects. In addition to the dose received during
their work, Belarusian workers who were residents of
contaminated settlements of the Gomel and Mogilev oblasts and were returning home every evening or after
AND
V. KRYUCHKOV
267
weekly shift work may also have received substantial dose
to the thyroid from 131I through consumption of locally
produced contaminated milk and/or vegetables. The socalled ‘‘residential’’ doses from intakes of 131I were estimated up to 20 June 1986, whereas those from external
irradiation were estimated for the entire period of work
as a liquidator. Residential doses were not calculated for
Russian and Baltic workers, as they are thought to have
consumed foodstuffs that were produced in and imported from non-contaminated areas. The estimated thyroid
doses from external and internal irradiation are presented
in Table 8. The average external dose received by the
Belarusian subjects, who worked away from the reactor
site in the Belarusian part of the 30-km zone, is much
smaller than that received by the Russian and the Baltic
subjects, who worked at the ChNPP site. On the other hand,
the average internal dose received by the Belarusian subjects was much higher than those due to the Russian and
the Baltic subjects, because the Belarusian subjects consumed locally produced milk and/or leafy vegetables that
were contaminated with 131I.
Fukushima accident
The Fukushima-Daiichi Nuclear Power Station (FDNPS)
of the Tokyo Electric Power Company (TEPCO) consisted
of six units of boiling-water reactors, with a total power
generating capacity of 4,700 MWof electricity. Unit 1 started
operating in 1971, Unit 2 in 1974, Unit 3 in 1976, Units 4
and 5 in 1978, and Unit 6 in 1979. On 11 March 2011, an
Table 8. Arithmetic means, medians and ranges of estimated external
doses, internal doses from 131I to the thyroid, and total doses (milligray)
for the recovery operation workers included in the thyroid study by
country and by case/control status (based on Kesminiene et al. 2012).
Belarus
Number of workers
Arithmetic mean
Median
Range
Russian Federation
Number of workers
Arithmetic mean
Median
Range
Baltic countries
Number of workers
Arithmetic mean
Median
Range
All countries
Number of workers
Arithmetic mean
Median
Range
External dose
Internal dose
Total dose
420
17
6.5
0.11Y454
412
183
63
0.2Y3253
420
191
68
0.24Y3307
77
101
63
0.13Y507
1
73
73
73
77
102
69
0.13Y507
33
79
55
1.9Y488
0
V
V
V
33
79
55
1.9Y488
530
33
10
0.11Y507
413
182
63
0.18Y3253
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530
171
68
0.13Y3307
268
Health Physics
earthquake of magnitude 9.0, the largest ever recorded in
Japan, occurred along the Japan Trench. The earthquake
created a series of tsunami waves that struck the east coast
of Japan. The earthquake and the tsunamis knocked out
the power supply to the FDNPS and, consequently, the
means to control and cool the reactor. In the days that followed, reactor meltdown and hydrogen gas explosions resulted in serious damage to the facility (WHO 2013).
Similarly to the Chernobyl nuclear reactor accident,
occupational exposure to radiation of the FDNPS workers
included external irradiation from radiation sources
within the damaged reactor and radioactive material deposited in the workplace, and internal irradiation from
inhalation of radioactive material. The assessment of
the radiation doses is under the responsibility of TEPCO.
Doses from external irradiation were mainly monitored
using alarm personal detectors that were changed every
day. However, because there was a shortage of monitoring equipment at the early stage of emergency response,
groups of workers were provided with a single personal
dosimeter, and the resulting measurements were taken
to be representative of the external doses received by all
members of the group (WHO 2013). Once monitoring equipment was available for all workers, external dose
assessment was based on the alarm personal detector
measurements. The distribution of the monthly effective
doses from external irradiation received by the TEPCO
workers and their contractors in March and April 2011
is presented in Table 9 (TEPCO 2011). The external effective doses are found to be, on average, lower in April
than in March by a factor of about three for the TEPCO
workers and two for the contractors. The average doses in
March 2011 were 19 mSv for the TEPCO workers and
6 mSv for the contractors, while the maximum dose was
200 mSv for both the TEPCO workers and the contractors. It is worth noting that other categories of workers
who may have been exposed to radiation during the response to the accident (e.g., rescue workers, firemen,
policemen, etc.) are not included in Table 9 (WHO 2013).
February 2014, Volume 106, Number 2
The internal dose assessment was based on in vivo
measurements performed with whole-body counters
(WBCs). The Japan Atomic Energy Agency and the
National Institute of Radiological Sciences cooperated
with TEPCO to assess the occupational doses during the
first few months after the accident (Kurihara et al. 2012;
Nakano et al. 2012; Takada et al. 2012). The initial screening
was performed using WBCs equipped with plastic scintillators. In a second step, WBCs with sodium iodide scintillators were used to identify the radionuclides present in
the body of workers with a predicted internal dose 920 mSv.
Finally, WBCs with germanium semiconductor detectors
were used for more precise measurements on workers with
predicted internal doses 9250 mSv (WHO 2013). The distribution of estimated internal effective doses is shown
in Table 10 (WHO 2013). Among the population of 23,000
workers that was considered, only 12 TEPCO workers
and no contractors were found with estimated effective
doses 9100 mSv. TEPCO concluded that workers with
the highest internal doses were those working in a central
control room; for these workers, 131I was by far the major
contributor to the internal dose. Consequently, the thyroid
doses are very high for those workers and are estimated to
be 910 Gy for two workers and in the range from 2Y10 Gy
for the other 10 (WHO 2013). Because of the short physical half-life of 131I (È1 wk), practically all of the thyroid
doses had been delivered by the end of April 2011.
The cumulative effective doses from external and
internal irradiation to the È25,000 Fukushima workers
(TEPCO and contractors) that were involved in mitigation
activities during the time period from March 2011 through
December 2012 are presented in Table 11 (TEPCO 2011).
Although many more contractors than TEPCO workers
had been involved in mitigation activities up to the end of
2012, the TEPCO workers received most of the effective
doses 9100 mSv (161 versus 20 for the contractors). The
average effective dose was 25 mSv among the TEPCO
workers and 10 mSv among the contractors. A comparison
of the effective dose estimates presented in Table 11 on
Table 9. Distribution of monthly effective doses (millisievert) from external irradiation to Fukushima workers: March
2011 and April 2011 (TEPCO 2011).
Effective dose
range (mSv)
0 Y 10
10 Y 20
20 Y 50
50 Y 100
100 Y 250
9250
All
Maximum
Average
TEPCO workers
Contractors
Total
March 2011
April 2011
March 2011
April 2011
March 2011
April 2011
689
560
274
108
26
0
1,657
182
19
1,397
173
58
1
0
0
1,629
60
6
1,553
323
146
95
11
0
2,088
199
9
3,479
485
234
24
0
0
4,123
85
5
2,242
883
420
163
37
0
3,745
199
14
4,876
658
193
25
0
0
5,752
85
5
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Increased occupational doses c A. BOUVILLE
the one hand and in Tables 9 and 10 on the other shows
that the highest doses were received during the first 2 mo
following the accident (March and April 2011).
AND
V. KRYUCHKOV
Table 11. Distribution of cumulative effective doses (millisievert)
to Fukushima workers: March 2011 through December 2012.
Effective doses from external and internal irradiation were added
(TEPCO 2013).
Dose range (mSv)
SUMMARY
Increased occupational exposures that resulted from
the four major reactor accidents that happened since the
beginning of the nuclear era and from the early operations
of two plutonium-production facilities for military purposes are presented and discussed in this paper. Particular
attention is paid to the increased occupational exposures
resulting from the Chernobyl nuclear reactor accident
that occurred in Ukraine in April 1986, the reactor accident of Fukushima that took place in Japan in March
2011, and the early operations in the 1940s and 1950s of
the MPA, which is located in Russia.
The Chernobyl nuclear reactor accident is the most
serious that ever occurred in the nuclear industry. In addition to the È800 emergency workers involved during
the first few days after the accident in firefighting and
closing down unaffected units of the power plant, 9500,000
cleanup workers took part in 1986Y1990 in the mitigation
of the consequences of the accident, including decontamination and construction of the sarcophagus. Among the
emergency radiation workers, special attention is paid to
the 134 persons who had been diagnosed with ARS: They
received bone marrow doses due to external gamma radiation ranging from 0.8 to 16 Gy. The average effective dose
received by the 530,000 cleanup workers, also called liquidators or recovery operation workers, was mainly due to
external irradiation and is estimated to have been È0.12 Sv.
The recorded worker doses varied from G0.01 to 91 Sv,
although È85% of the recorded doses were in the range
from 0.02Y0.5 Sv.
The accident at FDNPS was the consequence of an
earthquake of magnitude 9.0, which triggered a major
tsunami that submerged the emergency diesel generators,
resulting in serious damage to the reactor. From the time
of the accident until the end of 2012, È25,000 workers
were involved in activities on the reactor site; six of those
workers received effective doses (external + internal)
Table 10. Distribution of effective doses (millisievert) from internal
irradiation to Fukushima workers (WHO 2013).
Dose range (mSv)
0Y5
5 Y 10
10 Y 20
20 Y 50
50 Y 100
100 Y 250
9250
All
TEPCO workers
Contractors
Total
4,655
316
425
194
37
7
5
5,639
16,636
424
337
94
42
0
0
17,533
21,291
740
762
288
79
7
5
23,172
269
0Y1
1Y5
5 Y 10
10 Y 20
20 Y 50
50 Y 100
100 Y 250
9250
All
Maximum
Average
TEPCO workers
Contractors
Total
661
822
165
708
599
527
140
6
3,628
678
25
6,163
5,823
2,912
3,316
3,032
503
21
0
21,770
238
10
6,824
6,645
3,077
4,024
3,631
1,030
161
6
25,398
678
12
90.25 Sv, 167 workers received effective doses 90.1 Sv,
and about two-thirds of the workforce received effective
dose equal to or below 0.01 Sv.
The MPA was the first industrial complex in the former Soviet Union built for the production of plutonium.
The complex included reactors, chemical processing
plants, and plutonium production facilities. In the early
years, there was poor understanding of the consequences
of relatively high occupational radiation exposures. The
highest external gamma doses were recorded in 1948Y1952;
i.e., during the start-up and adaptation phase of the reactor and radiochemical plants. Average values of annual
doses amounted to 1 Gy, and maximum individual annual
doses were up to 8 Gy. High internal doses were due to the
exposure to plutonium.
Both for the Chernobyl nuclear reactor accident
and the routine operations at Mayak, the considerable efforts made to reconstruct individual doses from external
irradiation to a large number of workers revealed that
the recorded doses had been overestimated by a factor of
about two.
AcknowledgmentsVThe authors thank Ethel Gilbert and Vladimir Drozdovitch
(National Cancer Institute), Kazuo Sakai and Shin Saigusa (National Institute of
Radiological Sciences), Derek Hagemeyer (Oak Ridge Associated Universities), and Ellen Anderson (Nuclear Energy Institute) for providing information
on occupational doses that is not readily available in the open literature.
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