Results of the IAEA personal dosimetric measurements programme in support of the international Chernobyl project
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The film dosimeters produced by KODAK US Type 3 were distributed among residents of seven settlements of the three republics affected by the Chernobyl accident. Based on the testimony of 5641 dosimeters, it can be expected that 90% of the population received less than 1.2 mSv during 1990 - the year of measurement. Less than 2.5% should have received an annual dose of more than 6 mSv and only 11 people received irradiation at doses over 27 mSv. The data obtained on full-body counters for 9058 inhabitants show the range of average annual doses of internal exposure from 0.11 mSv to a maximum value of 0.99 mSv in a small settlement in Ukraine. The range reflects not only the variation in the values of local contamination with cesium, but also the differences between settlements in the restrictions on the consumption of certain products. The results indicate that during the measurement program, internal exposure made a major contribution to individual doses. Although the relevant data are not presented in the work, but the results obtained are generally consistent with the results published by Soviet scientists. The purpose of the comparison of indicators for total body counters was to assess the overall quality of the measurements, primarily 134Cs and 137Cs, produced on devices in the areas affected by the Chernobyl accident. It should be remembered that the comparison was carried out 4 years after the accident. Therefore, the results do not necessarily reflect the quality of measurements made during the first months after the accident. The IAEA does not have specific criteria for evaluating the functioning of total body counters. However, the quality of the comparison results can be correlated with the requirements contained in the IAEA Safety Series No. 84, Basic Principles for Occupational Radiation Monitoring [6], paragraph 4.1.5., Which states that: In the case of routine individual monitoring of external radiation, the relative uncertainty from -50% to + 100% at a 95% confidence level is acceptable for annual dose equivalents in the region of one-fifth of the required limit. If, however, the values are close to the annual limits, the relative uncertainties should not exceed -33% and + 50% at the 95% confidence level ..... Similar requirements should, in principle, also be applied in the case of routine individual monitoring for internal exposures, but in practice such small uncertainties as 50% are rarely possible. In 5 of the 36 measurements carried out at the institutes of the USSR (Table 8), the results were outside the range of ± 30% as compared with the reference values. One result was slightly more than 50% relative to the reference value. In the latter case, the measurement was made on an unshielded sample using a phantom twice the position of the Marinelli. Based on the distribution of the results of the comparison, it can be reasonably concluded that the institutions involved in the comparison are capable of measuring internal cesium with an accuracy that is acceptable and adequate for radiation protection purposes. It should also be noted that in 4 cases out of 5, when the results were outside the range of ± 30% of the reference value, measurements were made on phantoms representing children. Although the differences are not too large, they indicate the need for special attention to the calibration of meters intended for measuring children.
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Текст научной статьи Results of the IAEA personal dosimetric measurements programme in support of the international Chernobyl project
The accident at Unit 4 of the Chernobyl nuclear power plant occurred on 26 April, 1986. In October, 1989 the Government of the former USSR requested the International Atomic Energy Agency to organize and co-ordinate an assessment of the guidance given by the Soviet authorities following the accident to persons living in radiologically contaminated areas and an evaluation of measures to safeguard the health of the population.
As part of the international assessment programme, two projects were implemented by the IAEA to provide independent measurements of the external and internal doses received during the period of the assessment programme by individuals in selected settlements. A total of 12,000 personal dosimeters were distributed to evaluate the external dose to the population, and approximately 9,000 whole body measurements were made.
Moreover, during a mission to the Soviet Union in August 1990 it was concluded that an intercomparison of USSR, IAEA and of the Austrian whole body counters used for in vivo measurement of 137Cs would be valuable in corroborating large scale measurements of the USSR population. The IAEA arranged to obtain use of a standard, adult phantom from the United States for this intercomparison. Additional phantom sets from the Leningrad Institute of Sea Transport Hygiene and the All-Union Scientific Centre of Radiation Medicine, Kiev, were later included in the intercomparison.
External Dosimetry
External dosimetry was carried out under the limited project resources and within a short time scale. As a result, the measurements were not as extensive and sensitive as the Project scientists would have preferred. The results, therefore, only provide an indication of the maximum exposure that would have been received by the majority of the monitored population at the time of the measurements.
Film badge dosimeters were provided by the Service Central de Protection Contre les Rayonnements Ionisants (SCPRI) in France for use in the independent assessment of external doses. Three sets of dosimeters were distributed in the UkrSSR, BSSR, and RSFSR. The first set of 4000 dosimeters was distributed in seven settlements in May 1990 and collected in July 1990. At the time of collection an additional 4000 dosimeters were distributed. These were collected in October 1990. The third set was distributed in December 1990 and collected in February 1991.
The dosimeters were brought to each settlement by members of the international project, who explained the external dosimetry project to the population. The dosimeters were distributed to individuals selected by the local authorities in each settlement on the basis of predetermined criteria. In addition to the original seven settlements that were included in the first set, dosimeters were distributed in six settlements that were chosen as low radiation control areas in which independent project medical examinations were performed by the medical staff of the Chernobyl Project.
The most important criteria in measuring the dose to a representative sample of the population for each location surveyed were sex, age and occupation. The personal dosimeters were distributed to an approximately equal proportion of men and women. It was intended that individuals of all ages would be provided with dosimeters. However, at the first and second distribution periods most of the children were not present in the settlements because they had been sent to summer camps outside the affected areas. Occupation was also an important factor because of different times spent working indoors or outdoors.
The participants were instructed to carry the dosimeters in a pocket in their clothing in the upper half of the body, and to place it by the bedside while they slept. This procedure was to be continued until the dosimeters were collected.
The film type used in the dosimeters was KODAK US Type 3. This film has two emulsions on one film. Filters of copper (0.2 mm) and lead (1 mm) were used. The energy response is ±10% from 500 to 1000 keV. The detection limit of this method is 0.2 mSv, and the accuracy is consistent with ICRP recommendations (33% to +50%). The films were processed by conventional manual methods at SCPRI and the results were reported to the IAEA laboratory for evaluation.
For the first two sets, 7961 dosimeters were distributed to individuals or the local authorities and 5979 dosimeters were returned to SCPRI for reading. A set of dosimeters was used for background dose measurements or testing. In some cases, insufficient information on the individual was provided to allow inclusion of the datum. Table 1 presents the summary results for the 5641 personal dosimeters exposed for approximately two months in the selected settlements, divided into four ranges of dose.
The higher measurements, in most cases, could be attributed to the individual living in a slightly more contaminated area, or were due to their profession (i.e., forest worker). In five cases analysis of the patterns on the film indicated that the dosimeters had been exposed to x rays, probably intentionally. A few outliers of higher exposures were measured. These could not be explained by the external dose rate measured in the areas. It is suspected that these were from 'hot spots' in the environment or the individuals wearing them spent long periods of time outdoors.
Internal Dosimetry
During the period 5 July to 7 September 1990 a whole body counting campaign was conducted in the BSSR, RSFSR and UkrSSR. Over 9,000 measurements were made, using a mobile whole body counting van provided to the project by the SCPRI, France (Fig. 1). The measurements included (a) reference measurements of a calibration phantom, as well as of van staff, for quality control, and (b) replicate measurements. Some of the measurement results were found to be in error. Therefore, the results for 9,058 people are reported here. The counting locations, dates and number of people counted are summarized in Table 2.
The mobile van is equipped with four chair counters. Each counter has a 7.62 x 7.62 cm cylindrical NaI crystal housed in a collimated lead shield (Fig. 2). The person is positioned for counting so that the shield is centered on the chest, over the region of the lungs, and in contact with the body. The counting period was 5 minutes. The background counting rates of the counters were determined by inserting a conical plastic plug into the collimator.
During counting, the person being measured provided some self shielding, thus reducing the counter background. Unfortunately, the degree of self shielding was variable, depending on the mass of the person. This is a particularly a problem in evaluating results for small children. The counting procedure as established by SCPRI did not make provision for corrections based on body mass.
The data were processed with a Canberra S35 multichannel analyser connected to an Amstrad portable computer using software developed at SCPRI. Results were recorded on diskette and displayed on computer printout. The SCPRI data-unfolding process is intended to accommodate up to three radionuclides. However, the counting statistics were often poor, so only results for 137Cs are presented in this report. From information provided by whole body counting specialists in Kiev and Minsk, based on their counting results, the ratio of 137Cs to 134Cs was approximately 6.5. Results of this ratio for an environmental sample, dried green peas grown in the Chernobyl region and assayed in December 1990, ranged from 7.9 to 8.8. This would be equivalent to 7.2 in August 1990, the mid-point of the whole body counting project.
The limit of detection (LOD) as defined by the data-processing software of the mobile van is three times the background count standard deviation. Because of the high variability in background from one town to another, the LOD is variable. In addition, the efficiency depends significantly on the size of the person being counted. Therefore, a single value cannot be quoted. However, a typical LOD for an adult is about 0.74 kBq
(0.02 ^ Ci), while for a small child the value drops to about 0.19 kBq (0.005 ^ Ci).
Since the mobile van was designed for operational emergency-response applications, the calibration is based on a 70-kg reference man, 170 cm tall. The activity, A (in Bq), had normally been determined by the SCPRI staff using the following calibration:
Countrate
A = , (1)
0.0003 x CF where CF is the correction factor for body size:
CF = 2.088 - (1.695 x BSF), (2)
where BSF is the body size factor: [Weight/Height]1/2 (weight in kg and height in cm). CF = 1 for reference man.
The design of the counters in the counting van is such that only activity in the torso is detected. For adults this means that the mass of tissue seen by the counter is roughly constant. Following a review of the counting procedure, it was concluded that the calibration was reasonably accurate in the weight range 50 to 90 kg. However, for individuals outside that range the results could be significantly in error. For a child weighing 20 kg with a height of 100 cm, for example, the original calibration would overestimate the caesium burden by a factor of 2.6. Therefore, it was recommended that a modified correction factor based only on weight be used [2]:
CF = 70
Weight(kg)
.
The original and modified correction factors are displayed graphically in Fig. 3. All data presented in this report have been derived using the revised calibration factors.
During the measurement programme, a plastic cylindrical phantom provided by SCPRI was used to check the counting efficiency on a daily basis. In addition, a few members of the van staff had measurable levels of 137Cs. They were also counted at regular intervals. These results are presented in Table 3. Three of the people who were routinely counted had been checked by the IAEA whole body counter in Seibersdorf, and these results have been taken to be the reference values.
At the end of the counting programme, the van returned to Seibersdorf. At that time, the calibration of each counter was checked with (a) a standard bottle phantom obtained from the Battelle Pacific Northwest Laboratory in the USA and (b) two liquid filled manikins from Salzburg University simulating an adult female and a child. The bottle phantom contained 11.2 kBq 137Cs in a solid polyurethane tissue substitute. Since the Salzburg phantoms had activity levels that were too low to be useful, those additional results are not presented here.
Summary statistics and internal dosimetry results for the nine settlements in the BSSR, RSFSR and UkrSSR are presented in Tables 4-6. These include the number of persons counted, weight, age, total body burden, specific body burden (body burden/weight), and estimated annual dose based on specific body burden. Since the measurements were made at only one time, it is impossible to determine time dependent changes in the internal body levels. Therefore, a constant intake was assumed.
A conversion factor for specific body burden to dose rate of 2.5 µ Sv/a per Bq/kg was used to calculate annual dose [3]. The annual dose in future years will be reduced in proportion to the reduction in environmental caesium levels.
It can be expected that the results for a given population will have a log normal distribution, i.e., the log of the variable x is normally distributed. In this case, the variable is the specific body burden. Comparison of the distribution of specific body burden for each village with an estimate of the best fit for a log normal distribution is presented in Fig. 4. It is clear that the quality of agreement varies from village to village, and that, in some cases, non-statistical factors influence the results. One such influence is the assignment of the value of the detection limit to those measurements at or below that limit. The practice obviously biases the low activity results upwards.
This effect can be seen more clearly in the cumulative normal probability plots of the log of the specific body burden shown in Fig. 5. The overlying straight lines represent the distribution that would be expected for a population having a log normal distribution without additional influences. These plots also demonstrate deviation from the expected distribution at higher values of the specific body burden. The reason for this deviation has not been definitely identified. A possible explanation is that it is the result of a small subset of the population that does not observe the dietary restrictions imposed by local authorities, or that dietary habits (such as eating large quantities of forest mushrooms) predispose members of the population to higher body burdens.
The distributions of annual doses are derived from the specific body burden multiplied by the constant dose factor. There is, thus, no need to illustrate these results. Age-dependent distribution is an important concern. However, the scatter plots shown in Fig. 6 do not indicate a strong age dependence.
Intercomparison of Whole Body Counters
The IAEA arranged to obtain use of a standard, adult phantom from the Battelle Pacific Northwest Laboratories in the United States. The phantom is a "Bush" or "Bottle" type [4, 5] filled with solid, polyurethane tissue substitute and labelled uniformly with 137Cs. The total quantity at the time of the intercomparison was 11,170 Bq (0.3 µ Ci). A solid matrix was necessary to avoid the practical problems of handling radioactive liquids during transport. Although accurate measurement of the caesium level in children is a major concern, it was not possible to locate a standard child phantom with a solid matrix.
Counting was performed in the SCPRI counting van, with the IAEA chair counter and with a whole body counter of the Austrian Research Centre during the week starting 17 September 1990. The phantom was then taken to the USSR and used there in various institutes. The intercomparison programme was completed in December 1990 with a final counting of two USSR phantom systems at Seibersdorf. The USSR institutes that participated in the intercomparison programme and the counter characteristics are listed in Table 7.
At the invitation of the IAEA, two USSR phantoms were brought for counting to Seibersdorf in December 1990, namely a block phantom from the Leningrad Institute of Sea Transport Hygiene and another phantom in the form of a bag of dried peas from the All-Union Scientific Centre of Radiation Medicine. Equivalent configurations from both phantom systems were counted, representing (a) a small child, (b) a child of about age 10, and (c) a small adult. In addition, samples of the dried peas used in the Kiev phantom were assayed. Counting was done both in the IAEA chair counter and the counter of the Austrian Research Centre. It must be noted that the IAEA counter is intended only for adults, and it is not specifically calibrated for children. The Austrian Research Centre also normally only counts adults. Therefore, the results of the measurements made using the child phantoms should be viewed accordingly.
The results of the phantom intercomparison measurements are presented in Table 8. Under the conditions of the intercomparison, it was agreed that the results from the USSR counters would not be specifically identified. Therefore, the participating institutes in the USSR are indicated only by numbers in Table 8. However, each USSR facility has been provided a tabulation of the intercomparison results together with specific identification of its own data.
