Atomic Energy. Vol. 82. No. 2, 1997
COMMUTER
SYSTEM AT THE CHERNOBYL ATOMIC POWER
PLANT
E. A. lvanov, O. A. Kochetkov, A. G. Tsov'yanov, A. V. Nosovskii, and N. G. Chaban
UDC 621.039.8:614.8
Continued operation of the Chernobyl atomic power plant after the 1986 accident calls for appropriate radiation-safety measures, including the organization of a system to transport the workforce from Slavutich to the power plant and back, with minimal risk of exposing the city's population to unsafe radiation levels. It has been established that the radioactive contamination of the ground and the workers' protective clothing is significandy different within the Chernobyl plant itself and in the surrotinding exclusion area, especially within a 10-km radius of the plant. Radioactive contamination usually occurs predominandy through contact and less often through the incidence of liquid or solid particles containing radioactive materials from the air. In the exclusion area, however, the role of incidence from the air is significantly increased. Analysis of the complex radiation conditions in the region of the Chernobyl power station after the accident indicated that, to ensure radiational safety of the workforce in commuting between the power station and Slavutich, a full-scale radiational checkpoint meeting all health and safety standards should be built at the border of the exclusion zone. In this case, the workers traveling from Slavutich to the Chernobyl plant would leave the clean electric train at the Peresadochnaya station and then change into an intermediate set of safety clothing and continue to the station on so-called dirty trains. At the plant, the workers must work in the intermediate set of protective clothing or in the basic protective clothing after changing at a second set of safety checkpoints. This is the classical approach to protecting the population of Slavutich from exposure to radiation introduced from the Chernobyl plant, by means of an effective set of safety barriers. However, it has significant deficiencies associated with the lack of infrastructure at the Peresadochnaya station that would expedite the operation of the safety checkpoint. Accordingly, a system with two safety checlcpoints - at Slavutich and outside the power station site - was developed (Fig. 1). The construction of Slavutich for the Chernobyl staff and their families permitted the restoration of a standard powerstation operating schedule at the Chernobyl plant. Because of the distance between Slavutich and the plant, a special electrified rail line was reconstructed and went into operation in early 1988, conveying workers from Slavutich via Peresadochnaya to the Semikhody safety checkpoint and back. Technogertic transfer of radioactive material from the exclusion zone to Slavutich may occur by three means: on the outer surface of the train, on the workers' protective clothing, and on the workers' hair and skin. To ensure safe operation of the commuter line, five checkpoint modules with a total capacity of 5500 people were constructed at Slavutich at the end of 1988; in this facility, the workers changed into their traveling clothes, and four electric trains conveyed them at intervals to the Peresadochnaya station in Belarus. These are conventionally called the clean trains. The transfer took 5 minutes. So-called dirty trains carried the workers to Semikhody station, where they changed their traveling clothes to intermediate clothes in another radiation checkpoint. The Peresadochnaya station played an important role in the safety system, by obviating the need for the dirty trains to travel to Slavutich, except for occasional inspection and maintenance at the Chemigov railroad depot. To ensure that the trains leaving the exclusion zone were clean, the construction of a roiling-stock cleaning facility in Belarus was proposed. This facility became an important component of the safety system, and is still in operation today. All-Russian Scientific-Research Institute of Atomic Power Stations. Russian Institute of Biophysics. ChAES Production Combine. Slavutich Health Commission. Translated from Atomnaya t~nergiya, Vol. 82, No. 2, pp. 140-146, February, 1997. Original article submitted August 27, 1996. 140
1063-4258/97/8202-0140518.00
9
Plenum Publishing Corporation
Slavutich I
I
I
Fig. I. Safety system on the Chernobyl-Slavutich route in 1988. Thus, this system provided a reliable barrier to the transfer of radioactive material from the exclusion zone to Slavutich. Efforts to optimize the radiation-safety aspects of the commuter system must take into account that, thanks to natural processes and cleanup efforts, the dosimetric characteristics in the area around the plant are steadily improving; another important consideration is the role of workers' psychological, social, and behavioral patterns. Optimization may involve the introduction of more barriers in the system. Perhaps the most relevant factors in optimizing the system are the financial cost, the probability that individuals with above-threshold radiation contamination will slip through the system, subjective attitudes to radiation risk factors, and individual estimates of the effectiveness of the safety system. The effectiveness of the system may be evaluated by means of statistical estimates of the B contamination of workers' traveling clothes and skin in the cases with (1990) and without (1991) transfer at the Peresadochnaya station. It follows from Table 1 that B contamination of the traveling clothes is the same (to within 15-20%) in either case. Skin contamination (Table 2) in the case without transfer is approximately half that in the case with transfer. We would expect that the additional protective barrier provided by transfer should increase the overall effectiveness of the safety system and correspondingly reduce the radiation levels. However, the unfortunate location of the Peresadochnaya station at a point with high B levels (20-40 particles/min.cm2) not only eliminates the protective effect but can even serve as a source of additional B contamination of the external surfaces of the train and the workers' clothes, shoes, and skin. These results and analysis of the radiation conditions in the so-called clean and dirty trains indicate that eliminating the Peresadochnaya station is financially advantageous and consistent with safety considerations. Since 1991, the workers, after changing into traveling clothes at the Slavutich radiation checkpoint, have been taken by train directly to the radiation checkpoint at the Semikhody station. The safety system may be regarded as effective on the assumption that, in passing through all the safety barriers, the individual remains indifferent to the results of radiation monitoring. However, having received the information from the first dosimetric measurement, the individual will tend to use this information to formulate a strategy for the next radiation checkpoint. Depending on their own evaluation of the likelihood of radioactive contamination in that day's shift, some individuals will decide to slip through subsequent barriers; that is, they will contrive to skip the radioactive measurements. The action of these individuals is determined by the following adaptation mechanisms: eimitation (the strategy of slipping through the safety barriers based on imitation of some other individual); eadaptive imitation (the choice of a particular behavior as a result of observing the actions of several other individuals); eindividual adaptation (selection of a strategy for slipping through the safety barriers by testing various strategies). Dependent adaptation mechanisms in which the selection of a strategy is predetermined by the behavior of the individual ahead or behind also play a role. For example, the person ahead may remain in the open barrier or the person behind, anticipating its motion, may jump over the door without waiting for it to close. The choice of a strategy is determined by factors both internal and external to the individual. Changing the external conditions may have a considerable influence on the strategy chosen. Possible approaches are to have the dosimetric technician monitor workers" passage through the safety barrier; to introduce self-monitoring in rest and recreation areas; to induce greater concern about radioactive contamination of their
141
TABLE I. Statistical Characteristics of/3 Contamination of Workers' Travel Clothes in March 1990/July 1991, particles/(min-cm 2) Area
Mean
Abdomen
211--
Knee
23'23
Back
19/23
Chest
--/2a
Buttocks
--/23 201-23129
Trouser cuffs Shoes
Mean square deviation
Median
201-20120 20120 --/25 --120 20/-20125
a
5.9/-15/7,7 4.2/7.6 --17 --16,6
4.91-15124
TABLE 2. Statistical Characteristics of j Contamination of Workers' Skin in March 1990/July 1991, particles/(min.cm 2) Area
Mean
Median
[
o
Chest
4 0/I .4
4/I
3,6/1,7
Head Gonads
5,2/1.7
4/1.5 4/1,6 4/2
9,1/1,7 4.5/4.6 5.3/13
Arms
4.9/2,7 r 9/4,7
homes and greater confidence in the effectiveness of the safety system; and to introduce periodic dosimetric monitoring of buildings and streets in Slavutich to promote awareness of radiation hazards. The behavioral factors considered here help explain why there is still a pronounced probability of considerable contamination of workers' traveling clothes and skin even though the workers pass through at least three safety barriers on their way to work and back. In this context, statistical estimation of the effectiveness of the safety barriers was undertaken. The probability that the 13 contamination of workers' traveling clothes and skin will exceed the threshold level was taken as the measure of effectiveness (Table 3). Since ~ contamination of the workers' skin must be monitored 3-4 times, the probability that the corresponding levels are exceeded was calculated for each barrier (Table 4). The probability that a person with a skin ~ contamination of more than 10 particles/min'cm 2 will pass through the barrier is 35-56 %; this indicates low effectiveness of each safety barrier, an RZB-04-04 dosimetric monitoring unit. There is a 4-10 % probability that an individual with that level of contamination of some area of skin will pass through the whole system of barriers and monitoring units. Analysis of the /3 contamination of cars in the electric trains at the Semikhody station shows that the mean/3 flux density on the internal and external surfaces is 33 and 90 particles/(min.cm 2) with a mean-square deviation of 20 and 49 particles/(min.cm2), respectively. The ~ contamination of the workers' clothes differs by only 10-15% from that of the internal surfaces of the train. To estimate the possibility that radionuclides will enter the human organism in the course of the train journey, nose masks were analyzed; it was found that there is no accumulation of y- and ~-emitting radionuclides in the frontal passages of the nose. (The content of ~-emitting radionuclides in the masks was below the sensitivity threshold of the equipment.) The annual individual exposure to radionuclides is no more than 0.1-0.2 Bq. Research suggested the adoption of a simpler and more economical safety system; experiments have confirmed the validity of this approach. In addition, estimates were made of the probability of above-threshold/3 contamination on different sections of workers" skin resulting from the decision made in 1991 to remove one protective barrier from the system (Table 5). Comparative analysis of Tables 3 and 5 indicates a sharp reduction in efficiency of the safety system when one of the barriers is removed. In this case, the number of workers with above-threshold/3 contamination of various areas of skin is increased by a factor of 2-6. Obviously, this is not acceptable. At the end of 1991, taking account of the overall radiational conditions on the route from the Chernobyl plant to Slavutich, more rigorous monitoring levels for/3 contamination on clothing, shoes, the surface of the checkpoints, and the internal surfaces of the railroad cars were introduced. Then it was concluded that eliminating the safety checkpoint in Slavutich from the system and allowing the workers to wear their traveling clothes home (domestic storage) with mandatory monitoring on arrival at the city would be an acceptable means of hastening the workers' passage through the safety system. However,
142
TABLE 3. Probability of ~3 Contamination of Workers' Traveling Clothes and Skin
above Specified Levels [particles/(min.cm2)] in Safety System in 1991 Area Clothing: Chest
Back Buttocks Knee Shoes Skin: Head Chest Gonads
Alms
> 10
> 20
> 40
0,999 0.995 0,999 0,997 0,976
0.70 0.60 0.63 0.63 0.69
0,024 0,022 0,011 0.023 0,16
0.044 0.042 0.070 0.10
0.013 0,014 0.024 0,036
< 0.002 < 0,004 < 0.007 < 0,01
TABLE 4. Probability of ~ Contamination of Workers' Skin above Specified Levels [particles/(min.cm2)] in One Checkpoint in a System of Three/Four Barriers I
Area Head Chest Gonads
I Arms
> 10
> 20
> 40
0.35/0.46 0,35/(},45 0.41/0.51 0.46/0.57
0,24/0,34 0,24/0.34 0,29/0.39 t).33/0 aa
0,13/0.22 0.16/0.25 0,1910.29 t).22/0,32
TABLE 5. Probability of/3 Contamination of Workers' Skin above Specified Levels [particles/(min.cm2)] in a System of Two/Three Barriers Area
t
,
Head Chest Gonads Arms
> 10 0.12/0.10 0.12/0.10 0.17/0.13 0.21/0.19
> 20 0.058/0,039 0.058/0,039 0.08410,059 0,11/0,085
> 40 0.017/0.011 0.026/0.016 0.036/0.024 0.048/0.033
according to the results of a survey by O. V. Chinkina, most of the workers (82%) were psychologically unprepared for this step; appropriate informational and educational measures would be needed. Note that the specific proposals for increasing the efficiency of the safety system, especially in terms of increasing radiation monitoring, were based on the employees in the reactor shop and the radiation-safety shop in 80% of cases. On the basis of the available information, it might be supposed that the safety system serves to educate the power-plant personnel about the risks of radioactive contamination and that self-monitoring at the safety barriers is a viable approach. However, diagnostic research yields the following results: ea considerable proportion of the workers regard the transfer of radionuclides from the exclusion zone to Slavutich as unlikely and therefore neglect appropriate safety procedures; othe safety system is regarded as inconvenient and poorly designed; othe system is unable to provide the workers with complete information on the degree of radioactive contamination of their clothing and skin, although such information would help convince them of the need to observe safety rules. Thus, the survey reveals the workers' subjective opinion that the safety system should be strengthened, although their actual behavior at the barriers is the opposite. Most respondents (47 %) regarded the safety system as sufficiently effective, 35 % did not give an adequate response, and 18% regarded the system as ineffective. A poll of experts (specialists at the Chernobyl plant, the Ukrytie facility, and the Slavutich health commission) gave similar results. Whereas 61% of the respondents felt positively toward the safety system, more than 36% felt more negatively about it (2.1% declined to answer). This indicates that the human factor has not been adequately taken into account in the operation of the system.
143
TABLE 6. Probability of/3 Contamination of Workers' Traveling Clothes and Skin above Specified Levels Loarticles/min.cm2)] in Safety System in 1995 Area Clothing: Chest Back Buttocks Knee Shoes Skin:
>IO
> 20
> 40
0.89 0.84
0.65 0.51 0.57 0.53 0.62
0.017 0.014
0.93 0.89 0.94
Chest
0.034 0.033
Gonads Arms
0.054 0.0~,
Head
0.011 0.010 0.018 0.024
0.0084 0.014 0.11 < < < <
0.001 0.003 0.005 0 00.q
Analysis of responses regarding improvements in the system shows that 64% believed that radiation monitoring at the safety barriers should be stronger, 22% were satisfied with existing conditions, and only 11% regarded the barriers as unnecessary. Note that the latter answer was given mainly by those most likely to slip through the safety system. All the experts polled regarded the safety system as effective overall. However, 25 % of the experts felt the need for a three-stage system; the remainder supported the switch to a two-step system provided appropriate regulatory, informational, and monitoring measures were introduced to strengthen the system. About 25 % of the experts regarded the elimination of the checkpoint at Slavutich as a possibility. According to the survey of the workers, only 7.3% were ready to give up changing their clothes until they reached the checkpoint at Semikhody station; 86% were psychologically unready for this and would continue to change their clothes at Slavutich station. Nevertheless, at the beginning of 1995, the Slavutich checkpoint was eliminated from the safety system as an experiment. Radiation monitoring of the workers' clothing and skin in the experimental period permitted estimates of the likelihood of increased B contamination in the reduced system (Table 6). Comparison of Tables 3 and 6 reveals that, despite the removal of a safety barrier from the system in 1995, there was no reduction in its effectiveness relative to the 1991 level. In addition, the number of workers with above-threshold B levels on their skin was reduced by 30-50%. This is because the overall improvement in the radiation conditions at the Chernobyl site on account of natural processes and cleanup work was compensated and even exceeded by the reduced efficiency of the safety system after removing one of the barriers. At the end of 1995, more rigorous threshold levels for/~ contamination were introduced: 100 particles/min'cm2 for the traveling clothes, shoes, and any surfaces in the Semikhody checkpoint and 10 particles/(min.cm2) for the internal surfaces of the rail cars.
144
