S.S. Zolotarev1, M.A. Berberova2 zolotarev@phystech.edu|maria.berberova@gmail.com 1Moscow Institute of Physics and Technology, Moscow, Russia; 2International Nuclear Safety Center, Moscow, Russia Nuclear power plants, being complex technological systems, represent a source of increased risk, in particular, a specific risk of radiation exposure. Obtaining quantitative assessments of radiation risk is critical for risk reduction and accident prevention. Existing methods for assessing radiation risk do not take into account the influence of external factors, such as population composition, geographical features, anthropogenic environmental changes, etc. The result of the risk analysis is the assessment of physical and economic indicators for the Rostov and Kalinin NPPs, taking into account the age composition of the population, as the most significant parameter. Based on a comparison of the estimates obtained with the results without taking into account the age distribution, recommendations are given on the use of adjusted estimates when developing measures to reduce risk and mitigate the consequences for the most sensitive age groups of the population (1-12 years). The objective of the work is to modify the methodological approach to the calculation of radiation risk indicators of the population, taking into account the age composition and the practical application of the formulas for assessing the physical and economic indicators of damage to real objects.
As initial data for the calculation of risk for personnel and the population are used:
Frequencies of consequences in design and beyond basis accidents (calculated using probabilistic safety analysis (PSA)); Estimation of the number of radionuclides involved in the design and beyond design basis accidents; Data on climatic conditions in the area of the NPP location: repetition of wind directions, average annual wind speed; population distribution by points.
Data on the population in the 100-km NPP zone, carried out for two directions of external exposure: release of radioactive substances into the atmosphere; discharge of radioactive substances into surface waters (rivers, lakes, etc.).
In this study, only the first pathway is considered.
Estimation of population exposure doses is carried out for three age groups: for adults (over 18); for school-age children (from 7 to 12 years old); for preschoolers (from 1 to 2 years).
Evaluation of external exposure doses of the population is exposure from a radioactive cloud; irradiation from the underlying surface.
Release of radionuclides into the environment is considered short-lived. When estimating the population exposure doses, three methods of exposure are taken into account: direct inhalation; exposure from a radioactive cloud; exposure from a contaminated surface to the ground.
The assessment result of the accident radiation consequences on the population are: absorbed dose of external irradiation of the whole body and internal irradiation of the thyroid gland during inhalation at the initial stage (first 10 days) of the accident; annual effective radiation dose. population groups The following concepts are used for evaluation:
Absorbed dose in a tissue or organ - the amount of ionizing radiation energy transferred to the tissue or organ; Equivalent dose - absorbed dose in an organ or tissue multiplied by the corresponding weighting factor of this type of radiation, reflecting the ability to damage body tissues (the weighting coefficient of gamma radiation for biological tissue is taken to be unity); Effective dose - the value used as a measure of the risk of the occurrence of the remote effects of exposure of the entire human body and individual tissues and organs, taking into account their radiosensitivity. in the further chain of human exposure pathways: internal and
Early deaths among the population are excluded, because maximum individual absorbed doses do not reach the level of 1 Gray, and internal exposure of the thyroid gland during inhalation does not reach the level of 5 Gray, at which deterministic effects are possible.
In [5, 6], the assessment is made for an average adult with an average radiosensitivity. This provides a basis for studying the influence of age composition on estimates of radiation risk indicators of the population.
According to the results of calculations, indicators of radiation and economic risks from accidents for nuclear power plants are compared with [8, 9].
In [5, 6], the radiation risk assessment is made for an average adult with an average radiosensitivity. According to [4], which regulates the process of rapid assessment of doses to the public during radioactive contamination of the territory by air, there is a significant difference in the transition factors from the absorbed radiation dose to the effective dose, reflecting the risk of radiation effects for different age groups.
Estimation of the exposure level for different age specifically to the effects of air emissions on the population and external. The main ways of radioactive pollution of the personnel. environment are: population when exposed to a radioactive cloud Eie,xat (mSv) is defined as:
Eie,xat = Ki,a ∙ T ∙ ∑ =1 e ∙ , / m3;
where: e is the dose coefficient determining the absorbed dose rate of gamma radiation from the k-th radionuclide at a height of 1 m above the underlying surface from the source in the form of a radioactive cloud, (mGy / h) / (kBq / m3); − the average concentration of the -th radionuclide, kBq T − the time of exposure from this source, hour;
Ki,a is the coefficient of transition from the dose in the air to
the effective dose for the th group of the population, mSv / mGy.
(1)
(
Eie,xt = Ki,s ∙ T ∙ ∑ =1 e ∙ ,
Based on Formula 1, we can assume that the effective dose of external exposure from a radioactive cloud linearly depends on the coefficient Ki,a.
Similarly, the effective external dose of the i-th group of the population from a mixture of radionuclides dropped on the underlying surface, Eie,xt , linearly depends on Ki,s: where: e
is the dose coefficient determining the absorbed dose rate of gamma radiation from the k-th radionuclide at a height of 1 m from a flat isotropic source located at the air-toground boundary, (mGy/hour) / (kBq/m2); − surface activity of the k-th radionuclide on the soil, kBq/m2; hour;
T − the time elapsed since the end of the radioactive fallout, Ki,s is the transition coefficient from the dose in air at a height of 1 m above the underlying surface to the effective dose for the i-th group of the population, mSv / mGy. Ki,s is assumed to be: 0.75 mSv / mGy - for adults; 0.80 mSv / mGy - for schoolchildren; 0.90 mSv / mGy - for preschoolers.
When a population is exposed to a man-made environment, the characteristics of the radiation field change. It is possible to take into account this change in the calculations using the location factors Lj, defined as the ratio of the dose rate in the air at point j within the settlement or in its area due to man-made 6.
In this section, a quantitative assessment of the physical and economic indicators of radiation risk for the Rostov and Kalinin nuclear power plants taking into account the age composition of the population.
Rostov NPP is located in the Rostov region, 12 km from the city of Volgodonsk on the bank of the Tsimlyansk reservoir. Volgodonsk is located in the eastern part of the Rostov region, between two million-plus cities - Rostov-on-Don and Volgograd.
Rostov NPP is one of the largest energy companies in the south of Russia, providing about 15% of the annual electricity generation in the region.
The electrical capacity of the three existing power units is 3.1 GW. All reactors (four power units) are VVER-1000 water-towater power reactors. Power units of the Rostov NPP were commissioned in 2001, 2009, 2015 and 2018.
Estimation of the potential radiative forcing of radionuclide emissions (RS) is made on the basis of population data by points and distance from Rostov NPP from 3 km to 30 km, and the average population density outside the 30 km zone to 100 km is also estimated (Table 1 and Fig. 1). gamma radiation, to a similar value over the open virgin soil . Human behavior in the radiation field is described using the factors of behavior Fij, depending on the season and representing a fraction of the time during which representatives of the i-th population group are located at the j-th point of the locality.
In the study we neglect these parameters due to the lack of anthropogenic characteristics of the territory around the NPP. The study of the influence of place factors and factors of population behavior is also of scientific value and can be considered as part of a separate work.
The following initial data are used in assessing the internal exposure doses of the population: , (3) where: ℎ , ℎ is the dose coefficient for the -th group of the population and the -th radionuclide, Sv / Bq;
is the average concentration of the th radionuclide in the surface air layer during the passage of the radioactive cloud, kBq / m3; − time of exposure from the source, hour; − respiration intensity of representatives of the -th group of the population, m3 / hour.
Based on Formula 3, we can assume that the effective dose from inhalation depends linearly on ℎ , ℎ and .
Data on ℎ , ℎ are known only for the most irradiated critical age groups of the population [8], therefore, it is impossible to differentiate according to a given coefficient for different age
The value of the equivalent dose to the thyroid gland in the i-th group of the population from inhalation , ℎℎ is calculated , ℎ = · · ∑ =1 ∙ ℎ , ℎ ℎ (4) where: ℎ , ℎ − dose coefficient for the i-th population group and the k-th radionuclide (iodine or tellurium), mSv / kBq; is the average concentration of the th radionuclide in the surface air layer during the passage of the radioactive cloud, kBq − time of irradiation from this source, hour; − respiration intensity of representatives of the -th group of the population, m3 / hour.
On the basis of Formula 4, we can assume that the equivalent dose of irradiation of the thyroid gland linearly depends on ℎ , ℎ
The effective dose of internal exposure of the population due to the oral intake of the k-th radionuclide in the body is calculated by measuring its specific activity in the consumed food products. Sampling and measurements are carried out at times t1 and t2, where t1 and t2 are the time elapsed since t0 the start of consumption of contaminated food (it is assumed that this moment coincides with the end of radioactive fallout). In the study, we neglect this indicator, because We consider the indicators of the effective dose of radiation in the early phase of the accident (the first 10 days).
The total effective dose for the i-th group of the population living in the territory that has been contaminated with a mixture of radionuclides is equal to the sum of doses of external exposure from the radioactive cloud Eie,xat and deposition on the underlying surface Eie,xst , internal dose due to inhalation of radionuclides ℎ : = Eie,xat + Eie,xst + ℎ, мЗв (5)
The equivalent dose of the thyroid gland for the i-th group of the population is equal to the sum of doses due to inhalation of iodine radionuclides , ℎℎ , mSv. groups. as: / m3; and . groups in each of the ring segments of rumba during the most dangerous accident at the Rostov NPP 1000 type reactors. The 1st and 2nd power units were launched in 1984 and 1986, the 3rd and 4th power units in 2004 and 2012, respectively. The potential radiative forcing of radioactive substances is estimated on the basis of population data by points and the distance from Kalinin NPP from 3 to 30 km, and the average density of the population living at a distance of 30-100 km is estimated (Table 3 and Fig 3).
Calculating the coefficient ℎ , ℎ ∙ we get (Fig. 4):
Or the ratio relative to the indicators for the adult population Tver Region were used, according to which the adult population is 85%, schoolchildren (7-12 years old) - 8%, preschoolers (1-2 years) - 7%.
To assess the radiation effects of the accident on various groups of the population, the following calculations were performed at 8 points in the 100 km zone around the Kalinin NPP: collective dose , man · Sv / year; the number of late deaths, ℎ; collective risk of late death, ℎ.
The economic damage as a result of harm to the health of the population, which can manifest itself in the form of stochastic effects (radiogenic cancer and hereditary diseases), is estimated by the formula (9).
Comparison of the obtained damage indicators with those calculated without taking into account the age composition of the population confirms that the adjustment for the age composition gives an order of magnitude higher indicators of the risk of exposure of the population. The emergency exposure of the population, taking into account the adjustment, is below the levels that are subject to radiation safety requirements. Nevertheless, the results obtained make it possible to argue that the age composition of the population must be taken into account when developing measures to reduce the risk and mitigate the consequences for the population and, in particular, for the most sensitive age groups (1-12 years old).
According to the results of the study, the author achieved the following results: 1. Review of existing approaches to assessment of population radiation risk and risk indicators; 2. Identification of the degree of influence of age composition on the calculation of radiation doses of the population at different irradiation routes (external and internal); 3. Modification of the formula for calculating the annual effective dose of radiation of the population in the ring segment of the Rumba taking into account the age composition; 4. Calculation of the annual effective dose of different age groups for Rostov and Kalinin NPP with VVER-1000 Tapa reactors; 5. Calculation of physical and economic indicators of damage of different age groups of the population for Rostov and Kalinin NPP.
According to the results of calculations, indicators of radiation and economic risks from accidents for Rostov and Kalinin NPPs are within the limits allowed by the requirements [8] and [9]. Nevertheless, there is a significant deviation in the positive direction for all indicators in the calculation, taking into account the age composition. This determines the need to take into account the results obtained to assess the radiation risk.
The study makes a significant contribution to the development of methods for assessing the radiation risk of nuclear power plants and can serve as an incentive to further study the influence of external factors such as population composition, geographical features, anthropogenic environmental change, etc. on risk assessment.
Refined estimates of physical and economic indicators will significantly reduce planning errors in developing measures to reduce the risk and mitigate the consequences of accidents at nuclear power plants.
The study was carried out within the framework of grant 1907-00455 «Development of models, algorithms and software for solving the problems of safety and risk assessment at nuclear power plants during beyond design basis accidents with the release of sources of thermal neutrons with low flux density».