TY - JOUR
AU - Sharma, D, N
AB - Abstract Decommissioning of nuclear power plants is a multistage process involving complex operations like radiological characterization, decontamination and dismantling of plant equipment, demolition of structures, and processing and disposal of waste. Radioactive effluents released into the environment may result in exposure of population through various exposure pathways. The present study estimates the public dose due to atmospheric discharge of important radionuclides during proposed decommissioning activities of Indian Pressurized Heavy Water Reactors. This study shows that major dose contributing radionuclides are 60Co followed by 94Nb, 134Cs, 154Eu, 152Eu, 133Ba, 99Tc, 93Mo and 41Ca. It is found that infant dose is higher than adult dose and major fraction of total dose (~98%) is through ground shine and ingestion; other pathways such as inhalation and plume shine contribute only a small fraction. This study will be helpful in carrying out radiological impact assessment for decommissioning operations which is an important regulatory requirement. INTRODUCTION Decommissioning of a nuclear power plant (NPP) is a multistage operation and involves various complex tasks like radiological characterization of components, decontamination and dismantling of plant equipment, demolition of structures and buildings and processing and disposal of the resulting waste. Decommissioning is the final stage in the life cycle of a NPP, which refers to the technical and administrative actions to remove all or some of the regulatory controls from an authorized facility. Decommissioning strategies(1) adopted can be either immediate or deferred dismantling, and the latter case is the most preferred option for NPPs. During various decommissioning operations, radioactive wastes are generated in different physical forms(2, 3), for example, particulates (aerosols), vapors, gases are generated during cutting/dismantling of various metallic and concrete structures. These gaseous wastes are managed by the ventilation system and are ultimately discharged into the atmosphere through the facility stack after filtration through high-efficiency particulate air (HEPA) filters(4). The discharge of radioactive effluents into the atmosphere can result in the exposure of the nearby population through various pathways of exposure. A detailed radiological impact assessment (RIA) is necessary to show that the dose to members of the public during the normal decommissioning operations is within the accepted dose criteria. Shimada et al.(5) have computed the annual exposure dose to public and workers during decommissioning of light water reactors using a safety assessment code DecDose, and also estimated the public dose for accidental conditions. Bonavigo et al.(6) have evaluated the radiological impact due to decommissioning of a pressurized water reactor (Trino Vercellese NPP) using environmental monitoring data and estimated annual effective dose to the public. Tuca et al.(7) have estimated the derived emission limits for the discharge of gaseous and liquid effluents resulting from decommissioning of a light water reactor (VVR-S nuclear research reactor) by using dose assessment models and applying dose constraints. Ragaišis et al.(3) have estimated the release-to-dose calculation factors for additional significant radionuclides during decommissioning of water-cooled graphite-moderated channel-type power reactor (Ignalina NPP). Although many RIA studies are available for the decommissioning of various types of nuclear reactors, no detailed studies are existing for Indian Pressurized Heavy Water Reactors (PHWRs). The present study attempts to fill in the gap in estimation of public dose due to the atmospheric discharge of radionuclides that are significant in decommissioning activities for the Indian PHWRs. PHWRs(8) are the mainstays of Indian nuclear power program and there are 18 PHWRs out of 22 NPPs(9) currently operating at different parts of the country. Rajasthan Atomic Power Station (RAPS 1, 2) comprising of two 200 MWe units were the first PHWRs commissioned as a joint Indo–Canadian venture in the late 1960s. The Indian nuclear power program has come a long way since then with 14 units of 220 MWe PHWR operating at Kalpakkam, Narora, Kakrapar, Kaiga and Rajasthan sites and two units of 540 MWe PHWR operating at Tarapur site and four units of 700 MWe PHWR under construction at Kakrapar and Rajasthan sites. More details on the salient design features of Indian PHWRs, their evolution and operating experience are described systematically in the references(8, 10, 11). Some of the above-mentioned reactors in India have been in service for >30 years and will be completing their life time soon. Hence, full scale decommissioning of a PHWR may be taken up in future and a necessity arises for preparation of their decommissioning plan(12). Furthermore, our present day regulations insist that the decommissioning plan of NPPs should be provided to the regulator at the design stage of new NPPs(13). This study presents a methodology for carrying out RIA of a PHWR decommissioning by identifying important long-lived activation products in the various components of PHWR and assessing the dose to the public due to their discharges. MATERIALS AND METHODS The steps involved in dose assessment due to atmospheric discharges during a PHWR decommissioning are (1) estimation of source term, (2) calculation of air concentration using the appropriate dispersion models, (3) selection of various exposure pathways and (4) computation of doses using the appropriate dose assessment models. Source Term for Decommissioning Operations Source term denotes actual or potential releases of radioactive material from a given location which include amount, composition, rate and mode of release. In the present study, source term for dismantling operations will depend on many factors such as the activities of each isotope generated in each component, decay period, cutting technology used, the amount of material released and becoming airborne during cutting operations, plate out in structures, efficiency of the HEPA filters in building ventilation, etc. Various activation products are generated due to neutron activation of main and trace elements in different structural components of a PHWR during several years of reactor operation. PHWRs(8) are horizontal pressure tube reactors using natural UO2 as fuel and heavy water as the primary coolant and moderator. The reactor core consists of the horizontal calandria containing heavy water moderator and welded at both ends to end shields. The calandria and end shields are pierced by a large number of pressure tubes or coolant tubes which contain the fuel bundles and high pressure coolant flows through them. They are rolled into end fittings at each end. Coaxial calandria tubes surround the coolant tubes and garter springs are present in the annular gap to maintain the spacing between the two. The calandria along with all its internals is submerged in a concrete walled calandria vault filled with light water. The cutting and dismantling of these main components (in-core and out-of-the core structures of PHWR) that contribute to source term in the decommissioning operations are given in the following. Pressure tubes made up of cold worked Zircaloy-2 (old PHWRs) and Zr-2.5 Nb alloy (new PHWRs).(14) Calandria tubes and other in-core components like guide tubes of primary and secondary shut down system fabricated from Zircaloy-4.(14) End-fitting body made up of stainless steel (SS403). Calandria vessel and end shield components fabricated from stainless steel (SS304L). End shield filled with carbon steel balls. Biological shield of the reactor made up of Hematite concrete. The activation of these materials in the neutron flux have been studied for various Indian PHWRs like Rajasthan Atomic Power Station—1(15, 16) and Tarapur Atomic Power Station 3&4(17), particularly for the zirconium based alloys(18), stainless steel and carbon steel components. All these calculations are carried out for typical reactor operation and decay histories. Major radionuclides produced during the neutron induced activation of various reactor materials are identified(1, 19, 20, 21), as shown in Table 1. The tabulated radionuclides (Table 1) are selected based on their half-life and activity in structural components after a decay period of 50 years, which is generally considered as the time period for deferred dismantling. The radiological properties of these radionuclides are summarized in Table 2(1, 19, 20). The potential for atmospheric release of all the radionuclides may not exist, however all the radionuclides are considered to study the radiological impact in the public domain. Table 1 Long-lived radionuclides formed by neutron activation in various reactor materials. Material . Long-lived radionuclides formed by neutron activation . Zirconium Alloys (Zircaloy 2, Zircaloy 4, Zr2.5%Nb) 14C, 60Co, 59Ni, 63Ni, 93Zr,93mNb, 94Nb, 121mSn Stainless Steel and Carbon Steel 14C, 55Fe, 60Co, 59Ni, 63Ni, 93Mo, 99Tc Hematite concrete 3H, 41Ca, 55Fe, 60Co, 134Cs, 133Ba, 152Eu, 154Eu Material . Long-lived radionuclides formed by neutron activation . Zirconium Alloys (Zircaloy 2, Zircaloy 4, Zr2.5%Nb) 14C, 60Co, 59Ni, 63Ni, 93Zr,93mNb, 94Nb, 121mSn Stainless Steel and Carbon Steel 14C, 55Fe, 60Co, 59Ni, 63Ni, 93Mo, 99Tc Hematite concrete 3H, 41Ca, 55Fe, 60Co, 134Cs, 133Ba, 152Eu, 154Eu Open in new tab Table 1 Long-lived radionuclides formed by neutron activation in various reactor materials. Material . Long-lived radionuclides formed by neutron activation . Zirconium Alloys (Zircaloy 2, Zircaloy 4, Zr2.5%Nb) 14C, 60Co, 59Ni, 63Ni, 93Zr,93mNb, 94Nb, 121mSn Stainless Steel and Carbon Steel 14C, 55Fe, 60Co, 59Ni, 63Ni, 93Mo, 99Tc Hematite concrete 3H, 41Ca, 55Fe, 60Co, 134Cs, 133Ba, 152Eu, 154Eu Material . Long-lived radionuclides formed by neutron activation . Zirconium Alloys (Zircaloy 2, Zircaloy 4, Zr2.5%Nb) 14C, 60Co, 59Ni, 63Ni, 93Zr,93mNb, 94Nb, 121mSn Stainless Steel and Carbon Steel 14C, 55Fe, 60Co, 59Ni, 63Ni, 93Mo, 99Tc Hematite concrete 3H, 41Ca, 55Fe, 60Co, 134Cs, 133Ba, 152Eu, 154Eu Open in new tab Table 2 Radiological properties of long-lived radionuclides generated by neutron activation of reactor materials. Isotope . Half-life (years) . Means of production . Emission . Energy (MeV) . β . X, γ . 3H 12.32 6Li (n,α) β− 0.018592 – 14C 5700.0 14N (n, p) β− 0.156476 – 41Ca 9.94 x 104 40Ca (n, γ) EC – 0.42165 (K- X-rays) 55Fe 2.744 54Fe (n, γ) EC, X – 0.2311a 60Co 5.27 59Co (n, γ) β−, γ 0.318 1.17,1.33 59Ni 76000 58Ni (n, γ) EC, X – 1.073a 63Ni 101.2 62Ni (n, γ) β- 0.067 – 93Zr 1.61 x 106 92Zr (n,γ) β- 0.0908 – 93mNb 16.12 93Nb (n, n') X, γ – 0.0165, 0.030 94Nb 20300 93Nb (n, γ) β−, γ 0.472 0.703, 0.871 93Mo 4000 92Mo (n, γ) EC, X – 0.0165, 0.030 99Tc 211100 98Mo (n, γ) β− 0.2975 – 121mSn 43.9 120Sn (n, γ) β−, X 0.354 0.0263 134Cs 2.0652 133Cs (n, γ) β−, γ 0.658 0.605, 0.796 133Ba 10.551 132Ba (n, γ) EC, X, γ – 0.356, 0.303, 0.383 & 0.276 152Eu 13.517 151Eu (n, γ) EC, X, β−, γ 0.699 1.477 154Eu 8.601 153Eu (n, γ) β−, γ, X 0.571 1.85 Isotope . Half-life (years) . Means of production . Emission . Energy (MeV) . β . X, γ . 3H 12.32 6Li (n,α) β− 0.018592 – 14C 5700.0 14N (n, p) β− 0.156476 – 41Ca 9.94 x 104 40Ca (n, γ) EC – 0.42165 (K- X-rays) 55Fe 2.744 54Fe (n, γ) EC, X – 0.2311a 60Co 5.27 59Co (n, γ) β−, γ 0.318 1.17,1.33 59Ni 76000 58Ni (n, γ) EC, X – 1.073a 63Ni 101.2 62Ni (n, γ) β- 0.067 – 93Zr 1.61 x 106 92Zr (n,γ) β- 0.0908 – 93mNb 16.12 93Nb (n, n') X, γ – 0.0165, 0.030 94Nb 20300 93Nb (n, γ) β−, γ 0.472 0.703, 0.871 93Mo 4000 92Mo (n, γ) EC, X – 0.0165, 0.030 99Tc 211100 98Mo (n, γ) β− 0.2975 – 121mSn 43.9 120Sn (n, γ) β−, X 0.354 0.0263 134Cs 2.0652 133Cs (n, γ) β−, γ 0.658 0.605, 0.796 133Ba 10.551 132Ba (n, γ) EC, X, γ – 0.356, 0.303, 0.383 & 0.276 152Eu 13.517 151Eu (n, γ) EC, X, β−, γ 0.699 1.477 154Eu 8.601 153Eu (n, γ) β−, γ, X 0.571 1.85 aContinuous spectrum of X-ray energies below this number due to Bremsstrahlung. Open in new tab Table 2 Radiological properties of long-lived radionuclides generated by neutron activation of reactor materials. Isotope . Half-life (years) . Means of production . Emission . Energy (MeV) . β . X, γ . 3H 12.32 6Li (n,α) β− 0.018592 – 14C 5700.0 14N (n, p) β− 0.156476 – 41Ca 9.94 x 104 40Ca (n, γ) EC – 0.42165 (K- X-rays) 55Fe 2.744 54Fe (n, γ) EC, X – 0.2311a 60Co 5.27 59Co (n, γ) β−, γ 0.318 1.17,1.33 59Ni 76000 58Ni (n, γ) EC, X – 1.073a 63Ni 101.2 62Ni (n, γ) β- 0.067 – 93Zr 1.61 x 106 92Zr (n,γ) β- 0.0908 – 93mNb 16.12 93Nb (n, n') X, γ – 0.0165, 0.030 94Nb 20300 93Nb (n, γ) β−, γ 0.472 0.703, 0.871 93Mo 4000 92Mo (n, γ) EC, X – 0.0165, 0.030 99Tc 211100 98Mo (n, γ) β− 0.2975 – 121mSn 43.9 120Sn (n, γ) β−, X 0.354 0.0263 134Cs 2.0652 133Cs (n, γ) β−, γ 0.658 0.605, 0.796 133Ba 10.551 132Ba (n, γ) EC, X, γ – 0.356, 0.303, 0.383 & 0.276 152Eu 13.517 151Eu (n, γ) EC, X, β−, γ 0.699 1.477 154Eu 8.601 153Eu (n, γ) β−, γ, X 0.571 1.85 Isotope . Half-life (years) . Means of production . Emission . Energy (MeV) . β . X, γ . 3H 12.32 6Li (n,α) β− 0.018592 – 14C 5700.0 14N (n, p) β− 0.156476 – 41Ca 9.94 x 104 40Ca (n, γ) EC – 0.42165 (K- X-rays) 55Fe 2.744 54Fe (n, γ) EC, X – 0.2311a 60Co 5.27 59Co (n, γ) β−, γ 0.318 1.17,1.33 59Ni 76000 58Ni (n, γ) EC, X – 1.073a 63Ni 101.2 62Ni (n, γ) β- 0.067 – 93Zr 1.61 x 106 92Zr (n,γ) β- 0.0908 – 93mNb 16.12 93Nb (n, n') X, γ – 0.0165, 0.030 94Nb 20300 93Nb (n, γ) β−, γ 0.472 0.703, 0.871 93Mo 4000 92Mo (n, γ) EC, X – 0.0165, 0.030 99Tc 211100 98Mo (n, γ) β− 0.2975 – 121mSn 43.9 120Sn (n, γ) β−, X 0.354 0.0263 134Cs 2.0652 133Cs (n, γ) β−, γ 0.658 0.605, 0.796 133Ba 10.551 132Ba (n, γ) EC, X, γ – 0.356, 0.303, 0.383 & 0.276 152Eu 13.517 151Eu (n, γ) EC, X, β−, γ 0.699 1.477 154Eu 8.601 153Eu (n, γ) β−, γ, X 0.571 1.85 aContinuous spectrum of X-ray energies below this number due to Bremsstrahlung. Open in new tab Among these long-lived radionuclides, 3H, 14C, 55Fe, 60Co, 59Ni, 63Ni and 134Cs are the radionuclides found in routine discharges from PHWR during normal operation(22, 23). RIA for decommissioning operations is same as that carried out for the normal discharges during operation of a NPP; however, the form, quantity and radionuclide composition of the effluent discharges can be different. Remaining 10 radionuclides (41Ca, 93Zr, 93mNb, 94Nb, 93Mo, 99Tc, 121mSn, 133Ba, 152Eu and 154Eu) are specific to the present decommissioning study. Also, it is found that the total activity is mostly determined by 95Zr and 95Nb in the initial decay period (<0.5 years) and 14C, 60Co, 63Ni, 93Zr and 94Nb in the later stage (>50 years) for zirconium based alloys. In the case of stainless steel and carbon steel components, the activity in the initial period (<0.5 years) is mainly determined by 55Fe, 60Co and 51Cr; and by 63Ni, 59Ni, 60Co and 14C in the later stage (>50 years). For hematite concrete, the short-term decay is dominated by 3H, 55Fe, 54Mn, 65Zn, 60Co, and long-term decay by 3H, 14C, 41Ca, 63Ni, 152Eu, 154Eu isotopes. Estimation of atmospheric concentration The time-integrated sector-averaged ground level radionuclide concentration is estimated using the Gaussian plume model(24, 25) given by: $$\begin{eqnarray} \chi (x)&=&\frac{Q\times 2\times 3600}{3.1536\times{10}^7\times \sqrt{2\pi }\ x\ \Theta\ N}\ \frac{1}{\sigma_{zi}}\ \nonumber \\ &&\times\sum_{i,k}\frac{N_{ijk}}{U_k}\ \mathit{\exp}\left[-\frac{H^2}{2{\sigma}_{zi}^2}\right] \end{eqnarray}$$(1) where, χ is the concentration at the downwind distance x (m) in Bq/m3, Q is the release rate in Bq/s, Θ is the sector width in radian, N is the number of years for which the Triple Joint Frequency Distribution (TJFD) data are collected, σi (m) is the standard deviation of the concentration in the vertical direction, Nijk is the number of hours in particular wind speed class k, direction j and stability class category i, Uk is the wind speed in m/s and H is the release height in m. An in-house developed computer program(26) is utilized to generate TJFD using hourly meteorological data (wind direction, wind speed and stability class) obtained from a representative site. It is prepared by considering six stability categories (Pasquill-Gifford category: A through F), 10 wind speed classes and 16 directions starting at North with 0.39275 radian (22.5°) angular spacing. The TJFD is then used to compute dilution factor and time-integrated concentration for each sector at various downwind distances(27). Pathways of Exposure and Dose Assessment The pathways considered in the present study are inhalation, external plume shine dose from the plume, external dose due to ground deposition, ingestion dose due to consumption of milk, food crops and flesh food. The public dose assessment is carried out for these exposure pathways for all the selected long-lived radionuclides using the methodology given in IAEA Safety Series 19(28). The dose assessment for 3H and 14C is carried out based on the specific activity model(28), which does not consider the pathways explicitly. Finally, the total dose due to a particular radionuclide is obtained by summing the doses from all pathways for a hypothetical adult and infant at the site boundary. Dietary Intake Data Dietary intake rate of different food items is an important input parameter for the estimation of ingestion dose. In the present study, dietary intake data are obtained from National Nutrition Monitoring Bureau Technical Report No. 26(29). This report contains complete survey of diet and nutritional status of Indian population, categorized in different sections such as gender, age groups, regions, etc. The dietary intake data of an adult male (moderate activity) and infant in three different regions of the country and national average are presented in Table 3. We consider adult male with moderate activity and infant for dietary consumption rate to maximize the dose estimates. Dietary data for infants <1 year old are not available, hence the data for 1–3-year-old child are applied for infants in the present study. As the intake of a 1–3-year-old child is more than that of an infant, this can be considered a conservative assumption. Table 3 Intake data (g/d) for various food stuff. Food stuff . All India pooled data . Northern . Western . Southern . . Adult male . Infant . Adult male . Infant . Adult male . Infant . Adult male . Infant . Cereals 388 118 517 155 266 86 423 103 Millets 56 13 2 0 112 21 1 2 Pulses and Legumes 34 15 42 17 39 15 35 14 Green leafy vegetables 19 7 21 5 16 4 11 4 Other vegetables 48 13 47 13 38 7 55 12 Roots and tubers 63 21 118 40 20 4 65 18 Nuts and oil seeds 6 2 0 0 5 2 8 2 Condiments and spices 13 4 9 3 10 3 19 5 Fruits 26 12 20 6 9 6 45 15 Fish 9 2 3 0 0 0 14 3 Other flesh foods 7 4 3 5 5 1 12 11 Milk and milk products 78 86 65 115 52 106 98 185 Fats 16 6 19 7 18 5 16 5 Sugar and jaggery 13 10 15 9 20 7 11 13 Food stuff . All India pooled data . Northern . Western . Southern . . Adult male . Infant . Adult male . Infant . Adult male . Infant . Adult male . Infant . Cereals 388 118 517 155 266 86 423 103 Millets 56 13 2 0 112 21 1 2 Pulses and Legumes 34 15 42 17 39 15 35 14 Green leafy vegetables 19 7 21 5 16 4 11 4 Other vegetables 48 13 47 13 38 7 55 12 Roots and tubers 63 21 118 40 20 4 65 18 Nuts and oil seeds 6 2 0 0 5 2 8 2 Condiments and spices 13 4 9 3 10 3 19 5 Fruits 26 12 20 6 9 6 45 15 Fish 9 2 3 0 0 0 14 3 Other flesh foods 7 4 3 5 5 1 12 11 Milk and milk products 78 86 65 115 52 106 98 185 Fats 16 6 19 7 18 5 16 5 Sugar and jaggery 13 10 15 9 20 7 11 13 Open in new tab Table 3 Intake data (g/d) for various food stuff. Food stuff . All India pooled data . Northern . Western . Southern . . Adult male . Infant . Adult male . Infant . Adult male . Infant . Adult male . Infant . Cereals 388 118 517 155 266 86 423 103 Millets 56 13 2 0 112 21 1 2 Pulses and Legumes 34 15 42 17 39 15 35 14 Green leafy vegetables 19 7 21 5 16 4 11 4 Other vegetables 48 13 47 13 38 7 55 12 Roots and tubers 63 21 118 40 20 4 65 18 Nuts and oil seeds 6 2 0 0 5 2 8 2 Condiments and spices 13 4 9 3 10 3 19 5 Fruits 26 12 20 6 9 6 45 15 Fish 9 2 3 0 0 0 14 3 Other flesh foods 7 4 3 5 5 1 12 11 Milk and milk products 78 86 65 115 52 106 98 185 Fats 16 6 19 7 18 5 16 5 Sugar and jaggery 13 10 15 9 20 7 11 13 Food stuff . All India pooled data . Northern . Western . Southern . . Adult male . Infant . Adult male . Infant . Adult male . Infant . Adult male . Infant . Cereals 388 118 517 155 266 86 423 103 Millets 56 13 2 0 112 21 1 2 Pulses and Legumes 34 15 42 17 39 15 35 14 Green leafy vegetables 19 7 21 5 16 4 11 4 Other vegetables 48 13 47 13 38 7 55 12 Roots and tubers 63 21 118 40 20 4 65 18 Nuts and oil seeds 6 2 0 0 5 2 8 2 Condiments and spices 13 4 9 3 10 3 19 5 Fruits 26 12 20 6 9 6 45 15 Fish 9 2 3 0 0 0 14 3 Other flesh foods 7 4 3 5 5 1 12 11 Milk and milk products 78 86 65 115 52 106 98 185 Fats 16 6 19 7 18 5 16 5 Sugar and jaggery 13 10 15 9 20 7 11 13 Open in new tab RESULTS AND DISCUSSIONS Input Parameters The atmospheric discharges during decommissioning are assumed as continuous representing normal operations, and the radionuclides are released in the form of aerosols (except 3H and 14C) due to dismantling of various components by cutting operations. The present study is carried out for a typical coastal NPP site with 100 m stack, a typical stack height for Indian PHWRs, and the public doses are computed at a downwind distance of 1 km from the release location. The atmospheric dilution factor (χ/Q) is first estimated using site-specific hourly meteorological data and TJFD. From the 3 years meteorological data, χ/Q is estimated as 1.22E-07 s/m3 (average of the maximum of sectors for each year) for the given release height and downwind distance. Using this dilution factor, the ground level concentration (GLC) at 1 km is obtained for unit source term, i.e. 1 Bq/s release rate. The GLC is then used to estimate the adult and infant doses using standard dose evaluation models(28). The exposure models for the different pathways are same for all the radionuclides except 3H and 14C, where specific activity models (28) are expended. Hence, for these two radionuclides the contribution of each exposure pathway is not estimated explicitly. A total deposition velocity of 1000 m/d (28) is conservatively considered in the present calculation. For the default case study, the all India pooled dietary intake rates of various food stuffs are used and the dose coefficients for inhalation and ingestion for infants ≤1 year is used which give conservative dose estimates. The concentration ratio for pasture forage (Fv,1), human food crops (Fv,2), the fraction of the animals daily intake of radionuclide that appears in each litre of milk (Fm) and each kg of flesh (Ff) which are used in the present calculation are given in Table 4(28, 30). Table 4 Transfer factors for various elements. Element . Fv,1 forage (dry weight) . Fv,2 crops (fresh weight) . Fm milk (d/L) . Ff meat (d/kg) . Ca 8.7 8.7 0.01 0.013 Fe 0.1 0.001 0.0003 0.05 Co 2 0.08 0.01 0.07 Ni 1 0.3 0.2 0.05 Zr 0.1 0.001 6.00E-06 1.00E-05 Nb 0.2 0.01 4.00E-06 3.00E-06 Mo 1 0.2 0.005 0.01 Tc 80 5 0.001 0.001 Sn 1 0.3 0.001 0.01 Cs 1 0.04 0.01 0.05 Ba 0.1 0.05 0.005 0.002 Eu 0.1 0.002 6.00E-05 0.002 Element . Fv,1 forage (dry weight) . Fv,2 crops (fresh weight) . Fm milk (d/L) . Ff meat (d/kg) . Ca 8.7 8.7 0.01 0.013 Fe 0.1 0.001 0.0003 0.05 Co 2 0.08 0.01 0.07 Ni 1 0.3 0.2 0.05 Zr 0.1 0.001 6.00E-06 1.00E-05 Nb 0.2 0.01 4.00E-06 3.00E-06 Mo 1 0.2 0.005 0.01 Tc 80 5 0.001 0.001 Sn 1 0.3 0.001 0.01 Cs 1 0.04 0.01 0.05 Ba 0.1 0.05 0.005 0.002 Eu 0.1 0.002 6.00E-05 0.002 Open in new tab Table 4 Transfer factors for various elements. Element . Fv,1 forage (dry weight) . Fv,2 crops (fresh weight) . Fm milk (d/L) . Ff meat (d/kg) . Ca 8.7 8.7 0.01 0.013 Fe 0.1 0.001 0.0003 0.05 Co 2 0.08 0.01 0.07 Ni 1 0.3 0.2 0.05 Zr 0.1 0.001 6.00E-06 1.00E-05 Nb 0.2 0.01 4.00E-06 3.00E-06 Mo 1 0.2 0.005 0.01 Tc 80 5 0.001 0.001 Sn 1 0.3 0.001 0.01 Cs 1 0.04 0.01 0.05 Ba 0.1 0.05 0.005 0.002 Eu 0.1 0.002 6.00E-05 0.002 Element . Fv,1 forage (dry weight) . Fv,2 crops (fresh weight) . Fm milk (d/L) . Ff meat (d/kg) . Ca 8.7 8.7 0.01 0.013 Fe 0.1 0.001 0.0003 0.05 Co 2 0.08 0.01 0.07 Ni 1 0.3 0.2 0.05 Zr 0.1 0.001 6.00E-06 1.00E-05 Nb 0.2 0.01 4.00E-06 3.00E-06 Mo 1 0.2 0.005 0.01 Tc 80 5 0.001 0.001 Sn 1 0.3 0.001 0.01 Cs 1 0.04 0.01 0.05 Ba 0.1 0.05 0.005 0.002 Eu 0.1 0.002 6.00E-05 0.002 Open in new tab Total Effective Dose The total effective doses from individual long-lived radionuclides for a unit release rate are estimated and the results are presented in Figure 1. It is observed from Figure 1 that 60Co is the major contributor to the total adult dose, i.e. 8.69 × 10−6 mSv/y for a release rate of 1 Bq/s. This dose estimate is in close agreement with the study by Bonavigo et al.(6), i.e. an effective dose of 8.70 × 10−6 mSv/y for unit release rate of 60Co (1 Bq/s). The other major radionuclides contributing to the total dose are 94Nb, 134Cs, 154Eu, 152Eu, 133Ba, 99Tc, 93Mo and 41Ca. Similar trend is observed in the case of infant dose, with rearrangement of few radionuclides in the sequence, i.e. 60Co, 94Nb, 152Eu, 63Ni, 154Eu, 134Cs, 99Tc, 133Ba, 41Ca and 93Mo. Many of these radionuclides are β, γ emitters with strong γ energies, but few are pure β emitters (3H, 14C, 63Ni, 93Zr and 99Tc) among which 99Tc is major contributor to dose to an adult (Table 5) and 63Ni to an infant (Table 6). In case of infants, the maximum dose due to 63Ni among the pure β emitters is due to the higher ingestion dose coefficient and the air to milk transfer factor (Fm) which is highest for Nickel among all the elements considered (Figure 2). The percentage contribution of various pathways to total dose for adult population for 59Ni, 63Ni, 93Zr, 94Nb and 99Tc reported in the present study agrees well with the study by Ragaišis et al.(3) except for minor deviations. The differences can be attributed to the differences in the dietary intakes and other input parameters. Figure 1 Open in new tabDownload slide Dose to an adult and infant for the long-lived radionuclides (unit release rate) Figure 1 Open in new tabDownload slide Dose to an adult and infant for the long-lived radionuclides (unit release rate) Table 5 Percentage contribution of various pathways to dose to an adult. Radionuclide . Inhalation . Ingestion of . Ground shine . Plume shine . Total dose, mSv/y . Food crop . Milk . Flesh food . 3H – – – – – – 3.56E-11 14C – – – – – – 3.78E-08 41Ca 0.05 94.96 4.59 0.40 0.00 0.00 3.87E-07 55Fe 0.68 93.95 0.45 4.93 0.00 0.00 5.68E-08 60Co 0.12 6.86 1.44 0.67 90.90 0.02 8.69E-06 59Ni 0.23 24.79 73.62 1.24 0.12 0.00 5.75E-08 63Ni 0.36 24.68 73.33 1.23 0.40 0.00 1.37E-07 93Zr 5.30 94.35 0.01 1.02E-03 0.34 0.00 1.93E-07 93mNb 2.54 97.13 6.32E-03 3.18E-04 0.33 3.24E-03 2.05E-08 94Nb 0.16 3.94 0.00 0.00 95.90 0.01 7.22E-06 93Mo 0.09 91.39 7.45 1.00 0.06 5.47E-04 6.98E-07 99Tc 0.50 92.41 5.06 0.34 1.68 0.00E+00 8.19E-07 121mSn 4.62 85.67 1.27 0.86 7.57 8.74E-03 9.94E-08 134Cs 0.29 44.94 7.74 2.56 44.46 1.28E-02 6.94E-06 133Ba 0.62 15.80 1.20 0.03 82.33 0.01 1.64E-06 152Eu 0.91 4.89 4.69E-03 0.01 94.18 1.22E-02 4.72E-06 154Eu 1.10 6.65 0.01 0.01 92.22 1.26E-02 4.94E-06 Radionuclide . Inhalation . Ingestion of . Ground shine . Plume shine . Total dose, mSv/y . Food crop . Milk . Flesh food . 3H – – – – – – 3.56E-11 14C – – – – – – 3.78E-08 41Ca 0.05 94.96 4.59 0.40 0.00 0.00 3.87E-07 55Fe 0.68 93.95 0.45 4.93 0.00 0.00 5.68E-08 60Co 0.12 6.86 1.44 0.67 90.90 0.02 8.69E-06 59Ni 0.23 24.79 73.62 1.24 0.12 0.00 5.75E-08 63Ni 0.36 24.68 73.33 1.23 0.40 0.00 1.37E-07 93Zr 5.30 94.35 0.01 1.02E-03 0.34 0.00 1.93E-07 93mNb 2.54 97.13 6.32E-03 3.18E-04 0.33 3.24E-03 2.05E-08 94Nb 0.16 3.94 0.00 0.00 95.90 0.01 7.22E-06 93Mo 0.09 91.39 7.45 1.00 0.06 5.47E-04 6.98E-07 99Tc 0.50 92.41 5.06 0.34 1.68 0.00E+00 8.19E-07 121mSn 4.62 85.67 1.27 0.86 7.57 8.74E-03 9.94E-08 134Cs 0.29 44.94 7.74 2.56 44.46 1.28E-02 6.94E-06 133Ba 0.62 15.80 1.20 0.03 82.33 0.01 1.64E-06 152Eu 0.91 4.89 4.69E-03 0.01 94.18 1.22E-02 4.72E-06 154Eu 1.10 6.65 0.01 0.01 92.22 1.26E-02 4.94E-06 Open in new tab Table 5 Percentage contribution of various pathways to dose to an adult. Radionuclide . Inhalation . Ingestion of . Ground shine . Plume shine . Total dose, mSv/y . Food crop . Milk . Flesh food . 3H – – – – – – 3.56E-11 14C – – – – – – 3.78E-08 41Ca 0.05 94.96 4.59 0.40 0.00 0.00 3.87E-07 55Fe 0.68 93.95 0.45 4.93 0.00 0.00 5.68E-08 60Co 0.12 6.86 1.44 0.67 90.90 0.02 8.69E-06 59Ni 0.23 24.79 73.62 1.24 0.12 0.00 5.75E-08 63Ni 0.36 24.68 73.33 1.23 0.40 0.00 1.37E-07 93Zr 5.30 94.35 0.01 1.02E-03 0.34 0.00 1.93E-07 93mNb 2.54 97.13 6.32E-03 3.18E-04 0.33 3.24E-03 2.05E-08 94Nb 0.16 3.94 0.00 0.00 95.90 0.01 7.22E-06 93Mo 0.09 91.39 7.45 1.00 0.06 5.47E-04 6.98E-07 99Tc 0.50 92.41 5.06 0.34 1.68 0.00E+00 8.19E-07 121mSn 4.62 85.67 1.27 0.86 7.57 8.74E-03 9.94E-08 134Cs 0.29 44.94 7.74 2.56 44.46 1.28E-02 6.94E-06 133Ba 0.62 15.80 1.20 0.03 82.33 0.01 1.64E-06 152Eu 0.91 4.89 4.69E-03 0.01 94.18 1.22E-02 4.72E-06 154Eu 1.10 6.65 0.01 0.01 92.22 1.26E-02 4.94E-06 Radionuclide . Inhalation . Ingestion of . Ground shine . Plume shine . Total dose, mSv/y . Food crop . Milk . Flesh food . 3H – – – – – – 3.56E-11 14C – – – – – – 3.78E-08 41Ca 0.05 94.96 4.59 0.40 0.00 0.00 3.87E-07 55Fe 0.68 93.95 0.45 4.93 0.00 0.00 5.68E-08 60Co 0.12 6.86 1.44 0.67 90.90 0.02 8.69E-06 59Ni 0.23 24.79 73.62 1.24 0.12 0.00 5.75E-08 63Ni 0.36 24.68 73.33 1.23 0.40 0.00 1.37E-07 93Zr 5.30 94.35 0.01 1.02E-03 0.34 0.00 1.93E-07 93mNb 2.54 97.13 6.32E-03 3.18E-04 0.33 3.24E-03 2.05E-08 94Nb 0.16 3.94 0.00 0.00 95.90 0.01 7.22E-06 93Mo 0.09 91.39 7.45 1.00 0.06 5.47E-04 6.98E-07 99Tc 0.50 92.41 5.06 0.34 1.68 0.00E+00 8.19E-07 121mSn 4.62 85.67 1.27 0.86 7.57 8.74E-03 9.94E-08 134Cs 0.29 44.94 7.74 2.56 44.46 1.28E-02 6.94E-06 133Ba 0.62 15.80 1.20 0.03 82.33 0.01 1.64E-06 152Eu 0.91 4.89 4.69E-03 0.01 94.18 1.22E-02 4.72E-06 154Eu 1.10 6.65 0.01 0.01 92.22 1.26E-02 4.94E-06 Open in new tab Table 6 Percentage contribution of various pathways to dose to an infant. Radionuclide . Inhalation . Ingestion of . Ground Shine . Plume shine . Total dose, mSv/y . Food crop . Milk . Flesh food . 3H – – – – – – 3.56E-11 14C – – – – – – 3.78E-08 41Ca 4.98E-03 84.93 14.41 0.65 0.00 0.00 2.29E-06 55Fe 0.08 89.85 1.50 8.58 0.00 0.00 4.30E-07 60Co 0.05 19.77 14.55 3.53 62.09 0.01 1.50E-05 59Ni 0.05 8.67 90.46 0.79 0.03 0.00 2.78E-07 63Ni 0.01 8.68 90.52 0.79 0.01 0.00 7.00E-06 93Zr 0.88 97.84 0.03 1.93E-03 1.24 0.00 6.37E-08 93mNb 0.67 99.08 0.02 5.91E-04 0.23 2.53E-03 7.90E-08 94Nb 0.07 18.06 0.00 1.09E-04 81.85 0.01 1.02E-05 93Mo 0.06 76.37 21.88 1.53 0.17 1.72E-03 6.69E-07 99Tc 0.06 83.03 15.98 0.56 0.37 0.00 4.47E-06 121mSn 0.89 89.49 4.67 1.63 3.32 3.72E-03 3.62E-07 134Cs 0.20 22.29 13.48 2.31 61.71 0.02 6.01E-06 133Ba 0.21 25.37 6.77 0.09 67.53 0.01 2.57E-06 152Eu 0.21 38.78 0.13 0.15 60.72 8.14E-03 8.81E-06 154Eu 0.40 19.06 0.06 0.07 80.39 0.01 6.77E-06 Radionuclide . Inhalation . Ingestion of . Ground Shine . Plume shine . Total dose, mSv/y . Food crop . Milk . Flesh food . 3H – – – – – – 3.56E-11 14C – – – – – – 3.78E-08 41Ca 4.98E-03 84.93 14.41 0.65 0.00 0.00 2.29E-06 55Fe 0.08 89.85 1.50 8.58 0.00 0.00 4.30E-07 60Co 0.05 19.77 14.55 3.53 62.09 0.01 1.50E-05 59Ni 0.05 8.67 90.46 0.79 0.03 0.00 2.78E-07 63Ni 0.01 8.68 90.52 0.79 0.01 0.00 7.00E-06 93Zr 0.88 97.84 0.03 1.93E-03 1.24 0.00 6.37E-08 93mNb 0.67 99.08 0.02 5.91E-04 0.23 2.53E-03 7.90E-08 94Nb 0.07 18.06 0.00 1.09E-04 81.85 0.01 1.02E-05 93Mo 0.06 76.37 21.88 1.53 0.17 1.72E-03 6.69E-07 99Tc 0.06 83.03 15.98 0.56 0.37 0.00 4.47E-06 121mSn 0.89 89.49 4.67 1.63 3.32 3.72E-03 3.62E-07 134Cs 0.20 22.29 13.48 2.31 61.71 0.02 6.01E-06 133Ba 0.21 25.37 6.77 0.09 67.53 0.01 2.57E-06 152Eu 0.21 38.78 0.13 0.15 60.72 8.14E-03 8.81E-06 154Eu 0.40 19.06 0.06 0.07 80.39 0.01 6.77E-06 Open in new tab Table 6 Percentage contribution of various pathways to dose to an infant. Radionuclide . Inhalation . Ingestion of . Ground Shine . Plume shine . Total dose, mSv/y . Food crop . Milk . Flesh food . 3H – – – – – – 3.56E-11 14C – – – – – – 3.78E-08 41Ca 4.98E-03 84.93 14.41 0.65 0.00 0.00 2.29E-06 55Fe 0.08 89.85 1.50 8.58 0.00 0.00 4.30E-07 60Co 0.05 19.77 14.55 3.53 62.09 0.01 1.50E-05 59Ni 0.05 8.67 90.46 0.79 0.03 0.00 2.78E-07 63Ni 0.01 8.68 90.52 0.79 0.01 0.00 7.00E-06 93Zr 0.88 97.84 0.03 1.93E-03 1.24 0.00 6.37E-08 93mNb 0.67 99.08 0.02 5.91E-04 0.23 2.53E-03 7.90E-08 94Nb 0.07 18.06 0.00 1.09E-04 81.85 0.01 1.02E-05 93Mo 0.06 76.37 21.88 1.53 0.17 1.72E-03 6.69E-07 99Tc 0.06 83.03 15.98 0.56 0.37 0.00 4.47E-06 121mSn 0.89 89.49 4.67 1.63 3.32 3.72E-03 3.62E-07 134Cs 0.20 22.29 13.48 2.31 61.71 0.02 6.01E-06 133Ba 0.21 25.37 6.77 0.09 67.53 0.01 2.57E-06 152Eu 0.21 38.78 0.13 0.15 60.72 8.14E-03 8.81E-06 154Eu 0.40 19.06 0.06 0.07 80.39 0.01 6.77E-06 Radionuclide . Inhalation . Ingestion of . Ground Shine . Plume shine . Total dose, mSv/y . Food crop . Milk . Flesh food . 3H – – – – – – 3.56E-11 14C – – – – – – 3.78E-08 41Ca 4.98E-03 84.93 14.41 0.65 0.00 0.00 2.29E-06 55Fe 0.08 89.85 1.50 8.58 0.00 0.00 4.30E-07 60Co 0.05 19.77 14.55 3.53 62.09 0.01 1.50E-05 59Ni 0.05 8.67 90.46 0.79 0.03 0.00 2.78E-07 63Ni 0.01 8.68 90.52 0.79 0.01 0.00 7.00E-06 93Zr 0.88 97.84 0.03 1.93E-03 1.24 0.00 6.37E-08 93mNb 0.67 99.08 0.02 5.91E-04 0.23 2.53E-03 7.90E-08 94Nb 0.07 18.06 0.00 1.09E-04 81.85 0.01 1.02E-05 93Mo 0.06 76.37 21.88 1.53 0.17 1.72E-03 6.69E-07 99Tc 0.06 83.03 15.98 0.56 0.37 0.00 4.47E-06 121mSn 0.89 89.49 4.67 1.63 3.32 3.72E-03 3.62E-07 134Cs 0.20 22.29 13.48 2.31 61.71 0.02 6.01E-06 133Ba 0.21 25.37 6.77 0.09 67.53 0.01 2.57E-06 152Eu 0.21 38.78 0.13 0.15 60.72 8.14E-03 8.81E-06 154Eu 0.40 19.06 0.06 0.07 80.39 0.01 6.77E-06 Open in new tab Figure 2 Open in new tabDownload slide Dose to an infant due to ingestion of milk for various dietary data sets Figure 2 Open in new tabDownload slide Dose to an infant due to ingestion of milk for various dietary data sets Tritium (0.019 MeV) and 14C (0.156 MeV) are among the least contributors to the total dose as they are pure β emitters with low β energies. Dose from 152Eu is observed to be higher than 154Eu and 134Cs in infants because of the higher ingestion dose coefficient contributing to 38.8% of the total dose, whereas in adults, the contribution from 152Eu is only 4.9% (Tables 5 and 6). Overall comparison shows that the infant doses are always higher than the adult. The present study is based on unit release rate, however, in the real scenario, the relative proportion of the radionuclides will provide the realistic dose estimate. Contribution of Various Pathways of Exposure The percentage contribution of various pathways of exposure and the total dose for each radionuclide is given in Tables 5 and 6 for adult and infant, respectively. The major pathways contributing to the adult and infant doses are ground shine and ingestion (food crop and milk); other pathways such as inhalation, flesh food ingestion and plume shine pathways contribute only a small fraction to the total dose. Inhalation The inhalation dose is determined by the air concentration, breathing rate and the dose coefficient for inhalation(31). The adult dose is higher than the infant dose for all radionuclides as the effect of breathing rate(28) (adult—8400 m3/y and infant—1400 m3/y) is more as compared to the inhalation dose coefficient (which is higher for an infant). The inhalation dose coefficients for adult and infant are given for fast, moderate and slow lung absorption type for the selected radionuclides. The recommended lung absorption type for particulate aerosols of Iron, Cobalt, Nickel, Zirconium, Niobium, Molybdenum and Technetium is ‘moderate’ type(31) for members of the public when no specific information on their chemical nature is available. Hence, in the present study, we consider moderate type for 55Fe, 60Co, 59Ni, 63Ni, 93Zr, 93mNb, 94Nb, 93Mo and 99Tc and slow type for the remaining radionuclides. The percentage contribution of inhalation pathway to the total dose is less (< 1.1% for adult and < than 1% for infant) for all the radionuclides except 93Zr (5.3%), 121mSn (4.6%) and 93mNb (2.5%) for an adult. Ground shine The external exposure due to the radiations from ground deposited radionuclides is estimated using soil activity concentrations, external dose factor due to ground deposition(32) and occupancy factor. In the present calculation, the occupancy factor is conservatively assumed as 1 which is the default value recommended(28) and no credit is given for any shielding provided by the shelter. Age-specific external dose coefficients for ground contamination are used as provided in FGR 15(32) and this has resulted in a higher dose for infants as compared to the adults, which is an important result of the study. Till now most of the studies have used same dose coefficients for adult and infant. Also from Tables 5 and 6, it is found that the ground shine is the major pathway of exposure (> 90%) for high energy γ emitters like 94Nb (0.7877 MeV), 60Co (1.253 MeV), 154Eu (0.7445 MeV), 152Eu (0.7129 MeV), 134Cs (0.6976 MeV) and 133Ba (0.2663 MeV). Plume shine Plume dose is estimated using finite plume exposure model by assuming that the distribution of the radionuclides after release from stack is Gaussian in vertical and crosswind direction. The photons released from the radionuclides distributed in this plume reach the receptor location and contribute to the external plume dose. The external doses for different radionuclides are then estimated using a numerical program(33). The contribution from plume shine dose is found to be negligible (< 0.02%) for the various radionuclides considered for adult and infant, as shown in Tables 5 and 6. Ingestion Pathway The ingestion dose is estimated using activity concentrations in food crop (including vegetables), milk and flesh food, dietary consumption data and ingestion dose coefficient(31). From the results (Tables 5 and 6), it is found that the ingestion pathway is one of the dominant exposure route. Further, it is observed that the ingestion of food crops including vegetables is the dominant pathway of exposure for 41Ca, 55Fe, 93Zr, 93mNb, 93Mo, 99Tc and 121mSn among different ingestion intake routes; Ca and Tc have the highest Fv,2 values (Table 4), resulting in maximum contribution from ingestion dose. The milk ingestion pathway is the major pathway of exposure only for 59Ni and 63Ni, for adult (73%) and infant (90%) as Nickel has the highest value of Fm among the elements of selected radionuclides (Table 4 and Figure 2). The contribution from ingestion of flesh food is much less (< 2.5% for adult and infant except 55Fe (4.9%—adult and 8.6% infant)) compared to all other exposure pathways for all radionuclides. The infant dose for ingestion of food crop is always higher than the adult dose due to higher ingestion dose coefficient for infant. However, 134Cs is an exception, where adult ingestion dose is higher than the infant dose. This is due to higher consumption rate of food crop for an adult dominating the dose although the infant ingestion dose coefficient is higher than the adult by a factor of 1.37. Effect of dietary intake data The dietary intake plays an important role in the estimation of ingestion dose and due to vast geographical area of India with differences in climatic conditions there is variation in dietary pattern across the country. To study the effect of intake, we consider dietary data of three regions (northern, western and southern) other than the all India national pooled data for infants. With all these inputs, the total effective dose to an infant is calculated and the results are presented in Figures 2 and 3 for milk and food crop (including vegetable), respectively. From the results, it is observed that the dose variation is minimal in the case of food crop ingestion route (Figure 3). However, large variation is observed in the milk intake route (Figure 2), maximum observed in the case of southern region due to maximum milk consumption rate by the infants in that region. These variations in the dose estimates indicate that the site-specific dietary intake is better representative compared to the national average values. Figure 3 Open in new tabDownload slide Dose to an infant due to ingestion of food crop for various dietary data sets Figure 3 Open in new tabDownload slide Dose to an infant due to ingestion of food crop for various dietary data sets To summarize, the present work shows that 60Co, 94Nb, 59Ni, 63Ni, 93Zr, 121mSn are the important radionuclides present in structures made from zirconium-based alloys, such as pressure tube, calandria tube and guide tubes, contributing maximum exposure to the public. In the case of stainless and carbon steel structures, 60Co, 55Fe, 59Ni, 63Ni, 93Mo contribute maximum exposure while 41Ca, 55Fe, 60Co, 134Cs, 133Ba, 152Eu, 154Eu are important for hematite concrete structures. CONCLUSIONS The present study identifies important long-lived radionuclides in the structural materials that may be released during decommissioning stage of a PHWR, and assesses the public dose due to atmospheric releases of these radionuclides. 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TI - DOSE ASSESSMENT FOR ATMOSPHERIC DISCHARGE OF LONG-LIVED RADIONUCLIDES IN NUCLEAR POWER PLANT DECOMMISSIONING
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncaa088
DA - 2020-08-28
UR - https://www.deepdyve.com/lp/oxford-university-press/dose-assessment-for-atmospheric-discharge-of-long-lived-radionuclides-dH6IAFcJXP
SP - 139
EP - 149
VL - 190
IS - 2
DP - DeepDyve
ER -