ESTIMATION OF EXPOSURE DOSES FOR THE SAFE MANAGEMENT OF NORM WASTE DISPOSAL

ESTIMATION OF EXPOSURE DOSES FOR THE SAFE MANAGEMENT OF NORM WASTE DISPOSAL Abstract Naturally occurring radioactive materials (NORM) wastes with different radiological characteristics are generated in several industries. The appropriate options for NORM waste management including disposal options should be discussed and established based on the act and regulation guidelines. Several studies calculated the exposure dose and mass of NORM waste to be disposed in landfill site by considering the activity concentration level and exposure dose. In 2012, the Korean government promulgated an act on the safety control of NORM around living environments to protect human health and the environment. For the successful implementation of this act, we suggest a reference design for a landfill for the disposal of NORM waste. Based on this reference landfill, we estimate the maximum exposure doses and the relative impact of each pathway to exposure dose for three scenarios: a reference scenario, an ingestion pathway exclusion scenario, and a low leach rate scenario. Also, we estimate the possible quantity of NORM waste disposal into a landfill as a function of the activity concentration level of U series, Th series and 40K and two kinds of exposure dose levels, 1 and 0.3 mSv/y. The results of this study can be used to support the establishment of technical bases of the management strategy for the safe disposal of NORM waste. INTRODUCTION Naturally occurring radioactive materials (NORM) are materials that contain radionuclides that exist in the natural environment. Long-lived radioactive elements such uranium, thorium and potassium and any of their decay products such as radium and radon, are typical examples of NORM. These elements have always been present in the earth’s crust and within tissues of all living species. The amount of radioactivity in NORM can be increased or concentrated as a result of human activities and industrial processes, which are called Technologically Enhanced NORM (TENORM). TENORM can increase the potential for human and environmental exposure by concentrating or altering the radiological, physical and chemical properties of radioactive material. In Korea, there is no national guidance for enabling a uniform approach to establishing criteria for regulation of NORM, and there was no national guidance on the management of NORM waste until 2012. In 2012, the Korea Institute of Nuclear Safety (KINS) performed an assessment of the impact of environment radiation and the establishment of optimum management bases for natural radiation to investigate the current status of treatment processes and by-products in typical NORM/TENORM(1). Through this research, the features of high concentration and the effect on the environment were also investigated, and the exposure doses on workers were estimated. The Korean government promulgated and revised an act and an enforcement ordinance on the safety control of NORM around the living environments to protect citizen’s health and the environment(2, 3). The important articles of this act are: (1) a comprehensive plan for radiation protection resulting from radioactive materials around a living environment has to be made every 5 years; (2) the necessary safety guides for the treatment, reuse and disposal of NORM waste have to be made; (3) the regulatory body in Korea, Nuclear Safety and Security Commission (NSSC), should prepare necessary safety guidelines for the treatment, reuse and disposal of NORM waste; (4) all transactors of NORM should register all kinds and magnitudes of raw materials and NORM waste above the standard for registration with the NSSC; (5) the NSSC should install radiation monitoring devices in airports and harbors to secure safe management of NORM; and (6) the NSSC can launch several R&D works to efficiently make an integrated plan for radiation protection, and the government can donate part or all of these R&D funds. The standards for registration of raw materials and NORM waste are 10 Bq/g for 40K and 1 Bq/g for all other radionuclides of natural origin(3). The important articles of the enforcement ordinance of this act are: (1) methods and procedures should be established to reduce radiation exposure for workers in the fields of treatment, reuse and disposal of NORM wastes; (2) the radionuclide concentration can be reduced when NORM waste is reused and disposed; and (3) NORM waste must be disposed of in landfill sites or other disposal facilities, and they should not be reused. For the successful implementation of these acts, many R&D works are being performed to establish technical bases for securing safety control of NORM as follows: development of technical bases for safety regulation of NORM and NORM wastes; establishment of a framework and technical bases for safety assessments related to NORM; development and operation of a support system for the safe management of NORM; development of methods for the determination of NORM in raw materials and by-products; establishment of technical bases for securing radiological safety resulting from the disposal of NORM waste; development of assessment methodologies of exposure doses based on the characteristics of processed products; and qualification requirements for radiation exposure resulting from the use of NORM and building materials. In Korea, NORM waste can be produced by a range of industries such as coal-fired electrical power plants, aluminum production facilities, fertilizer production facilities, zirconium production facilities and steel industries. Activities in these industries can modify the NORM concentrations in products, by-products and waste (residues). However, management strategies and guidance for the safe disposal of NORM waste are not existent, and there are no official disposal facilities for NORM waste, which are temporarily stored near the NORM waste generation facility. For the safe disposal of NORM waste, securing radiological safety is a prerequisite for the safe management of NORM waste which cannot be reused. To secure the radiological safety for the disposal of NORM waste, the establishment of technical bases and procedures for the safe disposal of NORM is required. The International Atomic Energy Agency (IAEA) released several reports to help member states gain perspectives on the management of NORM waste(4–6). And there are other studies that calculate the peak-year dose and carcinogenic risk for disposal of NORM waste into a nonhazardous landfill(7), or recommend a generic upper limit on the annual mass of NORM waste that can be disposed of to a landfill site(8). Also, the masses that can be disposed without considerable radiological impact were derived for the disposal of NORM waste in hazardous and nonhazardous landfills(9) in Spain. In this study, we suggest a reference design for a landfill for the disposal of NORM waste by reinforcing a landfill facility for the disposal of industrial wastes. We estimate the exposure doses for several scenarios for NORM waste disposal into a reference landfill to verify radiological safety. The exposure dose used in this study is effective dose, which is also used in the regulation criteria for the general public in Korea. Also, we estimate the amount of NORM waste to be disposed of into a landfill for different activity concentration levels of important radionuclides in wastes based on exposure dose levels. The results of this study can be used to support the establishment of technical bases as safety guides for the safe disposal of NORM waste. MATERIALS AND METHODS A reference landfill for NORM waste disposal The management strategies for NORM waste disposal are diverse because the radiological, physical and chemical properties of the radioactive material in NORM waste are very different. Among the various kinds of disposal methods for NORM waste, we selected a landfill described in the act on the safety control of NORM(3). We suggested a reference design for a landfill for the disposal of NORM waste, which is shown in Figure 1. First, the landfill site must be located several kilometers from residential areas to minimize potential exposure to radiation. The landfill site also must not be susceptible to landslides and flooding during the rainy season. Figure 1. View largeDownload slide Conceptual design for landfill disposal of NORM waste. Figure 1. View largeDownload slide Conceptual design for landfill disposal of NORM waste. A reference landfill for the disposal of NORM waste was made by reinforcing a landfill for industrial or residential waste. The reinforced systems are impermeable layers and a clay layer at the side and bottom of the landfill, a leachate treatment system, and a system for the treatment and use of gas generated from waste. The impermeable layers at the side and bottom of the landfill site are made of high-density polyethylene (HDPE) which is chemically stable and has a high mechanical strength. Therefore, they can minimize the release and transport of leachate and can prevent the infiltration of water into the landfill site. The lifetime of HDPE is more than 50 years although it depends on the geochemical conditions of the landfill site. The radionuclides may be released from the landfill site due to the degradation of impermeable layer at the side and bottom of the landfill site. The degradation of the impermeable layer depends on the geochemical conditions around the landfill site. However, we assumed that the radionuclides may be released after the closure of the landfill site. A clay layer with a thickness of ~1 m at the side and bottom of the landfill site has to be installed to prevent and/or minimize infiltration of rainfall and release of leachate to neighboring environments. The clay layer also can retard the migration of radionuclides to the surrounding environment due to the high sorption capacity of the clay. The leachate treatment system can control the radioactivity in leachate by measuring and treating the radioactivity of radionuclides. If the radioactivity is higher than the standard for registration, they are treated by appropriate methods and returned to the landfill. If the radioactivity of the radionuclides are lower than the standard for registration, they are treated chemically and physically in the treatment system and are released into the environment. Scenarios for safety assessment of NORM waste disposal The landfill disposal of NORM waste may cause radiation exposure to residents near the site. Therefore, exposure doses must be estimated to secure radiological safety for several scenarios during landfill disposal of NORM waste. The estimation of exposure doses is made by RESRAD code which is a computer model designed to estimate radiation doses and risks from residual radioactive materials(10). This RESRAD code has been used widely by many government agencies and institutions in several countries including Korea as well as the USA. The exposure pathways for the critical population group in the RESRAD code are direct exposure to external radiation from contaminated soil material, internal doses from inhalation of airborne radionuclides, and internal doses from ingestion of plant foods, meat and milk, drinking water, fish and contaminated soil as in Figure 2. Dose coefficients are derived from the references(11–14) for each pathway as described in the user manual of the RESRAD code. Figure 2. View largeDownload slide Exposure pathways considered in RESRAD. Figure 2. View largeDownload slide Exposure pathways considered in RESRAD. We consider three scenarios for the estimation of maximum exposure doses: a reference scenario, an ingestion pathway exclusion scenario, and a low leach rate scenario. The reference scenario is a scenario that consider all exposure pathways and the derived input data. The ingestion pathway exclusion scenario is a scenario that does not consider the ingestion pathway to estimate exposure doses more realistically because the ingestion of plant foods, meat and milk, drinking water, fish, and soil contaminated by the radionuclides near the landfill site is very low. The low leach rate scenario is a scenario that uses a reduced rainfall rate. Almost all landfill sites have a facility that restricts the release of water produced autonomously in the landfill site and leachates to the soil. To consider the effect of this reduced release into the soil, we controlled the amount of water passing the contaminated zone by using 1% of the rainfall rate of the reference scenario. The important input data for the estimation of exposure doses are summarized in Tables 1–4. Table 1. Input data for primary contamination zone and soil cover. Parameter Unit Value Area of contaminated zone m2 400 000 Thickness of contaminated zone m 4 Length parallel to aquifer flow m 600 Depth of cover m 0.5 Density of cover material g/cm3 2.0 Erosion of contaminated zone m/y 0.001 Total porosity of contaminated zone – 0.4 Field capacity of contaminated zone – 0.2 Hydraulic conductivity of contaminated zone m/y 100 Evapotranspiration coefficient – 0.68 Wind speed m/s 1.9 Precipitation m/y 1.46 Runoff coefficient – 0.2 Watershed area for nearby stream or pond m2 1 000 000 Humidity in air g/cm3 7.5 Parameter Unit Value Area of contaminated zone m2 400 000 Thickness of contaminated zone m 4 Length parallel to aquifer flow m 600 Depth of cover m 0.5 Density of cover material g/cm3 2.0 Erosion of contaminated zone m/y 0.001 Total porosity of contaminated zone – 0.4 Field capacity of contaminated zone – 0.2 Hydraulic conductivity of contaminated zone m/y 100 Evapotranspiration coefficient – 0.68 Wind speed m/s 1.9 Precipitation m/y 1.46 Runoff coefficient – 0.2 Watershed area for nearby stream or pond m2 1 000 000 Humidity in air g/cm3 7.5 Table 1. Input data for primary contamination zone and soil cover. Parameter Unit Value Area of contaminated zone m2 400 000 Thickness of contaminated zone m 4 Length parallel to aquifer flow m 600 Depth of cover m 0.5 Density of cover material g/cm3 2.0 Erosion of contaminated zone m/y 0.001 Total porosity of contaminated zone – 0.4 Field capacity of contaminated zone – 0.2 Hydraulic conductivity of contaminated zone m/y 100 Evapotranspiration coefficient – 0.68 Wind speed m/s 1.9 Precipitation m/y 1.46 Runoff coefficient – 0.2 Watershed area for nearby stream or pond m2 1 000 000 Humidity in air g/cm3 7.5 Parameter Unit Value Area of contaminated zone m2 400 000 Thickness of contaminated zone m 4 Length parallel to aquifer flow m 600 Depth of cover m 0.5 Density of cover material g/cm3 2.0 Erosion of contaminated zone m/y 0.001 Total porosity of contaminated zone – 0.4 Field capacity of contaminated zone – 0.2 Hydraulic conductivity of contaminated zone m/y 100 Evapotranspiration coefficient – 0.68 Wind speed m/s 1.9 Precipitation m/y 1.46 Runoff coefficient – 0.2 Watershed area for nearby stream or pond m2 1 000 000 Humidity in air g/cm3 7.5 Table 2. Hydrological data for saturated zone. Parameter Unit Value Density of saturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 1 000 Hydraulic gradient of saturated zone – 0.02 Water table drop rate m/y 0.001 Well pump intake depth m/ 10 Well pumping rate m3/y 250 Parameter Unit Value Density of saturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 1 000 Hydraulic gradient of saturated zone – 0.02 Water table drop rate m/y 0.001 Well pump intake depth m/ 10 Well pumping rate m3/y 250 Table 2. Hydrological data for saturated zone. Parameter Unit Value Density of saturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 1 000 Hydraulic gradient of saturated zone – 0.02 Water table drop rate m/y 0.001 Well pump intake depth m/ 10 Well pumping rate m3/y 250 Parameter Unit Value Density of saturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 1 000 Hydraulic gradient of saturated zone – 0.02 Water table drop rate m/y 0.001 Well pump intake depth m/ 10 Well pumping rate m3/y 250 Table 3. Input data for uncontaminated unsaturated zone. Parameter Unit Value Thickness of unsaturated zone m 4 Density of unsaturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 100 Parameter Unit Value Thickness of unsaturated zone m 4 Density of unsaturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 100 Table 3. Input data for uncontaminated unsaturated zone. Parameter Unit Value Thickness of unsaturated zone m 4 Density of unsaturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 100 Parameter Unit Value Thickness of unsaturated zone m 4 Density of unsaturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 100 Table 4. Input data for occupancy, inhalation and external gamma. Parameter Unit Value Inhalation rate m3/y 8 400 Mass loading for inhalation g/m3 0.0001 Exposure duration y 30 Indoor dust filtration factor – 0.4 External gamma shielding factor – 0.7 Indoor time fraction – 0.5 Outdoor time fraction – 0.25 Parameter Unit Value Inhalation rate m3/y 8 400 Mass loading for inhalation g/m3 0.0001 Exposure duration y 30 Indoor dust filtration factor – 0.4 External gamma shielding factor – 0.7 Indoor time fraction – 0.5 Outdoor time fraction – 0.25 Table 4. Input data for occupancy, inhalation and external gamma. Parameter Unit Value Inhalation rate m3/y 8 400 Mass loading for inhalation g/m3 0.0001 Exposure duration y 30 Indoor dust filtration factor – 0.4 External gamma shielding factor – 0.7 Indoor time fraction – 0.5 Outdoor time fraction – 0.25 Parameter Unit Value Inhalation rate m3/y 8 400 Mass loading for inhalation g/m3 0.0001 Exposure duration y 30 Indoor dust filtration factor – 0.4 External gamma shielding factor – 0.7 Indoor time fraction – 0.5 Outdoor time fraction – 0.25 Exposure doses for activity concentration levels and disposal amount of wastes In the estimation of maximum exposure doses for three scenarios, we assumed that the NORM waste is uniformly distributed within the whole contaminated zone. However, the amount of NORM waste to be disposed of in a year may be dozens of tons or hundreds of tons, and NORM waste from several industries may be disposed of several times until the full capacity of a landfill is reached. In that case, the exposure doses may be close to the regulation criteria of exposure dose for the general public, 1 mSv/y, before the full capacity of the landfill is reached. Therefore, it is necessary to quantify the maximum amount of NORM waste to be disposed of based on the activity concentration level of each radionuclide in the NORM waste and the regulation criteria of exposure dose for the general public. If we have various values of exposure doses as a function of the activity concentration level of important radionuclides in NORM waste and the disposal amount of NORM waste for each level of radioactivity, we can derive guidelines for the safe management of NORM waste disposal. Therefore, we estimated exposure doses as a function of the activity concentration level and the disposal amount of waste containing U series, Th series and 40K, respectively. We quantified the disposal amount of NORM waste by comparing the regulation criteria of exposure dose for the general public in Korea, 1 mSv/y, and a dose constraint of 0.3 mSv/y as suggested by the IAEA for radiation protection in the post-closure period of a radioactive waste disposal facility(15). Although the occurrence probability of exposure doses from the ingestion pathway is very low, we consider two cases: with ingestion pathway (Case 1) and without ingestion pathway (Case 2). RESULTS AND DISCUSSION Estimation of maximum exposure doses for three scenarios The important input data for the estimation of exposure doses resulting from the disposal of NORM waste into a landfill include the size and shape of the landfill site, the amount of waste to be disposed, the activity concentration level of the radionuclides included in the waste, and the hydrogeological and atmospheric environmental characteristics, among others. The input data are derived from literature sources and the user manual of the RESRAD code. The radionuclides in NORM waste considered in this study are isotopes belonging to the radioactive series headed by the three long-lived isotopes 238U (uranium or U series), 232Th (thorium or Th series) and 40K, which are mentioned in the definition of raw materials of NORM and NORM waste in the act on safety control of NORM. For the conceptual design of the landfill site for NORM waste, we assumed that the area of the site is 400 000 m2, the thickness of the contaminated zone is 4 m, and the soil cover depth is 0.5 m. And it is assumed that the radionuclides in the NORM waste are uniformly distributed within the entire contaminated zone. All the radionuclides are assumed to be in radioactive equilibrium. According to an IAEA report(16), an institutional control period of 300 years is adopted for low and intermediate level radioactive waste repository in most countries. Therefore, we estimated the maximum exposure dose at 300 years, although the peak exposure dose times are different for U series, Th series and 40K. The maximum exposure doses for three scenarios are summarized in Table 5. The contribution of each pathway to exposure dose for the reference scenario and the low leach rate scenario are plotted in Figures 3 and 4. The maximum exposure doses are directly proportional to the activity concentration level of each radionuclide although they are different for each radionuclide due to the different physical, chemical and radiological properties of the radionuclides. Table 5. Maximum exposure doses for different scenarios (mSv/y). Activity concentration level (Bq/g) Radionuclides Exposure dose (mSv/y) Reference scenario Ingestion exclusion scenario Low leach rate scenario 0.1 U series 2.85 × 10−1 1.01 × 10−2 3.39 × 10−1 Th series 4.84 × 10−1 2.08 × 10−2 4.85 × 10−1 40K 2.56 × 10−1 7.69 × 10−5 4.72 × 10−2 0.3 U series 8.56 × 10+0 3.03 × 10−2 1.02 × 10+0 Th series 1.45 × 10+0 6.25 × 10−2 1.46 × 10+0 40K 7.69 × 10−1 2.31 × 10−4 1.42 × 10−1 1.0 U series 2.85 × 10+0 1.01 × 10−1 3.39 × 10+0 Th series 4.84 × 10+0 2.08 × 10−1 4.85 × 10+0 40K 2.56 × 10+0 7.69 × 10−4 4.72 × 10−1 3.0 U series 8.56 × 10+0 3.03 × 10−1 1.02 × 10+1 Th series 1.45 × 10+1 6.25 × 10−1 1.46 × 10+1 40K 7.69 × 10+0 2.31 × 10−3 1.42 × 10+0 10.0 U series 2.85 × 10+1 1.01 × 10+0 3.39 × 10+1 Th series 4.84 × 10+1 2.08 × 10+0 4.85 × 10+1 40K 2.56 × 10+1 7.69 × 10−3 4.72 × 10+0 Activity concentration level (Bq/g) Radionuclides Exposure dose (mSv/y) Reference scenario Ingestion exclusion scenario Low leach rate scenario 0.1 U series 2.85 × 10−1 1.01 × 10−2 3.39 × 10−1 Th series 4.84 × 10−1 2.08 × 10−2 4.85 × 10−1 40K 2.56 × 10−1 7.69 × 10−5 4.72 × 10−2 0.3 U series 8.56 × 10+0 3.03 × 10−2 1.02 × 10+0 Th series 1.45 × 10+0 6.25 × 10−2 1.46 × 10+0 40K 7.69 × 10−1 2.31 × 10−4 1.42 × 10−1 1.0 U series 2.85 × 10+0 1.01 × 10−1 3.39 × 10+0 Th series 4.84 × 10+0 2.08 × 10−1 4.85 × 10+0 40K 2.56 × 10+0 7.69 × 10−4 4.72 × 10−1 3.0 U series 8.56 × 10+0 3.03 × 10−1 1.02 × 10+1 Th series 1.45 × 10+1 6.25 × 10−1 1.46 × 10+1 40K 7.69 × 10+0 2.31 × 10−3 1.42 × 10+0 10.0 U series 2.85 × 10+1 1.01 × 10+0 3.39 × 10+1 Th series 4.84 × 10+1 2.08 × 10+0 4.85 × 10+1 40K 2.56 × 10+1 7.69 × 10−3 4.72 × 10+0 Table 5. Maximum exposure doses for different scenarios (mSv/y). Activity concentration level (Bq/g) Radionuclides Exposure dose (mSv/y) Reference scenario Ingestion exclusion scenario Low leach rate scenario 0.1 U series 2.85 × 10−1 1.01 × 10−2 3.39 × 10−1 Th series 4.84 × 10−1 2.08 × 10−2 4.85 × 10−1 40K 2.56 × 10−1 7.69 × 10−5 4.72 × 10−2 0.3 U series 8.56 × 10+0 3.03 × 10−2 1.02 × 10+0 Th series 1.45 × 10+0 6.25 × 10−2 1.46 × 10+0 40K 7.69 × 10−1 2.31 × 10−4 1.42 × 10−1 1.0 U series 2.85 × 10+0 1.01 × 10−1 3.39 × 10+0 Th series 4.84 × 10+0 2.08 × 10−1 4.85 × 10+0 40K 2.56 × 10+0 7.69 × 10−4 4.72 × 10−1 3.0 U series 8.56 × 10+0 3.03 × 10−1 1.02 × 10+1 Th series 1.45 × 10+1 6.25 × 10−1 1.46 × 10+1 40K 7.69 × 10+0 2.31 × 10−3 1.42 × 10+0 10.0 U series 2.85 × 10+1 1.01 × 10+0 3.39 × 10+1 Th series 4.84 × 10+1 2.08 × 10+0 4.85 × 10+1 40K 2.56 × 10+1 7.69 × 10−3 4.72 × 10+0 Activity concentration level (Bq/g) Radionuclides Exposure dose (mSv/y) Reference scenario Ingestion exclusion scenario Low leach rate scenario 0.1 U series 2.85 × 10−1 1.01 × 10−2 3.39 × 10−1 Th series 4.84 × 10−1 2.08 × 10−2 4.85 × 10−1 40K 2.56 × 10−1 7.69 × 10−5 4.72 × 10−2 0.3 U series 8.56 × 10+0 3.03 × 10−2 1.02 × 10+0 Th series 1.45 × 10+0 6.25 × 10−2 1.46 × 10+0 40K 7.69 × 10−1 2.31 × 10−4 1.42 × 10−1 1.0 U series 2.85 × 10+0 1.01 × 10−1 3.39 × 10+0 Th series 4.84 × 10+0 2.08 × 10−1 4.85 × 10+0 40K 2.56 × 10+0 7.69 × 10−4 4.72 × 10−1 3.0 U series 8.56 × 10+0 3.03 × 10−1 1.02 × 10+1 Th series 1.45 × 10+1 6.25 × 10−1 1.46 × 10+1 40K 7.69 × 10+0 2.31 × 10−3 1.42 × 10+0 10.0 U series 2.85 × 10+1 1.01 × 10+0 3.39 × 10+1 Th series 4.84 × 10+1 2.08 × 10+0 4.85 × 10+1 40K 2.56 × 10+1 7.69 × 10−3 4.72 × 10+0 Figure 3. View largeDownload slide Contribution of each pathway to exposure dose for the reference scenario (Case 1). Figure 3. View largeDownload slide Contribution of each pathway to exposure dose for the reference scenario (Case 1). Figure 4. View largeDownload slide Contribution of each pathway to exposure dose for the low leach rate scenario. Figure 4. View largeDownload slide Contribution of each pathway to exposure dose for the low leach rate scenario. According to the results from other studies(9, 10), the exposure doses to the general public after disposal of NORM waste into landfill sites are in the range of 1.0 × 10−8 to few mSv/y depending on the characteristics of NORM waste and the design of a landfill. And the exposure doses to workers and the general public play a very important role in deriving the masses and activity capacity of NORM waste that can be disposed without considerable radiological impact. For the reference scenario, the most important pathway is the ingestion of plant foods grown in contaminated soil and irrigated with contaminated water for the case of the U series and Th series. But the ingestion of fish from a contaminated pond is the most important pathway for 40K. This is due to the difference in the values of the distribution coefficient and solubility. The values of the distribution coefficients for U series, Th series and 40K used in this study are 50, 60 000 and 5.5 cm3/g, respectively. For the case of Th series, the concentration in the groundwater is very low due to the high value of the distribution coefficient. Therefore, the exposure pathway of direct ingestion of plant foods is the most important pathway. For the U series, there is a similar trend as with the Th series, although the value of the distribution coefficient of the U series is lower than that of the Th series. However, the ingestion of fish from a contaminated pond is the most important pathway for potassium because 40K is highly soluble and has the smallest distribution coefficient value. For the ingestion pathway exclusion scenario, exposure doses are much lower than that for the reference scenario. This is due to the fact that only external exposure from radionuclides in the contaminated soil can affect the exposure dose because there are no radiation effects due to the ingestion of foodstuffs affected by contaminated groundwater. Therefore, it is necessary to prohibit or minimize the consumption of foodstuffs grown near the NORM waste landfill site to protect people from the effects of ionizing radiation. For the low leach rate scenario, we applied a rainfall rate of 1% to the reference scenario to control the flowrate of water through the contaminated zone. For this scenario, the exposure doses have characteristics similar to the reference scenario; however, exposure dose levels are higher than those for other scenarios. The radionuclides in the contaminated zone for this scenario dissolve less and remain more than those in the reference scenario. The increased concentration of each radionuclide in the contaminated zone can have more influences on the root of plants, which are ingested by the exposed individuals. Therefore, the exposure dose by the U series and Th series slightly increase from the reference scenario after 300 years of landfill disposal. In the case of 40K, due to the same reason, the ingestion of contaminated plants has more influence on the total dose than that of the reference scenario, and the impact by the ingestion of groundwater and fish, which are caught in the surface water body affected by the contaminated groundwater, decreased. However, the total dose by 40K decreases up to ~20% of the reference scenario because the limited precipitation caused a lower amount of contaminated groundwater than the reference scenario, and the contaminated groundwater contributes to a major portion of the dose by 40K. Exposure doses for different activity concentration levels and disposal amounts of waste The results of exposure doses as a function of activity concentration level and the disposal amount of NORM wastes containing U series are plotted in Figures 5 and 6, respectively, for Cases 1 and 2, representing the highest and lowest exposure situations. As shown in Figure 5, the exposure dose for NORM wastes containing U series with an activity concentration level of 1 Bq/g does not exceed 1 mSv/y if the amount of wastes does not exceed 2000 tons. Therefore, we can safely dispose of a maximum 2 000 tons of NORM waste containing U series with an activity concentration level of 1 Bq/g. As shown in Figure 6 for Case 2, the exposure dose does not exceed 1 mSv/y for any amount of NORM waste containing U series if the activity concentration level of U series in NORM waste is below 10 Bq/g. This is because the contribution of the ingestion pathway to exposure dose increases as the disposal amount of waste increases. Figure 5. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing U series (Case 1). Figure 5. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing U series (Case 1). Figure 6. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing U series (Case 2). Figure 6. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing U series (Case 2). The exposure doses as a function of the activity concentration level and the disposal amount of NORM waste containing Th series are plotted in Figures 7 and 8, respectively, for Cases 1 and 2, representing the highest and lowest exposure situations. The overall trend of the results is very similar to the case of disposal of waste containing U series. However, compared with the disposal of U series, there is a lower activity concentration level and a lower amount of waste containing Th series. Therefore, more careful management strategies are required for the disposal of wastes containing Th series. According to the results of Case 1 with the ingestion pathway, the exposure dose may exceed 1 mSv/y which is the regulation criteria of exposure dose for the general public in Korea, if more than 600 tons of waste containing Th series is disposed. As shown in Figure 8, we can dispose of any amount of NORM wastes containing Th series if the activity concentration level of the waste is below 5 Bq/g, keeping the same 1 mSv/y of the regulation criteria of exposure dose for the general public. Figure 7. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing Th series (Case 1). Figure 7. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing Th series (Case 1). Figure 8. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing Th series (Case 2). Figure 8. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing Th series (Case 2). The exposure doses as a function of activity concentration level and the disposal amount of NORM wastes containing 40K are plotted in Figure 9 for the Case 1 scenario. If we do not consider the ingestion pathway, any amount and any activity concentration level of wastes containing 40K do not exceed 1 mSv/y. Therefore, the results of exposure doses for Case 2 are not plotted. The exposure doses for the disposal of waste containing 40K with an activity concentration level of 10 Bq/g exceed 1 mSv/y if the amount of waste exceeds 1000 tons. Figure 9. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing 40K (Case 1). Figure 9. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing 40K (Case 1). The other study for deriving the masses that can be disposed without considerable radiological impact showed that masses of NORM waste to be disposed in landfills are in the range of 2.7 × 103 to 1.2 × 106 tons depending on the characteristics of a landfill, the regulation criteria of exposure dose for the general public, activity concentration and total width of the landfill(10). CONCLUSIONS A reference landfill for the safe management of NORM waste disposal was suggested by adding reinforced systems to landfills for industrial and residential waste. Based on this reference landfill, we estimated maximum exposure doses and the relative impact of each pathway to exposure dose for the disposal of NORM waste. The most important pathway is the ingestion pathway, although the details of the impact of ingestion pathway on maximum exposure doses are different because of different distribution coefficients and solubility for each radionuclide in NORM waste. Therefore, the minimization of the impact of the ingestion pathway on exposure doses is very important for the safe management of NORM waste disposal. We estimated the disposal amount of NORM waste including U series, Th series and 40K separately as a function of activity concentration level and the two kinds of regulation criteria of exposure dose, 1 and 0.3 mSv/y. The results obtained in this study show that we can safely dispose maximum 2000 tons of NORM waste containing U series and 600 tons of NORM waste containing Th series with an activity concentration level of 1 Bq/g, if we apply the 1 mSv/y as the regulation criteria of exposure dose. And we can safely dispose ~1000 tons of NORM waste containing 40K with an activity concentration level of 10 Bq/g which is a standard for registration in Korea. And the results of the estimation of disposal amounts of NORM waste without ingestion pathway show that NORM wastes can be disposed of safely regardless of the regulation criteria of exposure dose if we can exclude or minimize the impact of the ingestion pathway on exposure dose. The results of this study can be used to secure the radiological safety of the disposal of NORM waste and to setup foundation for implementing the act on safety control of radioactive materials around the living environment. In addition, these results can be used as technical bases to support the establishment of a guide for the safe management of NORM waste disposal. However, the sensitivity analyses of important parameters such as erosion rate and leach rate need to be performed in order to suggest a suitable site for landfill disposal of NORM waste. FUNDING This work was supported by the Korea Institute of Nuclear Safety (KINS) and this work was performed under the support of Nuclear Research and Development Program (No. 2017M2A8A5014856) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Information and Communications Technology). REFERENCES 1 Chang , B. U. , Kwon , J. W. , Kim , K. H. , Kim , D. J. , Kim , Y. J. et al. . Nationwide surveillance on the environmental radiation. KINS/RR-939/2012 ( 2012 ). 2 Nuclear Safety and Security Commission . Enforce ordinances of the act on safety control of radioactive rays around living environment (in Korean). NSSC (Radiation Safety Division), A Presidential decree No. 27206, Korea ( 2015 ). 3 Nuclear Safety and Security Commission. Act on safety control of radioactive rays around living environment (in Korean). NSSC (Radiation Safety Division), Act No. 13542, Korea ( 2016 ). 4 International Atomic Energy Agency. Radiation protection and management of NORM residues in the titanium dioxide and related industries. IAEA Safety Reports Series No. 76. IAEA Vienna ( 2012 ). 5 International Atomic Energy Agency. Radiation protection and management of NORM residues in the phosphate industry. IAEA Safety Reports Series No. 78. IAEA Vienna ( 2013 ). 6 International Atomic Energy Agency. Management of NORM residues. IAEA-TECDOC-1712. Vienna ( 2013 ). 7 Smith , K. P. , Blunt , D. L. , Williams , G. P. , Arnish , J. J. , Pfingston , M. , Herbert , J. and Haffenden , R. A . An assessment of the disposal of petroleum industry NORM in nonhazardous landfills. DOE/BC/W-31-109-ENG-38-8. Argonne National Laboratory ( 1999 ). 8 Anderson , T. and Mobbs , S. Conditional exemption limits for NORM wastes. HPA-CRCE-001 ( 2010 ). 9 Juan , C. M. , Antonio , B. , Beatriz , R. and Javier , S. Assessment for the management of NORM wastes in conventional hazardous and nonhazardous waste landfills . J. Hazard. Mater. 310 , 161 – 169 ( 2016 ). Google Scholar CrossRef Search ADS PubMed 10 Yu , C. et al. . User’s Manual for RESRAD Version 6, ANL/EAD-4 ( Argonne, IL, USA : Argonne National Laboratory ) ( 2001 ). 11 Eckerman , K. F. et al. . Limiting values of radionuclide intake and air concentration and dose conversion factors for inhalation, submersion, and ingestion. EPA-520/1-88-020, Federal Guidance Report No. 11 ( 1988 ). 12 Eckerman , K. F. and Ryman , J. C. External exposure to radionuclides in air, water, and soil, exposure to dose coefficients for general application, based on the 1987 federal radiation protection guidance. EPA 402-R-93-076, Federal Guidance Report No. 12 ( 1993 ). 13 International Commission on Radiation Protection . Recommendations of the International Commission on Radiological Protection. ICRP Publication 60 ( 1990 ). 14 International Commission on Radiation Protection . Age-dependent doses to members of the public from intake of radionuclides: Part 5 compilation of ingestion and inhalation dose coefficients. ICRP Publication 72 ( 1996 ). 15 International Atomic Energy Agency . Disposal of radioactive waste. IAEA Specific Safety Requirements No. SSR-5 ( 2011 ). 16 International Atomic Energy Agency . Classification of radioactive waste. IAEA General Safety Guide No. GSG-1 ( 2009 ). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

ESTIMATION OF EXPOSURE DOSES FOR THE SAFE MANAGEMENT OF NORM WASTE DISPOSAL

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Abstract

Abstract Naturally occurring radioactive materials (NORM) wastes with different radiological characteristics are generated in several industries. The appropriate options for NORM waste management including disposal options should be discussed and established based on the act and regulation guidelines. Several studies calculated the exposure dose and mass of NORM waste to be disposed in landfill site by considering the activity concentration level and exposure dose. In 2012, the Korean government promulgated an act on the safety control of NORM around living environments to protect human health and the environment. For the successful implementation of this act, we suggest a reference design for a landfill for the disposal of NORM waste. Based on this reference landfill, we estimate the maximum exposure doses and the relative impact of each pathway to exposure dose for three scenarios: a reference scenario, an ingestion pathway exclusion scenario, and a low leach rate scenario. Also, we estimate the possible quantity of NORM waste disposal into a landfill as a function of the activity concentration level of U series, Th series and 40K and two kinds of exposure dose levels, 1 and 0.3 mSv/y. The results of this study can be used to support the establishment of technical bases of the management strategy for the safe disposal of NORM waste. INTRODUCTION Naturally occurring radioactive materials (NORM) are materials that contain radionuclides that exist in the natural environment. Long-lived radioactive elements such uranium, thorium and potassium and any of their decay products such as radium and radon, are typical examples of NORM. These elements have always been present in the earth’s crust and within tissues of all living species. The amount of radioactivity in NORM can be increased or concentrated as a result of human activities and industrial processes, which are called Technologically Enhanced NORM (TENORM). TENORM can increase the potential for human and environmental exposure by concentrating or altering the radiological, physical and chemical properties of radioactive material. In Korea, there is no national guidance for enabling a uniform approach to establishing criteria for regulation of NORM, and there was no national guidance on the management of NORM waste until 2012. In 2012, the Korea Institute of Nuclear Safety (KINS) performed an assessment of the impact of environment radiation and the establishment of optimum management bases for natural radiation to investigate the current status of treatment processes and by-products in typical NORM/TENORM(1). Through this research, the features of high concentration and the effect on the environment were also investigated, and the exposure doses on workers were estimated. The Korean government promulgated and revised an act and an enforcement ordinance on the safety control of NORM around the living environments to protect citizen’s health and the environment(2, 3). The important articles of this act are: (1) a comprehensive plan for radiation protection resulting from radioactive materials around a living environment has to be made every 5 years; (2) the necessary safety guides for the treatment, reuse and disposal of NORM waste have to be made; (3) the regulatory body in Korea, Nuclear Safety and Security Commission (NSSC), should prepare necessary safety guidelines for the treatment, reuse and disposal of NORM waste; (4) all transactors of NORM should register all kinds and magnitudes of raw materials and NORM waste above the standard for registration with the NSSC; (5) the NSSC should install radiation monitoring devices in airports and harbors to secure safe management of NORM; and (6) the NSSC can launch several R&D works to efficiently make an integrated plan for radiation protection, and the government can donate part or all of these R&D funds. The standards for registration of raw materials and NORM waste are 10 Bq/g for 40K and 1 Bq/g for all other radionuclides of natural origin(3). The important articles of the enforcement ordinance of this act are: (1) methods and procedures should be established to reduce radiation exposure for workers in the fields of treatment, reuse and disposal of NORM wastes; (2) the radionuclide concentration can be reduced when NORM waste is reused and disposed; and (3) NORM waste must be disposed of in landfill sites or other disposal facilities, and they should not be reused. For the successful implementation of these acts, many R&D works are being performed to establish technical bases for securing safety control of NORM as follows: development of technical bases for safety regulation of NORM and NORM wastes; establishment of a framework and technical bases for safety assessments related to NORM; development and operation of a support system for the safe management of NORM; development of methods for the determination of NORM in raw materials and by-products; establishment of technical bases for securing radiological safety resulting from the disposal of NORM waste; development of assessment methodologies of exposure doses based on the characteristics of processed products; and qualification requirements for radiation exposure resulting from the use of NORM and building materials. In Korea, NORM waste can be produced by a range of industries such as coal-fired electrical power plants, aluminum production facilities, fertilizer production facilities, zirconium production facilities and steel industries. Activities in these industries can modify the NORM concentrations in products, by-products and waste (residues). However, management strategies and guidance for the safe disposal of NORM waste are not existent, and there are no official disposal facilities for NORM waste, which are temporarily stored near the NORM waste generation facility. For the safe disposal of NORM waste, securing radiological safety is a prerequisite for the safe management of NORM waste which cannot be reused. To secure the radiological safety for the disposal of NORM waste, the establishment of technical bases and procedures for the safe disposal of NORM is required. The International Atomic Energy Agency (IAEA) released several reports to help member states gain perspectives on the management of NORM waste(4–6). And there are other studies that calculate the peak-year dose and carcinogenic risk for disposal of NORM waste into a nonhazardous landfill(7), or recommend a generic upper limit on the annual mass of NORM waste that can be disposed of to a landfill site(8). Also, the masses that can be disposed without considerable radiological impact were derived for the disposal of NORM waste in hazardous and nonhazardous landfills(9) in Spain. In this study, we suggest a reference design for a landfill for the disposal of NORM waste by reinforcing a landfill facility for the disposal of industrial wastes. We estimate the exposure doses for several scenarios for NORM waste disposal into a reference landfill to verify radiological safety. The exposure dose used in this study is effective dose, which is also used in the regulation criteria for the general public in Korea. Also, we estimate the amount of NORM waste to be disposed of into a landfill for different activity concentration levels of important radionuclides in wastes based on exposure dose levels. The results of this study can be used to support the establishment of technical bases as safety guides for the safe disposal of NORM waste. MATERIALS AND METHODS A reference landfill for NORM waste disposal The management strategies for NORM waste disposal are diverse because the radiological, physical and chemical properties of the radioactive material in NORM waste are very different. Among the various kinds of disposal methods for NORM waste, we selected a landfill described in the act on the safety control of NORM(3). We suggested a reference design for a landfill for the disposal of NORM waste, which is shown in Figure 1. First, the landfill site must be located several kilometers from residential areas to minimize potential exposure to radiation. The landfill site also must not be susceptible to landslides and flooding during the rainy season. Figure 1. View largeDownload slide Conceptual design for landfill disposal of NORM waste. Figure 1. View largeDownload slide Conceptual design for landfill disposal of NORM waste. A reference landfill for the disposal of NORM waste was made by reinforcing a landfill for industrial or residential waste. The reinforced systems are impermeable layers and a clay layer at the side and bottom of the landfill, a leachate treatment system, and a system for the treatment and use of gas generated from waste. The impermeable layers at the side and bottom of the landfill site are made of high-density polyethylene (HDPE) which is chemically stable and has a high mechanical strength. Therefore, they can minimize the release and transport of leachate and can prevent the infiltration of water into the landfill site. The lifetime of HDPE is more than 50 years although it depends on the geochemical conditions of the landfill site. The radionuclides may be released from the landfill site due to the degradation of impermeable layer at the side and bottom of the landfill site. The degradation of the impermeable layer depends on the geochemical conditions around the landfill site. However, we assumed that the radionuclides may be released after the closure of the landfill site. A clay layer with a thickness of ~1 m at the side and bottom of the landfill site has to be installed to prevent and/or minimize infiltration of rainfall and release of leachate to neighboring environments. The clay layer also can retard the migration of radionuclides to the surrounding environment due to the high sorption capacity of the clay. The leachate treatment system can control the radioactivity in leachate by measuring and treating the radioactivity of radionuclides. If the radioactivity is higher than the standard for registration, they are treated by appropriate methods and returned to the landfill. If the radioactivity of the radionuclides are lower than the standard for registration, they are treated chemically and physically in the treatment system and are released into the environment. Scenarios for safety assessment of NORM waste disposal The landfill disposal of NORM waste may cause radiation exposure to residents near the site. Therefore, exposure doses must be estimated to secure radiological safety for several scenarios during landfill disposal of NORM waste. The estimation of exposure doses is made by RESRAD code which is a computer model designed to estimate radiation doses and risks from residual radioactive materials(10). This RESRAD code has been used widely by many government agencies and institutions in several countries including Korea as well as the USA. The exposure pathways for the critical population group in the RESRAD code are direct exposure to external radiation from contaminated soil material, internal doses from inhalation of airborne radionuclides, and internal doses from ingestion of plant foods, meat and milk, drinking water, fish and contaminated soil as in Figure 2. Dose coefficients are derived from the references(11–14) for each pathway as described in the user manual of the RESRAD code. Figure 2. View largeDownload slide Exposure pathways considered in RESRAD. Figure 2. View largeDownload slide Exposure pathways considered in RESRAD. We consider three scenarios for the estimation of maximum exposure doses: a reference scenario, an ingestion pathway exclusion scenario, and a low leach rate scenario. The reference scenario is a scenario that consider all exposure pathways and the derived input data. The ingestion pathway exclusion scenario is a scenario that does not consider the ingestion pathway to estimate exposure doses more realistically because the ingestion of plant foods, meat and milk, drinking water, fish, and soil contaminated by the radionuclides near the landfill site is very low. The low leach rate scenario is a scenario that uses a reduced rainfall rate. Almost all landfill sites have a facility that restricts the release of water produced autonomously in the landfill site and leachates to the soil. To consider the effect of this reduced release into the soil, we controlled the amount of water passing the contaminated zone by using 1% of the rainfall rate of the reference scenario. The important input data for the estimation of exposure doses are summarized in Tables 1–4. Table 1. Input data for primary contamination zone and soil cover. Parameter Unit Value Area of contaminated zone m2 400 000 Thickness of contaminated zone m 4 Length parallel to aquifer flow m 600 Depth of cover m 0.5 Density of cover material g/cm3 2.0 Erosion of contaminated zone m/y 0.001 Total porosity of contaminated zone – 0.4 Field capacity of contaminated zone – 0.2 Hydraulic conductivity of contaminated zone m/y 100 Evapotranspiration coefficient – 0.68 Wind speed m/s 1.9 Precipitation m/y 1.46 Runoff coefficient – 0.2 Watershed area for nearby stream or pond m2 1 000 000 Humidity in air g/cm3 7.5 Parameter Unit Value Area of contaminated zone m2 400 000 Thickness of contaminated zone m 4 Length parallel to aquifer flow m 600 Depth of cover m 0.5 Density of cover material g/cm3 2.0 Erosion of contaminated zone m/y 0.001 Total porosity of contaminated zone – 0.4 Field capacity of contaminated zone – 0.2 Hydraulic conductivity of contaminated zone m/y 100 Evapotranspiration coefficient – 0.68 Wind speed m/s 1.9 Precipitation m/y 1.46 Runoff coefficient – 0.2 Watershed area for nearby stream or pond m2 1 000 000 Humidity in air g/cm3 7.5 Table 1. Input data for primary contamination zone and soil cover. Parameter Unit Value Area of contaminated zone m2 400 000 Thickness of contaminated zone m 4 Length parallel to aquifer flow m 600 Depth of cover m 0.5 Density of cover material g/cm3 2.0 Erosion of contaminated zone m/y 0.001 Total porosity of contaminated zone – 0.4 Field capacity of contaminated zone – 0.2 Hydraulic conductivity of contaminated zone m/y 100 Evapotranspiration coefficient – 0.68 Wind speed m/s 1.9 Precipitation m/y 1.46 Runoff coefficient – 0.2 Watershed area for nearby stream or pond m2 1 000 000 Humidity in air g/cm3 7.5 Parameter Unit Value Area of contaminated zone m2 400 000 Thickness of contaminated zone m 4 Length parallel to aquifer flow m 600 Depth of cover m 0.5 Density of cover material g/cm3 2.0 Erosion of contaminated zone m/y 0.001 Total porosity of contaminated zone – 0.4 Field capacity of contaminated zone – 0.2 Hydraulic conductivity of contaminated zone m/y 100 Evapotranspiration coefficient – 0.68 Wind speed m/s 1.9 Precipitation m/y 1.46 Runoff coefficient – 0.2 Watershed area for nearby stream or pond m2 1 000 000 Humidity in air g/cm3 7.5 Table 2. Hydrological data for saturated zone. Parameter Unit Value Density of saturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 1 000 Hydraulic gradient of saturated zone – 0.02 Water table drop rate m/y 0.001 Well pump intake depth m/ 10 Well pumping rate m3/y 250 Parameter Unit Value Density of saturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 1 000 Hydraulic gradient of saturated zone – 0.02 Water table drop rate m/y 0.001 Well pump intake depth m/ 10 Well pumping rate m3/y 250 Table 2. Hydrological data for saturated zone. Parameter Unit Value Density of saturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 1 000 Hydraulic gradient of saturated zone – 0.02 Water table drop rate m/y 0.001 Well pump intake depth m/ 10 Well pumping rate m3/y 250 Parameter Unit Value Density of saturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 1 000 Hydraulic gradient of saturated zone – 0.02 Water table drop rate m/y 0.001 Well pump intake depth m/ 10 Well pumping rate m3/y 250 Table 3. Input data for uncontaminated unsaturated zone. Parameter Unit Value Thickness of unsaturated zone m 4 Density of unsaturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 100 Parameter Unit Value Thickness of unsaturated zone m 4 Density of unsaturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 100 Table 3. Input data for uncontaminated unsaturated zone. Parameter Unit Value Thickness of unsaturated zone m 4 Density of unsaturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 100 Parameter Unit Value Thickness of unsaturated zone m 4 Density of unsaturated zone g/cm3 1.6 Total porosity of saturated zone – 0.4 Effective porosity of saturated zone – 0.2 Field capacity of saturated zone – 0.2 Hydraulic conductivity of saturated zone m/y 100 Table 4. Input data for occupancy, inhalation and external gamma. Parameter Unit Value Inhalation rate m3/y 8 400 Mass loading for inhalation g/m3 0.0001 Exposure duration y 30 Indoor dust filtration factor – 0.4 External gamma shielding factor – 0.7 Indoor time fraction – 0.5 Outdoor time fraction – 0.25 Parameter Unit Value Inhalation rate m3/y 8 400 Mass loading for inhalation g/m3 0.0001 Exposure duration y 30 Indoor dust filtration factor – 0.4 External gamma shielding factor – 0.7 Indoor time fraction – 0.5 Outdoor time fraction – 0.25 Table 4. Input data for occupancy, inhalation and external gamma. Parameter Unit Value Inhalation rate m3/y 8 400 Mass loading for inhalation g/m3 0.0001 Exposure duration y 30 Indoor dust filtration factor – 0.4 External gamma shielding factor – 0.7 Indoor time fraction – 0.5 Outdoor time fraction – 0.25 Parameter Unit Value Inhalation rate m3/y 8 400 Mass loading for inhalation g/m3 0.0001 Exposure duration y 30 Indoor dust filtration factor – 0.4 External gamma shielding factor – 0.7 Indoor time fraction – 0.5 Outdoor time fraction – 0.25 Exposure doses for activity concentration levels and disposal amount of wastes In the estimation of maximum exposure doses for three scenarios, we assumed that the NORM waste is uniformly distributed within the whole contaminated zone. However, the amount of NORM waste to be disposed of in a year may be dozens of tons or hundreds of tons, and NORM waste from several industries may be disposed of several times until the full capacity of a landfill is reached. In that case, the exposure doses may be close to the regulation criteria of exposure dose for the general public, 1 mSv/y, before the full capacity of the landfill is reached. Therefore, it is necessary to quantify the maximum amount of NORM waste to be disposed of based on the activity concentration level of each radionuclide in the NORM waste and the regulation criteria of exposure dose for the general public. If we have various values of exposure doses as a function of the activity concentration level of important radionuclides in NORM waste and the disposal amount of NORM waste for each level of radioactivity, we can derive guidelines for the safe management of NORM waste disposal. Therefore, we estimated exposure doses as a function of the activity concentration level and the disposal amount of waste containing U series, Th series and 40K, respectively. We quantified the disposal amount of NORM waste by comparing the regulation criteria of exposure dose for the general public in Korea, 1 mSv/y, and a dose constraint of 0.3 mSv/y as suggested by the IAEA for radiation protection in the post-closure period of a radioactive waste disposal facility(15). Although the occurrence probability of exposure doses from the ingestion pathway is very low, we consider two cases: with ingestion pathway (Case 1) and without ingestion pathway (Case 2). RESULTS AND DISCUSSION Estimation of maximum exposure doses for three scenarios The important input data for the estimation of exposure doses resulting from the disposal of NORM waste into a landfill include the size and shape of the landfill site, the amount of waste to be disposed, the activity concentration level of the radionuclides included in the waste, and the hydrogeological and atmospheric environmental characteristics, among others. The input data are derived from literature sources and the user manual of the RESRAD code. The radionuclides in NORM waste considered in this study are isotopes belonging to the radioactive series headed by the three long-lived isotopes 238U (uranium or U series), 232Th (thorium or Th series) and 40K, which are mentioned in the definition of raw materials of NORM and NORM waste in the act on safety control of NORM. For the conceptual design of the landfill site for NORM waste, we assumed that the area of the site is 400 000 m2, the thickness of the contaminated zone is 4 m, and the soil cover depth is 0.5 m. And it is assumed that the radionuclides in the NORM waste are uniformly distributed within the entire contaminated zone. All the radionuclides are assumed to be in radioactive equilibrium. According to an IAEA report(16), an institutional control period of 300 years is adopted for low and intermediate level radioactive waste repository in most countries. Therefore, we estimated the maximum exposure dose at 300 years, although the peak exposure dose times are different for U series, Th series and 40K. The maximum exposure doses for three scenarios are summarized in Table 5. The contribution of each pathway to exposure dose for the reference scenario and the low leach rate scenario are plotted in Figures 3 and 4. The maximum exposure doses are directly proportional to the activity concentration level of each radionuclide although they are different for each radionuclide due to the different physical, chemical and radiological properties of the radionuclides. Table 5. Maximum exposure doses for different scenarios (mSv/y). Activity concentration level (Bq/g) Radionuclides Exposure dose (mSv/y) Reference scenario Ingestion exclusion scenario Low leach rate scenario 0.1 U series 2.85 × 10−1 1.01 × 10−2 3.39 × 10−1 Th series 4.84 × 10−1 2.08 × 10−2 4.85 × 10−1 40K 2.56 × 10−1 7.69 × 10−5 4.72 × 10−2 0.3 U series 8.56 × 10+0 3.03 × 10−2 1.02 × 10+0 Th series 1.45 × 10+0 6.25 × 10−2 1.46 × 10+0 40K 7.69 × 10−1 2.31 × 10−4 1.42 × 10−1 1.0 U series 2.85 × 10+0 1.01 × 10−1 3.39 × 10+0 Th series 4.84 × 10+0 2.08 × 10−1 4.85 × 10+0 40K 2.56 × 10+0 7.69 × 10−4 4.72 × 10−1 3.0 U series 8.56 × 10+0 3.03 × 10−1 1.02 × 10+1 Th series 1.45 × 10+1 6.25 × 10−1 1.46 × 10+1 40K 7.69 × 10+0 2.31 × 10−3 1.42 × 10+0 10.0 U series 2.85 × 10+1 1.01 × 10+0 3.39 × 10+1 Th series 4.84 × 10+1 2.08 × 10+0 4.85 × 10+1 40K 2.56 × 10+1 7.69 × 10−3 4.72 × 10+0 Activity concentration level (Bq/g) Radionuclides Exposure dose (mSv/y) Reference scenario Ingestion exclusion scenario Low leach rate scenario 0.1 U series 2.85 × 10−1 1.01 × 10−2 3.39 × 10−1 Th series 4.84 × 10−1 2.08 × 10−2 4.85 × 10−1 40K 2.56 × 10−1 7.69 × 10−5 4.72 × 10−2 0.3 U series 8.56 × 10+0 3.03 × 10−2 1.02 × 10+0 Th series 1.45 × 10+0 6.25 × 10−2 1.46 × 10+0 40K 7.69 × 10−1 2.31 × 10−4 1.42 × 10−1 1.0 U series 2.85 × 10+0 1.01 × 10−1 3.39 × 10+0 Th series 4.84 × 10+0 2.08 × 10−1 4.85 × 10+0 40K 2.56 × 10+0 7.69 × 10−4 4.72 × 10−1 3.0 U series 8.56 × 10+0 3.03 × 10−1 1.02 × 10+1 Th series 1.45 × 10+1 6.25 × 10−1 1.46 × 10+1 40K 7.69 × 10+0 2.31 × 10−3 1.42 × 10+0 10.0 U series 2.85 × 10+1 1.01 × 10+0 3.39 × 10+1 Th series 4.84 × 10+1 2.08 × 10+0 4.85 × 10+1 40K 2.56 × 10+1 7.69 × 10−3 4.72 × 10+0 Table 5. Maximum exposure doses for different scenarios (mSv/y). Activity concentration level (Bq/g) Radionuclides Exposure dose (mSv/y) Reference scenario Ingestion exclusion scenario Low leach rate scenario 0.1 U series 2.85 × 10−1 1.01 × 10−2 3.39 × 10−1 Th series 4.84 × 10−1 2.08 × 10−2 4.85 × 10−1 40K 2.56 × 10−1 7.69 × 10−5 4.72 × 10−2 0.3 U series 8.56 × 10+0 3.03 × 10−2 1.02 × 10+0 Th series 1.45 × 10+0 6.25 × 10−2 1.46 × 10+0 40K 7.69 × 10−1 2.31 × 10−4 1.42 × 10−1 1.0 U series 2.85 × 10+0 1.01 × 10−1 3.39 × 10+0 Th series 4.84 × 10+0 2.08 × 10−1 4.85 × 10+0 40K 2.56 × 10+0 7.69 × 10−4 4.72 × 10−1 3.0 U series 8.56 × 10+0 3.03 × 10−1 1.02 × 10+1 Th series 1.45 × 10+1 6.25 × 10−1 1.46 × 10+1 40K 7.69 × 10+0 2.31 × 10−3 1.42 × 10+0 10.0 U series 2.85 × 10+1 1.01 × 10+0 3.39 × 10+1 Th series 4.84 × 10+1 2.08 × 10+0 4.85 × 10+1 40K 2.56 × 10+1 7.69 × 10−3 4.72 × 10+0 Activity concentration level (Bq/g) Radionuclides Exposure dose (mSv/y) Reference scenario Ingestion exclusion scenario Low leach rate scenario 0.1 U series 2.85 × 10−1 1.01 × 10−2 3.39 × 10−1 Th series 4.84 × 10−1 2.08 × 10−2 4.85 × 10−1 40K 2.56 × 10−1 7.69 × 10−5 4.72 × 10−2 0.3 U series 8.56 × 10+0 3.03 × 10−2 1.02 × 10+0 Th series 1.45 × 10+0 6.25 × 10−2 1.46 × 10+0 40K 7.69 × 10−1 2.31 × 10−4 1.42 × 10−1 1.0 U series 2.85 × 10+0 1.01 × 10−1 3.39 × 10+0 Th series 4.84 × 10+0 2.08 × 10−1 4.85 × 10+0 40K 2.56 × 10+0 7.69 × 10−4 4.72 × 10−1 3.0 U series 8.56 × 10+0 3.03 × 10−1 1.02 × 10+1 Th series 1.45 × 10+1 6.25 × 10−1 1.46 × 10+1 40K 7.69 × 10+0 2.31 × 10−3 1.42 × 10+0 10.0 U series 2.85 × 10+1 1.01 × 10+0 3.39 × 10+1 Th series 4.84 × 10+1 2.08 × 10+0 4.85 × 10+1 40K 2.56 × 10+1 7.69 × 10−3 4.72 × 10+0 Figure 3. View largeDownload slide Contribution of each pathway to exposure dose for the reference scenario (Case 1). Figure 3. View largeDownload slide Contribution of each pathway to exposure dose for the reference scenario (Case 1). Figure 4. View largeDownload slide Contribution of each pathway to exposure dose for the low leach rate scenario. Figure 4. View largeDownload slide Contribution of each pathway to exposure dose for the low leach rate scenario. According to the results from other studies(9, 10), the exposure doses to the general public after disposal of NORM waste into landfill sites are in the range of 1.0 × 10−8 to few mSv/y depending on the characteristics of NORM waste and the design of a landfill. And the exposure doses to workers and the general public play a very important role in deriving the masses and activity capacity of NORM waste that can be disposed without considerable radiological impact. For the reference scenario, the most important pathway is the ingestion of plant foods grown in contaminated soil and irrigated with contaminated water for the case of the U series and Th series. But the ingestion of fish from a contaminated pond is the most important pathway for 40K. This is due to the difference in the values of the distribution coefficient and solubility. The values of the distribution coefficients for U series, Th series and 40K used in this study are 50, 60 000 and 5.5 cm3/g, respectively. For the case of Th series, the concentration in the groundwater is very low due to the high value of the distribution coefficient. Therefore, the exposure pathway of direct ingestion of plant foods is the most important pathway. For the U series, there is a similar trend as with the Th series, although the value of the distribution coefficient of the U series is lower than that of the Th series. However, the ingestion of fish from a contaminated pond is the most important pathway for potassium because 40K is highly soluble and has the smallest distribution coefficient value. For the ingestion pathway exclusion scenario, exposure doses are much lower than that for the reference scenario. This is due to the fact that only external exposure from radionuclides in the contaminated soil can affect the exposure dose because there are no radiation effects due to the ingestion of foodstuffs affected by contaminated groundwater. Therefore, it is necessary to prohibit or minimize the consumption of foodstuffs grown near the NORM waste landfill site to protect people from the effects of ionizing radiation. For the low leach rate scenario, we applied a rainfall rate of 1% to the reference scenario to control the flowrate of water through the contaminated zone. For this scenario, the exposure doses have characteristics similar to the reference scenario; however, exposure dose levels are higher than those for other scenarios. The radionuclides in the contaminated zone for this scenario dissolve less and remain more than those in the reference scenario. The increased concentration of each radionuclide in the contaminated zone can have more influences on the root of plants, which are ingested by the exposed individuals. Therefore, the exposure dose by the U series and Th series slightly increase from the reference scenario after 300 years of landfill disposal. In the case of 40K, due to the same reason, the ingestion of contaminated plants has more influence on the total dose than that of the reference scenario, and the impact by the ingestion of groundwater and fish, which are caught in the surface water body affected by the contaminated groundwater, decreased. However, the total dose by 40K decreases up to ~20% of the reference scenario because the limited precipitation caused a lower amount of contaminated groundwater than the reference scenario, and the contaminated groundwater contributes to a major portion of the dose by 40K. Exposure doses for different activity concentration levels and disposal amounts of waste The results of exposure doses as a function of activity concentration level and the disposal amount of NORM wastes containing U series are plotted in Figures 5 and 6, respectively, for Cases 1 and 2, representing the highest and lowest exposure situations. As shown in Figure 5, the exposure dose for NORM wastes containing U series with an activity concentration level of 1 Bq/g does not exceed 1 mSv/y if the amount of wastes does not exceed 2000 tons. Therefore, we can safely dispose of a maximum 2 000 tons of NORM waste containing U series with an activity concentration level of 1 Bq/g. As shown in Figure 6 for Case 2, the exposure dose does not exceed 1 mSv/y for any amount of NORM waste containing U series if the activity concentration level of U series in NORM waste is below 10 Bq/g. This is because the contribution of the ingestion pathway to exposure dose increases as the disposal amount of waste increases. Figure 5. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing U series (Case 1). Figure 5. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing U series (Case 1). Figure 6. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing U series (Case 2). Figure 6. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing U series (Case 2). The exposure doses as a function of the activity concentration level and the disposal amount of NORM waste containing Th series are plotted in Figures 7 and 8, respectively, for Cases 1 and 2, representing the highest and lowest exposure situations. The overall trend of the results is very similar to the case of disposal of waste containing U series. However, compared with the disposal of U series, there is a lower activity concentration level and a lower amount of waste containing Th series. Therefore, more careful management strategies are required for the disposal of wastes containing Th series. According to the results of Case 1 with the ingestion pathway, the exposure dose may exceed 1 mSv/y which is the regulation criteria of exposure dose for the general public in Korea, if more than 600 tons of waste containing Th series is disposed. As shown in Figure 8, we can dispose of any amount of NORM wastes containing Th series if the activity concentration level of the waste is below 5 Bq/g, keeping the same 1 mSv/y of the regulation criteria of exposure dose for the general public. Figure 7. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing Th series (Case 1). Figure 7. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing Th series (Case 1). Figure 8. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing Th series (Case 2). Figure 8. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing Th series (Case 2). The exposure doses as a function of activity concentration level and the disposal amount of NORM wastes containing 40K are plotted in Figure 9 for the Case 1 scenario. If we do not consider the ingestion pathway, any amount and any activity concentration level of wastes containing 40K do not exceed 1 mSv/y. Therefore, the results of exposure doses for Case 2 are not plotted. The exposure doses for the disposal of waste containing 40K with an activity concentration level of 10 Bq/g exceed 1 mSv/y if the amount of waste exceeds 1000 tons. Figure 9. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing 40K (Case 1). Figure 9. View largeDownload slide Exposure doses as a function of activity concentration level and the disposal amount of wastes containing 40K (Case 1). The other study for deriving the masses that can be disposed without considerable radiological impact showed that masses of NORM waste to be disposed in landfills are in the range of 2.7 × 103 to 1.2 × 106 tons depending on the characteristics of a landfill, the regulation criteria of exposure dose for the general public, activity concentration and total width of the landfill(10). CONCLUSIONS A reference landfill for the safe management of NORM waste disposal was suggested by adding reinforced systems to landfills for industrial and residential waste. Based on this reference landfill, we estimated maximum exposure doses and the relative impact of each pathway to exposure dose for the disposal of NORM waste. The most important pathway is the ingestion pathway, although the details of the impact of ingestion pathway on maximum exposure doses are different because of different distribution coefficients and solubility for each radionuclide in NORM waste. Therefore, the minimization of the impact of the ingestion pathway on exposure doses is very important for the safe management of NORM waste disposal. We estimated the disposal amount of NORM waste including U series, Th series and 40K separately as a function of activity concentration level and the two kinds of regulation criteria of exposure dose, 1 and 0.3 mSv/y. The results obtained in this study show that we can safely dispose maximum 2000 tons of NORM waste containing U series and 600 tons of NORM waste containing Th series with an activity concentration level of 1 Bq/g, if we apply the 1 mSv/y as the regulation criteria of exposure dose. And we can safely dispose ~1000 tons of NORM waste containing 40K with an activity concentration level of 10 Bq/g which is a standard for registration in Korea. And the results of the estimation of disposal amounts of NORM waste without ingestion pathway show that NORM wastes can be disposed of safely regardless of the regulation criteria of exposure dose if we can exclude or minimize the impact of the ingestion pathway on exposure dose. The results of this study can be used to secure the radiological safety of the disposal of NORM waste and to setup foundation for implementing the act on safety control of radioactive materials around the living environment. In addition, these results can be used as technical bases to support the establishment of a guide for the safe management of NORM waste disposal. However, the sensitivity analyses of important parameters such as erosion rate and leach rate need to be performed in order to suggest a suitable site for landfill disposal of NORM waste. FUNDING This work was supported by the Korea Institute of Nuclear Safety (KINS) and this work was performed under the support of Nuclear Research and Development Program (No. 2017M2A8A5014856) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Information and Communications Technology). REFERENCES 1 Chang , B. U. , Kwon , J. W. , Kim , K. H. , Kim , D. J. , Kim , Y. J. et al. . Nationwide surveillance on the environmental radiation. KINS/RR-939/2012 ( 2012 ). 2 Nuclear Safety and Security Commission . Enforce ordinances of the act on safety control of radioactive rays around living environment (in Korean). NSSC (Radiation Safety Division), A Presidential decree No. 27206, Korea ( 2015 ). 3 Nuclear Safety and Security Commission. Act on safety control of radioactive rays around living environment (in Korean). NSSC (Radiation Safety Division), Act No. 13542, Korea ( 2016 ). 4 International Atomic Energy Agency. Radiation protection and management of NORM residues in the titanium dioxide and related industries. IAEA Safety Reports Series No. 76. IAEA Vienna ( 2012 ). 5 International Atomic Energy Agency. Radiation protection and management of NORM residues in the phosphate industry. IAEA Safety Reports Series No. 78. IAEA Vienna ( 2013 ). 6 International Atomic Energy Agency. Management of NORM residues. IAEA-TECDOC-1712. Vienna ( 2013 ). 7 Smith , K. P. , Blunt , D. L. , Williams , G. P. , Arnish , J. J. , Pfingston , M. , Herbert , J. and Haffenden , R. A . An assessment of the disposal of petroleum industry NORM in nonhazardous landfills. DOE/BC/W-31-109-ENG-38-8. Argonne National Laboratory ( 1999 ). 8 Anderson , T. and Mobbs , S. Conditional exemption limits for NORM wastes. HPA-CRCE-001 ( 2010 ). 9 Juan , C. M. , Antonio , B. , Beatriz , R. and Javier , S. Assessment for the management of NORM wastes in conventional hazardous and nonhazardous waste landfills . J. Hazard. Mater. 310 , 161 – 169 ( 2016 ). Google Scholar CrossRef Search ADS PubMed 10 Yu , C. et al. . User’s Manual for RESRAD Version 6, ANL/EAD-4 ( Argonne, IL, USA : Argonne National Laboratory ) ( 2001 ). 11 Eckerman , K. F. et al. . Limiting values of radionuclide intake and air concentration and dose conversion factors for inhalation, submersion, and ingestion. EPA-520/1-88-020, Federal Guidance Report No. 11 ( 1988 ). 12 Eckerman , K. F. and Ryman , J. C. External exposure to radionuclides in air, water, and soil, exposure to dose coefficients for general application, based on the 1987 federal radiation protection guidance. EPA 402-R-93-076, Federal Guidance Report No. 12 ( 1993 ). 13 International Commission on Radiation Protection . Recommendations of the International Commission on Radiological Protection. ICRP Publication 60 ( 1990 ). 14 International Commission on Radiation Protection . Age-dependent doses to members of the public from intake of radionuclides: Part 5 compilation of ingestion and inhalation dose coefficients. ICRP Publication 72 ( 1996 ). 15 International Atomic Energy Agency . Disposal of radioactive waste. IAEA Specific Safety Requirements No. SSR-5 ( 2011 ). 16 International Atomic Energy Agency . Classification of radioactive waste. IAEA General Safety Guide No. GSG-1 ( 2009 ). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Radiation Protection DosimetryOxford University Press

Published: Mar 16, 2018

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