Radionalytical chemistry in support of the naturally occurring radioactive material (NORM) industries in South Africa

Abstract

Humans are exposed to ionizing radiation from a number of sources of which the largest proportion (about 75%) comes from natural sources. Exploitation of Uranium- and Thorium bearing mineral resources may result in elevated concentrations of the naturally occurring radionuclides (238U, 234U, 230Th, 226Ra, 210Pb, 210Po, 235U, 231Pa, 227Ac, 232Th, 228Ra, 228Th) in products, residues, waste and the environment at industries involved with processing of these raw materials. This can result in exposure to the public via a number of pathways. In South Africa the main industries of concern include the building-, coal-, phosphate-, and mineral sands industry, as well as gold mining. The industry operator is responsible for radiation protection of both people and the environment in relation to all industrial and mining activities. The radiological impact of all operations in South Africa is monitored as part of the license obligations, imposed by the South African Nuclear Energy Act and enforced by the South African National Nuclear Regulator (NNR). Accurate and sensitive measurements are required to provide data for Naturally Occurring Radioactive Material (NORM) industries to prove compliance with regulations in order to minimize the radiological impact on humans and the environment. In this study the application of radiometric techniques in support of environmental monitoring programmes of NORM industries, were demonstrated. Suitable radiometric techniques were evaluated and implemented in the laboratory. Radio-analytical data for environmental media from various NORM industries were collected and evaluated. Results of more than 1000 samples analysed over a period of 5-7 years were used in this study. Although the focus of the presented work was on the analytical techniques, measurement and the results obtained, is also gave some indication of the radiological impact of NORM industries in South Africa, how they contribute to radioactivity in the environment and how this can be accurately measured and assessed. The method performance parameters indicated suitability of the implemented methods for quantification of naturally occurring radionuclides in water, food and solids at levels well below internationally accepted limits. It was demonstrated how incorrect and insensitive analytical methodology could result in incorrect conclusions, usually leading to over-estimation of the dose and impact of the industry. For analysis of foodstuff it was shown that measurement of ashed samples greatly improved the limit of detection (LOD) of the instrumental techniques. However, to prevent overestimation of dose due to a high LOD, nuclides such as 230Th and 210Po had to be analysed by radiochemical separation after microwave digestion. The radiochemical methods were adequate for determination of individual nuclides of interest at a level of 0.005-0.01 Bq.L-1 with a precision and accuracy of better than 20%, which met the demand for more sensitive radiometric techniques for lower maximum limits in radioprotection regulations. Since equilibrium between the nuclides within the natural decay series may be disturbed as a consequence of physical and chemical processes in the extraction of minerals and concentrates from ores, it was important to analyse the individual radionuclides of these series and not only the parent nuclides. Analytical data for environmental samples obtained from routine monitoring programmes of NORM industries were used to evaluate the potential impact of the industries in South Africa by application in several case studies. The ERICA tool was used to assess the radiological risk to biota, but the main emphasize was on the impact to humans. For this purpose the pathways of internal ingestion and external gamma dose radiation were considered, as well as some secondary pathways such as consumption of meat and milk of cattle grazing on contaminated crops. The results presented for solids gave an indication of the distribution of radionuclides associated with solids from different NORM industries in SA. In general, results obtained were comparable to similar studies undertaken in other countries. As long as activity concentrations remained below 500 Bq.kg-1, the reference dose level of 1 mSv per year was not exceeded. Higher activity concentrations observed in solid samples from some industries was found to be of limited concern to the public. Based on the results presented for selected samples of building materials and constituents used in the manufacture of cement, it was concluded that the building industry can be exempted from nuclear regulatory control in South Africa, even though some raw material (slag, fly ash) exceeded the recommended exemption levels for exposure. The results presented for selected samples from the phosphate industry also suggested that it was unlikely for any member of the public to receive a significant dose from the use of phosphate rocks for fertilizer production. Essential basic data were provided for environmental radiological risk analysis for Coal Fired Power Plants which are not currently regulated in South Africa. Since the activity concentrations in fly ash and coal samples were all lower than the 500 Bq.kg-1 exemption limit set by the NNR, it was found not to be a source of alarm. Natural radioactivity in coal samples was not significantly higher than in soils and rocks and of the same order of magnitude as in other parts of the world, whereas fly ash showed some enrichment, again similar to the trend observed in worldwide studies on coal and ashes. Utilization of fly-ash as an additive in cement should be possible without any special restrictions based on the regulations dealing with building material in the European directive.

The results for environmental media showed elevated concentrations compared to the baseline data in some areas. Measured soil concentrations from all areas were similar to background, whereas some sediment samples showed enhanced concentrations. On average the concentrations in sediment and tailings samples from the specific gold mine area investigated did not exceed the 500 Bq.kg-1 exemption level and no urgent countermeasures are required. The radiological impact may become important if new uses for these areas are considered. The bottled water analysed was radiologically safe and posed no significant hazard to the consumer. For the majority of environmental water sources sampled by the Department Water and Sanitation (DWS) the screening results indicated good or ideal quality water, with only some radioactive ―hot spots‖ indicated; for which future monitoring should include nuclide specific analysis. Dose assessment based on nuclide specific analysis of environmental surface- and groundwater showed that the radiological quality of most of the sources was good resulting in a dose of less than 1 mSv.a-1 if consumed. Water from some sites exceeded the NNR dose constraint of 0.25 mSv.a-1 and might have health effects to the most sensitive age group (infant) if used for domestic purposes. The quality of water from mining- and NORM industries varied, with fairly high activities measured in some surface water samples from the gold mine areas, indicating a possible radiological health risk from ingestion of this water which was clearly impacted by process water. However, for the majority of sources from other industries the results indicate insignificant levels of public exposure and suggest only limited radiological impact of the industrial activities on communities in the area. In water impacted by mines the ratio of 234U/238U was found to be equal to one, whereas in environmental groundwater the concentration of 234U was found to be higher than 238U up to a factor three. Measurement of only 238U was shown to result in an underestimation of the total U activity in these water samples with observed disequilibrium. However, for surface water sources from the DWS study future radiological assessment may only require determination of the chemical concentration of uranium, based on the correlation found between the U concentration and gross alpha activity. With single exceptions, the results did not indicate a significant impact of the industrial activities on food grown in any of the areas. The estimated dose for adults was well below the ―single facility‖ dose constraint of 0.25 mSv.a-1. For many samples where the dose limit was exceeded it could be explained by either poor sensitivity of the analytical technique resulting in high LODs, assumption of equilibrium for some nuclides with high dose conversion coefficients which were not measured (e.g. 230Th and 210Po) or unrealistic high consumption rates used for the dose estimation. The high concentrations measured in some samples cannot be ignored since it might indicate contamination in some areas. More work has to be done to determine the impact of the specific NORM industries. The activity concentrations of 210Po in food confirmed that ingestion of food is an important route vi

for intake of this nuclide. The choice of parameters used in dose assessment models can have a huge impact on results. Discrepancies were observed between measured and modelled values. The parameters suggested for use by the NNR for assessments were in most cases found to be conservative, resulting in overestimation of dose. Unfortunately, overestimation due to conservative assumptions is usually not communicated to the public, leading to unnecessary concern. It was therefore recommended that site-specific parameters are collected whenever possible. The ability of the radio analytical laboratory to provide accurate and reliable data at the required sensitivity level in a timely fashion is essential for this purpose. NORM industries in South Africa are well regulated; operating under a Certificate of Registration issued by the NNR, and optimizing practises to keep dose As Low as Reasonably Achievable (ALARA). Industries are assuming responsibility and control releases to the environment. The results presented suggested that members of the public were unlikely to receive any significant dose from any of the NORM industries. The high concentrations measured in some of the products confirmed the need for continuous control and monitoring of the industry, especially where byproducts are utilized elsewhere. To prevent unnecessary concern by the public it is important that enough scientific data are published and available in the public domain. Access to data similar as presented in this thesis will help to provide clarity and transparency. Finally it was emphasized that no evaluation should be based on results from a single grab sample, but that an effort should be made to obtain samples representative of a long period.