Assessment of radiological impact on the surrounding environmental and biota in NORM-relate facility through analysis of uranium-238 and thorium-232 decay series

Abstract

In general, minerals or soil containing natural radionuclides at a low concentration level mainly exist, but certain minerals or raw materials (building materials), including some commercial minerals, have a fairly high concentration of natural radionuclides. In addition, as it is unevenly distributed to various substances generated during the water extraction process and subsequent physical and chemical processes, it has become a subject to consider exposure to the environment or people. It is important to first determine the concentration of a natural radionuclide in order to conduct an assessment of radiological effects on the environment (including organisms) or humans. The main objective of this study is to investigate an efficient and accurate method for quantitative analysis of uranium and thorium and assess for the radiological impact on the surrounding NORM-related facility. In Chapter 2, a separation method for efficiently analyzing the concentration of uranium and thorium is investigated and the results of a comparative study of the three separation methods are presented. As a result, the separation method using resin showed that the matrix effect was mostly removed, but the recovery rate was lower than that of other separation methods. The Fe co-precipitation method was the easiest and simplest method and showed a high recovery rate at low cost. The PEG method has shown excellent ability to remove Si from some samples and has a high recovery rate, but is quite time consuming. This study can be useful in determining the separation method required for matrix removal depending on the type of NORM sample to determine the concentrations of uranium and thorium through ICP-MS. Chapter 3 describes a standard addition (SA) calibration method for the accurate analysis of uranium and thorium utilizing ICP-MS in naturally occurring radioactive materials (NORM). A pretreatment method using Fusion was established and methodologically optimized. The standard addition method was validated (LOD, LOQ, linearity, selectivity and accuracy) using certified reference materials (SRM 2709a, RM-ZR and RM-BX). In addition, the results of the analysis for the validated method were evaluated for bias and a correction equation including contributions to the extended uncertainty was used. All performance criteria were satisfied, and the results were good agreement with the certification and reference values. In addition, the standard addition method was applied to 7 samples collected from facilities handling NORM, and the results of gamma technique analysis for these samples were compared. Through the study results, it was confirmed that the standard addition method can compensate for the bias that cannot be corrected for the influence of the medium through external correction methods or internal standards. In Chapter 4, the effects of the deposited gypsum residue on the surrounding environment under various exposure conditions and the radiation exposure of animals and plants were evaluated. The waste stockyard site at the Korean facility was used to store phosphogypsum, a by-product of the phosphoric acid process, in the form of a slurry in a large gypsum storage facility. Accordingly, considering the diffusion to the environment by rain or wind, etc., the effect of radiation on the non-human environment was evaluated through the ERICA tool. The effect of radiation on the environment due to the phosphate gypsum reservoir was negligible with a screening dose of less than 10 μGy h −1. However, in the phosphogypsum accumulation field, the estimated dose rate of the shrub was evaluated considering the soil at the boundary of the accumulation field as an input value. The effects of phosphogypsum reservoirs on the surrounding environment accounted for 95–100% of the total dose for internal exposure to biota. In general, radium was found to be the largest contributor to biota, followed by lead and polonium to radiation dose. The results of the study are expected to contribute to understanding the radioactive impact of waste stored and disposed of at the facility on the environment and biota (all exposure pathways), and to develop sustainable operation and pollution monitoring policies.