Variation of natural radionuclides in non-ferrous fayalite slags during a one-month production period. (June 2017) - Croymans, Tom
Abstract: The European Basic Safety Standards (EU-BSS) describes a set of NORM (Naturally Occurring Radioactive Material)-processing industries which produce residues known to be possibly enriched in NORs (Naturally Occurring Radionuclides). These residues can be used as a component in building materials aimed for public usage. The industrial processes, in which the residues are produced, are often complex and total monitoring can be challenging especially when the origin of the used raw materials varies. In this study the NORs present in non-ferrous fayalite slags of a secondary smelter facility, a NORM-processing industry according to the EU-BSS, were monitored daily during a one-month production period. In addition flue dust samples and feedstock samples, known to contain elevated levels of NORs, of the same period were measured. The survey involved the gamma-ray spectrometric analysis of the decay products from the 238 U and 232 Th decay chains, 235 U and 40 K using HPGe detectors. Secular equilibrium was observed for the slags, flue dust and feedstock samples in the 232 Th decay chain, in contrast to the 238 U decay chain. During the month in question the ratios of maximum over minimum activity concentration were 3.1 ± 0.5 for 40 K, 4 ± 1 for 238 U, 6 ± 1 for 226 Ra, 13 ± 7 for 210 Pb, 4.5 ± 0.6 for 228 Ra and 4.7 ± 0.7 for 228 Th for the slags. Even with the activity concentration of the feedstock material ranging up to 2.1 ± 0.3 kBq/kg for 238 U, 1.6 ± 0.2 kBq for 226 Ra, 22 ± 7 kBq/kg for 210 Pb, 2.1 ± 0.2 kBq/kg for 228 Ra and 2.0 ± 0.4 kBq/kg for 228 Th, none of the slag samples exceeded the exemption/clearance levels of the EU-BSS and RP-122 part II, which can respectively provide guidance under equilibrium and in absence of equilibrium. As each NORM-processing industry has its own complexity and variability, the observed variations point out that one should approach one-time measurements or low frequency monitoring methods cautiously. Low frequency measurements should be optimised depending on the discharge of the batches. A follow up of the industrial process and its output can provide important insights to assure a limited public exposure upon application of these industrial residues. Finally a comparison is made with reported data on other metallurgical slags and the use of the slags in building materials is evaluated using the Activity Concentration Index (ACI) proposed by the EU-BSS. Highlights: Natural radionuclides monitored in non-ferrous slag during 1 month production. Large activity concentration fluctuations of natural occurring radionuclides. Disequilibrium in the 238 U decay series and equilibrium in the 232 Th decay series. Extensive comparison between different types of metallurgical slags. Continuous monitoring of the produced slag is advised to assure safe application.