Variation of natural radionuclides in non-ferrous fayalite slags during a one-month production period

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 ²³⁸U and ²³²Th decay chains, ²³⁵U and ⁴⁰K using HPGe detectors. Secular equilibrium was observed for the slags, flue dust and feedstock samples in the ²³²Th decay chain, in contrast to the ²³⁸U decay chain. During the month in question the ratios of maximum over minimum activity concentration were 3.1 ± 0.5 for ⁴⁰K, 4 ± 1 for ²³⁸U, 6 ± 1 for ²²⁶Ra, 13 ± 7 for ²¹⁰Pb, 4.5 ± 0.6 for ²²⁸Ra and 4.7 ± 0.7 for ²²⁸Th for the slags. Even with the activity concentration of the feedstock material ranging up to 2.1 ± 0.3 kBq/kg for ²³⁸U, 1.6 ± 0.2 kBq for ²²⁶Ra, 22 ± 7 kBq/kg for ²¹⁰Pb, 2.1 ± 0.2 kBq/kg for ²²⁸Ra and 2.0 ± 0.4 kBq/kg for ²²⁸Th, 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.

Keywords: Gamma-ray spectrometry; NORM (naturally occurring radioactive materials); Naturally occurring radionuclides; Non-ferrous metallurgical slag; Radiological equilibrium; Variation over time.