Equilibrium in segments of natural decay series in NORM samples from the Czech Republic: 238U - 234U - 230Th and 232Th - 228Ra - 228Th.

Industrial processing of mineral resources can lead to the redistribution and concentration of naturally occurring radioactive materials (NORM). In the Czech Republic, hazards associated with the release of NORM from industrial facilities, its handling, and long-term exposure are assessed by comparing activity concentrations of 10 long-living radionuclides with the associated clearance levels. Six radionuclides are determined by gamma-ray spectrometry: 238U, 226Ra, 210Pb, 228Ra, 228Th, and 40K. Activity concentrations of four radionuclides - 234U, 230Th, 210Po, and 232Th - are derived using a set of estimative equations. These equations frequently lead to overestimation of 230Th and 232Th, and underestimation of 234U. The aim of this work was to study the equilibrium in the first segments of the uranium (238U -234U - 230Th) and thorium (232Th - 228Ra - 228Th) decay series, and to propose an updated set of estimative equations for 234U, 230Th, and 232Th for materials that satisfy the equilibrium condition in the given segments. Importantly, materials originating from different industrial processes were assessed individually, and all materials included in this study were released from workplaces in the Czech Republic. In total, 48 samples from 7 workplace types were split into 8 material categories. For each sample, gamma-ray spectrometry was used to determine 228Ra, and 228Th, and alpha spectrometry to determine 238U, 234U, 230Th, and 232Th. Activity ratios for each radionuclide pair within the given decay segment were assessed using orthogonal distance regression and/or by evaluating the activity ratios. Material categories exhibiting radioactive equilibrium in both decay segments were ashes and slag from coal combustion plants, samples utilized in cement manufacturing, old building materials, drilling muds, and waste from zircon and zirconia processing. In contrast, sludges, sinters and filter materials from water treatment facilities, along with waste from oil and gas transportation and processing, were identified as materials with disturbed equilibrium in the 238U -234U - 230Th and 232Th - 228Ra - 228Th decay segments.