Development of in vitro testing methods with materials suitable for radiation exposure and internal contamination assessment including NORM materials

The main purpose of this study was to determine the solubility and distribution of particles forming slag and ash containing naturally occurring radioactive materials, in simulated lung fluids (SLF). The experiments were performed to simulate the behaviour of radionuclides deposited in workers' lungs at sites, where the high risk of dust particles and aerosol inhalation is expected, especially at coal power plants. Several samples of slags and fly ash were taken from fossil fuel power stations throughout the Czech Republic. The samples were sieved to obtain particle size <40 μm. Afterwards, the slag or fly ash aliquot was inserted between two cellulose nitrate filters of pore size 0.05 μm with additional layer of filter paper on each side. All the layers were then fixed in a Teflon disc-shape holder, where one side was exposed to 80 mL of filtrated SLF (Gamble's solution) under stable conditions of pH 7.3. The whole assembly was left in a sealed beaker for a period of 30 days, while the solution was replaced by the fresh portion in planned intervals. The extraction time ranged from 1 hour up to 7 days. Prior to subsequent analysis extracts were acidified by a few millilitres of HNO₃, wet ashed, uranium and thorium were separated and measured using alpha spectrometry with PIPS detector. The dissolution rates and fractions of rapid and slow components were assessed by using a triexponential approximation equation. The data for a slag sample containing uranium isotopes are presented in Table 1. For Th and Ra isotopes, no relevant data were obtained as the concentration of thorium and ²²⁶Ra in solutions was less than the limit of quantification of the methods. Comparing the results with the data in ICRP Publication 71, both radionuclides are assigned to solubility class M, where the lung retention (the undissolved fraction of material) is considered to be from 13% to 87% between 30 and 180 days. Furthermore, detailed results from solubility of other NORM materials originating from different parts of power stations will be presented in this contribution as well as an advanced method of in vitro testing of NORM behaviour in lungs and determination of the internal radiation dose for workers