Bioassay methods for monitoring of thorium intakes

'Full text:' Studies have shown that radiation exposure following inhalation of thorium and its decay products could lead to an increased risk of developing cancers of the liver, lung, pancreas, blood and bone, etc. Several internal monitoring techniques could be used to provide dosimetry for workers involved in the production and handling of thorium fuels and other thorium materials, including in vivo measurements of thorium decay products (e.g., ²¹²Pb, ²⁰⁸Tl and ²²⁸Ac) using whole body or lung counting, and in vitro measurements of ²²⁰Rn in breath, and thorium bioassays in urine and faecal samples. Based on the international radiological protection standards for occupational exposure, the goal for the thorium bioassay programme design is to be capable of detecting at least 1 mSv of committed effective dose. In order to identify the most appropriate dosimetry monitoring methods, the sensitivity and limitation of the bioassay techniques have been evaluated and compared. It was concluded that the bioassay methods for thorium in urine and faeces are sufficiently sensitive for assessing dose intakes below the exposure limit, whereas the lung and breath monitoring are of little value for dose calculation, other than perhaps to confirm or screen larger intakes. A new rapid thorium urinalysis method is currently in development at Chalk River Laboratories, AECL. In this method, thorium isotopes (²³²Th, ²³⁰Th and ²²⁸Th) are co-precipitated with hydrous titanium oxide from a 24-hour urine sample, separated by anion exchange chromatography and measured using inductively coupled plasma mass spectrometry and alpha spectrometry. The preliminary results from the development tests will also be presented and discussed. (author)