Determination of Boron in ThO2 Powder by ICP-AES After Matrix Separation Using Solvent Extraction

Abstract

Thorium-based nuclear reactors use thorium as fuel through breeding to uranium-233. The analysis of boron level in thorium fuels is essential due to the stringent specifications for boron. A method has been developed for the determination of trace boron in high-purity ThO2 powders. About 0.4000 g of ThO2 powder was added to a mixture of 4 mL concentrated HNO3 and a few drops of 0.5 % HF, then dissolved in a microwave digester. The solution was then heated to evaporate surplus HNO3 and HF, and then an amount of 3 M HNO3 was added and the solution was subsequently used for the chemical separation. By spiking a known amount of boron before the evaporation process, it was observed that recoveries of boron were near 100 % in this open heating step. 50 % (v/v) tri-n-butyl phosphate (TBP) using carbon tetrachloride as the diluent was used as the extractant. The number of extractions was optimized to reduce the thorium concentration in the aqueous phase to avoid spectral interference of thorium in the determination of boron by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The results of blank experiments showed that the limit of detection of the developed method was about 0.03 µg B/g ThO2. Samples of a high-purity ThO2 powder (ThO2 > 99.99 % by weight) were analyzed, and the results showed that the boron level was about 0.24 µg/g ThO2 and that the relative standard deviation (RSD) was about 10.5 % (n = 6). By spiking 0.10 µg boron (corresponding to 0.25 µg/g ThO2) into the ThO2 powders before digestion, it was found that the recoveries were in the range of 88–108 % (n = 6). These results indicated that the proposed method could meet the requirements for the determination of trace boron in high-purity ThO2 powders.