Fig.8-9 System of extraction chromatography
Fig.8-10 β-ray spectrum of 242Am and so on
The radioactive inventory of low-level radioactive waste (LLW) must be evaluated to dispose of LLW. However, some nuclides that are important for safety are difficult to analyze, such as americium-242m (242mAm). Thermal ionization mass spectrometry (TIMS) is a conventional method to determine the content of 242mAm. 242mAm can also be analyzed by α-ray of curium-242 (242Cm), which is the progeny nuclide of 242mAm (242Cm method). Although the TIMS measuring time is short, more sample is required than for 242Cm method. However, less sample is available for the 242Cm method, it takes several months to determine the 242mAm content. In addition, both methods require separation of Am from Cm.
In the present study, we developed a new method to determine the 242mAm content that involves measuring β-ray emission from 242Am, which is the progeny nuclide of 242mAm. To measure β-rays from 242Am, Am must be separated from β-ray emitting nuclides in LLW. Therefore, we used a separation method based on extraction chromatography (Fig.8-9). First, trivalent actinides (An3+) and lanthanides (Ln3+) were separated from the major elements in LLW by using transuranic resin (Eichrom Technologies), which has selectivity for transuranic elements. Next, An3+ was separated from Ln3+, which has similar chemical properties and contains some β-ray emitting nuclides, by using a tetravalent actinide resin (Eichrom Technologies). This An3+ fraction contains Cm, but the influence of Cm on the β-ray measurement can be subtracted because there is no β-ray emitting Cm in LLW. Conversely, β-rays from neptunium-239 (239Np), which is a progeny nuclide of 243Am, have to be detected and considered. Therefore, a 243Am source was prepared to measure β-ray spectra of 239Np, which were subtracted from those of An3+ to obtain the radioactivity of 242Am (Fig.8-10). As a verification, the value obtained was compared with that determined by the 242Cm method.
The separation step in the new method to determine the radioactivity of 242Am is simpler than that for the TIMS and 242Cm methods. Less sample is required by this new method than by the TIMS method and is comparable to that required by the 242Cm method. The measuring time for this new method is considerably shorter than that for the 242Cm method.
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