8-15 Toward the Rapid Analysis of Plutonium Isotopes

－Analytical Technique Without Pretreatments－

Pu isotopes are important radionuclides to manage radiation exposure control and radioactive waste at the TEPCO’s Fukushima Daiichi Nuclear Power Station. Measurement of Pu isotopes (²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴²Pu, and ²⁴⁴Pu) is necessary to obtain information on distribution and contamination levels. However, the conventional method using alpha-ray spectrometry requires a considerably long analytical time and high levels of efforts and techniques for separating radionuclides with energy similar to that of the alpha rays emitted by Pu.

Inductively coupled plasma-mass spectrometry (ICP-MS) allows measuring multiple radionuclides simultaneously and rapidly via mass spectrometric discrimination without any separation steps requiring handling by humans. However, the quantification is affected by co-existing isobaric interferences. ²³⁹Pu and ²⁴⁰Pu overlap with ²³⁸U hydride (²³⁸UH⁺ and ²³⁸UH₂⁺) and ²⁴¹Pu and ²⁴⁴Pu with ²⁴¹Am and ²⁴⁴Cm, respectively. In this study, we developed a discrimination method to detect multiple Pu isotopes simultaneously in a single data acquisition using ICP-tandem mass spectrometry (ICP-MS/MS), in which the instrument is equipped with a collision reaction cell (CRC) between two quadrupoles.

First, we introduced several oxidizing gases individually (O₂, CO₂, and NO) into the CRC and investigated the oxidation behaviors of Pu, Am, and Cm. Pu was converted to dioxides (PuO₂⁺) and Am and Cm were converted to monoxides (AmO⁺ and CmO⁺) by CO₂ (Fig.1), making mass separation of ²⁴¹Pu and ²⁴⁴Pu from ²⁴¹Am and ²⁴⁴Cm possible. The ²³⁸UO₂⁺ ion generated from ²³⁸U⁺ by oxidation in the CRC creates hydrides (²³⁸UO₂H⁺ and ²³⁸UO₂H₂⁺) that interfere with ²³⁹PuO₂⁺ and ²⁴⁰PuO₂⁺. Therefore, ²³⁸U⁺ was removed at the quadrupole before CRC to suppress the formation of ²³⁸UO₂ hydride. The combined separation of isobaric interferences (²⁴¹Am⁺, ²⁴⁴Cm⁺, and ²³⁸U⁺) achieved remarkably low background counts for the mass of ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, and ²⁴⁴Pu (as PuO₂⁺). From these results, it is clear that Pu isotopes (²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴²Pu, and ²⁴⁴Pu) can be measured simultaneously in one-shot data acquisition (Fig.2). Note that ²³⁸Pu is out of scope of this determination because it is removed along with ²³⁸U.

This method discriminates target Pu isotopes and their isobaric interferences without any human-handling separation process. Moreover, the simultaneous determination of Pu isotopes will lead to the development of a high-throughput analysis and contribute to the decommissioning of nuclear power stations.

(Makoto Matsueda)

<Previous: 8-14 | Next: 8-16>