Direct Analysis of Zirconium-93 in Nuclear Site Decommissioning Samples by ICP-QQQ

Zirconium-93 is a long-lived radionuclide that is produced by nuclear fission of uranium and plutonium. It is also formed by neutron activation of stable zirconium in nuclear fuel cladding in pressurized water reactors. Therefore, 93Zr is an important element to monitor during the decommissioning of nuclear sites. The long half-life of 93Zr (1.61 × 106 years (1)) means it is a significant contributor to the total waste inventory over long timescales. Clearly, there is a need to accurately quantify 93Zr in various complex decommissioning wastes as part of the initial site characterization process. Also, monitoring of waste repositories and the environment around nuclear sites following decommissioning is needed.

Zirconium-93 decays to stable 93Nb by beta emission, but with a low decay energy that makes measurement by decay counting methods challenging. With its long-half-life, 93Zr is well suited to measurement by ICP-MS (1 Bq/g is equivalent to 1.1 × 104 pg/g), which offers a high throughput alternative to decay counting techniques. However, accurate measurement by ICP-MS is affected by the isobaric interference from 93Nb (100% abundance), as well as potential radioactive 93Mo, and polyatomic ion interference from 92Mo1H+ and 92Zr1H+. The removal of these interferences traditionally requires time-consuming, multistage extraction and/or chromatographic separation before measurement, using a significant number of reagents and materials (2, 3). Also, since 93Nb is monoisotopic, it is challenging for the analyst to be confident that complete interference removal has been achieved before measurement.

In this study, triple quadrupole ICP-MS (ICP-QQQ) was used for the rapid and direct low-level measurement of 93Zr in decommissioning samples below International Atomic Energy Agency (IAEA) regulatory limits (4). Reactive gases were used in the collision/reaction cell (CRC) of the ICP-QQQ to separate the isobaric overlap from 93Nb on 93Zr, eliminating the need for chromatographic separation. Isobaric ion interferences can be separated using ICP-QQQ when the cell gas reacts quickly with one of the elements to form a product ion, while the other element does not react (or reacts slowly). This “chemical” resolution method significantly reduces the procedural time and secondary waste associated with decay counting and alternative mass spectrometric procedures. With its fast analysis times, ICP-QQQ offers a cost-effective method for the analysis of nuclear decommissioning samples.

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