Determination of 135Cs and 137Cs in environmental samples: A review

Radionuclides of caesium are environmentally important since they are formed as significant high yield fission products (Cs-135 and Cs-137) and activation products (Cs-134 and Cs-136) during nuclear fission. They originate from a range of nuclear activities such as weapons testing, nuclear reprocessing and nuclear fuel cycle discharges and nuclear accidents. Whilst Cs-137, Cs-134 and Cs-136 are routinely measurable at high sensitivity by gamma spectrometry, routine detection of long-lived Cs-135 by radiometric methods is challenging. This measurement is, however, important given its significance in long-term nuclear waste storage and disposal. Furthermore, the Cs-135/Cs-137 ratio varies with reactor, weapon and fuel type, and accurate measurement of this ratio can therefore be used as a forensic tool in identifying the source(s) of nuclear contamination. The shorter-lived activation products Cs-134 and Cs-136 have a limited application but provide useful early information on fuel irradiation history and have importance in health physics. Detection of Cs-135 (and Cs-137) is achievable by mass spectrometric techniques; most commonly inductively coupled plasma mass spectrometry (ICP-MS), as well as thermal ionisation (TIMS), accelerator (AMS) and resonance ionisation (RIMS) techniques. The critical issues affecting the accuracy and detection limits achievable by this technique are effective removal of barium to eliminate isobaric interferences arising from Ba-135 and Ba-137, and elimination of peak tailing of stable Cs-133 on Cs-135. Isobaric interferences can be removed by chemical separation, most commonly ion exchange chromatography, and/or instrumental separation using an ICP-MS equipped with a reaction cell. The removal of the peak tailing interference is dependent on the instrument used for final measurement. This review summarizes and compares the analytical procedures developed for determination of Cs-135/Cs-137, with particular focus on ICP-MS detection and the methods applied to interference separation. (C) 2015 Elsevier B.V. All rights reserved.