Marine radioactivity of Cs-134 and Cs-137 in the Malaysian Economic Exclusive Zone after the Fukushima accident

Development of a Novel DGT Passive Sampler for Measuring Cs-137 In Situ in Marine Environments

Cs-137 is the most released fission product in the marine environment. It is important to develop a robust in situ technique for its monitoring. The existing diffusive gradients in thin films (DGT) passive sampling techniques for in situ measurement of Cs+ have some limitations due to the ion competition and high pH of seawater. A new DGT sampler based on potassium zinc hexacyanoferrate (KZFCN) as a binding layer has been developed and investigated for the measurement of the time-weighted average concentration of Cs-137 in seawater. This binding layer proved a working pH range of 2-12 and an ionic strength of up to 0.75 M. Two types of diffusive gels were tested and agarose gel (AGE) was chosen for the KZFCN-DGT sampler. The measured Cs+ diffusion coefficient (1.71 × 10-5 cm2·s-1 at 25 °C) in the diffusive gel from seawater was within the expected range published in the literature. The measured concentrations of Cs-137 in seawater obtained by laboratory deployments of the KZFCN-DGT samplers for up to 4 weeks showed good precision (RSD = 13%) and accuracy (relative error = 8.5%) values. The performance test results demonstrated that the KZFCN-DGT sampler is suitable for long-term monitoring of Cs-137 in seawater due to its high capacity and resistance to ion competition and high pH.

Identification of the origin of radiocesium released into the environment in areas remote from nuclear accident and military test sites using the 135Cs/137Cs isotopic signature

The isotopic signature of radionuclides provides a powerful tool for discriminating radioactive contamination sources and estimating their respective contributions in the environment. In this context, the ¹³⁵Cs/¹³⁷Cs ratio has been tested as a very promising isotopic ratio that had not been explored yet in many countries around the world including France. To quantify the levels of radioactivity found in the environment, a new method combining a thorough radiochemical treatment of the sample and an efficient measurement by ICP-MS/MS has been recently developed. This method was successfully applied, for the first time, to soil and sediment samples collected in France in two mountainous regions preferentially impacted either by global fallout from nuclear weapons testing (i.e., the Pyrenees) or by the Chernobyl accident (i.e., the Southern Alps). The ¹³⁵Cs/¹³⁷Cs ratios measured on twenty-one samples ranged from 0.66 ± 0.04 and 4.29 ± 0.21 (decay-corrected to January 1st, 2022) corresponding to the characteristic signatures of the fallout from Chernobyl and global fallout associated with the nuclear weapons testing, respectively. Moreover, large variations of both the ¹³⁷Cs mass activity and the studied isotopic ratio recorded by most samples from the southern Alps suggest varying proportions of these two ¹³⁷Cs sources. For these samples, the contribution of each source was estimated using this new tracer (¹³⁵Cs/¹³⁷Cs) and compared with the mixing contribution given by activity ratio: ^239+240Pu/¹³⁷Cs. This work has successfully demonstrated the applicability of the ¹³⁵Cs/¹³⁷Cs isotopic signature to nuclear forensic studies and could be extended to better evaluate the environmental impact of nuclear facilities (i.e., NPP, waste reprocessing).

Innovative ICP-MS/MS Method To Determine the 135Cs/137Cs Ratio in Low Activity Environmental Samples

The ¹³⁵Cs/¹³⁷Cs isotopic ratio is a powerful tool for tracing the origin of radioactive contamination. Since the Fukushima accident, this ratio has been measured by mass spectrometry in several highly contaminated environmental matrices mainly collected near nuclear accident exclusion zones and former nuclear test areas. However, few data were reported at ¹³⁷Cs environmental levels (<1 kBq kg^-1). This is explained by the occurrence of analytical challenges related to the very low radiocesium content at the environmental level with the large presence of mass interferences, making ¹³⁵Cs and ¹³⁷Cs measurements difficult. To overcome these difficulties, a highly selective procedure for Cs extraction/separation combined with an efficient mass spectrometry measurement must be applied on a quantity of ca. 100 g of soil. In the current research, an innovative inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS) method has been developed for the ¹³⁵Cs/¹³⁷Cs ratio measurement in low activity environmental samples. The use of ICP-MS/MS led to a powerful suppression of ¹³⁵Cs and ¹³⁷Cs interferences by introducing N₂O, He, and, for the first time, NH₃, into the collision-reaction cell. By adjusting the flow rates of these gases, the best compromise between a maximum signal in Cs and an effective interference elimination was achieved allowing a high Cs sensitivity of more than 1.10⁵ cps/(ng g^-1) and low background levels at m/z 135 and 137 lower than 0.6 cps. The accuracy of the developed method was successfully verified by analyzing two certified reference materials (IAEA-330 and IAEA-375) commonly used in the literature as validation samples and three sediment samples collected in the Niida River catchment (Japan) impacted by the Fukushima fallout.