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.

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).

240Pu/239Pu signatures allow refining the chronology of radionuclide fallout in South America.

Atmospheric nuclear tests (1945-1980) have led to radioactive fallout across the globe. French tests in Polynesia (1966-1974) may influence the signature of fallout in South America in addition to those conducted by USA and former USSR until 1963 in the Northern hemisphere. Here, we compiled the 240Pu/239Pu atom ratios reported for soils of South America and conducted additional measurements to examine their latitudinal distributions across this continent. Significantly lower ratio values were found in the 20-45° latitudinal band (0.04 to 0.13) compared to the rest of the continent (up to 0.20) and attributed to the contribution of the French atmospheric tests to the ultra-trace plutonium levels found in these soils. Based on sediment cores collected in lakes of Chile and Uruguay, we show the added value of measuring 240Pu/239Pu atom ratios to refine the age models of environmental archives in this region of the world.

Europe-Wide Atmospheric Radionuclide Dispersion by Unprecedented Wildfires in the Chernobyl Exclusion Zone, April 2020.

From early April 2020, wildfires raged in the highly contaminated areas around the Chernobyl nuclear power plant (CNPP), Ukraine. For about 4 weeks, the fires spread around and into the Chernobyl exclusion zone (CEZ) and came within a few kilometers of both the CNPP and radioactive waste storage facilities. Wildfires occurred on several occasions throughout the month of April. They were extinguished, but weather conditions and the spread of fires by airborne embers and smoldering fires led to new fires starting at different locations of the CEZ. The forest fires were only completely under control at the beginning of May, thanks to the tireless and incessant work of the firefighters and a period of sustained precipitation. In total, 0.7-1.2 TBq 137Cs were released into the atmosphere. Smoke plumes partly spread south and west and contributed to the detection of airborne 137Cs over the Ukrainian territory and as far away as Western Europe. The increase in airborne 137Cs ranged from several hundred µBq·m-3 in northern Ukraine to trace levels of a few µBq·m-3 or even within the usual background level in other European countries. Dispersion modeling determined the plume arrival time and was helpful in the assessment of the possible increase in airborne 137Cs concentrations in Europe. Detections of airborne 90Sr (emission estimate 345-612 GBq) and Pu (up to 75 GBq, mostly 241Pu) were reported from the CEZ. Americium-241 represented only 1.4% of the total source term corresponding to the studied anthropogenic radionuclides but would have contributed up to 80% of the inhalation dose.

U isotopes distribution in the Lower Rhone River and its implication on radionuclides disequilibrium within the decay series.

The large rivers are main pathways for the delivery of suspended sediments into coastal environments, affecting the biogeochemical fluxes and the ecosystem functioning. The radionuclides from 238U and 232Th-series can be used to understand the dynamic processes affecting both catchment soil erosion and sediment delivery to oceans. Based on annual water discharge the Rhone River represents the largest river of the Mediterranean Sea. The Rhone valley also represents the largest concentration in nuclear power plants in Europe. A radioactive disequilibrium between particulate 226Ra(p) and 238U(p) was observed in the suspended sediment discharged by the Lower Rhone River (Eyrolle et al. 2012), and a fraction of particulate 234Th was shown to derive from dissolved 238U(d) (Zebracki et al. 2013). This extensive study has investigated the dissolved U isotopes distribution in the Lower Rhone River and its implication on particulate radionuclides disequilibrium within the decay series. The suspended sediment and filtered river waters were collected at low and high water discharges. During the 4-months of the study, two flood events generated by the Rhone southern tributaries were monitored. In river waters, the total U(d) concentration and U isotopes distribution were obtained through Q-ICP-MS measurements. The Lower Rhone River has displayed non-conservative U-behavior, and the variations in U(d) concentration between southern tributaries were related to the differences in bedrock lithology. The artificially occurring 236U was detected in the Rhone River at low water discharges, and was attributed to the liquid releases from nuclear industries located along the river. The (235U/238U)(d) activity ratio (=AR) in river waters was representative of the 235U natural abundance on Earth. The (226Ra/238U)(p) AR in suspended sediment has indicated a radioactive disequilibrium (average 1.3 ± 0.1). The excess of 234Th in suspended sediment =(234Thxs(p)) was apparent solely at low water discharges. The activity of 234Thxs(p) was calculated through gamma measurements and ranged from unquantifiable to 56 ± 14 Bq kg-1. The possibility of using 234Th as a tracer for the suspended sediment dynamics in large Mediterranean river was then discussed.

Empirical calibration of uranium releases in the terrestrial environment of nuclear fuel cycle facilities.

In the present paper the activity of uranium isotopes measured in plants and aerosols taken downwind of the releases of three nuclear fuel settlements was compared between them and with the activity measured at remote sites. An enhancement of 238U activity as well as 235U/238U anomalies and 236U are noticeable in wheat, grass, tree leaves and aerosols taken at the edge of nuclear fuel settlements, which show the influence of uranium chronic releases. Further plants taken at the edge of the studied sites and a few published data acquired in the same experimental conditions show that the 238U activity in plants is influenced by the intensity of the U atmospheric releases. Assuming that 238U in plant is proportional to the intensity of the releases, we proposed empirical relationships which allow to characterize the chronic releases on the ground. Other sources of U contamination in plants such as accidental releases and "delayed source" of uranium in soil are also discussed in the light of uranium isotopes signatures.

Behaviour of radiocaesium in coastal rivers of the Fukushima Prefecture (Japan) during conditions of low flow and low turbidity--Insight on the possible role of small particles and detrital organic compounds.

To investigate riverine transfers from contaminated soils of the Fukushima Prefecture in Japan to the marine environment, suspended sediments, filtered water, sediments and detrital organic macro debris deposited onto river beds were collected in November 2013 within small coastal rivers during conditions of low flow rates and low turbidity. River waters were directly filtered on the field and high efficiency well-type Ge detectors were used to analyse radiocaesium concentrations in very small quantities of suspended particles and filtered water (a few mg to a few g). For such base-flow conditions, our results show that the watersheds studied present similar hydro-sedimentary behaviours at their outlets and that the exports of dissolved and particulate radiocaesium are comparable. Moreover, the contribution of these rivers to the instantaneous export of radiocaesium to the ocean is similar to that of the Abukuma River. Our preliminary results indicate that, in the estuaries, radiocaesium concentrations in suspended sediments would be reduced by more than 80%, while radiocaesium concentration in filtered waters would be maintained. Significant correlations between radiocaesium concentrations and radiocaesium inventories in the soils of the catchments indicate that there was at that time little intra and inter-watershed variability in the transfer processes of radiocaesium from lands to rivers at this regional scale. The apparent liquid-solid partition coefficient (KD) values acquired for the lowest loads/finest particles complement the values acquired by using sediment traps and highlight the strong capacity of the smallest particles to transfer radiocaesium. Finally, but not least, our observations suggest that there could be a significant transfer of highly contaminated detrital biomass from forest litter to the downstream rivers in a rather conservative way.

Environmental releases from fuel cycle facility: part 1: radionuclide resuspension vs. stack releases on ambient airborne uranium and thorium levels.

Airborne activity levels of uranium and thorium series were measured in the vicinity (1.1 km) of a uranium (UF4) processing plant, located in Malvési, south of France. Regarding its impact on the environment, this facility is characterized by its routine atmospheric releases of uranium and by the emission of radionuclide-labelled particles from a storage pond filled with waste water or that contain dried sludge characterized by traces of plutonium and thorium ((230)Th). This study was performed during a whole year (November 2009-November 2010) and based on weekly aerosol sampling. Thanks to ICP-MS results, it was possible to perform investigations of uranium and thorium decay product concentration in the air. The number of aerosol filters sampled (50) was sufficient to establish a relationship between airborne radionuclide variations and the wind conditions. As expected, the more the time spent in the plume, the higher the ambient levels. The respective contributions of atmospheric releases and resuspension from local soil and waste ponds on ambient dust load and uranium-bearing aerosols were estimated. Two shutdown periods dedicated to facility servicing made it possible to estimate the resuspension contribution and to specify its origin (local or regional) according to the wind direction and remote background concentration. Airborne uranium mainly comes from the emission stack and, to a minor extent (∼20%), from wind resuspension of soil particles from the surrounding fields and areas devoted to waste storage. Moreover, weighed activity levels were clearly higher during operational periods than for shutdown periods.

Theoretical study of the uranyl complexation by hydroxamic and carboxylic acid groups.

A theoretical study on the complexation of uranyl cation (UO2(2+)) by three different functional groups of a calix[6]arene cage, that is, two hydroxamic and a carboxylic acid function, has been carried out using density functional theory calculations. In particular, interaction energies between the uranyl and the functional groups have been used to determine their affinity toward uranyl, whereas pKa calculations give some information on the availability of the functional groups in the extraction conditions. On the one hand, calculations of the interaction energies have pointed out clearly a better affinity with the hydroxamic groups. The stabilization of this complex was rationalized in terms of a stronger electrostatic interaction between the uranyl cation and the hydroxamic groups. The presence of a water molecule in the first coordination sphere of uranyl does not destabilize the complex, and the most stable complex is obtained with two functional groups and two water molecules, leading to a coordination number of 8 for the central uranium atom. On the other hand, pKa theoretical evaluation shows that both hydroxamic (deprotonated on the oxygen site) and carboxylic groups are potential extractants in aqueous medium with a preference for carboxylic functions at low pH. Moreover, these data allowed to unambiguously identify the oxygen of the alcohol function as the favored deprotonation site on the hydroxamic function.

A combined experimental and theoretical study on the conformational behavior of a calix[6]arene.

An experimental and theoretical study on the conformational behavior of the 1,3,5-OMe-2,4,6-OCH(2)CONHOH-p-tert-butylcalix[6]arene has been carried out. In particular, semiempirical (AM1) and density functional theory (DFT) calculations have been performed in order to identify the possible conformers. The obtained results show that the cone structure is the most stable conformer at any level of theory, even if significant differences have been obtained for the other species. The inclusion of solvent effect, through a continuum model, also points out the relevant role played by the solvent in the stabilization of the cone structure in solution. These latter results have been confirmed by NMR experiments, which clearly show the presence of only the cone conformer in a polar solvent, such as DMSO. Finally, (1)H and (13)C NMR spectra on model systems, i.e., two successive phenol rings (Ar(1)-CH(2)-Ar(2)), have been computed at the DFT level and compared with the experimental spectra of the complete molecule. The results show an overall good agreement with the experimental data, thus leading to an unambiguous assignment of the experimental spectra.