Determination of 93Mo in Radioactive Samples of Sulfuric Acid Media from Nuclear Facilities.

93Mo is an important radionuclide in view of radioactive waste repository because of its long half-life and high mobility in the environment. 93Mo decays by electron capture without any measurable gamma ray emission. The concentration of 93Mo in most of the radioactive waste is many orders of magnitude lower than the major activation product radionuclides, which makes the accurate determination of 93Mo a big challenge. A new analytical method for the determination of 93Mo in sulfuric acid media from nuclear power reactor was developed. 93Mo was separated from most of the radionuclides by cation exchange chromatography followed by the removal of sulfate by CaSO4 precipitation. A further purification of 93Mo, especially from anion species of 51Cr and 125Sb, was achieved by anion exchange chromatography and a short alumina column separation. The chemical yield of 93Mo in the entire separation procedure reached about 75%, and the decontamination factors for all potential interfering radionuclides were 1.5 × 106-1.6 × 108. The purified 93Mo was measured by liquid scintillation counting through counting its low-energy Auger electrons. A detection limit of 2 mBq/g for 93Mo in 50 g sample was achieved by this method, which enables the quantitative determination of 93Mo in most of the radioactive samples in the decommissioning waste and coolant water of nuclear power reactors. The developed method has been successfully applied to determine 93Mo in coolant water of nuclear power reactors, providing a robust analytical approach of 93Mo for the radiological characterization of radioactive wastes.

Plutonium isotopes in the northwestern South China Sea: Level, distribution, source and deposition.

The spatial distribution of plutonium isotopes (239Pu, 240Pu) in the surface sediments collected from the northwestern South China Sea (SCS) in 2018 was investigated. The 239,240Pu concentrations in surface sediments vary from 0.048 to 0.960 mBq/g (with mean of 0.282 ± 0.242 mBq/g) depending on the geographical feature of the sampling location such as the river estuary, continental shelf, slope and deep basin. Higher 240Pu/239Pu atomic ratios (0.24-0.31) in the surface sediment of the SCS compared to the global fallout value of 0.18 were observed, this is attributed to the input of close-in fallout of the Pacific Proving Ground (PPG) transported by the North Equatorial Current and Kuroshio Current to the northern SCS. The contribution of the PPG derived plutonium in the SCS sediment was estimated to be 39%-78% using a simple two-end member mixing model based on the measured 240Pu/239Pu atomic ratios in the sediment. Besides the soluble 239,240Pu level in seawater, load of suspended particulate matter from the river runoff and biological debris, hydrographic and hydrodynamic conditions are key parameters influencing the deposition process of plutonium to the sediment.

70-Year Anthropogenic Uranium Imprints of Nuclear Activities in Baltic Sea Sediments.

A strongly stratified water structure and a densely populated catchment make the Baltic Sea one of the most polluted seas. Understanding its circulation pattern and time scale is essential to predict the dynamics of hypoxia, eutrophication, and pollutants. Anthropogenic 236U and 233U have been demonstrated as excellent transient tracers in oceanic studies, but unclear input history and inadequate long-term monitoring records limit their application in the Baltic Sea. From two dated Baltic sediment cores, we obtained high-resolution records of anthropogenic uranium imprints originating from three major human nuclear activities throughout the Atomic Era. Using the novel 233U/236U signature, we distinguished and quantified 236U inputs from global fallout (45.4-52.1%), Chernobyl accident (0.3-1.8%), and discharges from civil nuclear industries (46.1-54.3%) to the Baltic Sea. We estimated the total release of 233U (7-15 kg) from the atmospheric nuclear weapon testing and pinpointed the 233U peak signal in the mid-to-late 1950s as a potential time marker for the onset of the Anthropocene Epoch. This work also provides fundamental 236U data on Chernobyl accident and early discharges from civil nuclear facilities, prompting worldwide 233U-236U tracer studies. We anticipate our data to be used in a broader application in model-observation interdisciplinary research on water circulation and pollutant dynamics in the Baltic Sea.

Determination of low-level 135Cs and 135Cs/137Cs atomic ratios in large volume of seawater by chemical separation coupled with triple-quadrupole inductively coupled plasma mass spectrometry measurement for its oceanographic applications.

Radioisotopes of cesium are powerful tracer for oceanographic studies. In this work, a novel method was developed for determination of ultra-low level 135Cs and 137Cs in seawater using triple-quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS). Cesium was pre-concentrated from up to 45 L seawater samples using ammonium molybdophosphate (AMP) adsorption, following a selective leaching of cesium using Sr(OH)2. The cesium was further purified from interfering elements using AMP-PAN and cation-exchange chromatography. Sr(OH)2 leaching was found to be an effective approach for selective exchange of cesium from the AMP sorbent without dissolution, which avoids the problem of separation of huge amount of NH4+ and MoO42- in the following steps. The decontamination factors for barium and rubidium with the developed method were more than 4 × 107 and 800, respectively. The separated 135Cs and 137Cs were measured using ICP-MS/MS by employing N2O as reaction gas to further elimination of isobaric (i.e. 135Ba and 137Ba) and polyatomic ions interferences. A detection limit of 1.5 × 10-16 g L-1 for 135Cs in seawater was achieved. The concentrations of 135Cs in seawater from Baltic Sea, Danish straits and Roskilde Fjord were determined using the developed method to identify the sources of 135Cs, the water masses exchange in this region was investigated using 135Cs and 137Cs.

Determination of 135Cs concentration and 135Cs/137Cs ratio in waste samples from nuclear decommissioning by chemical separation and ICP-MS/MS.

Determination of 135Cs concentration and 135Cs/137Cs atomic ratio is of great importance in characterization of radioactive waste from decommissioning of nuclear facilities. In this work, an effective analytical method was developed for simultaneously determination of 135Cs and 137Cs in different types of waste samples (steel, zirconium alloy, reactor coolant, ion exchange filter paper and spent ion exchange resin) by coupling AMP-PAN, AG MP-1M and AG 50 W-X8 chromatographic separation with ICP-MS/MS measurement. Decontamination factors of 7.0 × 106 for Co, 6.0 × 106 for Ba, 4.2 × 105 for Mo, 3.2 × 105 for Sn and 2.1 × 105 for Sb were achieved using the chemical separation procedure. The overall chemical yields of cesium were higher than 85%. A detection limit of 3.1 × 10-14 g/g for 135Cs was achieved for 0.2 g stainless steel sample or spent resin. The developed method was validated by analysis of standard reference materials (IAEA-375) and successfully applied for analysis of zirconium alloy, steel, ion exchange filter paper and spent ion exchange resin from nuclear power reactors. The obtained 135Cs can be used to evaluate the long-term environmental impact and provide useful information for waste disposal. The measured 135Cs/137Cs ratio in reactor coolant, as a characteristic information, might be useful for source identification and localization of leaked fuel element. The neutron flux of the leaked fuel element can be estimated based on the measured 135Cs/137Cs atomic ratios in the reactor coolant water. The developed method is simple and rapid (8 samples/day) for the determination of 135Cs concentrations and 135Cs/137Cs ratios in various waste samples from nuclear decommissioning.

Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS.

An analytical method was developed for the determination of ultralow level 135Cs in environmental samples by chromatographic separation of cesium with AMP-PAN and AG50W-X8 columns and sensitive measurement of cesium isotopes with triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS). Cesium was simply released by acid leaching using aqua regia from environmental solid samples and preconcentrated on AMP-PAN column. The cesium adsorbed on the column was effectively eluted with NH4Cl solution without dissolving the AMP. The excessive amount of NH4Cl in the eluate was removed by sublimation in the presence of small amount of LiCl. The remaining barium and other interfering elements such as Mo, Sn, Sb, and Li were efficiently removed using cation exchange chromatography (AG50W-X8). The decontamination factors of this procedure are above 4 × 107 for barium and 4 × 105 for molybdenum; the chemical yields of cesium are more than 85% for samples of less than 10 g. This method enables to separate cesium from large size of samples for the determination of ultralow level 135Cs, avoiding the problem of removal of a huge amount of Mo in the dissolved AMP. Intrinsic 137Cs in the environmental samples measured by gamma spectrometry before and after separation was used as internal isotope dilution standard for quantitative determination of 135Cs without complete release and recover of radiocesium. The interference of barium (135Ba and 137Ba) to the ICP-MS measurement of 135Cs and 137Cs was further suppressed to 8 × 10-5 by using N2O as the reaction gas in ICP-MS/MS at a flow rate of 0.7 mL/min, so a total suppression of 2 × 10-12 for Ba was achieved, making the isobaric interference of Ba isotopes to the measurement of 135Cs and 137Cs in environmental samples negligible. A detection limit of 9.1 × 10-17 g/g for 135Cs and 137Cs was achieved for 60 g samples. The developed method was validated by analysis of standard reference materials (IAEA-375, IAEA-330, and IAEA-385) and successfully applied for the determination of 135Cs concentrations and 135Cs/137Cs ratios in soil samples collected from Denmark, Sweden, and Ukraine. The 135Cs/137Cs isotopic ratios in Danish soil (2.08-2.68) were significantly higher than that from Sweden and Ukraine (0.65-0.71), indicating different sources of radiocesium. This work demonstrated the application of 135Cs/137Cs as a unique fingerprint for discriminating the sources of radioactive contamination and estimating their contribution to the total inventory, which will be useful for nuclear forensics and environmental tracer studies.

Determination of Ultratrace Level 135Cs and 135Cs/137Cs Ratio in Small Volume Seawater by Chemical Separation and Thermal Ionization Mass Spectrometry.

The atomic ratio of 135Cs/137Cs is a powerful fingerprint for distinguishing the source terms of radioactive contamination and tracing the circulation of water masses in the ocean. However, the determination of the 135Cs/137Cs ratio is very difficult due to the ultratrace level of 135Cs (<0.02 mBq/m3) and 137Cs (<2 Bq/m3) in the ordinary seawater samples. In this work, a sensitive method was developed for determination of 135Cs concentration and 135Cs/137Cs ratio in seawater using chemical separation combined with thermal ionization mass spectrometry (TIMS) measurement. Cesium was first preconcentrated from seawater using ammonium molybdophosphate-polyacrylonitrile column chromatography and then purified using cation exchange chromatography to remove the interferences. With this method, decontamination factors of 6.0 × 106 for barium and 1800 for rubidium and a chemical yield of more than 60% for cesium were achieved. By using glucose as an activator, the ionization efficiency of cesium was significantly improved to 50.6%, and a constant high current of Cs+ (20 V) can be maintained for more than 180 min, which ensures sensitive and reliable measurement of low level 135Cs and 137Cs. Detection limits of 4.0 × 10-17 g/L for both 135Cs and 137Cs for 200 mL seawater were achieved, which enables the accurate determination of 135Cs concentration and 135Cs/137Cs ratio in a small volume of seawater samples (<200 mL). The developed method has been validated by analysis of seawater reference material IAEA-443. Seawater samples collected from the Greenland Sea, Baltic Sea, and Danish Straits have been successfully analyzed for 135Cs concentrations and 135Cs/137Cs ratios, and the results showed that 135Cs concentrations in the seawater of the Baltic Sea is much higher than that in the Greenland Sea, which is attributed to the high deposition of Chernobyl accident derived radiocesium in the Baltic Sea region.