Linear Fitting of Time-Varying Signals in Static Noble Gas Mass Spectrometry Should Be Avoided.

This Perspective highlights a malpractice during data processing in static noble gas mass spectrometry, viz. retrodicting the noble gas signals at "time zero" based on the linear fitting results of the time-varying analytical responses. Linear fitting is the most commonly used by noble gas analysts mainly on the consideration of high coefficient of determination (R2), while it poses risks of inaccurate estimation of required information. Here, we appeal to re-examining the signal variation mechanisms before deciding the way of fitting instead of merely choosing one with good R2. The primary processes during the static mass spectrometry measurement are considered, and a mechanism-based exponential fitting is recommended for the relevant data processing. For the same given data set, exponential fitting is a more robust processing strategy than linear fitting because not only does the former show perfect goodness of fitting but also it contributes to better consistence of retrodicted signals. For the sake of maintaining a high level of data quality in analytical science, we propose to apply the mechanism-based exponential fitting to retrodict the "time zero" signal in static noble gas mass spectrometry, and the commonly used linear fitting should be avoided.

High-Performance Method for Determination of Pu Isotopes in Soil and Sediment Samples by Sector Field-Inductively Coupled Plasma Mass Spectrometry.

Plutonium is extensively studied in radioecology (e.g., soil to plant transfer and radiological assessment) and geochemistry (e.g., sediment dating). Here, we reported a new chemical separation method for rapid determination of Pu in soil and sediment samples, based on the following investigations: extraction behaviors of interfering elements (IEs, for inductively coupled plasma mass spectrometry (ICPMS) measurement) on TEVA resin; decontamination of U using TEVA, UTEVA, and DGA resins; and the impact of coprecipitation on Pu determination. The developed method consists of four steps: HNO3 leaching for Pu release; CaF2/LaF3 coprecipitation for the removal of major metals and U; the proposed TEVA + UTEVA + DGA procedure for the removal of U, Pb, Bi, Tl, Hg, Hf, Pt, and Dy; and ICPMS measurement. The accuracy of this method in determining 239+240Pu activity and 240Pu/239Pu and 241Pu/239Pu isotopic ratios was validated by analyzing five standard reference materials (soil, fresh water sediment, and ocean sediment). This method is characterized by its stable and high Pu recovery (90-97% for soil; 92-98% for sediment) and high decontamination factor of U (1.6 × 107), which is the highest reported for soil and sediment samples. In addition, the short analytical time of 12 h and the method detection limits, which are the lowest yet reported in literature, of 0.56 µBq g-1 (0.24 fg g-1) for 239Pu, 1.2 µBq g-1 (0.14 fg g-1) for 240Pu, and 0.34 mBq g-1 (0.09 fg g-1) for 241Pu (calculated on the basis of a 1 g soil sample) allow the rapid determination of ultratrace level Pu in soil and sediment samples.

Plutonium in the coastal seawater around Chinese nuclear power plants: Sources, distribution, and environmental implications.

Coastal surface seawater samples within 30 km around ten Chinese nuclear power plants (NPPs) were systematically investigated. The 239+240Pu activity concentration in the samples varied from 0.226 mBq/m3 to 3.098 mBq/m3, meanwhile the 240Pu/239Pu atom ratios ranged from 0.151 to 0.353. Besides, the Pacific Proving Ground (PPG) close-in fallout and the global fallout were recognized as two primary sources of Pu in these samples. The 239+240Pu activity concentration as well as the PPG contribution showed similar trends as the Kuroshio intrusion path and the coastal currents in the China Seas, illustrating long-range transport and consuming of PPG derived Pu in the coastal China Seas. Moreover, accumulation of PPG sourced Pu in the Beibu Gulf were observed and was attributed to the continuous invasion of the high isotopic Pu that remobilized from the South China Sea (SCS).

A novel strategy for Pu determination in water samples by automated separation in combination with direct ICP-MS/MS measurement.

Methods for Pu determination in water samples has been longtime studied but they generally involved tedious manual operations. In this context, we proposed a novel strategy for accurate determination of ultra-trace Pu in water samples by the combination of fully automated separation with direct ICP-MS/MS measurement. A recently commercialized extraction resin TK200 was used for single-column separation due to its distinctive nature. Acidified waters up to 1 L were directly loaded to the resin at high flow rate (15 mL min-1) with omitting the frequently used co-precipitation process. Small volumes of dilute HNO3 were used for column washing, and Pu was efficiently eluted within only 2 mL 0.5 mol L-1 HCl-0.1 mol L-1 HF with a stable recovery (65%). This separation procedure was fully automated under the control of user program, meanwhile the final eluent was compatible for direct ICP-MS/MS measurement without extra sample treatment. In that way, both the labor intensity and reagent consumption were minimized compared with existing methods. With the high decontamination (104 to 105) of U in the chemical separation and the further elimination of uranium hydrides under oxygen reaction model during ICP-MS/MS measurement, the overall interference yields of UH+/U+ and UH2+/U+ were down to 10-15. The limits of detection (LODs) of this method reached 0.32 µBq L-1 for 239Pu and 2.00 µBq L-1 for 240Pu, which were much lower than those stipulated in the general guidelines for drinking water standards, suggesting this method was promising in routine or emergency radiation monitoring. Furthermore, the established method was successfully applied in a pilot study to determine global fallout derived Pu in surface glacier samples with extremely low concentrations of 239+240Pu, which suggested the method would also be feasible in glacial chronology studies in the future.

Distributions of fallout 137Cs, 239+240Pu and 241Am in a soil core from South Central China.

The source and vertical distribution of 137Cs,239+240Pu and 241Am activity concentrations in a soil core from Hunan Province, China were investigated. The maximum 137Cs and 239+240Pu activity concentrations were 15.45 ± 0.76 mBq/g and 0.819 ± 0.066 mBq/g, respectively. While the maximum 241Am activity concentration in samples obtained from the core was 0.341 ± 0.019 mBq/g. The 240Pu/239Pu atom ratio and the 137Cs/239+240Pu activity ratio were 0.183 ± 0.011 and 19.5 ± 1.8, respectively, and both were consistent with the characteristic value of global fallout. The integrated 241Am/239+240Pu activity ratio for global fallout was also re-estimated. The measured 241Am/239+240Pu activity ratio (average 0.43 ± 0.07) in the samples was very close to the estimated value (0.45), which suggested their 241Am also came from the global fallout. Regarding the vertical distribution of 137Cs, 239+240Pu and 241Am in these red soil samples, all these radionuclides had higher concentrations in upper layers of several centimeters of soil while they had slightly lower concentrations in lower soil layers down to 30 cm. Vertical distributions of 137Cs/239+240Pu and 241Am/239+240Pu activity ratios indicated the migration velocity was Am ≈ Pu > Cs. The intrinsic chemical properties of the radionuclides as well as soil type and properties (acidic, nutrient-deficient and low in organic matter and cation exchange capacity) might be reasons for the differences in their migration behaviors.

First study of 237Np in Chinese soils: Source, distribution and mobility in comparison with plutonium isotopes.

In this study, the distribution and migration of 237Np and 239+240Pu in soils in the vicinity (<5 km) of Qinshan and Tianwan Nuclear Power Plants in China were studied, which is the first specific study of global fallout 237Np in Chinese soils. The 237Np and 239+240Pu concentrations in surface soils showed large spatial inhomogeneity. A remarkable 239+240Pu concentration (4.783 mBq/g) was observed in a surface soil near Qinshan NPP and stands for the ever reported highest value in the Chinese soils. The inventories of 239+240Pu in two Qinshan and Tianwan soil cores were estimated to be 128.8 Bq/m2 and 121.0 Bq/m2, respectively; while the 237Np inventories were 0.039 Bq/m2 and 0.035 Bq/m2 at these sites, respectively. The 240Pu/239Pu atomic ratios in these soils indicated that the global fallout is the main source of Pu in these regions. However, the non-isotopic 237Np/239Pu atomic ratio in environmental soil is not a sensitive indicator for source identification. Furthermore, we conducted pilot study on the migration behaviors of 237Np and 239+240Pu in soil core at Qinshan site with the Convection-Dispersion Equation (CDE) model. The obtained apparent dispersion coefficients of 237Np (2.82 ± 2.06 cm2/y) was 5 times higher than that of 239+240Pu (0.57 ± 0.16 cm2/y), proving that 237Np has stronger migration ability than Pu isotopes in the Qinshan soil. Finally, we predicted that with the increase of migration time, both 237Np and 239+240Pu concentration in the soil will gradually become more evenly distributed among different soil layers due to the dominant dispersion effects.

Low-temperature fusion using NH4HSO4 and NH4HF2 for rapid determination of Pu in soil and sediment samples.

Two fusion methods were established for rapid determination of Pu in soil and sediment samples. The methods consisted of NH4HSO4 or NH4HF2 fusion procedures incorporated with procedures for CaF2/LaF3 co-precipitation, extraction chromatography and SF-ICP-MS measurement. The fusion procedures were done on a portable hot plate instead of in a cumbersome muffle furnace and took only 15 min heating-up time from room temperature to 250 °C and 15 min fusion time at 250 °C. Chemical recoveries of Pu after completing the NH4HSO4 and NH4HF2 fusion methods for 0.5-1 g sample were approximately 70% and more than 90%, respectively, and decreased with the increase of sample weight from 0.5 g to 5 g. Sediment samples were pre-ignited at different temperatures ranging from 450 °C to 1000 °C to form refractory fractions of Pu, with which the dissolution rates of Pu by the NH4HSO4 and NH4HF2 fusion were investigated. With the increase of pre-ignition temperature of the sediment samples, the dissolution rates of Pu from the samples prepared by NH4HSO4 fusion declined dramatically from near 100% for 450 °C to 8% for 1000 °C. In contrast, the NH4HF2 fusion was capable of completely releasing Pu from samples that were pre-ignited at temperatures over 450 °C to 1000 °C, which was comparable to releases obtained by the hazardous and time-consuming HNO3-HF digestion, and was superior to the conventional HNO3 digestion. Additionally, because HF is not used in any procedure of the NH4HF2 fusion, a safer and greener alternative to NH4HSO4 fusion and HNO3-HF digestion is realized for rapid Pu determination in environmental samples for nuclear emergency response and application in environmental studies.

The transfer of fallout plutonium from paddy soil to rice: A field study in Japan.

Reported transfer factor (TF) values of Pu from paddy soil to rice are rather scarce, despite the radiotoxicity of Pu and the irreplaceable role of rice in Asian peoples' diets. Here, we conducted a field study to investigate the transfer of global fallout Pu from paddy soil to rice grain (hulled rice) in Japan. The 240Pu/239Pu atomic ratios in two rice grain samples out of 16 samples were determined and the ratios corresponded well with the global fallout value. The soil-to-rice TFPu in 12 Japanese prefectures ranged from 4.5 × 10-6 to 1.2 × 10-4 with a geometric mean of 3.3 × 10-5. The TFs of rice obtained in this study were compatible to the TFs for the broad heading "cereals" compiled in the IAEA Technical Report Series No. 472. Weak correlations were found between the TF and the investigated soil characteristics such as soil pH and loss on ignition. Regarding the TFs for cerium (Ce) and thorium (Th) which are commonly considered as Pu analogues, we observed no significant correlations between the log(TFPu) and log(TFCe) or log(TFPu) and log(TFTh). On the other hand, interestingly, a significantly positive correlation (r = 0.795, p < 0.001) was observed between log(TFPu) and log(TFU). In view of the observed similarity of TF values for U and Pu from soil to rice, we thought that using the easy-to-measure TFU to estimate TFPu from soil to rice might be suggested although the mechanism was unclear.

Pu isotopes in the seawater off Fukushima Daiichi Nuclear Power Plant site within two months after the severe nuclear accident.

The marine environment is complex, and it is desirable to have measurements for seawater samples collected at the early stage after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident to determine the impact of Fukushima-derived radionuclides on this environment. Here Pu isotopes in seawater collected 33-163 km from the FDNPP site at the very early stage after the accident were determined (May 2011, within two months after the accident). The distribution and temporal variation of 239Pu and 240Pu were studied. The results indicated that both 239+240Pu activity concentrations (from 0.81 ±â€¯0.16 to 11.18 ±â€¯1.28 mBq/m3) and 240Pu/239Pu atom ratios (from 0.216 ±â€¯0.032 to 0.308 ±â€¯0.036) in these seawater samples were within the corresponding background ranges before the accident, and this suggested that Fukushima-derived Pu isotopes, if any, were in too limited amount to be distinguished from the background level in the seawater. The analysis of Pu isotopic composition indicated that the major sources of Pu in the seawater after the accident were still global fallout and the Pacific Proving Ground close-in fallout. The contribution analysis showed that the contributions of the Pacific Proving Ground close-in fallout in the water column of the study area ranged from 26% to 77% with the average being 48%.

First determination of Pu isotopes (239Pu, 240Pu and 241Pu) in radioactive particles derived from Fukushima Daiichi Nuclear Power Plant accident.

Radioactive particles were released into the environment during the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Many studies have been conducted to elucidate the chemical composition of released radioactive particles in order to understand their formation process. However, whether radioactive particles contain nuclear fuel radionuclides remains to be investigated. Here, we report the first determination of Pu isotopes in radioactive particles. To determine the Pu isotopes (239Pu, 240Pu and 241Pu) in radioactive particles derived from the FDNPP accident which were free from the influence of global fallout, radiochemical analysis and inductively coupled plasma-mass spectrometry measurements were conducted. Radioactive particles derived from unit 1 and unit 2 or 3 were analyzed. For the radioactive particles derived from unit 1, activities of 239+240Pu and 241Pu were (1.70-7.06) × 10-5 Bq and (4.10-8.10) × 10-3 Bq, respectively and atom ratios of 240Pu/239Pu and 241Pu/239Pu were 0.330-0.415 and 0.162-0.178, respectively. These ratios were consistent with the simulation results from ORIGEN code and measurements from various environmental samples. In contrast, Pu was not detected in the radioactive particles derived from unit 2 or 3. The difference in Pu contents is clear evidence towards different formation processes of radioactive particles, and detailed formation processes can be investigated from Pu analysis.

Rapid determination of ultra-trace plutonium isotopes (239Pu, 240Pu and 241Pu) in small-volume human urine bioassay using sector-field inductively coupled plasma mass spectrometry.

A rapid method with enhanced 238U decontamination was developed for ultra-trace Pu analysis in small-volume urine bioassays. This method consists of acid digestion, co-precipitation, extraction chromatography and sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS) measurement. Parameters that may influence the analytical performance were studied systematically. This method achieved a high 238U decontamination factor (3.8 × 106) and the 242Pu recovery was stable for 20 mL and 100 mL urine bioassays with an average value of 72.7 ±â€¯5.5%. The limits of detection for 239Pu, 240Pu and 241Pu by the method were 0.016 fg mL-1, 0.016 fg mL-1 and 0.019 fg mL-1 for 20 mL urine samples and 0.003 fg mL-1, 0.002 fg mL-1 and 0.003 fg mL-1 for 100 mL urine samples, respectively. Considering the small volume of urine employed in this study, the absolute detection limits of the method were comparable or even better than those measured with thermal ionization mass spectrometry and accelerator mass spectrometry. All procedures for 20 mL and 100 mL urine bioassays were completed in 9.5 h and 11 h, respectively, and analysis of 10 samples could be finished within one day. With the considerably low detection limits of Pu isotopes and high sample throughput, this method would be a promising tool for the quick response to radiological emergencies and for rapid screening of unexpected occupational exposures of workers involved in the future FDNPP reactor decommissioning operations.

Establishing rapid analysis of Pu isotopes in seawater to study the impact of Fukushima nuclear accident in the Northwest Pacific.

In order to assess the impact of the Fukushima derived Pu isotopes on seawater, a new analytical method to rapidly determine Pu isotopes in seawater by SF-ICP-MS including Fe(OH)2 primary co-precipitation, CaF2/LaF3 secondary co-precipitation and TEVA+UTEVA+DGA extraction chromatographic separation was established. High concentration efficiency (~100%) and high U decontamination factor (~107) were achieved. The plutonium chemical recoveries were 74-88% with the mean of 83 ± 5%. The precisions for both 240Pu/239Pu atom ratios and 239+240Pu activity concentrations were less than 5% when 15 L of seawater samples with the typical 239+240Pu activity of the Northwest Pacific were measured. It just needs 12 hours to determine plutonium using this new method. The limit of detection (LOD) for 239Pu and 240Pu were both 0.08 fg/mL, corresponding to 0.01 mBq/m3 for 239Pu and 0.05 mBq/m3 for 240Pu when a 15 L volume of seawater was measured. This method was applied to determine the seawater samples collected 446-1316 km off the FDNPP accident site in the Northwest Pacific in July of 2013. The obtained 239+240Pu activity concentrations of 1.21-2.19 mBq/m3 and the 240Pu/239Pu atom ratios of 0.198-0.322 suggested that there was no significant Pu contamination from the accident to the Northwest Pacific.

Distinctive distributions and migrations of 239+240Pu and 241Am in Chinese forest, grassland and desert soils.

The vertical distributions and downward migrations of the global fallout derived 239+240Pu and 241Am in diverse types of Chinese soils (forest, grassland and desert) were studied. The mean 239+240Pu and 241Am activity concentrations in the investigated soil cores were 0.28-0.69 mBq/g and 0.13-0.37 mBq/g, respectively, while the accumulative inventories were 61.53-138.99 Bq/m2 for 239+240Pu and 28.29-61.05 Bq/m2 for 241Am. The convection-dispersion equation (CDE) was used to calculate the migration parameters of 239+240Pu and higher apparent dispersion coefficients (D) were observed for the acidic forest soils compared with the alkaline grassland and desert soils; meanwhile a compartment model was employed to compare the migration of 239+240Pu and 241Am in successive soil layers which showed that the migration behaviors of 239+240Pu and 241Am were rather similar; both velocities were less than 0.3 cm/y in diverse types of soils and these findings were compatible with those of short-term laboratory simulation experiments in China.

Pu isotopes in soils collected downwind from Lop Nor: regional fallout vs. global fallout.

For the first time, soil core samples from the Jiuquan region have been analyzed for Pu isotopes for radioactive source identification and radiological assessment. The Jiuquan region is in downwind from the Lop Nor Chinese nuclear test (CNT) site. The high Pu inventories (13 to 546 Bq/m(2)) in most of the sampling locations revealed that this region was heterogeneously contaminated by the regional fallout Pu from the CNTs. The contributions of the CNTs to the total Pu in soils were estimated to be more than 40% in most cases. The (240)Pu/(239)Pu atom ratios in the soils ranged from 0.059 to 0.186 with an inventory-weighted average of 0.158, slightly lower than that of global fallout. This atom ratio could be considered as a mixed fingerprint of Pu from the CNTs. In addition, Pu in soils of Jiuquan region had a faster downward migration rate compared with other investigated places in China.