On the Quality Control for the Determination of Ultratrace-Level ²³⁶U and ²³³U in Environmental Samples by Accelerator Mass Spectrometry

Abstract

Our recent attempt to determine ultra-trace level 236 U and 233 U in small volume seawater samples was challenged by high and unstable procedure blanks in our environmental radioactivity laboratory, which used to be a spent fuel research facility. Through intercomparison experiments with different laboratories and background checks on the chemical reagents and laboratory dust, the resuspended U-bearing dust was identified as the dominating source of the 236 U and 233 U contamination.With the implementation of background control (especially dust control) measures, the procedure blanks and detection limits of 236 U and 233 U for the radiochemical separation procedure has been significantly improved by three orders of magnitudes. With well-controlled blanks, the analytical precision for 236 U and 233 U predominantly relies on the AMS counting statistics. Background check and dust control are strongly recommended before the analyses of environmental level long-lived radionuclides (such as 236 U and 233 U) that are conducted in the former or active nuclear facilities, even if clearance of radioactivity relevant for radioprotection was achieved.The long-lived uranium isotope 236 U (t1/2 = 23.4 Myr) has become a popular tracer in oceanic studies because of improved measurement techniques by accelerator mass spectrometry (AMS) and its excellent properties, including high solubility, long residence time, and conservative features. [1][2][3][4] Besides a minor amount of naturally produced 236 U, 5 the majority of environmental 236 U was released through anthropogenic nuclear activities, including atmospheric nuclear weapon tests, 1,6-8 authorized discharges of nuclear reprocessing plants 9 and nuclear accidents [10][11][12] . The pulse injection of anthropogenic 236 U results in a wide scale of 236 U/ 238 U atomic ratios (10 -13 -10 -6 ) in the global oceanic systems (Figure 1), 1,7,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]29,30 which roughly corresponds to a range of (10 3 -10 9 ) atoms/kg for 236 U concentrations in seawater.This makes 236 U a suitable transient tracer to investigate the oceanic circulation patterns, water masses mixing, and transit time.Recent developments of the AMS technique enable the measurement and tracer application of another long-lived anthropogenic uranium isotope 233 U (t1/2 = 0.16 Myr) in the environment. 3,30,31 Different from 236 U, the only massive emission of anthropogenic 233 U occurred during the global fallout of nuclear weapons testing (GF). In contrast, little 233 U is produced by nuclear reactors and released from reprocessing plants or nuclear accidents. 3 The representative level of the 233 U/ 236 U atomic ratio in natural reservoirs exposed to GF is estimated to be (1.40 ± 0.15) × 10 -2 by a recent study on a peat core (Black forest, Germany), 3 while the documented levels in the discharges of the nuclear reprocessing plant at La Hague are 10 -7 -10 -6 (HELCOM MORS Discharge database). 32 In the Irish Sea, an average 233 U/ 236 U atomic ratio of (0.12 ±...

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

“…An optimized radiochemical procedure modified from our previous work was utilized for the determination of 236 U/ 238 U, 233 U/ 238 U, and 233 U/ 236 U atomic ratios in sediments by accelerator mass spectrometry (AMS), including acid digestion, coprecipitation, and extraction. , As strong acids are able to leach 236 U from soil and sediments quantitatively, , acid digestion with aqua regia (HCl:HNO 3 = 3:1, v/v) rather than total dissolution was adopted in our procedure to deal with 1–10 g of sediment samples. About 93 and 84% of 238 U in the Gotland Basin and Landsort Deep cores (obtained by total dissolution using HNO 3 -HClO 4 -HF) was leached by aqua regia digestion, respectively.…”

“…Nine procedure blanks were prepared following the same analytical protocol along with the sediment samples. Special measures presented in the Supporting Information were implemented for the background control of 236 U and 233 U . The procedure blanks contained less than 5.3 ng of 238 U, 1.8 × 10 6 atoms of 236 U, and 9.6 × 10 4 atoms of 233 U, which were negligible compared with the minimum amounts of 238 U, 236 U, and 233 U in the sediment samples except for the lowermost layer.…”

“…Special measures presented in the Supporting Information were implemented for the background control of 236 U and 233 U. 37 The procedure blanks contained less than 5.3 ng of 238 U, 1.8 × 10 6 atoms of 236 U, and 9.6 × 10 4 atoms of 233 U, which were negligible compared with the minimum amounts of 238 U, 236 U, and 233 U in the sediment samples except for the lowermost layer. To locate the layer recording the Chernobyl accident signal, 137 Cs and 241 Am activities in the sediment were directly measured by gamma spectrometry.…”

Simple and convenient preconcentration procedure for the isotopic analysis of uranium in seawater

“…In recent years, mainly due to the development of accelerator mass spectrometry (AMS), anthropogenic 233 U can be reliably measured in environmental samples. [5][6][7][8][9][10][11] The major routes for the production of 233 U are 232 Th(n, g) 233 Th / 233 Pa / 233 U and 235 U(n, 2n) 233 U in nuclear facilities and nuclear testing, respectively, 6 where the ratio 233 U/ 236 U is about 10 −2 in the environment. Thus, the measurement and acquisition of combinations of U isotope ratios such as 236 U/ 238 U, 233 U/ 238 U and 233 U/ 236 U represent a powerful strategy for clarifying the origins of U in the environment.…”

Rapid and Precise Uranium Isotopic Screening in Natural Water Using Bis(2-ethylhexyl)phosphoric Acid-Modified Membranes

“…Most of the natural water samples contain detectable amounts of uranium: the average concentration in seawater is around 3 ppb, whereas groundwater may contain varying concentrations of uranium, ranging from less than 0.1 ppt to several ppm. , Because of the associated toxicity, uranium assay in natural water samples is of prime importance, and highly sensitive methods are required for monitoring trace or ultra-trace levels of uranium in water samples which are collected for prospective purposes. Although various instrumental techniques including spectrophotometry, fluorimetry, radiometry, atomic absorption/emission spectrometry, and XRF analyses are available for the determination of uranium, − mass spectrometric techniques, especially thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICP-MS), and accelerator mass spectrometry (AMS) are known to provide the most accurate and precise data for uranium measurement. − …”