Temporal evolution of plutonium concentrations and isotopic ratios in the Ukedo - Takase Rivers draining the Difficult-To-Return zone in Fukushima, Japan (2013-2020).

In 2011, the Fukushima Dai-Ichi Nuclear Power Plant (FDNPP) accident released significant quantities of radionuclides into the environment. Japanese authorities decided to progressively reopen the Difficult-To-Return Zone after the decontamination of priority reconstruction zones. These areas include parts of the initially highly contaminated municipalities located to the north of the FDNPP, including Namie Town, an area drained by the Ukedo and Takase Rivers. Eleven years after the accident, research focused on the spatial distribution of plutonium (Pu) and radiocesium (Cs) isotopes at contrasted individual locations. To complement previous results, the current research was conducted on flood sediment deposits collected at the same locations after major flooding events during eleven fieldwork campaigns organised between 2013 and 2020 at the outlet of the Ukedo and Takase Rivers (n = 22). The results highlighted a global decrease of the Pu and 137Cs contents in sediment with time during the abandonment phase in the region, from 2013 (238.20 fg g-1) to 2020 (4.28 fg g-1). Furthermore, based on the analysis of the 240Pu/239Pu isotopic ratios, the plutonium transiting these rivers (range: 0.166 - 0.220) essentially originated from the global fallout (0.180 ± 0.014 (Kelley et al., 1999)). Sediment showed contrasted properties in the two investigated rivers, which is likely mainly the result of the occurrence of Ogaki Dam on upper sections of the Ukedo River as it strongly impacts the material supply from this river to the Pacific Ocean. A statistical analysis highlighted the strong correlation between Pu activity concentrations and 137Cs activities in both rivers, confirming that both radionuclides are transported with a similar pathway. Despite it was detected early after the accident (2011-2013), the current research demonstrates that plutonium originating from FDNPP is no longer detected in these rivers draining the Difficult-To-Return Zone at the onset of the reopening of the area to its former inhabitants.

240Pu/239Pu isotopic ratio measurements in micrometric Pu and MOX particles using Secondary Ion Mass Spectrometry.

Every accident affecting industrial or nuclear facilities emits micrometric fragments of material into the environment whose elemental and isotopic compositions are characteristic of the process or event. Particle analysis, mainly implemented in the framework of the Non Proliferation Treaty to detect clandestine nuclear activities, provides a powerful tool to identify the origin of the nuclear particulate matter and to assess the environmental impact of nuclear accidents. Initially, particle-scale isotopic analyses aimed at the determination of the U isotopic composition. Now, focus is increasingly given on Pu isotopic measurements to address its origin and potential use. Such measurements are more challenging because of isobaric interferences, including those induced by hydride ions, like 239PuH+ on 240Pu+ and 238UH+ on 239Pu+ in Mixed Oxide (MOX). Such ions are generated during ionization processes by Secondary Ion Mass Spectrometry. Based on a parametric study aiming at the measurement of uranium oxide, uranium carbide and uranium single and double hydride rates, we determined that Pu and U should be detected as elementary ions to limit the impact of such interferences, although mono-oxide ions are more abundant. Thus, we developed an analytical methodology to obtain accurate 240Pu/239Pu atomic ratios both for weapon grade Pu and MOX materials. Hydride rate is first measured in U oxide particles and then applied to correct 240Pu+ and 239Pu+ signals. The relative difference of corrected 240Pu/239Pu isotopic ratios with expected values is reduced by a factor of 4 when measuring weapon grade Pu particles and by a factor of 10-100 when measuring MOX particles containing 1 to 10 wt% of Pu. We also proposed a method to determine the Relative Sensitivity Factor (RSF) based on the decay of Pu in order to quantify the Pu content in MOX samples. The estimated lowest measurable 239Pu/238U atomic ratio in MOX particles is ∼1.6 × 10-3.

More than Five Percent Ionization Efficiency by Cavity Source Thermal Ionization Mass Spectrometry for Uranium Subnanogram Amounts.

Numerous applications require the precise analysis of U isotope relative enrichment in sample amounts in the subnanogram to picogram range; among those are nuclear forensics, nuclear safeguards, environmental survey, and geosciences. However, conventional thermal ionization mass spectrometry (TIMS) yields U combined ionization and transmission efficiencies (i.e., ratio of ions detected to sample atoms loaded) of less than 0.1% or 2% depending on the loading protocol, motivating the development of sources capable of enhancing ionization. The new prototype cavity source TIMS at ETH Zürich offers improvements from 4 to 15 times in combined ionization and transmission efficiency compared to conventional TIMS, yielding up to 5.6% combined efficiency. Uranium isotope ratios have been determined on reference standards in the 100 pg range bound to ion-exchange or extraction resin beads. For natural U standards, n(235U)/ n(238U) ratios are measured to relative external precisions of 0.5-1.0% (2RSD, 2 < n < 11, conventional source) or 2.0% (2RSD, n = 6, cavity source) and accuracies of 0.2-0.7% (conventional source) or 0.4-0.9% (cavity source). Meanwhile, n(234U)/ n(238U) ratios are determined to relative external precisions of 1.7-3.6% (2RSD, 2 < n < 11, conventional source) or 5.6% (2RSD, n = 6, cavity source) and accuracies of 0.1-2.5% (conventional source) or 0.5-8.3% (cavity source), which would benefit further from in-run organic interference and peak tailing corrections.

Age Dating of Individual Micrometer-Sized Uranium Particles by Secondary Ion Mass Spectrometry: An Additional Fingerprint for Nuclear Safeguards Purposes.

A direct and simultaneous analysis of the age and isotopic composition of nuclear material at the particle scale is described in this study. By comparison with other conventional techniques such as inductively-coupled plasma mass spectrometry or thermal-ionization mass spectrometry, secondary ion mass spectrometry enables one to determine the ages of individual particles in a mixture of nuclear materials. Having access to the purification date can give precious information on the history of a nuclear facility or nuclear material for safeguarding purposes. The high sensitivity of this technique combined with its imaging capabilities enables one to detect and to sort out all particles according to their isotopic composition in one analysis. The succession of two microbeam analyses on an individual particle allows the determination of the precise 235U abundance and the model age. The methodology was successfully applied to a mixture of uranium particles coming from certified reference material with a 235U abundance ranging from 10% to 97%: the accuracy on the 235U abundance is greater than 0.5% and the accuracies on the 235U abundance and the model age are better than 0.5% and 3%, respectively.