Extraction and analysis of nuclear fuel fragments from the Chernobyl exclusion zone

Abstract

To this day, the radioactive fallout from the Chernobyl Nuclear Power Plant accident contaminates the Chernobyl Exclusion Zone (CEZ) in the form of spent fuel fragments, so called hot particles. For precise prediction of particle leaching and subsequent mobilization of radioecologically relevant radionuclides, such as U, Pu, 137Cs, 90Sr or 241Am, in-depth knowledge about these particle’s nature needs to be gathered. This work consists of several parts. First, methods for particle separation from soil and sediment samples are tested and developed into an optimized routine sequence, which is then used to identify a number of hot particles. In the following step, a Scanning Electron Microscopy (SEM)-based micromanipulation technique is applied to image and extract single micron-sized particles. Individual specimen are glued to fine tungsten needles for easier and safer handling. Using such separately mounted particles, a range of non-destructive analytical techniques is applied to achieve a wide set of data for each particle. These techniques include SEM imaging and Energy Dispersive X-ray Spectroscopy (EDS) analysis and γ spectrometry for quantification of 137Cs, 241Am and 154Eu in single particles. To analyze isotopic ratios with lateral resolution, static Time Of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) is applied, which can also be used to image the isotope distribution on a particle’s surface. General feasibility and capability of resonant Laser-Secondary Neutral Mass Spectrometry (SNMS) analysis on individual particles is demonstrated. To investigate uranium speciation non-destructively within the samples, three particles were analyzed at the Swiss Light Source (SLS) beamline at the Paul Scherrer Institut (PSI) in Switzerland. Here, µ-focus X-ray Fluorescence (µ-XRF), µ-focus X-ray Absorption Near Edge Spectroscopy (µ-XANES) and finally µ-focus X-ray Diffraction (µ-XRD) were applied to yield extensive data about oxidation states of uranium and the internal structure of the particles. Finally, an incremental dissolution sequence was tested for feasibility. A single tungsten needle-mounted particle was subsequently exposed to increasingly aggressive acidic leaching steps. After each step, the leaching progress was monitored by measuring the leachant with γ spectrometry and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for 137Cs and plutonium quantification as well as particle SEM imaging after each step.