During the Chernobyl accident about 6-8 tonnes of uranium fuel were released and fuel particles were identified up to 2000 km from the site (e.g. Norway). During the initial explosion mechanical destruction of fuel occurred and fuel particles were deposited to the West of the reactor. During the subsequent reactor core fire, however, fuel particles were also deposited to the North. To assess the long term consequences, information on source and release dependent characteristics of the particles is needed. Thus, microscopic X-ray techniques such as µ-X-ray absorption tomography, µ-X-ray absorption near edge spectroscopy (µ-XANES) and µ-X-ray diffraction (µ-XRD) have been employed at beamline ID22. A combination of these techniques enables the determination of the uranium distribution, crystallographic structure and oxidation states, characteristics essential for understanding weathering and subsequent mobilisation of associated radionuclides.

Computed µ-tomography data collection and reconstruction techniques have provided 3-D images of uranium within individual particles, characterised by gamma spectrometry and SEM [1]. During rotation, images were recorded at 17 keV with a high resolution, cooled CCD-based X-ray detector. Due to the high coherence of the beam, image contrast is improved compared to pure absorption as the phase contrast caused by the phase shift of the plane incoming wave at edges or interfaces invokes a different increment of the refractive index. Tomographic reconstruction (Figure 25a) and computerised slicing (Figure 25b) of the 3-D image demonstrated inhomogeneous distribution of U within the particles. Channels and cavities are probably caused by formation of volatile fission products during reactor operations.

Differences in U-oxidation states are manifested by shifts in pre-edge and bound-state edge features in µ-XANES spectra due to an increase in the binding energy of the core electron levels. Well-defined standards: U-metal (oxidation state 0), UO2 (+4), U3O8 (i.e. UO2x2UO3), and UO2Ac2 (+6) were used for calibration. Metallic U and UO2Ac2 served as the oxidation state scale endpoints. The µ-XANES spectra were obtained at 0.8 eV increments over a 160 eV energy range extending from about 40 eV below and 120 eV above the U LIII absorption edge at 17.163 keV. The µ-XANES spectra of the standards (Figure 26a) reflected a shift of the inflection point energy (central location of edge positions) with oxidation state, a near-linear relationship, having a slope of about 1 eV/oxidation state unit (Figure 26b).

µ-XANES spectra of particles collected at the Chernobyl North region were close to the standard U3O8 spectra. The inflection point energies corresponded to oxidation state 5.0±0.5 (Figure 26b), in agreement with U2O5/U3O8. Using line scan XANES, particles released during the reactor fire were characterised by UO2 cores surrounded by U2O5/U3O8 layers (Figure 26c). Surprisingly, µ-XANES spectra of West particles (Figure 26b) occurred as an intermediate between metallic U and UO2 with inflection point energies corresponding to the oxidation state 2.5±0.5. This finding is both novel and surprising. Using line scan XANES, particles released during the initial explosion were characterised by a UO2 core surrounded by a layer of reduced U, probably due to interactions with graphite from the damaged moderator. Although the presence of UC2 and UO was indicated by µ-XRD, further analysis is needed to establish the phases. Although UO2 fuel is the common source, particle characteristics such as oxidation states depend on the release scenario (explosion or fire). The differences in oxidation states of uranium in fuel particles explain well the observed differences in weathering kinetics, mobility and soil-to-vegetation transfer coefficients of particle associated radionuclides West and North of the reactor.

[1] B. Salbu, T. Krekling, D.H. Oughton, Analyst, 123, 843-849, 1998.

B. Salbu (a), K. Janssens (b), T. Krekling (a), A. Simionovici (c), M. Drakopoulos (c), C. Raven (c), I. Snigireva (c), A. Snigirev (c), O.C. Lind (a), D.H. Oughton (a), F. Adams (b), V.A. Kashparov (d).

(a) Department of Chemistry and Biotechnology, Agricultural University of Norway, Aas (Norway)
(b) Department of Chemistry, University of Antwerp (Belgium)
(c) ESRF
(d) Ukrainian Institute of Agricultural Radiology, Kiev (Ukraine)