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Experimental investigations and atomistic simulations are combined to study the cesium diffusion processes at high temperature in UO2. After 133Cs implantation in UO2 samples, diffusion coefficients are determined using the depth profile evolution after annealing as measured by secondary ion mass spectrometry. An activation energy of 1.8 ± 0.2 eV is subsequently deduced in the 1300–1600 °C temperature range. Experimental results are compared to nudged elastic band simulations performed for different atomic paths including several types of uranium vacancy defects. Activation energies ranging from 0.49 up to 2.34 eV are derived, showing the influence of the defect (both in terms of type and concentration) on the Cs diffusion process. Finally, molecular dynamics simulations are performed, allowing the identification of preferential Cs trajectories that corroborate experimental observations. [ABSTRACT FROM AUTHOR]

Cesium and iodine, which are formed during a fission process in a nuclear reactor, are considered as major fission products responsible for the environmental burden in case of a nuclear accident. From the safety point of view, it is thus important to understand their release mechanism when overheating of the reactor core occurs. This work presents an experimental investigation of the behaviour of caesium iodide and caesium fluoride in fluoride based molten salt reactor fuel during high temperature events. It has been demonstrated that CsF will be retained in the fuel salt and thus its volatility will be significantly reduced, while CsI will not dissolve in the fluoride-based fuel matrix and will thus remain more volatile. The influence of the presence of CsI and CsF on the melting behaviour of the fuel has been investigated using calorimetry, revealing their negligible effects. [ABSTRACT FROM AUTHOR]

The article deals with some fundamental aspects of the fission energy dispersion in nuclear reactor fuel. The analysis starts from the transmission electron microscope observation of tracks produced by energetic fission fragments in thin foils of UO[SUB2] single crystals. On the one hand, these tracks appear on the free surface as explosive material displacements. On the other, the passage of fission fragments in the bulk does not leave visible, continuous traces. Furthermore, irradiated U[SUB4]O[SUB9], which consists of a UO[SUB2] lattice hosting a temperature sensitive superstructure of interstitial oxygen, persists after experiencing the near-field action of fission fragment thermal spikes. These seemingly inconsistent phenomena have been explained by showing that a large fraction of the fission fragment electronic losses is converted in strong shock waves whose passage in the solid is too fast for producing atomic displacements, but which can release high energies by unloading on free surfaces. The consequent matter displacements have been shown to have important effects on the structural properties of the nuclear fuel. [ABSTRACT FROM AUTHOR]

A multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) coupled to a 213 nm ns-laser was used to measure uranium isotope ratios (234U/238U, 235U/238U, and 236U/238U) in six solid nuclear certified reference materials (CRMs). The powdered CRMs, containing depleted, natural and low enriched uranium (U), were pressed into pellets, for straightforward handling of the nuclear material. Being analysed without dissolution, the experimental U isotope ratios of the solid materials were in good agreement with the corresponding certified values. The measured U isotope ratios in the samples were corrected for mass bias using a pelletised CRM containing nominally 2% enriched 235U. Scanning electron microscopy investigations of an actual sample from which material was previously ablated revealed that only 5 ng of sample is consumed during a single measurement. As such laser ablation MC-ICP-MS can be regarded as quasi-non-destructive and therefore is an ideal technique for the rapid but still accurate and precise determination of the uranium isotopic composition in solid samples for nuclear safeguards and forensic purposes. The proposed analytical procedure essentially assists the characterisation of the illicit nuclear material for hazard and origin assessment. Moreover, as the sample is also an evidence, it can be analysed with further techniques. [ABSTRACT FROM AUTHOR]

A new double neptunium zirconium phosphate of the type MxZr2(PO4)3 (M = Np), crystallizing in the structure type NaZr2(PO4)3 (NZP, NASICON), was synthesized by solid state reactions at high temperatures and characterized by X-ray diffraction, infrared spectroscopy and Mössbauer spectroscopy. The Rietveld refinement of the XRD pattern together with the analysis of the IR spectra of the sample confirmed the space group P3̅c, the same as that for the lanthanide analogues Ln0.33Zr2(PO4)3. However, Mössbauer studies revealed the presence of neptunium in the two oxidation states +3 and +4, indicating a two-phase NZP system with different crystallographic environments of the neptunium atoms. The thermal behaviour of the sample was followed up to 1400 °C by thermogravimetric analysis. [ABSTRACT FROM AUTHOR]

During irradiation UO2 nuclear fuel experiences the development of a non-uniform distribution of porosity which contributes to establish varying mechanical properties along the radius of the pellet. Radial variations of both porosity and elastic properties in high burnup UO2 pellet can be investigated via high frequency acoustic microscopy. For this purpose ultrasound waves are generated by a piezoelectric transducer and focused on the sample, after having travelled through a coupling liquid. The elastic properties of the material are related to the velocity of the generated Rayleigh surface wave ( \mathrm V \mathrm {R} ). A UO2 pellet with a burnup of 67 GWd/tU was characterized using the acoustic microscope installed in the hot cells of the JRC-ITU at a 90 MHz frequency, with methanol as coupling liquid. \mathrm V \mathrm {R} was measured at different radial positions. A good agreement was found, when comparing the porosity values obtained via acoustic microscopy with those determined using SEM image analysis, especially in the areas close to the centre. In addition, Young’s modulus was calculated and its radial profile was correlated to the corresponding burnup profile and to the hardness radial profile data obtained by Vickers micro-indentation [ABSTRACT FROM PUBLISHER]

Recent electron microscopy investigations are presented with an emphasis on properties related to the safe extended operation of nuclear fuel. In particular, this article considers the formation of the high burnup structure (HBS), the prevailing microstructural aspects associated to spent fuel aging, and the release of fission products under accidental condition. Examples of microstructures associated with transient-tested samples but also with very high burnup fuel or heterogeneous mixed oxides (MOX) are presented together with results on damage formation in UO samples doped with Pu to study the specific effect of alpha damage on the microstructure during the cooling/storage time of irradiated fuel. Examples of single-effect studies (e.g., on the behavior of tellurium, a typical volatile fission product) using ion implantations are also presented. [ABSTRACT FROM AUTHOR]

Since the fall of the Iron Curtain illicit trafficking of nuclear and radioactive material has become an issue of concern both on the political and the scientific level. Seized material may be analysed in order to obtain clues on its origin and intended use and to prevent diversion of material from the same source in the future. Nuclear materials (uranium or plutonium) are of particular worry due to the nuclear proliferation risk associated with the material. Nuclear forensic investigations are aimed at the fact that nuclear material carries (inherent) information on its history, including on its origin and the processes applied for its production. Important conclusions can be drawn from decay products, activation products and fission products. Chemical impurities and the isotopic composition of certain major and minor constituents may provide additional information. Comparison of the measured results with nuclear material databases may yield evidence on the production site. The paper will describe the methodologies developed for addressing the above issues, focussing on radiochemical methods. Examples of nuclear forensic casework will illustrate the experience gathered in these areas. [ABSTRACT FROM AUTHOR]

BaxMIV xCe2−2 x (PO4)2 [MIV=Zr, Hf] monazite-like compounds were succesfully synthesized by solid state reaction for x≤0.2 (MIV=Zr) and x≤0.1 (MIV=Hf). The low miscibility of BaMIV(PO4)2 (MIV=Zr, Hf) compounds in CePO4 was explained on the basis of the monoclinic-to-trigonal phase transition that occurs at 733 K in BaZr(PO4)2 and at 798 K in BaHf(PO4)2. The hydrothermal alteration of these compounds was tested using a modified MCC-1 static leaching test in acid (1 mol·dm−3 HCl) and basic (1 mol dm−3 KOH) solutions at 373 K, 473 K and 573 K; both the experimental fluids and the reacted solid specimens were analyzed by different analytical techniques and the reaction mechanisms were elucidated. All the tested compounds are stable in 1 mol·dm−3 HCl until 573 K. The stability of the monazites in 1 mol·dm−3 KOH is a function of the temperature. [ABSTRACT FROM AUTHOR]