S. Brémier

Microstructure of irradiated SBR MOX fuel and its relationship to fission gas release

Abstract SEM and EPMA examinations of the microstructure and microchemistry of British Nuclear Fuel’s quasi-homogeneous SBR MOX fuel following irradiation suggests behaviour which is very similar to that observed in UO2. Most significantly, a fission gas release of 1% in three-cycle SBR MOX PWR rods is associated with the development of a well-defined intergranular bubble network, which has not been seen previously in the more heterogeneous MOX fuels irradiated under similar conditions. The contrast between the observations is attributed to the relatively low volume fraction and small size of the Pu rich inhomogeneities in the SBR fuel which generate only 4% of the total fission gas and eject most of this into the surrounding mixed oxide matrix. The resulting perturbation in the Xe distribution has a negligible influence on the evolution of the microstructure. A key observation is made from the results of recent post-irradiation annealing experiments performed on SBR MOX and UO2. These confirm near identical fission gas behaviour in the two fuel types when the influence of thermal conductivity and rod rating are removed.

Properties of the high burnup structure in nuclear light water reactor fuel

Abstract The formation of the high burnup structure (HBS) is possibly the most significant example of the restructuring processes affecting commercial nuclear fuel in-pile. The HBS forms at the relatively cold outer rim of the fuel pellet, where the local burnup is 2–3 times higher than the average pellet burnup, under the combined effects of irradiation and thermo-mechanical conditions determined by the power regime and the fuel rod configuration. The main features of the transformation are the subdivision of the original fuel grains into new sub-micron grains, the relocation of the fission gas into newly formed intergranular pores, and the absence of large concentrations of extended defects in the fuel matrix inside the subdivided grains. The characterization of the newly formed structure and its impact on thermo-physical or mechanical properties is a key requirement to ensure that high burnup fuel operates within the safety margins. This paper presents a synthesis of the main findings from extensive studies performed at JRC-Karlsruhe during the last 25 years to determine properties and behaviour of the HBS. In particular, microstructural features, thermal transport, fission gas behaviour, and thermo-mechanical properties of the HBS will be discussed. The main conclusion of the experimental studies is that the HBS does not compromise the safety of nuclear fuel during normal operations.

Microbeam analysis of irradiated nuclear fuel

Microbeam analysis is widely used in the nuclear power industry. It is used to characterise the as-fabricated fuel, for routine post-irradiated examination and for research into the mechanisms of phenomena that limit the energy production of the fuel. The techniques most commonly used are wavelength-dispersive electron probe microanalysis, scanning electron microscopy and secondary ion mass spectrometry. Other microbeam analysis techniques that have been successfully applied to irradiated nuclear fuel are transmission and replica electron microscopy, X-ray fluorescence and micro X.-ray diffraction. Specific examples illustrating the past and present use of microbeam analysis in nuclear research establishments are presented with emphasis on the unique results they provide. As an aid to understanding, some basic facts about nuclear fuel rods and their irradiation are first given. This is followed by a description of features that set apart the microbeam analysis of high radioactive materials from standard practice.

Calibration of a Cameca SX100 microprobe for the measurement of retained xenon in nuclear fuels

In 2007 a new fully shielded Cameca SX100 electron microprobe was installed at the Institute for Transuranium Elements in Karlsruhe, Germany, for the measurement of irradiated nuclear fuels. For the future analysis of the fission gas xenon, it was necessary to calibrate the machine for the use of antimony as a standard, due to the lack of suitable xenon standards. This paper describes the procedure and results of the calibration process and shows an example of a measured Xe profile.

Analysis of Pu by Virtual-standard WDS-EPMA. Results of an Interlaboratory Round-robin Test

* GM, CNRS, Universite de Montpellier II, Pl. E. Bataillon. FR-34095 Montpellier Cedex 5. France. ** SCT, Universitat de Barcelona. Lluis Sole i Sabaris, 1-3. ES-08028 Barcelona. Spain. CEA Cadarache, DEN/DEC, FR-13108 St-Paul-Lez-Durance. France. **** EC, JRC, Institute for Transuranium Elements, P.O. Box 2340, DE-76125 Karlsruhe, Germany. ***** CEA Marcoule BP 17171, FR-30207 Bagnols sur Ceze. France. ****** Paul Scherrer Institut, CH-5232 PSI Villigen, Switzerland. ******* SCK CEN, Boeretang 200, BE-2400 Mol, Belgium

Electron probe microanalysis of a high burnup (Th,Pu)O 2 fuel section

Electron probe micro-analysis (EPMA) is an important technique for a broad range of applications in nuclear sciences. One main target is to improve the safety of the nuclear fuel cycle, by studying the chemical and physical properties of spent nuclear fuel and its fission products, either solids, volatiles, or gases, after the irradiation [1]. Of particular interest for the nuclear scientist is the distribution and quantity of actinides in the fuel, before and after irradiation. The fuel types most commonly studied by EPMA are uranium oxide and Mixed Oxide Fuel (MOX) containing a mixture of uranium and plutonium oxide. Minor actinides (Np, Am, Cm) are produced during irradiation by neutron capture and/or alpha-decay of uranium, and can be added during fuel fabrication (usually containing the fissile material) in the case of transmutation schemes.

Improved Background Correction for the Quantification of Actinide M-lines in EPMA

Electron probe micro-analysis (EPMA) is an important technique for a broad range of applications in nuclear sciences. One main target is to improve the safety of the nuclear fuel cycle, by studying the chemical and physical properties of spent nuclear fuel and its fission products, either solids, volatiles, or gases, after the irradiation [1]. Of particular interest for the nuclear scientist is the distribution and quantity of actinides in the fuel, either before or after irradiation. Actinides in nuclear fuel can be added during fuel fabrication (usually containing the fissile material), or are being produced during irradiation by neutron capture and/or alpha-decay of existing actinides. The fuel types most commonly studied by EPMA are uranium oxide and Mixed Oxide Fuel (MOX) containing a mixture of uranium and plutonium oxide.

Microbeam analysis techniques for the characterisation of irradiated nuclear fuel

Microbeam analysis is widely used in the nuclear power industry. It is used for routine postirradiation examination and for research into the mechanisms affecting safe operation of the nuclear fuel. The techniques most commonly used are wave-length dispersive electron probe microanalysis (WDSEPMA), scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). Other microbeam analysis techniques that have been successfully applied to irradiated nuclear fuel are transmission and replica electron microscopy (TEM and REM), micro X-ray fluorescence (micro-XRF) and micro X-ray diffraction (micro-XRD). SEM, TEM and REM have been mainly used to study the evolution of fission gas bubbles, which cause the fuel to swell during irradiation [1–3], and micro-XRD has been used to investigate the change in lattice parameter caused by irradiation damage and the buildup of fission products during irradiation and to assess their influence on the transformation of the fuel microstructure after prolonged irradiation [4].

Micro-analytical Investigations on Actinide (Am, Cm) Reference Materials

Actinides are extremely toxic and highly radioactive (especially the transuranic elements) hence difficult to handle. Two Am oxide compounds are currently investigated in the shielded facilities of the Institute for Transuranium Elements (ITU): (U0.9Am0.1)O2 and (U0.8Am0.2)O2. These ceramics are being characterized by inductively coupled plasma mass spectrometry (ICP-MS) for their bulk chemistry and impurities, by EPMA (Cameca SX100R) for their precise chemical composition and lateral homogeneity, and by secondary mass spectrometry (SIMS) (Cameca 6fR) depth-profiling for their in-depth homogeneity. The Am content of the two compounds is cross-checked by EPMA by analytical determination and using a calibration-curve approach described in [2]: the X-ray Ma and M ß intensities of Th, U, Np, and Pu are measured, and the resulting curve fitted. The Am intensity can then be linked to the U intensity; this allows using the U signal as a standard for Am. Both the analytical characterization and the calibration curve method are expected to yield the same result.

Determination of the Oxidation State of Irradiated Nuclear Fuel Using SIMS

During the fission of UO2 nuclear fuel oxygen is set free. Part of the oxygen liberated reacts with the fission products and forms oxides of these elements. The balance dissolves in the fuel matrix increasing its stoichiometry as indicated by the oxygen to metal (O/M) ratio, or is absorbed by the Zircaloy tube which contains the fuel. The O/M ratio is generally acknowledged to be the most important chemical property of UO2 nuclear fuel during irradiation. This is because the oxygen potential, ( ) 2 O G Δ , and the related O/M ratio of the fuel, affect diffusion controlled processes such as grain growth [1], creep [2] and fission gas release [1], the thermal conductivity of the fuel [3] and the chemical state and hence the behaviour of the fission products. The ( ) 2 O G Δ and O/M ratio of the fuel are not constant during irradiation but change with burn-up (time) due to the incorporation of fission products.