P. Pöml

Reassessing the melting temperature of PuO2

The melting behavior is a fundamental property of a material, closely related to its structure and thermodynamic stability, and has therefore been a crucial subject of research for ages. The melting point is also an important engineering parameter, as it defines the operational limits of a material in its application environment. This point becomes critical in nuclear engineering where the thermo-mechanical stability of a nuclear fuel element is a key factor determining fuel performance and safety. However, experimental difficulties stemming from the extreme temperatures, complex pressure-temperature-composition relations, and the high radioactivity make the study of melting of refractory actinide compounds particularly challenging. As a consequence, experimental data are rare and subject to large uncertainties, and more reliable experimental techniques are badly needed. A novel experimental approach is presented here, yielding new data and allowing a re-assessment of the PuO 2 melting behaviour.

Contrasting immobilization behavior of Cs+ and Sr2+ cations in a titanosilicate matrix

ETS-10 titanosilicate was tested as an adsorbent for the removal of Cs+ and Sr2+ cations from radioactive waters, considering both the ion exchange and the behaviour of the loaded adsorbent during thermal conditioning. The studies indicate that ETS-10 has a high and similar affinity for both Cs+ and Sr2+ cations reaching the ion exchange capacity of ETS-10 at a concentration of about 50 milliequivalents gram per liter. Thermal treatment of Cs+- and Sr2+-exchanged ETS-10 materials results in melting at approximately 700 °C. The melting temperature increases with the initial Cs+ and Sr2+ concentration and is higher for Sr2+ than for Cs+ exchanged ETS-10. Recrystallisation occurs only in the presence of Sr2+ as evidenced by the exothermic effects between 800 and 900 °C. After calcination of Cs+- and Sr2+-exchanged ETS-10 in air at 800 °C, two types of materials were obtained: an amorphous glass material with homogeneous Cs+ distribution and a strontium fresnoites glass–ceramic material.

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.

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.

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.