Rationale
Knudsen effusion mass spectrometry (KEMS) has been a powerful tool in physical chemistry since 1954. There are many excellent reviews of the basic principles of KEMS in the literature. In ...this review, we focus on the current status and potential growth areas for this instrumental technique.
Methods
We discuss (1) instrumentation, (2) measurement techniques, and (3) selected novel applications of the technique. Improved heating methods and temperature measurement allow for better control of the Knudsen cell effusive source. Accurate computer models of the effusive beam and its introduction to the ionizer allow optimization of such parameters as sensitivity and removal of background signals. Computer models of the ionizer allow for optimized sensitivity and resolution. Additionally, data acquisition systems specifically tailored to a KEMS system permit improved quantity and quality of data.
Results
KEMS is traditionally utilized for thermodynamic measurements of pure compounds and solutions. These measurements can now be strengthened using first principles and model‐based computational thermochemistry. First principles can be used to calculate accurate Gibbs energy functions (gefs) for improving third law calculations. Calculated enthalpies of formation and dissociation energies from ab initio methods can be compared to those measured using KEMS. For model‐based thermochemistry, solution parameters can be derived from measured thermochemical data on metallic and nonmetallic solutions. Beyond thermodynamic measurements, KEMS has been used for many specific applications. We select examples for discussion: measurements of phase changes, measurement/control of low‐oxygen potential systems, thermochemistry of ultrahigh‐temperature ceramics, geological applications, nuclear applications, applications to organic and organometallic compounds, and thermochemistry of functional room temperature materials, such as lithium ion batteries.
Conclusions
We present an overview of the current status of KEMS and discuss ideas for improving KEMS instrumentation and measurements. We discuss selected KEMS studies to illustrate future directions of KEMS.
The fission product release of two irradiated mixed oxide (MOX) fuel variants from the high burn-up disk irradiation test IFA-655 in the Halden reactor was investigated by Knudsen Effusion Mass ...Spectrometry (KEMS). The fresh samples were provided and characterised within the NFIR11NFIR: https://www.epri.com/portfolio/programs/108031 program. Both fresh fuel variants contained 16 wt.% plutonium but differed principally by the distribution of the plutonium: in one it was heterogeneously dispersed, in the other one homogeneously. The fission products release (Kr, Xe, I, Cs, Te, Sr, Nd) measured by KEMS as a function of annealing temperature is reported. The gas release (including helium) has been measured quantitatively. The interpretation of the release measurements is supported by microstructure and elemental analyses (SEM, TEM, EPMA). A correlation has been made with results from thermal diffusivity measurements obtained on the same fuels.
General interest in the deployment of molten salt reactors (MSRs) is growing, while the available data on uranium-containing fuel salt candidates remains scarce. Thermophysical data are one of the ...key parameters for reactor design and understanding reactor operability. Hence, filling in the gap of the missing data is crucial to allow for the advancement of MSRs. This study provides novel data for two eutectic compositions within the NaF-KF-UF4 ternary system which serve as potential fuel candidates for MSRs. Experimental measurements include their melting point, density, fusion enthalpy, and vapor pressure. Additionally, their boiling point was extrapolated from the vapor pressure data, which were, at the same time, used to determine the enthalpy of vaporization. The obtained thermodynamic values were compared with available data from the literature but also with results from thermochemical equilibrium calculations using the JRCMSD database, finding a good correlation, which thus contributed to database validation. Preliminary thoughts on fluoride salt reactor operability based on the obtained results are discussed in this study.
In search for chemically stable americium compounds with high power densities for radioisotope sources for space applications, AmVO3 and AmVO4 were prepared by a solid-state reaction. We present here ...their crystal structure at room temperature solved by powder X-ray diffraction combined with Rietveld refinement. Their thermal and self-irradiation stabilities have been studied. The oxidation states of americium were confirmed by the Am M5 edge high-resolution X-ray absorption near-edge structure (HR-XANES) technique. Such ceramics are investigated as potential power sources for space applications like radioisotope thermoelectric generators, and they have to endure extreme conditions including vacuum, high or low temperatures, and internal irradiation. Thus, their stability under self-irradiation and heat treatment in inert and oxidizing atmospheres was tested and discussed relative to other compounds with a high content of americium.
Detailed understanding about the chemical and physical properties of the Chernobyl “lava” and other radioactive meltdown products is of paramount importance for the support of decommissioning ...operations and nuclear accident modelling. In this study, we provide new results about the chemical composition and structural properties of the Chernobyl “lava” matrix obtained by electron microprobe analysis and confocal Raman spectroscopy. Based on the compositional data, a principal component analysis (PCA) was conducted to visualize compositional features of the black and brown “lava”, considering data from previous studies. In addition, an inverse modelling approach was performed to assess fractional contributions of construction materials that potentially contributed to the “lava” formation process. The results of the PCA show three varieties of “lava”. Different fractional contributions of UO2-fuel and Zr-cladding indicate the formation of at least two distinct sources of coium melt, respectively, for the black and brown “lava”. For the brown “lava”, the high concentration of Mg is explained by the melting and assimilation of 23% serpentine stemming from the lower reactor shield. The black “lava” shows a high contribution of concrete (43%). Significant differences in the Fe concentration of the black “lava”, as well as macroscopic flow patterns are indicative for a progressive melt formation. However, the cooling of the “lava” occurred relatively fast, forming a metaluminous glass with local variations in the degree of polymerization. Sub-microscopic inclusions of (U1-xZrx)O2 and (Zr1-xUx)O2 solid solutions point to a fractionation of U from an highly oversaturated melt. Based on the new results, the current hypothesis about the “lava” formation process is discussed and reviewed, questioning the existence of one homogenous source of melt and its stratification into layers of black and brown “lava”, before spreading and solidification of the melt.
Actinide-based nuclear ceramics, oxides particularly, are not only used as fuel in nuclear power reactors (uranium and plutonium) but are also used/envisaged as materials for electrical power sources ...in space probes (plutonium or americium). These actinides are all alpha-emitters, some having rather short half-lives. As a result of their strong alpha-activity, the actinide-based materials cumulate radiation damage and radiogenic helium. The stability of such materials needs to be assessed and understood for predicting the long-term stability of not only spent fuel in storage/disposal conditions but also of electrical power sources to be used in space probes. This paper describes the specific transmission electron microscope microstructure analyses of aged 238PuO2, 238Pu-doped UO2 (to simulate aged spent nuclear fuel), and of 241AmO2 samples (candidate electrical power source) and makes the correlation of the observed defects with other properties like helium thermal desorption and lattice parameter. It is shown that these fluorite structured materials resist to high alpha-damage levels and can accommodate large quantities of helium.
Significant amounts of spent uranium dioxide nuclear fuel are accumulating worldwide from decades of commercial nuclear power production. While such spent fuel is intended to be reprocessed or ...disposed in geologic repositories, out-of-reactor radiation damage from alpha decay can be detrimental to its structural stability. Here we report on an experimental study in which radiation damage in plutonium dioxide, uranium dioxide samples doped with short-lived alpha-emitters and urano-thorianite minerals have been characterized by XRD, transmission electron microscopy, thermal desorption spectrometry and hardness measurements to assess the long-term stability of spent nuclear fuel to substantial alpha-decay doses. Defect accumulation is predicted to result in swelling of the atomic structure and decrease in fracture toughness; whereas, the accumulation of helium will produce bubbles that result in much larger gaseous-induced swelling that substantially increases the stresses in the constrained spent fuel. Based on these results, the radiation-ageing of highly-aged spent nuclear fuel over more than 10,000years is predicted.
Aerosols were produced from a MOX (Mixed Oxide of uranium and plutonium) sample by laser heating up to 4000 K under atmospheric conditions. They were collected with a cascade impactor as function of ...their aerodynamic size, and studied by SEM, EDS, TEM, EELS, SAED and ICPMS. The uranium and plutonium concentrations were related to the aerosol morphology, size and formation temperature. A clear decrease of the Plutonium concentration was observed as function of the Aerodynamic Equivalent Diameter (AED). Some bigger aerosols enriched in Plutonium were found which originated from solidification of ejected droplets of the liquid phase at the surface of the sample.
•In sub micrometric aerosols produced from laser heated MOX the plutonium concentration decrease with aerosol size.•The specific properties of UO2 and PuO2 affects the formation of aerosols leading to elemental enrichment in the aerosols.•The collected EELS data on the smallest aerosols <20 nm shows that they are composed of pure uranium oxide.
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