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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.
The long-term behavior of nuclear glass subjected to alpha radiation by minor actinides must be investigated with a view to geological disposal. This study focuses on the effect of alpha radiation on the chemical reactivity of R7T7 glass with pure water, mainly on the residual alteration rate regime. A glass specimen doped with 0.85 wt% 239PuO2 (α emitter) is leached under static conditions in argon atmosphere at 90°C and at a high surface-area-to-volume ratio (S/V = 20 cm−1). The alteration rate is monitored by the release of glass alteration tracer elements (B, Na and Li). Radiation effects on the leached glass and its gel network are characterized by SEM and TEM analyses. Plutonium release is also measured by radiometry and its chemical oxidation state is assessed by measuring the pH and redox potential of the leachates. The results do not highlight any significant effect of alpha radiation on the residual alteration of this doped glass. This observation is consistent with SEM and TEM characterizations, which show that a protective layer can be formed under alpha radiation. Very low concentrations of soluble plutonium are measured in the leachate. These Pu releases are three orders of magnitude lower than the boron release, indicating strong plutonium retention.
The use of X-ray elemental analysis tools like energy dispersive X-ray
(EDS) is described in the context of the investigation of nuclear
materials. These materials contain radioactive elements, particularly
alpha-decaying actinides that affect the quantitative EDS measurement by
producing interferences in the X-ray spectra. These interferences
originating from X-ray emission are the result of internal conversion by
the daughter atoms from the alpha-decaying actinides. The strong
interferences affect primarily the L X-ray lines from the actinides (in
the typical energy range used for EDS analysis) and would require the use
of the M lines. However, it is typically at the energy of the
actinide's M lines that the interferences are dominant. The artifacts
produced in the X-ray analysis are described and illustrated by some
typical examples of analysis of actinide-bearing material.
So-called alpha-doped UO2 (i.e. UO2 containing short-lived alpha-emitters) can reproduce the activity levels of spent fuel at different ages. Previous experiments showed significant effects of alpha-radiolysis at relatively high alpha-activity levels (e.g., in the range 108 – 1010 Bq/g). This study shows the results of leaching tests done on low alpha-activity material (106 – 107 Bq/g). UO2 containing ∼10 and ∼1 wt. % 233U was leached at room temperature in deionized and carbonated water under deaerated conditions. Higher release in carbonated water was observed. A clear radiolytic enhancement of the concentration of uranium in solution was observed for the material containing 10 wt. % 233U. The dissolved U concentration was very close to the values previously observed for higher activity alpha-doped UO2 and confirmed the finding that under relatively high surface/volume condition the measured amount of U in solution is essentially independent of the alpha-activity level. No significant radiolysis effect was detected for the material doped with 1% 233U compared to undoped UO2 after ∼4 months of leaching. The post- leaching SEM examination revealed no pronounced etching of the surfaces.
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