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Abstract: Dense and tailored porosity U1− y Am y O2− x (y =0.10; 0.15) pellets were fabricated to determine the influence of the microstructure on the self-irradiation effect at room temperature due to Am α decay. A first positron annihilation lifetime spectroscopy (PALS) approach showed the presence of vacancy type defects due to the cumulative decay dose. When He fills these vacancies, a void growth and swelling can occur. In addition, accurate diameter measurements with time were performed showing that a macroscopic swelling of 1% could be attained for the high density material. The stage value seems to depend on both type and porosity level. Swelling laws describing the diameter variation have been determined. [Copyright &y& Elsevier]

Abstract: Leaching experiments were performed on UO2 pellets doped with alpha-emitters (238/239Pu) and on spent fuel, in the presence of an external gamma irradiation source (A 60Co=260 Ci, Gyh−1). The effects of α, β, γ radiation, the fuel chemistry and the nature of the cover gas (aerated or Ar+4%H2) on water radiolysis and on oxidizing dissolution of the UO2 matrix are quantified and discussed. For the doped UO2 pellets, the nature of the cover gas clearly has a major role in the effect of gamma radiolysis. The uranium dissolution rate in an aerated medium is 83mgm−2 d−1 compared with only 6mgm−2 d−1 in Ar+4%H2. The rate drop is accompanied by a reduction of about four orders of magnitude in the hydrogen peroxide concentrations in the homogeneous solution. The uranium dissolution rates also underestimate the matrix alteration rate because of major precipitation phenomena at the UO2 pellet surface. The presence of studtite in particular was demonstrated in aerated media; this is consistent with the measured H2O2 concentrations (1.2×10−4 molL−1). For spent fuel, the presence of fission products (Cs and Sr), matrix alteration tracers, allowed us to determine the alteration rates under external gamma irradiation. The fission product release rates were higher by a factor of 5–10 than those of the actinides (80–90% of the actinides precipitated on the surface of the fragments) and also depended to a large extent on the nature of the cover gas. No significant effect of the fuel chemistry compared with UO2 was observed on uranium dissolution and H2O2 production in the presence of the 60Co source in aerated conditions. Conversely, in Ar+4%H2 the fuel self-irradiation field cannot be disregarded since the H2O2 concentrations drop by only three orders of magnitude compared with UO2. [Copyright &y& Elsevier]

Controlled Potential Coulometry (CPC) is generally regarded as one of the most accurate analytical methods available for the quantification of plutonium (Pu) in acidic aqueous solutions. With a growing use of mixed oxide nuclear fuel, there has been an interest in the technique's applicability to the analysis of Pu in the presence of uranium (U). Expanding on previous studies, the work presented herein demonstrates the highly accurate CPC analysis of Pu in mixed solutions (Pu + U) containing large quantities of U (U:Pu up to 100:1) as well as establishing the role played by sulfate anions during the analyses. By combining experimental results with speciation simulations, it is shown that the introduction of U in mixed Pu + U solutions is accompanied by increases in sulfate ions in the nitric acid medium which complex unequally with the different Pu cations in solution. The anions' stronger affinity of for Pu(IV) rather than Pu(III) causes a decrease in the formal potential of the Pu(IV)/Pu(III) redox couple, directly affecting the degree of completion of the electrochemical reactions performed during CPC. A correction factor, f, is applied to account for the degree of completion of the analysis' electrochemical steps and compensate for the effects of complexation. Thereby, a level of accuracy comparable to that achieved during the analysis of pure Pu solutions, is attained. Ultimately, a series of recommendations are put forward such that it becomes possible to maintain CPC's good analytical performance for the measurement of Pu content in mixed Pu + U solutions., (Copyright © 2020 Elsevier B.V. All rights reserved.)