Wolfgang Dorr

Control of UO2 microstructure by oxidative sintering

Sintering of UO2 under oxidative conditions enables more easily than conventional sintering the control of the density, of the open porosity and of the grain structure. Coarsening and spheroidization as well as closure of pores are obtained by “solarization”. Large grains and desired grain size distributions are realized by “controlled path sintering” related to the U-O phases in the hyperstoichiometric range of the U-O phase diagram.

Bestimmung der dichte, offenen porosität, porengrössenverteilung und spezifischen oberflache von UO2-Tabletten

Abstract Basically, the quantitative characterization of structural parameters of UO2 pellets is necessary for optimizing their microstructure with respect to in-service performance. Dimensional behaviour (swelling, densification, plasticity) as well as fission gas and fission product release are dependent on density, open porosity, pore size distribution and specific surface area of the pellets. Measurement techniques for one or several of these properties are reported together with typical results on UO2 pellets from the AUC conversion process.

Resinter testing in relation to in-pile densification

Abstract There are two ways to predict dimensional behaviour during irradiation: 1. (i) quantitative analysis of the density and pore structure and model calculation, and 2. (ii) thermal resinter testing and application of a predicting correlation between measured density changes and in-pile densification. This paper gives a comparison between predicted in-pile densification and actual densification as determined by post-irradiation examination. A mechanistic model for thermal and fission-induced densification was applied. It was shown that the model predictions and the experimental results agreed well. Thus it was possible to derive a relation for the out-pile simulation of in-reactor behaviour which is applicable in quality control practice. However, long resintering periods are necessary for the simulation of high burnup.

Zur wärmeleitfähigkeit und plastizität von UO2 mit Gd-zusätzen

The thermal diffusivity and the thermal conductivity were measured for UO2-Gd2O3 with Gd2O3 ⩽ 10 W/O in the temperature range 25 °C ⩽ ⩽ 500 °C. An evaluation of the experimental data has shown, that the thermal conductivity is mainly influenced by microstructural changes in the mixed oxide structure due to Gd2O3-content. This result was also confirmed by detailed investigations of the structure of UO2-Gd2O3. Measurements of the stationary creep rate at Δ = 1500 °C and 100 N/mm2 from UO2 and UO2-6,5 W/O Gd2O3, showed, that the plasticity of such fuel is scarcely influenced by the Gd2O3, content.

Einstellung und charakterisierung der strukturgrössen von lwr-brennstoff

Zusammenfassung LWR fuel for high-burn up has initiated some efforts at the accommodation of the microstructural fuel properties. The adaption of the pore size can easily be performed by the addition of U 3 O 8 in well defined powder fractions. The adjustment of a coarse grain structure without the addition of oxidic dopants can be reached by using an oxidative sintering process. This process utilizes the diffusion enhancing effect of interstitial oxygen in UO 2 . Both measures do not only guarantee a reliable adjustment of the fuel properties, but also meet the conditions for a well controlled manufacturing procedure.