Young Woo Lee

Non-stoichiometry, electrical conductivity and defect structure of hyper-stoichiometric UO2+x at 1000°C

Abstract The oxygen non-stoichiometry (x) and electrical conductivity (σ) of hyper-stoichiometric UO2+x have been measured as a function of partial pressure (PO2) at 1000°C by a solid-state coulometric titration technique and a dc 4-probe method, respectively. Both of the properties were found to be proportional to PO21/2 at the high oxygen partial pressure regime, and PO21/5 at the low oxygen partial pressure regime. These PO2-dependencies of the non-stoichiometry and the electrical conductivity are well explained with the (2:2:2) cluster model: (2Oia2Oib2VO)′ and (2Oia2Oib2VO)″″ are predominant at high and low PO2, respectively. The electron–hole mobility of UO2+x at 1000°C has been determined by the combination of the non-stoichiometry and electrical conductivity combined on the basis of the (2:2:2) cluster model.

Effects of Nb2O5 addition on grain growth and densification in UO2 pellets under reducing and/or oxidizing atmospheres

Abstract Grain growth and densification in Nb 2 O 5 -doped UO 2 pellets have been investigated under reducing and/or oxidizing atmospheres. Pellets are sintered at 1300°C in oxidizing (CO 2 ) atmosphere or at 1700°C in reducing (H 2 ) atmosphere and then annealed for 4 to 40 hours in reducing or oxidizing atmosphere. The addition of Nb 2 O 5 causes the formation of large pores in mirostructure, and the resultant decrease in density is smaller in reducing atmosphere than in oxidizing. Grain growth in Nb 2 O 5 -doped UO 2 is much more significant in reducing atmosphere than in oxidizing. Thus the enhancement of uranium diffusion due to Nb 2 O 5 addition is thought to be larger in reducing atmosphere. When an atmosphere is changed from oxidizing to reducing or vice versa, the effect of Nb 2 O 5 addition is accordingly enhanced or reduced. Possible formation of defects due to Nb 2 O 5 addition is discussed in oxidizing and/or reducing atmospheres.

Redox reaction kinetics of UO2+x

Abstract Oxidation and reduction kinetics of UO 2+ x was investigated in CO 2 /CO gas mixtures at elevated temperatures of 1000°C to 1300°C by monitoring the electrical conductivity relaxation induced by an abrupt change of ambient oxygen partial pressure. By fitting the experimental relaxation data to theoretical equations, the oxidation and reduction kinetics was found to be controlled mostly by the surface reaction step in the ranges of oxygen partial pressure and temperature examined and the surface reaction rate constants were derived therefrom.

Pore growth in sintered UO2

Abstract The density of UO 2 pellets decreases with sintering time during the sintering at 1500°C for 0.1 to 40 h in CO 2 and H 2 -CO 2 atmospheres. The density decrease is caused by pore growth. The microstructure developed in CO 2 atmosphere has many intragranular pores, which are mostly annihilated for further sintering, while intergranular pores grow significantly. The coalescence of intergranular pores occurs considerably even under negligible grain growth. In the uniform grain structure developed in H 2 -CO 2 atmosphere, the average pore size is proportional to the grain size as long as grain growth proceeds. The mechanisms of pore growth are discussed on the basis of the pore coalescence due to grain growth and the interlinkage of pores.

Density decrease of oxidatively sintered UO2–5wt%CeO2 pellets during resintering in H2 atmosphere

Abstract Pellets admixed with zinc stearate showed density decrease of 3%T.D. during resintering in H 2 atmosphere, but density change was below 0.4%T.D. in the pellets admixed with Acrawax and stearic acid when all the powder compacts were sintered in CO 2 atmosphere. Experiments revealed that the density of the pellets doped with zinc compounds and sintered in CO 2 also displayed a density decrease of 3%T.D. after resintering. Therefore, it was considered that the major cause of the large swelling of the pellets sintered in CO 2 atmosphere after resintering was Zn residues.

Characteristic Changes of (U, Ce)O2 Pellet Sintered by Addition of MO2 or M3O8 Scrap Powder and Different Sintering Processes

MO2 scrap powder was recovered by the mechanical crushing-milling or by the oxidation-reduction of a (U, Ce)O2 pellet, and M3Q5 powder by the oxidation of the pellet. The MQ2 or M3O8 scrap powder was reused up to 30wt% by adding it to UO2+5wt%CeO2 with two different powder treatment routes. The morphology and the addition effects of each scrap powder on sinterability were investigated with the powder routes and the sintering processes. Densification behaviors of the scrap-added powders were studied in an oxidizing atmosphere. A step was observed in each shrinkage curve between 900 and 1060°C as a result of shrinkage measurement up to 1500°C in CO2. Both the sintered density and the pore distribution could be controlled by the powder treatment method and the adding amount of scrap powder. Grain size of the pellet does not depend mainly on the powder process but on the sintering process.