3.1 Precise Measurement of the Oxygen Potential of Uranium Oxides

Fig. 3-1
Apparatus of a solid state electrolytic cell for precisely measuring the oxygen potential of uranium oxides
This apparatus is designed to be able to measure the EMF of specimens smaller than the ordinary ones, in order to search for the oxygen potential g(O2) of oxide fuels. As a specimen is to be 3 mm in diameter and 2-3 mm in length, it is possible to easily transport a certain amount of oxygen into the specimens from the reference substance through a solid electrolyte barrier of zirconia by the coulometric titration method and to continuously measure the oxygen potential of the specimens of various non-stoichiometric compositions (x).

Fig. 3-2
Relation between the non-stoichiometric composition (x) and the activity(aO2) of oxygen in the U1-yGdyO2+x system
The relationship between the non-stoichiometric composition of oxygen (log x) and the oxygen potential g(O2), i.e. the oxygen activity (log aO2) for the U1-yGdyO2+x with different values of y at 1,000 degrees is represented. The values of y described in this figure are related to the contents of U and Gd. The solid lines are theoretical curves deduced from experimental data and the broken ones are deduced from those results.

In recent years the use of urania-gadolinia fuels in light water reactors has been promoted significantly because of the social demand for higher-burnup fuels. The U1-yGdyO2+x fuels containing several mol% Gd (y<0.1) as the burnable-neutron poison are very attractive for improving the power mismatch between fresh and partially burned fuel assemblies. Many other rare-earth elements (La, Nd, Sm, Ce, Pr, etc.) and their homologues Y are the major fission products resulting from the irradiation of UO2 fuel. These rare-earth elements similar to Gd dissolve in the host UO2 lattice and affect significantly the high temperature irradiation behavior of UO2 fuel such as the pellet-clad interaction, the release of fission products, the change of the oxygen potential g (O2) under the burnup and so on. The irradiation behavior is also crucially influenced by the initial values of g (O2) and the composition of oxygen (x) in the starting fuel. Thus, precise oxygen potential g (O2) data for the U1-yGdyO2+x system are indispensable for a better understanding and for controlling not only the fabrication process but also the irradiation process of the fuel. Yet, the experimental data for the U1-yGdyO2+x system are still fragmented and partly inconsistent, in contrast to the base material UO2+x for which very extensive and thorough experimental data are available. Using the solid state electromotive force (EMF) measurement as shown in Fig. 3-1, new concise thermodynamic quantities of the U1-yGdyO2+x have been obtained for wide ranges of the non-stoichiometric composition of oxygen (x), the content of Gd (y) and temperature (T) (1,500 degrees). One of the experimental results is given in Fig. 3-2. The oxygen potential g(O2), i.e. oxygen activity (aO2), steadily shifts to higher values with increasing content of Gd (y). This information on thermodynamics is very useful to analyze the behavior of the U1-yGdyO2+x fuels under a high-burnup and high temperature condition.

Reference
A. Nakamura, Thermodynamic Study of U1-yGdyO2+x by Solid State EMF Measurements, Z. Phys. Chem., 207, 223 (1998).

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