Oxidation behaviour

3.1 Definition of the test atmosphere

Under the dry conditions, the pure reaction of pellets with oxygen has been investigated. The appropriate test conditions were defined so as to achieve a sufficient oxidation for representative measurements but without a bulk oxidation of pellets. This was done by a test series with standard UO2 at different oxygen partial pressures in argon inert atmosphere.

Standard UO2 pellets were subjected to four atmosphere variants made of argon with oxygen content from 0.01% to 1%. After 20 h testing it was observed that the atmosphere containing 0.1% oxygen already led to a significant attack of the pellet, at 1% oxygen, the pellet was almost completely pulverized. The pellets tested in atmospheres above 0.05% oxygen have shown already a significant amount of U3O8 powder on their surfaces, whereas the pellet tested in the atmosphere with 0.01%

oxygen show more or less intact surface with only few traces of oxidation (U3O8 grey structures in white UO2 phase). Figure 4 shows the polished cross-sections of the samples. In all atmospheres, the oxidation took place in an intergranular mode, where the oxygen attacked the grain boundaries first.

The consequent volume increase of the matrix when oxidizing produced stresses in the structure leads to propagation of cracks and to strip deeper grain layers.

FIG. 4. Ceramography of UO2 samples tested under [Ar-O2] atmospheres (×300).

The thermogravimetric records on Figure 5 show that the weight gain logically increases when rising the oxygen concentration in Ar. It is also observed that the oxidation kinetic varies according to the oxygen concentration. Under Ar+0.01% O2, a period of low weight gain, often called induction period, is followed by a rapidly accelerating rate up to a linear portion. The induction period reveals that the oxidation is governed by surface processes mainly. For the other test conditions, the oxidation process is initiated faster and the kinetic is continuously linear. The dependence of the oxidation rate on the oxygen partial pressure was of potential character with an exponent ~0.72. This result is consistent with that obtained by Tucker [12] who determined values of 0.65 and 0.68 at 350 and 400 °C respectively. The total sample weight change after 20 h indicated that an atmosphere with 1% oxygen led to an almost complete oxidation of UO2 to U3O8.

Based on these series, further tests were performed applying an Ar atmosphere containing 0.01% O2. Under this condition, the mass changes are reliably measurable, the pulverization of the pellet surface occurs relatively late so the ceramographic examinations are less affected by the loss of material.

Moreover, such low oxygen content allows distinguishing, possible behavioural differences of samples during the initial surface oxidation process, before the bulk oxidation begins.

0.001 0.01 0.1 1 10

1 10 100 1000 10000

Time (min)

Weight gain (%)

1%O2 0.1%O2 0.01%O2

FIG. 5. Thermogravimetric records of UO2 samples tested under Ar-containing different oxygen concentrations.

3.2 Influence of pellet characteristics 3.2.1 Influence of UO2 powder source

In this series, all variants have a sintered density of ~96.5% TD and an average grain size of about 15 µm. From one sample to the other, the initial oxidation phases are only slightly different. Despite,

some kinetic changes during the oxidation process, all samples reached the same final linear-rate curve; with comparable final weight changes (see Figure 6). All together, one can conclude that the type of UO2-source powder doesn't deeply affect the long-term oxidation behaviour of pellets The variability observed in the oxidation kinetics is difficult to explain a posteriori. One hypothesis to put forward is the respective impurity amounts for each UO2 powder source. Impurities can concentrate up to high amounts in grain boundaries due to local favourable atomic structure. On these locations the grain boundary oxidation rate can thus slow down more or less temporarily.

3.2.2 Influence of pelletdensity

Whatever is their initial density, the UO2 variants exhibit a very slow non-linear oxidation in the early 250 min followed by a fast linear mass increase and roughly the same final weight change (see Figure 7). In the domain investigated, the pellet density doesn’t play a fundamental role in the fuel oxidation.

0.0001 0.001 0.01 0.1 1

1 10 100 1000 10000

Time (min)

Weight gain (%)

ex-DC powder ex-ADU powder ex-AUC powder

FIG. 6. Influence of the powder source on the oxidation behaviour of UO2 samples (Ar+0.01% O2 — 380 °C).

0 1 2 3 4

0 500 1000 1500 2000 2500

Time (min)

Weight change (mg/cm²)

d = 94.6% TD d = 96.5% TD

FIG. 7. Influence of pellet density on the oxidation behaviour of UO2 samples (Ar+0.01% O2 — 380°C).

The ceramographic examinations performed after testing indicate that the behaviour is again an intergranular oxidation mode. XRD examination of the material spalled and scrapped-off from the surface revealed three phases: U3O8, U3O7 and UO2 phases (see Figure 8). This is consistent with the broad consensus for which the oxidation of uranium dioxide below 400 °C is a two-step reaction [13]:

UO2 Š U4O9/U3O7 Š U3O8. Moreover, on the microphotographs, the different oxide phases can be identified by colour variation: dark U3O8 layer on the outer surface, bright U4O9/U3O7 as intermediate layer and the grey UO2 meat. Bringing together the phase identification and thermogravimetric records indicated that the formation of intermediate U4O9/U3O7, i.e. UO2+x corresponds to the initial non-linear oxidation before intergranular cracks formation [14].

90

FIG. 8. Identification of phases involved during the oxidation of UO2 (Ar+0.01% O2 — 380 °C).

3.2.3 Influence of chromia doping

From the thermogravimetric records of Figure 9 it appears that chromia doping distinctively enhances the resistance of fuel pellets against oxidation. Similar to trends observed above with non-doped fuel, increasing the density of doped pellets leads to some limited improvement. On the other hand, it appears that the matrix grain size has a more decisive effect on the oxidation behaviour. This is clearly revealed by the result obtained for the doped sample having a grain size of 26 µm which exhibits roughly a similar behaviour to non-doped UO2. On the contrary, 33% is gained on the resistance to oxidation with an average grain size of 55 µm what is typical of the AREVA 0.16 wt% Cr2O3-doped UO2.

0 500 1000 1500 2000 2500 3000

Time (min)

0 500 1000 1500 2000 2500 3000

Time (min)

FIG. 9. Influence of chromia doping on the oxidation behaviour of UO2 samples (Ar+0.01% O2 — 380

°C).

The ceramographic examinations on Figure 10 reveal different features in the oxidation modes of doped and non-doped samples. For pure UO2 pellets, the surface oxidation is followed by intergranular cracking and later spallation of oxidized grains [15]. No intergranular cracks are formed with the chromia-doped samples and the oxidation proceeds by a surface mode. Here the oxidized layers offer a certain protection for oxygen diffusion and decelerate so the oxidation rate. However, it is still not clear, why the initial oxidation period is short with the high dense and large-grained doped fuel compared to standard UO2. This goes contrary to general understanding for which longer induction times are observed with doped fuels due to a lower rate of U3O8 formation [14]. In the case of the chromia-doped fuel, the total thickness of the attacked layer is thinner by a factor up to 2.5 compared to the standard pellets. A separation of the individual grains did not occurred in the doped UO2, but the stresses induced by the lattice changes on the pellet surface were released in the form of a few deep cracks. Close to the pellet surface, big pores due to calibrated poreformer addition seem to promote those cracks. However, their contribution to the oxidation is limited to the nearest region of the crack and the surface.

(top: after 20h bottom: after 40 h)

Standard UO2 pellets Chromia-doped UO2 pellets

FIG. 10. Oxidation modes of Cr2O3-doped and non-doped UO2 samples tested under Ar+0.01% O2 at 380 °C (×300).