Experimental details 1 Experimental means

The test program consists of two parts characterizing first the oxidation behaviour of the fuel pellet samples by means of thermogravimetry under dry conditions and second, the corrosion behaviour by means of autoclave leaching tests of the fuel under representative conditions for in-reactor operation (pressure, temperature and oxygen potential).

2.1.1 Thermogravimetry

Thermogravimetry tests were carried out in a Netzch STA409C device under an Ar-oxygen atmosphere with well defined oxygen contents up to 40 h at 380 °C. The samples have been heated-up under pure Ar and then the desired amount of oxygen has been admixed to the gas stream. For each fuel variant, the absolute mass change and the kinetics of the mass change during a test have been evaluated and compared. The penetration depth of oxygen (reaction front) was determined by means of ceramographic examinations and the corresponding O-U phase was clarified by X ray diffractometry.

2.1.2 Autoclave testing

Data from the thermogravimetric study are not sufficient to describe the situation under wet conditions. Therefore, autoclave leaching tests have been performed to simulate a long-term exposure of the fuel pellets to the in-reactor environment in the case of a cladding failure.

The autoclave tests have been performed in a refreshing autoclave with a steel pressure vessel having a volume of 2.3 l and a water/steam flow rate of 1.5 kg/h (see Figure 1). All along the tests, the autoclave and feed water temperatures were monitored, as well as the autoclave pressure and the pH, oxygen and conductivity values in the feed water tank. De-ionized water has been used as the basis of the feed water.

The autoclave leaching tests included two runs under simulated BWR conditions –in water and in steam with very pure water chemistry and higher oxygen content and two runs under simulated PWR conditions –in water with boron and lithium additions and in steam at elevated temperatures for accidental case simulation. The typical BWR and PWR conditions under which the tests were carried out are summarized in Table 1. The samples have been exposed to these conditions for 6 days. After the test the samples have been dried at 60 °C in vacuum for 24 h and the absolute mass change was measured and compared. As for thermogravimetry, the penetration depth of oxygen was determined by ceramographic examinations and the evolution of fuel phases by X ray diffractometry.

FIG. 1. Schematic view of the autoclave testing equipment.

TABLE 1. TYPICAL BWR AND PWR AUTOCLAVE TESTING CONDITIONS Study case Pressure

(bar)

Temperature (°C)

Water chemistry

70 290 (water) 70 ppm H2O2

BWR

70 360 (steam) 70 ppm H2O2

180 360 (water) 650 ppm B by H3BO3, 2 ppm Li by LiOH (pH ~7.4) PWR

100 400 (steam) No additives

2.2 Fuel specimens

The AREVA chromia-doped UO2 fuel was developed in order to combine a large grain microstructure with improved viscoplastic fuel characteristics [7]. This goal is achieved with an optimum chromium oxide concentration of 0.16 wt% and fine tuned manufacturing conditions. The AREVA Cr2O3-doped UO2 fuel is characterized by a matrix grain size of 50 to 60 µm (linear intercept method). As a consequence of the grain growth activation, a better densification of the matrix occurs what allows reaching high densities as well as very stable UO2 fuel. Otherwise, no large Cr2O3 free particles are observed in the fuel matrix since the additive content selected is compatible with the solubility limit of chromium in UO2 [8].

Following the chromia-doped UO2 fuel technology, AREVA has also launched the development of Cr2O3-doped UO2 gadolinia fuel. At first, the influence of chromia doping on the sinter kinetic of (U-Gd)O2 was determined by means of dilatometric tests. Compared to pure UO2, the densification curves of the (U-Gd)O2 fuels are shifted to higher temperatures due to the formation of the (U-Gd)O2 solid solution at about 1100–1300 °C. This shift is all the more important than the gadolinia content is high.

Compared to non-doped (U-Gd)O2, significant enhanced densification behaviour is present with Cr2O3

doping. Whereas, the sintering begins ~ 100–200 °C later, a significant acceleration of the

densification rate takes place above ~ 1600 °C, in particular for chromia addition larger than 0.075 wt%. Such trends are typical of chromium oxide doping effect as already observed with UO2

fuel [9]. However, with very high Cr2O3 contents (> 0.3 wt%), detrimental effects are noticed:

decrease of fuel density due to solarization effect [10] and dopant precipitations. Figure 2 shows that no significant influence of the chromia doping on the U/Gd phase structure is observed and accordingly on the formation of (U–Gd)O2 solid solution. On the other hand for the higher Cr2O3

additions tested, an increase of dopant precipitates is seen. This reveals a clear excess of dopant with regard to its solubility limit into the fuel matrix.

Microstructural examinations of the samples show a significant development of the gadolinia fuel matrix polycrystalline structure from a chromia addition of 0.16 wt% with average grain sizes larger than 40 µm (see Figure 3). Considering these positive trends and by reference to the chromia-doped UO2 product, a doping level of 0.16 wt% Cr2O3 has been also selected for the gadolinia fuel.

Un-doped (U-Gd)O2 fuel Chromia doped (U-Gd)O2 fuel 0.16 wt%

Chromia doped (U-Gd)O2 fuel 1 wt%

Cr₂O₃-free particles free UO₂

FIG. 2. Cr2O3 doping effect on the U/Gd phase structure revealed by colour etching.

Standard (U-Gd)O2 pellets Chromia-doped (U-Gd)O2 pellets 50 µm 20 µm

FIG. 3. Microstructure of the Cr2O3-doped (U-Gd)O2 fuel in comparison to standard (U-Gd)O2 fuel.

The Cr2O3-doped gadolinia fuel thermal conductivity was determined by means of laser flash method up to 1500 °C under controlled atmosphere to avoid phase transformation. As commonly reported in the literature, it was found that the addition of Gd2O3 deteriorates the thermal conductivity of UO2

[11]. On the other hand, Cr2O3 doping has no significant influence on the thermal conductivity of the (U-Gd)O2 fuel within the margin of measurement uncertainty. Finally, the viscoplastic properties were characterized by creep testing means at high temperature (1500 °C) and compression stresses in the range of 30 to 60 MPa. Compared to non-doped (U-Gd)O2, doping with chromia allows enhancing distinctly the fuel plastic behaviour with an improvement factor up to 10 on the deformation rate.

This effect, also observed in case of chromia-doped UO2 fuel, is attributed to grain size enlargement what allows compensation of the matrix hardening which results from a solid solution formation [7].

The Cr2O3 addition leads to an increase in the density of mobile dislocations what provides a lower-stress resistance capability to the fuel and accordingly faster strains rates. As-fabricated data of all pellet samples tested in the present study are summarized in Table 2. For a better understanding of the mechanisms involved variants with and without dopant having different density, grain size characteristics or UO2 powder source have been prepared in the lab in addition to the industrially produced pellets. Allowing for exceptions, the chromia doping level is of 0.16 wt%.

TABLE 2. FUEL SAMPLE CHARACTERISTICS FOR THERMOGRAVIMETRY AND LEACHING TESTING

Pellet type Pellet density (% TD)

Pellet grain size^a (µm)

Remark

Non-doped UO2 94.6 10 DC powder

96.5 14 DC powder

97.0 12 DC powder

96.7 13 ex-AUC powder

96.7 19 ex-ADU powder

Cr2O3-doped UO2 97.1 57 DC powder

98.2 55 DC powder

96.8 28 ex-AUC powder — 0.07 wt%

Cr2O3

Non-doped (U-Gd)O2 95.9 25 DC powder — 6 wt% Gd2O3

Cr2O3-doped (U-Gd)O2 95.9 44 DC powder — 6 wt% Gd2O3 a Linear intercept method.