**2.4. Oxidation rates at higher temperatures and Zircaloy-4**

Although the TREAT design basis accident shows that the cladding temperature does not exceed 820°C, higher temperature data are also of interest. The above three correlations were extrapolated to 900°C and are shown in **Figure 6** along with other data.

**Figure 6.** Comparison of correlations and data in the high temperature range.

Hayes [19] made measurements on zirconium in moist air up to and including 900°C. Hayes was only able to obtain oxidation data at 900°C for 1 hour whereas his 500 to 800°C data were gathered over a 24 hour period. As shown in **Figure 6**, except for the 800°C data point, the oxidation rate was higher for zirconium oxidation in moist air than the Zircaloy-2 rates of Kendall. Other 900°C data include measurements at Argonne reported by Natesan [4] on bare Zircaloy-4 up to 900°C, which is also shown in **Figure 6**. Again, this is higher than the Zircaloy-2 data.

Hayes [19] concluded that the increase in the oxidation rate at 900°C is due to the change from alpha phase zirconium to beta phase zirconium at 862°C. He states "Microscopic examination of all specimens shows that there is no evidence of oxygen or nitrogen penetration into the body of the metal below 700°C. Short time exposures of less than 1 hour at 800°C produced only oxide surface layers and showed no evidences of further penetration. The specimens exposed for 1 hour and longer showed the intragranular rods and coarsened grain boundaries found in the oxygen series. In the 900°C series, it was found that the diffusion was extremely rapid. In one test with a thicker section, oxides were observed at the center of a quarter-inch (6.3 mm) sheet, which had been held at this temperature only 30 min. It appears probable that this sudden vulnerability to oxygen penetration can be ascribed to the change in volume that takes place at 862°C when the close-packed, hexagonal, alpha phase transforms to the bodycentered, cubic beta form."

This large increase in the rate is not seen for Zircaloy-4, but perhaps this one metal sample was not representative. The design basis reactivity insertion accident maximum temperature is 820°C and this is below the zirconium phase change of 862°C. Although zirconium data in moist air and the Zircaloy-4 data are higher than the 1955 Kendall Zircaloy-2 correlation, the experience with the TREAT cladding is that the Kendall correlation in the dry climate of the INL and the less reactive Zircaloy-3 cladding has conservatively bounded the oxide growth. If there has been any oxide growth so far on the fuel, it is less than the uncertainty in the measurements of 0.5 mils [20]. Based on the continued low oxide buildup reported in Mouring [21] and Kramer [20], the conservative method of estimating oxide buildup is sufficient with visual observation if the computed buildup is greater than 3 mils or if the temperature of a transient exceeds 600°C.

#### **2.5. Corrosion rate summary**

correlation which is used by TREAT to estimate the amount of oxide which has formed on the fuel does so conservatively. Note no transients performed so far have brought the fuel or

**Figure 5.** Comparison of the Kendall and Sandia correlations and Argonne Zircaloy-3 values.

extrapolated to 900°C and are shown in **Figure 6** along with other data.

Although the TREAT design basis accident shows that the cladding temperature does not exceed 820°C, higher temperature data are also of interest. The above three correlations were

Hayes [19] made measurements on zirconium in moist air up to and including 900°C. Hayes was only able to obtain oxidation data at 900°C for 1 hour whereas his 500 to 800°C data were gathered over a 24 hour period. As shown in **Figure 6**, except for the 800°C data point, the oxidation rate was higher for zirconium oxidation in moist air than the Zircaloy-2 rates of

**2.4. Oxidation rates at higher temperatures and Zircaloy-4**

**Figure 6.** Comparison of correlations and data in the high temperature range.

cladding above 600°C.

68 Nuclear Material Performance

In summary, to conservatively calculate oxide buildup on Zircaloy-3, use the oxide buildup rate equation for Zircaloy-2 which is

$$M\_{\text{gain}} = 8.5 \times 10^6 \text{ \* } t \text{ \* } e^{\left(\frac{-3.1 \times 10^4}{1.9872 \text{ \*} (T + 273)}\right)}\tag{15}$$

Where *M*gain is in mg/cm2 and *T* is temperature in °C and *t* in hours.

The relations between the weight gain due to oxide buildup, *M*gain, the metal loss thickness, and the oxide thickness,*TZrO*<sup>2</sup> , are:

$$T\_{\rm z} = 0.172 \, ^\ast \overline{M}\_{\rm gain} \, T\_{\rm zO\_2} = 0.253 \, ^\ast \overline{M}\_{\rm gain} \, T\_{\rm z} = 0.680 \, ^\ast T\_{\rm zO\_2} \tag{16}$$

Zirc Lost 0.680 Oxide Thickness <sup>=</sup>
