**6. Summary of conclusions/recommendations**

**Figure 17.** Growth rate of Zirconium-3.

**Maximum Temperature of**

84 Nuclear Material Performance

**Transient**

The lateral growth for the TREAT 4 in. with fuel elements was calculated for 600°C transient presented in the previous section and the growth is shown in **Figure 18**. Similar calculations were done for 500 and 400°C. The resultant growth for each transient is shown in **Table 7**.

**is Used**

**Temperature Range Where the TREAT Metal Loss**

**Lateral Calculated Growth,**

**mils**

**Table 7.** Calculated lateral growth of TREAT fuel elements.

**Figure 18.** Lateral growth of fuel element during a 600°C transient.

600 0.011096 500–600°C 500 0.000949 400–500°C 400 0.000041 <400°C

> An equation based upon Zircaloy-2 oxidation rate is developed from the literature and compared to literature data that covers the range of temperatures from room temperature to 1100°C. This equation is shown to be conservative when predicting the oxidation rate of Zircaloy-3 based on the Zircaloy-3 data available. This equation is recommended to conser‐ vatively predict oxide growth, metal loss, and remaining Zircaloy metal.

> An evaluation of the minimum temperature at which Zircaloy-3 transition between alpha and beta phase occurs concluded that 857°C is consistent and conservative with the data reported in the literature. The beta phase shows a rapid increase in oxygen inclusion in the grain boundaries with a corresponding increase in the brittleness of the Zircaloy-3. A safety limit of 820°C was chosen to avoid this undesirable behavior.

> An equation was developed that expresses the relationship between the conservatively calculated oxide growth and measured data on 1 in. Zircaloy-3 samples. This equation was

used to predict the actual oxide growth. A chart is provided on the use of Zircaloy-3 color to estimate oxide thickness in the range of interest for operation (<600°C).

A corroded cladding thickness of 14.45 mils is recommended as the limit below which fuel assemblies should be removed from service to ensure that fuel may be handled without damage. It was concluded that, based on historical data and conservative calculations, fuel assembly cladding would only develop another 1 mil of oxide for the next 35 years of operation if the cladding temperatures are maintained below 600°C. The total fuel growth was calculated for 150 transients conducted in TREAT. The total calculated growth was 0.36 mils. This is a value too small to measure accurately. It is recommended that the effects of cladding growth be evaluated if fuel assembly temperatures ever exceed 600°C.
