**3. Maximum recommended operating temperature for Zirconium-3**

The maximum cladding oxidation which is allowed in a light water reactor (LWR) is 0.17 times the total cladding thickness. This limit is imposed to prevent the cladding from becoming brittle, which occurs due to the incursion of hydrogen and oxygen into the grain boundaries in the unreacted Zircaloy-2 cladding. Oxygen incursion into the remaining metal is also a problem with the oxidation of Zircaloy-3. This problem is circumvented by limiting the maximum cladding temperature. Lustman [12] states that: "below 900°C no evidence was found for the penetration of oxygen or nitrogen into zirconium. Above the alpha-to-beta phase transformation the diffusion of oxygen into the metal was rapid." Alpha zirconium transforms to beta zirconium above 862°C as reported in Hayes, 1949. Both confirm that oxygen does not diffuse into zirconium below 862°C. So, the zirconium which remains under the oxidation layer will not become brittle as long as it remains below 862°C. Note, there is some variation in the literature on the alpha-to-beta temperature (for example, Lyman [22] reports 872°C). All, however, are equal to or above 862°C.

The zirconium transformation temperature is changed when it is alloyed into Zircaloy-3. The major additives in Zircaloy-3 are reported in Gibbons [1] as 0.2–0.3% Sn and 0.2–0.3% Fe. The only other trace element is a maximum of 0.05%. Lyman [22] and Hanson [23] report an increase in the transformation temperature with tin (Sn) concentration so that the tin addition does not lower this temperature. Hanson [23] shows a decrease in this temperature when Fe is added. A portion of the phase diagram boundary is shown and magnified in **Figure 7**. It is seen that with 0.3% addition of iron, the transformation temperature remains above 857°C.

Therefore, the transformation temperature for Zircaloy-3 is 857°C or above. Since Lustman [5] states that no penetration of oxygen or nitrogen was observed below 900°C for exposure up to 6 hours, this penetration is assumed to be very small between 850 and 900°C. Taking all of the above information into account leads to the conclusion that as long as the maximum limiting temperature is selected at or below 900°C, the disintegration of the metal due to nil ductility will not be a problem nor will there be a rapid increase in oxidation rate over that described above.

#### Oxidation, Embrittlement, and Growth of TREAT Zircaloy-3 Cladding http://dx.doi.org/10.5772/62708 71

**Figure 7.** Partial phase diagram for the Zr Fe binary alloy.

2 2 gain gain = == 0.172 \* 0.253\* 0.680 \* *T MT MT T Zr ZrO Zr ZrO* (16)

Zirc Lost 0.680

Oxide Thickness <sup>=</sup>

**3. Maximum recommended operating temperature for Zirconium-3**

however, are equal to or above 862°C.

70 Nuclear Material Performance

described above.

The maximum cladding oxidation which is allowed in a light water reactor (LWR) is 0.17 times the total cladding thickness. This limit is imposed to prevent the cladding from becoming brittle, which occurs due to the incursion of hydrogen and oxygen into the grain boundaries in the unreacted Zircaloy-2 cladding. Oxygen incursion into the remaining metal is also a problem with the oxidation of Zircaloy-3. This problem is circumvented by limiting the maximum cladding temperature. Lustman [12] states that: "below 900°C no evidence was found for the penetration of oxygen or nitrogen into zirconium. Above the alpha-to-beta phase transformation the diffusion of oxygen into the metal was rapid." Alpha zirconium transforms to beta zirconium above 862°C as reported in Hayes, 1949. Both confirm that oxygen does not diffuse into zirconium below 862°C. So, the zirconium which remains under the oxidation layer will not become brittle as long as it remains below 862°C. Note, there is some variation in the literature on the alpha-to-beta temperature (for example, Lyman [22] reports 872°C). All,

The zirconium transformation temperature is changed when it is alloyed into Zircaloy-3. The major additives in Zircaloy-3 are reported in Gibbons [1] as 0.2–0.3% Sn and 0.2–0.3% Fe. The only other trace element is a maximum of 0.05%. Lyman [22] and Hanson [23] report an increase in the transformation temperature with tin (Sn) concentration so that the tin addition does not lower this temperature. Hanson [23] shows a decrease in this temperature when Fe is added. A portion of the phase diagram boundary is shown and magnified in **Figure 7**. It is seen that

Therefore, the transformation temperature for Zircaloy-3 is 857°C or above. Since Lustman [5] states that no penetration of oxygen or nitrogen was observed below 900°C for exposure up to 6 hours, this penetration is assumed to be very small between 850 and 900°C. Taking all of the above information into account leads to the conclusion that as long as the maximum limiting temperature is selected at or below 900°C, the disintegration of the metal due to nil ductility will not be a problem nor will there be a rapid increase in oxidation rate over that

with 0.3% addition of iron, the transformation temperature remains above 857°C.

The brittleness of the cladding material was also investigated experimentally. The approach to nil ductility is accompanied by an increase in hardness. Microhardness tests on a control sample and on each of the above samples showed an average hardness increase of about 45 Knoop hardness points for the heated samples. There was no clear pattern established between the center and the edges of the samples. If oxygen had diffused into the surface metal, one would expect a significantly harder material at the edge than in the center [15].

The weld material was also tested to determine if there was any preferential oxide formation in this area. A section of a sample that contained a weld was bent on a 3/8-in. diameter mandrel after the 600°C thermal cycling test without cracking the weld or the oxide layer over the weld. Metallographic examination of the welded area of a sample heated at 600°C did not show any greater oxide penetration than in the adjacent areas of the base metal [15].

The value of 820°C is recommended as the safety limit based on the above discussion and data. This value is sufficient to allow margin for operation, but is below the zirconium phase change of 862°C. This is a temperature limit at which further accident analysis would predict to not be exceeded with a large degree of certainty. Data for highly oxidized samples at 800°C do exhibit embrittlement and these are discussed in the next section.
