Radionuclides in the fuel rod cladding

In light water reactors, Zircaloy is commonly used as the fuel rod cladding material, a zirconium alloy with various metallic constituents. Pressurized water reactors use Zircaloy-4 (Zry-4), while in boiling water reactors Zircaloy-2 (Zry-2) is the preferred cladding material the compositions of both alloys are shown in Table 1.2. In German PWRs the mass of Zircaloy amounts to about 290kg/Mg HM (heavy metal), in BWRs to about 320 kg/Mg HM (including the fuel assembly channels). 

The oxide layers mentioned above are generated by oxidation of the Zircaloy base material by high-temperature water or steam. They must not be confused with the layers formed by deposition of corrosion product oxides originating from the surfaces of the other primary circuit components and structures these corrosion 

As can be seen from Table 3.11., in a typical LWR fuel rod about one-half to two-thirds of the produced by ternary fission is released from the fuel to the Zircaloy cladding. The resulting axial profile in the cladding is virtually parallel to the y-scan data this means that the relative fraction of total production 

In the course of the dissolution of irradiated Zircaloy cladding specimens in mineral acids, the present is liberated almost quantitatively as HT (Goode and Cox, 1970 Neeb et al., 1980), indicating that the chemical form of in the metal is dissolved hydrogen or ZrH. Small fractions of HTO observed in these experiments are probably due to secondary chemical reactions which occur during the dissolution process. 

The probabilities and the rates of both reactions under the prevailing conditions are still not known and, therefore, it is not possible to identify the predominant mechanism. 

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