Effect of Low DH Concentration on Crud Deposition on Heated Zircaloy-4 in Simulated PWR Primary Water

In order to mitigate PWSCC initiation and propagation of Ni base alloy, Japanese PWR utilities desire to employ optimized dissolved hydrogen (DH) control operation in the near future. Prior to the application of the optimized DH control operation to PWR, the effect of DH concentration on the fuel crud deposition should be clarified. Crud deposition tests were carried out in 1200 ppm as B + 2.2 ppm as Li + 5 to 25 cm³.STP/kg.H₂O solutions at 325 deg. C under sub-cooled boiling and non-irradiated condition. The corrosion resistance of zircaloy-4 was also investigated. From the test results, it was revealed that the crud layer composed of NiFe₂O₄ and NiO was formed on zircaloy-4 fuel cladding. NiO was easy to form in the crud layer under the 5 and 7 cm³.STP/kg.H₂O condition. The amounts of deposited crud layer, boron contents incorporated into the crud layer and the corrosion resistances of cladding were not affected by DH concentration. (authors)