Effects of Ni, Fe chemicals and hydrogen on deposits formed on Zirlo fuel cladding material in nuclear primary water chemistry conditions at 325 °C

• Deposit formation was tested by the use of a loop simulating axial offset anomaly.
• Porous deposits with whiskers are formed in the solution with Ni and Fe chemicals.
• Polyhedral structures are formed depending on the Ni/Fe ratio.
• Whiskers are observed at a hydrogen concentration below a critical value.
• Hydrogen concentration does not greatly affect the amount of deposit on the surface.

The morphology and amount of deposits formed on the surface of a Zirlo fuel cladding material were investigated in terms of the concentration of Ni, Fe chemicals and hydrogen after testing using a test loop simulating an AOA, since these deposits can have an influence on an AOA. Porous deposits with some needle-like whiskers were observed after testing in a solution containing 3.5 ppm Li as LiOH and 1500 ppm B as H3BO3. Ni and Fe acetates were added for this test intermittently to increase the deposits formed on the surface of the fuel cladding material. The pore sizes are from 0.1 to 0.3 μm. To minimize the pH change during the tests, Ni EDTA and Fe EDTA were used in the same solution instead of Ni and Fe acetates. Polyhedral structures were found, when both of Ni EDTA and Fe EDTA coexist significantly. These polyhedral structures are nickel ferrites analyzed by SEM–EDS. A number of whiskers with about a 0.05 μm diameter were detected, when the ratio of Ni/Fe increased due to decrease of Fe concentration during the test, even though the initial Fe concentration was high. However, some short whiskers are formed, when initial Ni concentration was high. These whiskers seem to be NiO by SEM–EDS. It was reported that the formation of fine whiskers might be one of the major causes for an AOA in connection with SNB. These whiskers were observed, when the hydrogen concentrations were 5 and 25 cc/kg H2O in the test conditions at 325 °C. No whiskers were found in the conditions at above 25 cc H2/kg H2O. In addition, it was found that the concentration of hydrogen does not greatly affect the amount of deposit formed on the surface during testing.