Diametral strain of fast reactor MOX fuel pins with austenitic stainless steel cladding irradiated to high burnup

Uwaba, Tomoyuki ; Ito, Masahiro*; Maeda, Koji  

The C3M irradiation test, which was conducted in the experimental fast reactor in Joyo, demonstrated that mixed oxide (MOX) fuel pins with austenitic steel cladding could attain a peak pellet burnup of 130 GWd/t safely. The irradiated fuel pins exhibited diametral strain due to cladding void swelling and irradiation creep. Contributions to the cladding irradiation creep strain were made by the pellet-cladding mechanical interaction (PCMI) as well as the internal gas pressure. From the fuel pin ceramographs and Cs scanning, it was found that the PCMI was associated with the pellet swelling which was enhanced by the rim structure formation or by cesium uranate formation. The PCMI due to cesium urinate, which occurred near the top of the MOX fuel column, significantly increased the cumulative damage fraction (CDF) of the cladding though the CDF indicated that the cladding still had some margin to failure due to the creep damage.