Oxidation of spent UO₂ fuel stored in moist environment

Spent fuel remnants, cut from BWR fuel rods were retrieved for microscopic investigations after 25 years of storage at ambient temperature. The storage atmosphere of the samples studied here was dry air for 10 years and a moist environment for the remaining 15 years. The comparison with earlier experiments conducted on samples that were stored in dry air for the full period demonstrates that moisture accelerates the oxidation process also at the low temperatures considered here. The composition of the atmosphere in contact with the fuel has been analysed by mass spectrometry and the microstructure of the samples has been investigated by optical and scanning electron microscopy. The microscopic observations evidence that grain boundaries are affected and different stages in grain boundary alteration could be evidenced. Oxidation first induces a weakened intergranular bond followed by increased sensitisation to chemical attack and finally decohesion and onset of bulk oxidation.