14 C content in CANDU spent fuel cladings and its release under alkaline conditions

The total 14 C content and its partition between inorganic and organic species were measured on irradiated Zy-4 samples from a CANDU spent fuel rod transferred from Cernavoda Nuclear Power Plant (NPP). Long-term leaching tests and accelerated corrosion tests were carried out to measure the 14 C release and corrosion rate, respectively, in chemical conditions relevant to cementitious environment. Experimentally measured 14 C inventory was compared to the theoretically one predicted based on the irradiation history and impurity content of Zy-4 by means of ORIGEN computations. CANDU SF claddings have a 14 C content of around 2 × 10 4 Bq/g of Zy-4, mainly as organic compounds (more than 99%). The total 14 C content measured by acid dissolution/wet oxidation method is in good agreement with the value estimated by ORIGEN simulations for an average burn-up of 7 MWd/kgU. The total 14 C released as dissolved species after 18 days and 18 months of Zy-4 immersing in alkaline solution are similar, indicating that a small amount of 14 C was available as instant release fraction (0.05% from the initial 14 C content) followed by a very low release rate that could not be measured by liquid scintillation counting. In alkaline conditions, 14 C is released predominantly (∼70%) as soluble species, but also inorganic 14 C was measured as gaseous species. From the soluble 14 C released during leaching test, more than 60% was found to be as organic species. Generally, corrosion rates values ranging between 46 and 130 nm/yr were measured by the linear polarization resistance method. In addition, defects and cracks were observed on the oxide layer by scanning electron microscopy (SEM) investigation.