Life prediction of intermediate-level radioactive waste disposal vaults

The Argentine Atomic Energy Commission (CNEA) is the responsible for developing a management nuclear waste disposal programme. This programme contemplates the design and construction of a facility for the final disposal of medium level radioactive wastes (MLRW). The proposed model is the near-surface monolithic repository similar to those in operation in El Cabril, Spain. The design of this type of repository is based on the use of multiple, independent and redundant barriers. Since the vault and cover are major components of the engineered barriers, the durability of these concrete structures is an important aspect for the facilities integrity. This work introduces a laboratory and field investigation performed on reinforced concrete specimens in order to predict the service life of the MLRW from data obtained from electrochemical techniques. On the other hand, a full scale (1:1) container was constructed and instrumented in order to monitor the evolution of the rebars corrosion rate in time. From the present work, the following conclusions can be drawn: - The concrete made of sulphate resistant cement provides to steel rebars a passive status of corrosion characterised by corrosion rates lower than 0.1 pA.cm-2 (∼ 1 μm.year-1). - The values for the chloride effective diffusion coefficient (Def) and its surface concentration when reinforced concrete is exposed to extreme conditions (in contact with sea water), are low enough and the carbonation ratio is also adequate to comply with the foreseen specifications. - To guarantee a 300 years service life, the rebars corrosion rate on medium level radioactive waste disposal containers, must be lower than 0.01 μA.cm2 (0.1 μm.year-1). The measured data show that this durability criterion is fulfilled. - The electric resistivity of concrete has a tendency to increase with time, which reflects the continuous curing process showed by concrete at an early stage. - In order to fully evaluate the behaviour of concrete durability from the rebar corrosion point of view, after the selection of the site for the near surface disposal facility was made, the medium and long term evolution of the electrochemical parameters under study must be followed. (authors)