Long term thermomechanical stability of a spent fuel final repository located in granite at large depth

The research program at INR Pitesti includes among other issues, the item of final disposal of the spent fuel from Cernavoda NPP. The heat release by the spent fuel and high radioactive waste leads to an increase of the temperature around each canister and in the granite rock. A series of processes such as canister's corrosion are dependent on the temperature. On the other hand, the temperature distribution inside the repository affects the stress and deformation state in different zones of the repository and implicitly its stability. The present work reports the results of a 3-D thermomechanical analysis carried out by means of ANSYS code for the final disposal of CANDU spent fuel from the Cernavoda NPP. The repository proposed consists in 72 parallel tunnels drilled in the granite rock at a 500 m depth. The spent fuel canisters are placed in parallel wells located at a 6 m distance from tunnel axis. The 3-D distribution of the temperature increase inside the repository after placing the fuel containers and tunnel closing are presented. The peak temperatures reached at the container's center and on its surface amount about 134 deg. C and 116 deg. C, respectively. These temperature values were reached at 25 years after containers' disposal inside the repository. Also, the stress distribution directly after excavation and after 25 years from containers' disposal was evaluated. The highest values of the axial stress were obtained in the canister's vicinity and at the tunnel ends. To evaluate the rock stability directly after excavation and at 25 years after containers' disposal the Hoek and Brown braking criterion was used