Thermally Induced Rock Stress Increment and Rock Reinforcement Response

The thermal heating caused by the deposition of spent nuclear fuel containers increases the in situ rock stress during disposal time. The thermal stress increase was modelled using thermo-mechanical modelling. The numerical codes used to establish the effects of heating on the in situ stress field are outlined, together with the rock mass parameters, in situ stress values, radiogenic temperatures and reinforcement structures. This is followed by a study of the temperature and stress evolution during the repository's operational period and the effect of the heating on the reinforcement structures. It is found that, during excavation, the maximum principal stress is concentrated at the transition areas where the excavation profile changes and that, due to the heating from the deposition of spent nuclear fuel, the maximum principal stress rises significantly in the tunnel arch area of NWISW oriented central tunnels. However, it is predicted that the rock's crack damage (CD, short term strength) value of 99 MPa will not be exceeded anywhere within the model. An additional study of the radiogenic heating effect on the brittle deformation zones is included. The main conclusion is that, despite deep reaching damage potential in all the load cases studied the currently designed and used reinforcement types and configurations (rock bolts, shotcrete) are capable of handling the dead weight of the damaged rock should this occur, with damage occurring on the shotcrete liner. The long term safety and stability of the repository during its lifetime can be guaranteed by perceiving the reinforcement strategy in two stages. Firstly, by installing the rock reinforcement to sustain the initial stresses and short term increases from the start of deposition with a monitoring programme in place. Secondly, by installing additional reinforcement, if found necessary through monitoring and observation of the underground facilities. In this way, the effect of any time dependent rock stress increase affecting the reinforcement structures can be observed, in addition to creep based damage, thus providing a better level of safety than a single stage design.