Seismic Behaviour of Large Rock Caverns Used for Small Modular Reactors
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Nuclear power is an alternative power source that accounts for 11% of the world s energy generation. While nuclear power has negligible CO2 emissions, there are several other concerns. Nuclear power also creates radioactive waste which is hard to store safely, and a number of severe accidents have questioned the safety of nuclear power. Locating nuclear power plants underground, using small modular reactors is an attractive option, as underground caverns can protect nuclear power plants for earthquakes and other natural hazards. From case histories and empirical correlations between earthquake parameters and damage states of tunnels after earthquakes, underground structures have been found to experience significantly less damage than surface structures. In the ground, earthquake waves travel as P-waves and S-waves. These waves can cause compression and tension along the length of the tunnel, ovaling of the tunnel cross section, and curvature deformation along the tunnel. An engineering approach to seismic analysis and to design a systematic approach can be followed: definition of seismic environment and determining seismic parameters for the analysis; evaluation of how the ground responses to shaking; and evaluation of how the structure (tunnel) responses to shaking. Earthquakes cause cyclic loading on both the rock mass and rock support. Cyclic loading can cause a degradation in shear strength of joints and cyclic tests on unreinforced and fibre reinforced concrete show that the strength reduces due to increasing numbers of cycles. From empirical studies, it is found that damage reduces with reduced magnitude and increases distance from the earthquake epicentre to the tunnel. Also, damage reduced when the depth of the tunnel is increased. Cracking of lining, such as longitudinal cracks, inclined cracks, and transverse cracks, have been found in tunnels after earthquakes. Heaving of tunnel invert has also been found as earthquake damage to tunnels.Numerical modelling was performed using the numerical modelling software Phase2. A proposed simplified design of underground caverns for small modular reactors was investigated for a fictitious, good quality rock mass. The results showed a maximum increase in maximum total displacement of 330% for a shaking of 0.5 g. The stress field changes around underground openings, and the axial force on the support lining is redistributed causing both increased tensile stresses and compressive stresses. Underground caverns were found to be an attractive option for earthquake resistant nuclear power plants, as the rock mass generally suffers less damage from earthquakes than surface structures. In addition, earthquake induced shaking reduces with depth which results in less damage to any installations in the caverns.