A probabilistic seismic hazard assessment for Wylfa Newydd, a new nuclear site in the United Kingdom

This study presents the probabilistic seismic hazard assessment (PSHA) for a nuclear site in the UK, Wylfa Newydd. The PSHA was defined at a bedrock horizon with shear wave velocity of 3000 m/s and developed to capture the centre, body and range of the technical defensible interpretations. Although not a formal SSHAC (Senior Seismic Hazard Assessment Committee) process, the PSHA met all the SSHAC Level 2 requirements with some aspects of SSHAC Level 3 and 4. Two base source models were developed: SM1 mainly driven by the observed seismicity and SM2 guided by the geology and tectonics. To capture epistemic uncertainty, variations of boundaries were included in the logic tree, resulting in five different models. Based on the selection discussed by Villani et al. (Bull Seismol Soc Am 109(4):1378-1400, 2019), four empirical models adjusted to the hard rock site conditions of Wylfa Newydd and a site-specific stochastic GMPE, developed within this project, were adopted in the logic tree for the median prediction. For all GMPEs a logic tree for the single-station sigma model was adopted. The PSHA resulted in a mean peak ground acceleration value for annual probability of exceedance of 10(-4) of 0.192 g. The comparison of the mean 10(-4) uniform hazard spectrum with the traditional piecewise linear design spectrum showed significant conservatism up to 20 Hz and underestimation at higher frequencies. To overcome this limitation, a performance-based design spectrum, following US practice (ASCE/SEI 43-05), was therefore recommended; this marks a significant difference to the traditional UK design practice.