A 3D Geological Fault Model for Characterisation of Geological Faults at the Proposed Site for the Wylfa Newydd Nuclear Power Plant, Wales

Nuclear energy is an important part of a sustainable, economic and secure energy balance for the United Kingdom (UK). Horizon Nuclear Power commissioned Arup to provide seismic hazard assessment (SHA) consultancy services for the proposed Wylfa Newydd Nuclear Power Station in Anglesey, Wales, UK. Advanced Boiling Water Reactors are proposed to provide at least 5400 MW, enough to power around 10 million homes. The SHA comprises a probabilistic seismic hazard assessment of ground motion, a tsunami hazard assessment and a capable faulting assessment. The capable faulting assessment included thoroughly identifying and investigating geological faults at the site, with due consideration of the geological, seismological and tectonic setting. The objective of the capable faulting assessment was to demonstrate that faults are not capable of surface rupture within the current tectonic setting. Should evidence suggest that faults with the potential to affect the safety of the nuclear installation are capable, IAEA guidance dictates that an alternative site shall be considered. A project 3D Geological Fault Model was developed to characterise geological faulting at the site for the capable faulting assessment. A systematic methodology that incorporates expert judgment was developed and applied to the investigation and correlation of geological faults between site investigation data points. A total of 36 geological faults were mapped across the site and three fault sets were identified. Absolute age dating of the fault gouge on a representative selection of samples was subsequently undertaken to demonstrate that faults at the site are not capable in accordance with IAEA guidelines.