The seismic assessment of British energy nuclear power stations and some pragmatic solutions to seismic modifications

British Energy owns and operates 8 Nuclear Power Stations in the United Kingdom. These include seven Advanced Gas-cooled Reactor (AGR) sites and one Pressurised Water Reactor (PWR) site at Sizewell B. As part of the site licence conditions the structures, plant and systems have to be maintained throughout their operational life in such a manner as to maintain their fitness for purpose and to carry out the role allocated to them by the reference safety case. A review, referred to as the Periodic Safety Review of the plant, is carried out every 10 years. The original design intent, changes to codes of practices and the effects of any ageing and/or deterioration are considered and any remedial action necessary is identified. The effects of external hazards are considered as part of that review. Many of the older stations were not designed for, and had never been assessed for the effects of earthquakes. As a consequence, major review work against site-specific seismic hazards has been carried out. In general, the seismic assessment of the plant, systems and structures relies on 'seismic walk-down' techniques, seismic qualification databases, similarity arguments, mathematical models and code of practice comparisons. These techniques are applied to the 'success paths' set out for the two lines of protection required under British Energy 'Nuclear Safety principles'. Where any of the above arguments result in plant modifications, these are implemented on site. The assessment process is described in this paper and some pragmatic solutions to the retrofitting of restraints to, amongst others, electrical cabinets, pipework, masonry walls, and tanks are discussed. Some novel techniques such as the use of structural adhesives are described in detail. (author)