Treatment of natural external hazards in the probabilistic safety assessment of a nuclear submarine refit programme

摘要

The Royal Navy's fleet of Vanguard Class nuclear submarines are undergoing a major programme of refit and overhaul at Devonport Dockyard, Plymouth. Implementation of the programme is subject to the approval of a safety case by military and civil regulators. In support of the safety case, WS Atkins Consultants Ltd. (Atkins) has undertaken a Probabilistic Safety Assessment (PSA) of the parts of the refit programme relevant to the refuelling of the submarines' nuclear reactors. This paper outlines the Probabilistic Safety Assessment (PSA) modelling work undertaken by Atkins and DML for the evaluation of natural external hazards. The Natural External Hazards Assessment (NEHA) is required in support of the refit programme and contributes to the 9 Dock LOP(R) pre operational safety case report (POSR). The natural hazards assessment (NEHA) has subjected the following potential natural hazards to detailed assessment: seismic, extreme tide, high wind, lightning strike and extreme temperature events. The NEHA has been fully incorporated into the pre operational safety case PSA model. The ability of the Vanguard class submarine and associated 9 Dock facilities to withstand the above set of natural hazards has been assessed. For all hazards the PSA assessment considers hazard induced failures of the dock facilities and for the seismic hazard only, dock facilities and submarine components. Traditionally, natural hazards are represented in a PSA at a design basis event (DBE), for example at a 10~(-4)/yr event and, in the case of seismic hazards, an additional seismic margin event (SME) (e.g. at 40% above DBE). This paper presents a novel "decade step" approach, in which the robustness of the dock facilities are examined for specific hazard events with return frequencies that decrease in decade steps e.g. seismic events with return frequencies of 1E-02, 1E-03, 1E-04, 1E-05, 1E-06 and 1E-07. As the submarine is being refitted in a Dockyard facility a wider range of hazards could potentially affect the submarine than a well sited civil nuclear facility. A decade step approach would therefore enable a comprehensive assessment of the risks and robustness of the facilities against the hazards at different hazard intensities and frequency levels.