An active facility management strategy for containment of intermediate level waste using risk-based service life modelling of reinforced concrete

BNFL Environmental Services are planning the on-site storage of Intermediate Level Waste (ILW) arising from historic operations of the Hunterston 'A' Nuclear Power Station, West Kilbride, Ayrshire UK within a shielded store. This paper proposes a proactive strategy of durability management for reinforced concrete structures used in long-term storage (at least 100 years) of nuclear waste. A critique is presented on the common approach of durability design using Standards compared with an 'active' performance-based risk approach. The active approach is thought to give greater flexibility to the decision-making process for management of concrete structures depending on the balance chosen between acceptable risk and maintenance philosophy. The management proposed uses a reliability-based risk approach to concrete degradation whose behavioural models are updated, with increasing accuracy, as new monitoring results from the structures are generated with time. Hunterston 'A' Nuclear Power Station, is in the process of being decommissioned. Operational wastes from the decommissioning process are to be stored in an on-site Intermediate Level Waste Store (ILWS) built in reinforced concrete and clad externally with aluminium that will create an environment to keep the waste packages in such a condition that they will be acceptable for final disposal. The facility will have a design life of at least 100 years. Probabilistic models have been developed for the durability design of the reinforced concrete structure subject to chloride- or carbonation-induced corrosion of the reinforcement. These have been used to confirm that the design will be durable for longer than the design life even under the most adverse environmental conditions. They have also shown that the recommendations of British Standards could lead to corrosion initiation well within the design service life under extreme conditions and would not provide sufficient margin for such an important structure. An integrated programme of inspection and maintenance is proposed with routine inspections anticipated every five years, with extended inspections due every 25 years. The data from the inspection testing will enable the models to be updated and allow statistical updating of input distributions using Bayesian techniques of inference for more accurate predictions of service life and a pro-active approach to future maintenance. In considering the implementation of the proposed monitoring regime, the site licensee will require to assess cost benefit and ALARP considerations when finally accepting an inspection and maintenance regime. The probabilistic approach affords the site licensee a greater understanding of the performance of the structure through time, allows timely intervention should the need arise and an improved ability to predict the residual life of the structure. (authors)