Seismic Fragility Analysis of Containment Structure of Nuclear Reactor

Abstract

Containment structure is the most important safety related civil engineering structure of the nuclear power plant (NPP), as it forms the ultimate barrier for release of radioactivity to the surrounding environment in the event of a radiation accident. It is a leak tight structure and houses the reactor and other systems connected with nuclear steam supply, steam generation, and also provide structural support to various structures and equipment housed within it. Structural configurations for containment system have evolved over the years. Adopted structural configurations include single containment system is prestressed or reinforced concrete, and double containment system with prestressed inner and reinforced concrete outer containments. Be it prestressed or reinforced concrete, containment structure generally consists of a cylindrical wall capped by a hemispherical/segmental dome. The structure is supported on a reinforced concrete raft. Present regulatory requirement warrants that the, containment structure of NPP be designed for two levels of earthquake viz. operating basis earthquake and safe shut down earthquake. However, the older generation reactors were not designed for such a requirement. The regulatory requirements for seismic design of NPP have evolved over the years. Also the methods for evaluating the seismic hazard of a site have improved considerably. In such a situation to ascertain the safety of older generator nuclear reactors with respect to current regulatory requirements, seismic reevaluation is assorted to. Generally, seismic reevaluation is performed by either Seismic margin assessment (SMA) or Seismic probabilistic safety analysis (PSA). Evaluation of seismic fragility of structures, systems and components is an integral part of seismic PSA. Seismic fragility of structures, systems and components (SSC) is defined by the curve that gives conditional probability of failure for a given value of seismic input motion parameter. Seismic fragility curves are developed for different possible failure modes of the SSC. In the present work, it is proposed to evaluate the seismic fragility of containment structure of a nuclear reactor. Major activities envisaged during the IV work includes review of literature to understand the different methods available for deriving seismic fragility of structures and possible failure modes of containment structure under seismic excitation, 3-D finite element modeling of containment structure, linear seismic analysis of containment structure using response spectrum method and derivation of fragility curves by methods available in published literature.