Seismic re-evaluation program of the Armenia nuclear power plant. Results from an international co-operation project

The Armenian Nuclear Power Plant, constituted by two WWER-440 type reactor, was shutdown in 1989 after the destructive Spitak earthquake (December 1988). Following the decision to re-start the operation of Unit 2 (April 1993), Armenian authorities launched a complete program for enhance the plant safety. A co-operation effort, financed by a number of different Organisations, among which EC through TACIS Program, IAEA and U.S. DOE, aiming at improving the seismic safety of the plant was undertaken in accordance to methods and criteria as generally accepted by the international community, taking into account that no regulatory guides exist on seismic re-evaluation of existing NPP. An important milestone of this co-operation effort was the conference hosted by IAEA in Vienna in 1999 among the donor Institutions and involved Organisations where a joint work-plan for the plant seismic re-evaluation, to be completed soon on the base of the results achieved, was agreed. On this basis different Organisations, with very different approaches to the assistance to developing countries (e.g.: bilateral, UN, EC, etc.), cooperated within the same framework in order to provide the best safety improvement to the plant with a global resource optimisation. As a result, the planned tasks are under implementation and safety review, with a very important contribution by the Armenian Nuclear Power Plant (ANPP) personnel which has been trained and involved since the very beginning of the project. The agreed technical approach among the different Organisations was codified in a IAEA Technical Guidelines Document, endorsed by both the ANPP and the Armenian Safety Authority (ANRA). In the above mentioned framework three main aspects were addressed: - re-assessment of the site geological stability and seismic hazard aimed to define the site specific maximum earthquake (Review Level Earthquake) to be used for plant re-evaluation purposes; - assessment of the seismic capacity of as-built structures, systems and components (SSC) essential to achieve the plant safe shutdown; - plant upgrading. The first aspect was developed in compliance with criteria established for new nuclear power plants (IAEA Safety Guides 50-SG-S1). Earthquake peak ground accelerations of 0.21 g and 0.35 g were obtained for two different fractile values, respectively 50% and 84%. For re-evaluation purposes a broader band ground response spectrum than the site specific one with a free field horizontal PGA of 0.35 g was chosen. A detail work plan for the seismic capacity assessment of SSC has been set-up. The seismic response evaluation of buildings and structures of Unit 2 is firstly performed to determine seismic forces and the in-structure input motion to systems and components. Seismic resistance of distribution systems and components is then assessed basically relying on the seismic margin assessment method. The scope of this safety re-evaluation process is determined by the Safe Shutdown Equipment List (SSEL) and plant walk-downs. The seismic re-evaluation of Medzamor NPP started in 1993 and since then different Organisations have been involved to provide the best safety improvement to the plant. To date a complete seismic hazard of the site was carried out and it was finalised in the assessment of Review Level Earthquake to be used in the re-evaluation process. The evaluation of the seismic response of the NPP Unit 2 and consequently the capacity of buildings and structures is presently underway and its completion is envisaged by the end of 2001. The results of the seismic response analysis together with the final development of the SSEL and plant walk-down evaluations will permit to carry out the seismic capacity assessment of systems and components whose development is envisaged in 2002. It is worth to mention that the availability of the above mentioned results will have an impact also on the implementation of safety related projects currently in progress. Potential modifications to SSCs will be prioritised for design and implementation, giving considerations to risk/safety, but also to economic and ease of implementation. Detailed criteria on this issue as well as design codes and quality assurance have still to be assessed, basing on an agreement between all involved parties regulator, operator and donor Institutions. (authors)