Safety Assessment Papers

 

In 2010, the International Atomic Energy Agency (IAEA) published its Safety Guide on Development and Application of Level 1 Probabilistic Safety Assessment for Nuclear Power Plants (i.e. SSG-3). Although it was aimed at covering state-of-the-art recommendations for the development of PSA of Nuclear Power Plants (NPP), the guidance was deemed to be applicable to other nuclear installations as well. In order to get insights regarding the applicability of SSG-3 for a research reactor, and at the same time to achieve the goal of building competence and capacity for PSA in Malaysia, in December 2012, the IAEA started an extra budgetary project entitled “Applying PSA to Existing Facilities to Develop Transferable Skills in the Use of PSA to Evaluate NPP Safety.” This project has been funded by Norway. The facility selected for the PSA study was the TRIGA Puspati Research Reactor (1 MW) which has been in operation in Malaysia since 1982. All major PSA tasks have been performed in accordance with the recommendations provided in SSG-3 (e.g. initiating events analysis, systems analysis, component data evaluation, human reliability analysis, etc). The design specifics of the research reactor under consideration have been addressed in the PSA model (e.g. four end states have been defined, detailed consideration of initiating events induced by human errors, several operational states, etc.). The paper provides an overview of the methodology applied and discusses specific features of PSA tasks for the research reactor. Preliminary results and insights obtained are presented. In addition, insights for guidance in developing a research reactor PSA are highlighted in the paper.

The vast majority of Level-1 Probabilistic Safety Assessment (PSA) studies for operating Nuclear Power Plants (NPPs) were completed within the last twenty years. The experience shows that PSA projects for NPPs represent major commitments in terms of resources and time. Many PSA specialists developed their skills and knowledge through the performance of practical work on PSA modeling within those PSA projects. Practical experience in PSA performance is deemed to be very important for building an in-depth understanding of PSA results and insights that are necessary prerequisites for consistent use of PSA, by both the utility and the nuclear safety regulatory body. However, currently there are few opportunities for inexperienced engineers to receive on-the-job training and thereby develop their PSA modeling competence. To address this challenge, the International Atomic Energy Agency (IAEA) has developed, through an extra-budgetary project funded by Norway aimed at building competence and capacity for safety assessment in countries embarking on nuclear power  programmes, and successfully implemented a special PSA practical training program as a means of compensating for the lack of hands on opportunities to build competence in PSA through performance of real PSA studies. The training includes lecturing on PSA methodologies, followed by practical training in performance of a PSA for a simplified NPP design using commercial PSA software. The developed model enables training in PSA concepts, such as minimal cutsets, importance measures, etc. As a result of this simulation-type training, the trainees obtain not only basic knowledge in PSA techniques as well as practical skills with the PSA software, but also a feeling of confidence that the development of a PSA is a manageable task and that the PSA represents a valuable safety analysis tool. The simulation-type training has been successfully conducted by the IAEA in training for three countries introducing nuclear power programmes, and the effectiveness of this training has been clearly demonstrated. The paper provides details of the training approach and discusses some relevant organizational issues to be noted, as well as outputs and outcomes.

The International Atomic Energy Agency (IAEA) through an extra-budgetary project funded by Norway has launched a research activity to develop a complementary analysis method and supporting tools to assist in a systematic review of the impact of potential extreme events on protection provided at a nuclear facility in terms of safety features. The developed method encompasses the principles of the ‘stress test’ being performed within the European Union and provides a possibility to assess the robustness of the protection of nuclear power plants (NPPs) and other nuclear installations against extreme events and their combinations considering sufficiency of defence-in-depth provisions, including various dependencies, safety margins, application of specific design features, cliff edge effects, multiple failures, prolonged loss of support systems and the capability of safety important systems for long term operation. The method utilizes the qualitative information obtained from Level-1 internal initiating events probabilistic safety assessment studies (e.g. minimal cut sets), information on the operability limits of structures, systems and components and feasibility of operator actions under different severe conditions caused by extreme events. An advantage of the new method in comparison to traditional safety analysis is the direct consideration of combined load conditions resulting from the simultaneous occurrence of extreme external events. The paper discusses the method, the features of the software tool developed by the IAEA to support the application of the method, as well as required input information and typical outcomes of the assessment and insights for safety. Some preliminary results of the application of the method and the software tool at NPP Goesgen-Daeniken are also discussed in the paper. The method and the software tool developed may be utilized through the existing IAEA’s Design and Safety Assessment Review Services.

It is internationally recognized that to develop a technical safety infrastructure for a nuclear power programme, a high level of safety assessment competence is needed for successful planning, licensing, construction and operation of new nuclear facilities. The International Atomic Energy Agency (IAEA) has established the Safety Assessment Education and Training (SAET) programme to support the development of this competence within Member States embarking on a nuclear power programme. Recipients of the SAET programme include national decision-makers as well as organizations developing/reviewing safety assessment documentation. The SAET programme includes a competence evaluation methodology to assist in the identification of training needs, a safety assessment syllabus, based on IAEA Safety Requirements, and the delivery of safety assessment training modules, which assists in the development of the required knowledge, skills and attitudes necessary to support a nuclear power programme. The SAET programme is delivered via a two tiered training approach, with essential knowledge elements needed by all experts involved in safety assessment, as well as more specialized training in practical applications and calculation methods for safety assessment analyses. The SAET programme has been developed for use by nuclear regulatory organizations, owners/operators of nuclear power plants, as well as technical and scientific support organizations (TSO’s), and has been successfully applied within an IAEA extra-budgetary programme, ensuring the harmonized applications of the IAEA’s Safety Standards.

The methodology for Probabilistic Safety Assessment (PSA) of Nuclear Power Plants (NPPs) was created in mid-70s of the last century and has been maturing within more than three decades. Currently, PSA studies are performed for practically all NPPs around the world. There are many guidelines and standards for PSA methodology (e.g. IAEA Safety Guides and TECDOCs, ASME/ANS PRA Standard). The PSA technology was judged to be well matured. However, the accident happened at Fukushima NPP in Japan in March 2011 highlighted some issues in the PSA methodology that need more emphasis or further development and/or adjustment. In addition, the application of PSA methodology requires a more rigorous control and independent review. These notes provide a summary of some major considerations in Level-1 and Level-2 PSAs that may require enhancement based on the preliminary lessons learned from the Fukushima accident regarding the impact of external hazards.