The approaches presented in short in the previous section were discussed by their proponents and other experts at a dedicated IAEA technical meeting, convened on 12-16 June 2006 in Vienna (Austria) with experts from interested Member States and international organizations — Argentina, Brazil, China, France, India, Italy,

Japan, the Russian Federation, the USA, and the European Commission as an observer. In the conclusions to this meeting, it was noted that the APSRA and the RMPS methodologies are complementary in the following:

• APSRA incorporates an important effort to qualify the model and use available experimental data. These aspects have not been studied in the RMPS, given the context of the RMPS project;

• APSRA includes, within the PSA model, failure of those components which cause a deviation of key parameters resulting in a system failure, but does not take into account the fact that the probability of success of a physical process could be different from unity;

• RMPS proposes to take into account, within the PSA model, failure of a physical process. It is possible to treat such data, e. g., the best estimate code plus the uncertainty approach is suitable for this purpose;

• In fact, two different philosophies or approaches have been used in the RMPS and in the APSRA and the two developed methodologies are, therefore, different. At the same time, proponents of the RMPS conclude that certain parts of the APSRA and the RMPS could be merged in order to obtain a more complete methodology.

During the IAEA technical meeting mentioned above — and after it — several other distinct approaches for reliability assessment of passive safety system performance were noted [14, 15], and the consensus was that a common analysis and test based approach would be helpful to the design and qualification of future advanced nuclear reactors. The inclusion of tests appears to be a must for new designs of passive systems and, especially, when non-water-cooled reactors are considered, for which validated codes and sufficient data for validation of the codes might be a priori not available. The approach itself is expected to streamline and speed up the process, and improve the quality of validation and testing of passive safety system performance.

Reflecting on these developments in Member States, the IAEA is implementing a CRP on Development of Methodologies for the Assessment of Passive Safety System Performance in Advanced Reactors in 2008­2012. The objective is to determine a common method for reliability assessment of passive safety system performance. Such a method would facilitate application of risk informed approaches in design optimization and safety qualification of future advanced reactors, contributing to their enhanced safety levels and improved economics.

In addition to the above discussed topics, it will likely be necessary to confirm that over a plant’s lifetime passive safety systems retain the capability to perform safety functions as designed. As it has already been mentioned, such confirmation would be facilitated if possible ageing effects on passive safety systems are considered in plant design and if passive safety systems are designed with a provision for easy in-service inspection, testing, and maintenance. In addition to this, new approaches might be needed to perform this confirmation, different from those used with active safety systems. One possible approach to deal with this issue is outlined in a short paper contributed by D. C. Wade of the Argonne National Laboratory (USA), enclosed as Appendix II.

REFERENCES TO APPENDIX I

[1] INTERNATIONAL ATOMIC ENERGY AGENCY, Innovative Small and Medium Sized Reactors: Design Features, Safety Approaches and R&D Trends, IAEA-TECDOC-1451, IAEA, Vienna (2005).

[2] INTERNATIONAL ATOMIC ENERGY AGENCY, Status of Innovative Small and Medium Sized Reactor Designs 2005: Reactors with Conventional Refuelling Schemes, IAEA-TECDOC-1485, IAEA, Vienna, (2006).

[3] INTERNATIONAL ATOMIC ENERGY AGENCY, Status of Small Reactor Designs Without On-site Refuelling, IAEA-TECDOC-1536, IAEA, Vienna, (2007).

[4] INTERNATIONAL ATOMIC ENERGY AGENCY, Status of Advanced Light Water Reactor Designs, IAEA-TECDOC-1391, IAEA, Vienna (2004).

[5] INTERNATIONAL ATOMIC ENERGY AGENCY, Advanced Nuclear Power Plant Design Options to Cope with External Events, IAEA-TECDOC-1487, IAEA, Vienna (2006).

[6] INTERNATIONAL ATOMIC ENERGY AGENCY, Safety Related Terms for Advanced Nuclear Plants, IAEA-TECDOC-626, IAEA, Vienna (1991).

[7] INTERNATIONAL ATOMIC ENERGY AGENCY, Natural Circulation in Water Cooled Nuclear Power Plants. Phenomena, Models and Methodology for System Reliability Assessments, IAEA-TECDOC-1474, IAEA, Vienna (2005).

[8] AMERICAN SOCIETY OF MECHANICAL ENGINEERS, Standard for Probabilistic Risk Assessment for Nuclear Power Plant Applications, ASME RA-S-2002, ASME, New York (2002).

[9] INTERNATIONAL ATOMIC ENERGY AGENCY, Development and Application of Level-1 PSA for Nuclear Power Plants, IAEA, Vienna (2007).

[10] INTERNATIONAL ATOMIC ENERGY AGENCY, Proposal for a Technology-Neutral Safety Approach for New Reactor Designs, IAEA-TECDOC-1570, IAEA, Vienna (2007).

[11] GENERATION-IV INTERNATIONAL FORUM, A technology roadmap for Generation-IV Nuclear Energy Systems, US Department of Energy, Nuclear Energy Research Advisory Committee, Washington, DC (2002).

[12] MARQUES, M. et al., Methodology for the reliability evaluation of a passive system and its integration into a Probabilistic Safety Assessment, Nucl. Eng. Des. 235 (2005) 2612-2631.

[13] NAYAK, A. K., et al., “Reliability analysis of a boiling two-phase natural circulation system using the APSRA methodology”, paper 7074. Proc. ICAPP’07, Nice, France, 13-18.

[14] DELANEY, M. J., APOSTOLAKIS, G. E., DRISCOLL, M. J., Risk Informed Design Guidance for Future Reactor Systems, Nucl. Eng. Des. 235 (2005) 1537-1556.

[15] BURGAZZI, L., State of the Art in Reliability of Thermal-Hydraulic Passive Systems, Reliab. Eng. Sys. Saf. 92 (2007) 671-675.