Status Report on Hydrogen Management and Related Computer Codes

In follow-up to the Fukushima Daiichi NPP accident, the Committee on the Safety of Nuclear Installations (CSNI) decided to launch several high priority activities. At the 14. plenary meeting of the Working Group on Analysis and Management of Accidents (WGAMA), a proposal for a status paper on hydrogen generation, transport and mitigation under severe accident conditions was approved. The proposed activity is in line with the WGAMA mandate and it was considered to be needed to revisit the hydrogen issue. The report is broken down into five Chapters and two appendixes. Chapter 1 provides background information for this activity and expected topics defined by the WGAMA members. A general understanding of hydrogen behavior and control in severe accidents is discussed. A brief literature review is included in this chapter to summarize the progress obtained from the early US NRC sponsored research on hydrogen and recent international OECD or EC sponsored projects on hydrogen related topics (generation, distribution, combustion and mitigation). Chapter 2 provides a general overview of the various reactor designs of Western PWRs, BWRs, Eastern European VVERs and PHWRs (CANDUs). The purpose is to understand the containment design features in relation to hydrogen management measures. Chapter 3 provides a detailed description of national requirements on hydrogen management and hydrogen mitigation measures inside the containment and other places (e.g., annulus space, secondary buildings, spent fuel pool, etc.). Discussions are followed on hydrogen analysis approaches, application of safety systems (e.g., spray, containment ventilation, local air cooler, suppression pool, and latch systems), hydrogen measurement strategies as well as lessons learnt from the Fukushima Daiichi nuclear power accident. Chapter 4 provides an overview of various codes that are being used for hydrogen risk assessment, and the codes capabilities and validation status in terms of hydrogen related phenomena (e.g., generation, distribution, combustion and mitigation). Chapter 5 summarizes the findings identified in Chapters 1 to 4 and discusses the remaining open issues. Appendix A presents the detailed descriptions of various containment designs. The main focus is to summarize the containment design features and systems to preserve containment integrity in case of an accidents, and in particular the description of systems foreseen for severe accident mitigation (e.g., hydrogen mitigation, containment venting). Appendix B includes detailed descriptions of various codes with focus on code capabilities and validation status in terms of hydrogen generation, distribution, combustion and mitigation. The present report is an adequate basis for reviewing SAM strategies for hydrogen management. It is recommended that assessment of the SAM strategies or guidelines as well as advantages and drawbacks of the various hydrogen mitigation approaches implemented by different countries be pursued as a follow-up activity